# Glass

Venetian and German glasses and an oriental glass vase made of milk and opal glass, with burned-in gilding and paintings from the 16th, 17th and 18th centuries (photo by Ludwig Belitski, 1855)

Glass (from Germanic glasa "the shiny, shimmering", also for " amber ") is a collective term for a group of amorphous solids . Most glasses consist mainly of silicon dioxide , such as drinking glasses or window glasses ; These - mostly translucent - silicate glasses have by far the greatest economic importance of all glasses. Also amorphous solidified metals are glasses. Glasses made from organic materials are, for example, natural amber or many plastics such as acrylic glass . By cooling down very quickly from the liquid or gaseous state, almost any substance can be converted into a ( metastable ) glass. There is a very large number of glasses of various compositions which, due to their properties, are of economic or scientific interest. Because of the wide range of applications for glasses, there are also diverse techniques for their production and shaping. Many of these techniques are very old and - unchanged in their basic principle - are still being implemented industrially today.

Glass facade of the UNIQA Tower in Vienna

## definition

SiO 2 as a crystal: quartz crystal
SiO 2 as glass: quartz glass

Glass is an amorphous substance that is created by melting . Glass can also be made by heating sol-gel and shock waves . Thermodynamically , glass is called a frozen, supercooled liquid . This definition applies to all substances that are melted and cooled accordingly quickly. This means that in the solidification of the melt to the glass while crystal nuclei form, for the crystallization process , however, not enough time remains. The solidifying glass solidifies too quickly to allow the building blocks to be rearranged to form a crystal. To put it simply, the atomic structure of a glass roughly corresponds to that of a liquid. The transformation area , that is the transition area between melt and solid, is around 600 ° C for many types of glass.

Despite the undefined melting point, glasses are solids . However, they are referred to as "non-ergodic" in specialist terminology . That is, their structure is not in thermodynamic equilibrium . Many plastics , such as plexiglass , also fall into the glass category because of their amorphous structure and a glass transition , although they have a completely different chemical composition than silicate glasses. They are therefore often referred to as organic glass.

The difference between glasses and other amorphous solids is that when heated, glasses change to a liquid state in the region of the glass transition temperature , while non-glassy amorphous substances crystallize in the process.

From the observation of the properties of glasses and their structure, many attempts have been made to give a comprehensive definition for the term glass. The recognized glass scientist Horst Scholze carried out an evaluation of the most common attempts to define the term glass. G. Tamman defined the state of glass in 1933 as follows: “ The state of glass contains solid, non-crystallized substances. “, While ASTM 1945 defines“ Glass is an inorganic melt product that solidifies essentially without crystallization. " proposed. F. Simon gave a definition from a thermodynamic point of view as early as 1930: “ In the physicochemical sense, glass is a frozen, supercooled liquid. ". According to Scholze, all of these definitions have their merits, but also their weaknesses. The Tamman definition is too general and does not exclude silica gel , which is also a non-crystalline solid, as glass. The restriction of the ASTM definition to inorganic substances was assessed by Scholze as questionable, as some organic glasses are now known. A comprehensive definition was given by the Terminology Commission of the USSR : “ Glasses are all amorphous bodies obtained by supercooling a melt, regardless of their chemical composition and the temperature range of their solidification, and which, due to the gradual increase in viscosity, are mechanical Assume properties of solid bodies. The transition from the liquid to the glass state must be reversible. “The restriction of glasses to solids obtained from a melting phase is also questionable from today's point of view, since the sol-gel process can also produce amorphous solids or glasses. The special feature of the glassy state of matter goes so far that some researchers it as a "fourth state of aggregation looked between the solid and liquid."

## Classification of glasses

Natural glass: a moldavite ; the green color is mainly due to the iron oxide in the melted sand.
According to the genesis
In addition to artificial ones, there are also natural glasses: Obsidian and pumice stone are of volcanic origin, impact glasses and tektites are created by a meteorite impact , fulgurite by lightning strikes , trinitite by an atomic bomb explosion and the frictionite köfelsite by landslides. These glasses are made by melting sand . When exposed to a shock wave , a crystal lattice can lose its regulated structure and thus transform itself into an amorphous solid. Glasses created in this way are called diaplectic . This includes maskelynite , which is made from feldspar . Artificial glasses are mainly produced by melting raw materials in a wide variety of melting units. Another synthetic route for the production of glasses is the sol-gel process , with which thin layers or aerogels can be produced.
According to the type of "chemism"
Most of the glasses produced today are soda-lime glasses , which belong to the group of silicate glasses. All glasses in this group have in common that their network is mainly formed from silicon dioxide (SiO 2 ). Adding further oxides such as aluminum oxide or various alkali oxides results in the aluminum or alkali silicate glasses. The decisive factor for the classification is which oxide is the second most common in terms of quantity in the silicate base glass. A silicate glass without further components - i.e. pure SiO 2 - is referred to as silica or quartz glass. Due to its high chemical resistance and thermal resistance as well as the low coefficient of thermal expansion, it is often used in special technical applications. If phosphorus pentoxide or boron trioxide appears as the main network-forming agent in a glass , one speaks of phosphate or borate glasses, the properties of which can also be adjusted by adding further oxides. All of the glasses mentioned above consist largely of oxides, which is why they are collectively referred to as oxide glasses. If the anion of a glass is a halide ion , one speaks of halide glass or of a chalcogenide glass if it is mainly sulfur , selenium or tellurium as the anion in the glass network. These glasses are characterized by high transparency, far beyond the visible range of light, and are therefore used in infrared optics. In addition to these inorganic-non-metallic glasses, there are also organic glasses, for example amorphous plastics , which can be combined with the aforementioned non-metallic glasses and are opposed to metallic glasses. The boundaries between the individual types of glass are fluid and there are numerous sub-types. An example of this are the oxy-nitride glasses in which some of the oxygen ions have been replaced by nitrogen in order to create specific properties. As a result, this glass is to be understood as a hybrid between oxide and non-oxide glasses. Glasses that only consist of one component, i.e. the network builder, are called single-component glasses. The typical example of this is quartz glass. By adding further components, what are known as two-component glasses such as alkali borate glass or three-component glasses such as soda-lime glass are obtained. As a rule, glasses contain more than just three components, but only the main components are mentioned, since the glasses in these compositions are largely similar in terms of their properties and areas of application. The hierarchical relationship between the glasses is shown in the figure below.
Artful Roman slide glass
Goblet made of milk glass (cloudy glass)
According to the basic shape of the product and the production process
The glass industry is usually divided into hollow glass , flat glass and specialty glass production , although this simple breakdown does not cover all areas of the glass industry. Hollow glass usually refers to containers for food, such as bottles and canning jars. These mass-produced products are manufactured by machine using the press-blow or blow-blow process . Glass blocks and drinking glasses are only formed by a pressing process . High-quality products such as wine glasses are referred to as so-called tableware and are usually manufactured in a complex, multi-stage process. In contrast to the glass bottles, they are not produced with the help of IS machines , but so-called rotation blow molding machines. A special process is necessary for incandescent lamps , which is particularly characterized by the high production speeds of the ribbon machine . Tubular glass can be manufactured using various processes, which differ in terms of the different dimensions of the semi-finished product to be manufactured. Flat glass is called float glass or rolled glass, depending on the production process . The basic product is a pane of glass. End products are e.g. B. automotive glass , mirrors , tempered glass or laminated glass , which have been reworked in various ways. Applications in the form of fibers include fiber optics , glass wool and glass fiber reinforced plastic, as well as textile glass . Hand-blown glasses practically only exist in the arts and crafts, as well as in expensive vases and wine glasses.
Antique glass , diatret glass , optical glasses such as crown glass and flint glass (lead glass), hyalith glass (opaque glass, used in the 19th century for table and pharmaceutical glass), cryolite glass (opaque, white fluoride glass).
According to their brand names as a generic term
The brand name of a glass manufacturer has often become a collective term for various products from one or even several glass manufacturers. Ceran is very often used as a synonym for glass ceramics or hobs. Jenaer Glas often stands for all types of heat-resistant borosilicate glass. In the Anglo-Saxon region, the brand name Pyrex from Corning has become established for this type of glass.
After their use
The most important optical glasses for the production of lenses , prisms , mirrors and other optical components for microscopes , binoculars , objectives etc. are quartz glass , crown glass , flint glass and borosilicate glass . The glass-ceramic material Zerodur (Schott) is used as the substrate material for optical elements in astronomy and space travel . This has an extremely low coefficient of expansion and is therefore suitable, for. B. excellent as a mirror carrier for large astronomical telescopes . Another example is device glass as a generic term for all types of glass in the field of technical laboratory glass. A similar generic term for various processed glasses is architectural or building glass .

## properties

### structure

SiO 4 tetrahedra
Soda lime glass

Although glass is one of the oldest materials known to mankind, many questions about the atomic structure and its structure are still unclear. The now generally accepted interpretation of the structure is the network hypothesis , which was put forward by WH Zachariasen in 1932 and BE Warren experimentally confirmed in 1933. This means that the same bond states or basic building blocks must exist in the glass as in a crystal . In the case of silicate glass so the SiO 4 - tetrahedrons , but which form as opposed to a quartz crystal, a random network. In order to be able to predict the glass formation of further chemical compounds, Zachariasen established further rules in his network hypothesis. Among other things, a cation in a compound must be relatively small in relation to the anion . The polyhedra formed from the anions and cations may only be connected via their corners. If the compounds under consideration are limited to oxides, phosphorus pentoxide (P 2 O 5 ), silicon dioxide (SiO 2 ) and boron trioxide (B 2 O 3 ) meet these conditions for network formation and are therefore referred to as network formers.

As the two-dimensional images of the quartz and quartz glass show, the difference lies in the regularity of the atomic structure. With quartz, which is a crystal, there is a lattice structure - with quartz glass, on the other hand, there is a random network of SiO 4 tetrahedra arranged in a row . For the sake of clarity, the fourth oxide bond, which would protrude from the plane of the drawing, is not shown. The bond angles and distances in the glass are not regular and the tetrahedra are also distorted. The comparison shows that glass only has a short-range order in the form of the tetrahedron, but has no crystalline long-range order. This lack of long-range order results in a very difficult analysis of the glass structure. In particular, the medium-range analysis, i.e. the connections of several basic forms (here the tetrahedra), is the subject of research and is one of the greatest problems in physics today. This is due, on the one hand, to the fact that glasses are very difficult to access for X-ray examinations and, on the other hand, to the fact that the structure-forming processes sometimes begin in the melt, with the temperatures at hand making a precise examination even more difficult.

The material that determines this basic structure of the glass is called a network builder . In addition to the silicon oxide mentioned, other substances can also be network formers, such as boron trioxide and also non-oxidic substances such as arsenic sulfide . However, single-component glasses are very rare. This also applies to pure quartz glass, which is the only single-component glass that has economic significance. The reason for this are the extremely high temperatures (over 2000 ° C) which are necessary to generate it.

Other substances are integrated differently into the glass network of the network builders. A distinction is made here between network converters and stabilizers (or intermediate oxides).

Network converters are built into the framework formed by the network builder. For ordinary glass for everyday use - lime-alkali glass (but the narrower term lime-soda glass is more common ) - these are sodium or potassium oxide and calcium oxide . These network converters tear open the network structure. In the process, bonds of the bridging oxygen in the silicon oxide tetrahedra are broken. Instead of the atomic bond with the silicon, the oxygen enters into a significantly weaker ionic bond with an alkali ion.

Intermediate oxides such as aluminum oxide can act as network formers and converters, that is, they can strengthen (stabilize) a glass network or, just like the network converters, weaken the structures. Their respective effect in a glass is always dependent on a number of factors. However, intermediate oxides alone are not capable of glass formation.

### Transition from the melt to the solid glass

While in the case of crystalline materials the transition from the melt to the crystal occurs through slow cooling, in the case of glasses this process is so rapid that no crystal structure can form. The transition area from a melt to the glass is called the transformation area . In the course of cooling, the viscosity of the material increases significantly. This is the external sign of an increasing internal structure. Since this structure does not have a regular pattern, the state of the melt in the transformation area, like that of the solidified glass, is called amorphous . At the cool end of the transformation area there is a thermodynamic transition, which is characteristic of glass and therefore bears the name glass transition . The melt changes into the solid, vitreous state on it, which the glass retains even when it cools down further. The glass transition is characterized by a sudden change in the coefficient of thermal expansion and a decrease in the specific heat . This sequence of transformation area and glass transition is characteristic of all glasses, including those made of hydrocarbons like Plexiglas . The amorphous, viscous state of the melt in the transformation area is used for processing glass by glass blowing . It allows any deformation without surface tension and gravity causing the workpiece to melt immediately. ${\ displaystyle C _ {\ mathrm {p}}}$

### Physical Properties

property value unit
Density of a soda-lime glass 2500 kg / m³
Density of a heavy flint glass (SF59) 6260 kg / m³
Thermal conductivity of soda-lime glass 0.80 W / (K m)
Thermal conductivity quartz glass 1.38 W / (K m)
Zerodur thermal conductivity 1.46 W / (K m)
Electric conductivity up to approx. 600 ° C isolator
Thermal expansion soda-lime glass 9.0 · 10 −6 1 / K
Thermal expansion borosilicate glass 3.3 3.3 · 10 −6 1 / K
Thermal expansion quartz glass 0.57 · 10 −6 1 / K
Zerodur thermal expansion <0.1 · 10 −6 1 / K
tensile strenght 30th MPa
Compressive strength 900 MPa
Modulus of elasticity 70,000 MPa
Heat capacity 0.8 kJ / (kg K)
Transmission (physics) 0-100 %
Refractive index (see optical glass ) 1.5 to 1.9

The characteristic of glass in common parlance is its optical transparency . The optical properties are as varied as the number of glasses. In addition to clear glasses, which allow light to pass through a wide range , the addition of special materials to the melt can block the transparency. For example, this makes optically clear glasses impenetrable for infrared light , and heat radiation is blocked. The most well-known control of permeability is coloration. A wide variety of colors can be achieved. On the other hand, there are opaque glass that already due to its main components or the addition of opacifiers opaque is.

Utility glass has a density of approx. 2500 kg / m³ ( soda-lime glass ) . The mechanical properties vary greatly. The fragility of glass is proverbial. The breaking strength is largely determined by the quality of the surface. Glass is largely resistant to chemicals. Hydrofluoric acid is an exception ; it dissolves the silicon dioxide and converts it to hexafluorosilicic acid . Weathering, e.g. storage for decades in the ground, creates microscopic cracks on the surface of the glass, the so-called glass disease . Clear glass then appears cloudy to the human eye. At room temperature, soda-lime glass has a high electrical resistance , which, however, drops sharply with increasing temperature. Quartz glass (pure silicon dioxide that has solidified like a glass) is an insulator even at significantly higher temperatures. In addition to silicate glasses, there are also so-called metallic glasses such as Fe 80 B 20 , which already have higher conductivities at room temperature because they behave in a similar way to frozen liquid metals.

Because of its supercooled melt nature, glass can also flow to a very limited extent. However, this effect only becomes noticeable at higher temperatures. The frequent assertion that church windows are thicker at the bottom because the glass has flowed downwards through the force of gravity over the centuries is wrong; such flow processes would have taken millions of years at room temperature. The thickening is due to the production process at that time (cylinder bubbles).

## Production processes

### mixture

Quartz sand as a raw material
Insertion machine for a float glass plant

The soda-lime glass is the predominant mass of glass and approximately accounts for 90% of the glass produced worldwide from. Basically, this glass consists of silicon dioxide (SiO 2 ), sodium oxide (Na 2 O) and calcium oxide (CaO). In everyday glass, which still belongs to the family of soda-lime glasses, various other ingredients are added in order to optimize the properties and manufacturing conditions. Minor impurities in the raw materials, which are compatible with the normal quality requirements for utility glass, are also sources of other (unintended) glass components. In normal glass, such as is used for the production of colorless containers or flat glass, there are often certain amounts of aluminum oxide, Magnesium oxide and potassium oxide, which are deliberately added. Small amounts of iron oxides, titanium oxide and, for example, chromium (III) oxide can still be found in the glass due to impurities. The most common raw materials in mass glass production can be found in the following list:

• Quartz sand is an almost pure SiO 2 carrier for network formation. It is important that the sand may only contain a small amount of Fe 2 O 3 (<0.05%), since otherwise annoying green colorations will occur on white glass. This raw material makes up the largest part of the mixture with over 70% by mass and is one of the main sources of contamination.
• Soda (Na 2 CO 3 ) serves as a sodium oxide carrier, which serves as a network converter and as a flux and lowers the melting point of the SiO 2 . Carbon dioxide is released in the melt and dissolves from the glass as a gas. Soda is the most expensive raw material in the mass glass sector because it is hardly available as a naturally occurring mineral. Sodium can also be added to the melt as nitrate or sulphate ( sodium sulphate is a refining agent to reduce the bubble content).
• Potash (K 2 CO 3 ) supplies potassium oxide for the melt, which, like sodium oxide, serves as a network converter and flux.
• Feldspar (NaAlSi 3 O 8 ) adds not only SiO 2 and Na 2 O but also alumina (Al 2 O 3 ) to the mixture. This leads to an increase in chemical resistance to water, food and environmental influences.
• Lime serves as a network converter. During the melting process, it decomposes into carbon dioxide and calcium oxide. A moderate addition (10–15%) of CaO increases the hardness and chemical resistance of the end product.
• Dolomite is a carrier for CaO and MgO. Magnesium oxide has a similar effect on the melt as calcium oxide. However, an excessively high MgO content in the glass can undesirably increase the liquidus temperature and lead to devitrification .
• Old glass or cullet from broken production are also added back to the batch. Old glass from glass recycling is mainly used in the container glass industry, because today glass bottles consist on average of around 60% old glass, green bottles up to 95%, and in the production of glass wool , where their share is up to 80%. This saves raw materials and energy, as the cullet melts more easily than the mixture and the chemical reactions such as the decarbonation of soda, lime and dolomite no longer have to take place. Recycled cullet is another main source of contamination, as the sorting of colors poses problems when recycling old glass and other undesirable foreign substances such as metals, ceramics or special glasses can only have been insufficiently read out. The foreign substances cause glass defects due to incomplete melting or unwanted coloration of the glass and damage to the glass melting tank, as metals eat into the refractory floor.

Red lead , borax , barium carbonate and rare earths are also used for special glasses .

### melt

Doghouse of the melting tank with inserter

The glass melt can be divided into three phases:

• It begins with the rush melt, which includes the melting of the mixture and its homogenization.
• This is followed by purification , in which the gases are expelled.
• Finally, the refined melt is cooled to the desired molding temperature ("the glass protrudes").

In the case of batch -type day tubs and port furnaces, these steps take place one after the other in the same basin. Today, this historical production process only takes place in small quantities for handicraft production and special optical glasses. On an industrial scale, only continuously operating furnaces are used. Here the sequence of the above steps is not temporally but spatially separated, even if the individual sections flow into one another. The amount of batch added must correspond to that of the glass removal. The energy required to melt the glass can be provided by fossil fuels or electrical energy, by means of the passage of electricity through the melt.

The batch is fed into the melting tank with an inserting machine on the inserting front, the doghouse. Since the batch has a lower density than the glass melt, it floats on the melt and forms the so-called batch carpet. At temperatures of approx. 1400 ° C and more, the various components slowly melt. Some of the components of the mixture can together form eutectics and form the first melting phases even at significantly lower temperatures. The convection in the glass bath causes a continuous removal of material that is detached from the batch carpet. At the same time, it causes the mixture to warm up. It thus creates both thermal and chemical homogeneity of the melt. This can be supported by a bubbling , the injection of air or gases into the melt.

In the refining area, which immediately follows the melting area and is often separated from it by a wall in the melt, gases remaining in the melt are expelled. For this purpose, a so-called refining agent is added to the mixture beforehand. This refining agent decomposes at a certain temperature with the formation of gas. Due to partial pressure differences, gases from the melt now diffuse into the refining agent gas bubbles, which grow and rise as a result. In order to be able to carry out this process in economically justifiable times, the refining part of a glass melting tank has similarly high temperatures as in the melting part, because too high a viscosity of the melt would greatly slow down the rise of the gas bubbles. Since the refining is decisive for the quality of the glass, there are various supportive measures.

The structurally clearly separated working tub adjoins the lauter area . Since lower temperatures are required for shaping than for melting and refining, the glass must protrude beforehand, which is why the vessel is also called a holding tank . The channel that connects the melting tank and working tank is called the passage and works according to the siphon principle. In the case of flat glass tanks, the melting and working tanks are only separated by a constriction, as a passage would create an optical disorder in the finished product.

The glass flows from the working tub to the removal point, where it is then shaped. In the production of hollow glass, these are the risers or feeders . Drops are generated here, which are fed into glass machines below via a channel system . When producing flat glass using the float glass process, the glass flows over a lip stone into the float bath .

### Shaping

IS machine in bottle production

Glass is shaped differently depending on the product. Shaping takes place by pressing, blowing, centrifuging, spinning, rolling or drawing:

• Hollow glass is made in several processes by pressing, blowing, suction and combinations of these techniques. The IS machine , which works with the blow-blow or press-blow process, dominates here. For high-quality tableware, press-blow processes are used, which work in the shape of a carousel and are referred to as rotary or rotary blow molding machines.
• Continuous glass fibers are produced by spinning in the so-called TEL process
• Glass fibers for, for example, glass wool are produced by being thrown through a sieve
• Flat glass is mainlyproduced usingthe float process, but it can also be drawn, rolled or cast using various older processes. For some time now, manufacturers have been increasingly offering hand-blown flat glass, which is called antique glass (or cylinder glass, based on its manufacturing method).
• Tubular glass is produced using various continuous drawing processes, while large-format glass tubes are produced using a special centrifugal process.

### cooling

Tensions in glass made visible with the help of the birefringence of polarized light
Hollow glass production: canning jars after leaving the cooling track

In every glass object, mechanical stresses arise during shaping as a result of forced shaping or expansion differences in the material due to temperature gradients . These voltages can be measured geometrically with optical voltage testers under polarized light ( stress birefringence ). The susceptibility to stress depends on the expansion coefficient of the respective glass and must be thermally compensated.

A cooling area can be defined for each glass, which is limited by the so-called upper and lower cooling temperature . The position of these temperatures is defined according to the viscosity, so the upper cooling temperature is that temperature at which the glass has a viscosity of 10 12 Pa · s. At the lower cooling temperature there is a viscosity of 10 13.5 Pa · s. As a rule, the cooling range for most commercially used glasses extends between 590 ° C and 450 ° C. The stresses are reduced through tempering , i.e. through defined slow cooling, since with the prevailing viscosities, stress relaxation is just possible and permanent stresses in the glass body are avoided.

The time in which a glass object can pass through the cooling area depends largely on the temperature to be bridged and the thickness (thickness) of the object, depending on the type of glass. In the hollow glass sector, this is between 30 and 100 minutes; for large optical lenses with a diameter of 1 m and more, a slow cooling of a year may be necessary in order to avoid visible tension and thus image distortion of the lens. In the case of optical glasses, the cooling rate is the second important parameter for setting the refractive index or dispersion, after the chemical composition, and is therefore generally of particular importance in the production process.

There are two types of cooling units that can be used for relaxation cooling of glass objects: the intermittent cooling furnaces and continuously operating cooling tracks . In practice, however, there is usually no clear distinction between these two cases, for example the continuously operated cooling unit in the flat glass industry is often referred to as a roller cooling furnace.

Cooling furnaces are only suitable for special productions and very small batches, as the furnace has to be brought back up to temperature each time the workpieces are removed. Cooling tracks are used industrially. In the hollow glass industry, the glass objects are transported on steel mats or chain belts through the cooling path, while the continuous glass ribbon in the flat glass industry is transported through the cooling path by means of rollers. In front of the cooling tracks (regionally also called cooling belts), so-called draft ovens were used for medium-sized assortments. After the train in the furnace was filled with glasses, one car was driven out of the furnace and an empty car was driven in. The hot car was covered with insulated metal sheets and was allowed to cool slowly before it was emptied. Usually three car changes were carried out per shift.

The processes described so far can be summarized under the concept of relaxation cooling , i.e. the cooling of a glass body with the purpose of avoiding permanent tension. The reverse case can be seen in the thermal toughening of glass for the production of, for example, single-pane safety glass . The glass is cooled so quickly from a temperature above its transformation temperature that the thermally generated stresses can no longer be relieved. As a result, tensile stresses arise in the glass volume and compressive stresses in the glass surface, which are the cause of the increased strength and thermal shock resistance of the glass body.

### Surface finishing

Coating of a substrate with gold by sputtering
A disc artistically decorated by etching.

A surface refinement is created by applying layers or removing layers, as well as modifying the structure or the chemistry of the glass surface. The purpose of such measures is to improve the existing properties of a glass object or to create new areas of application for a glass object.

• By chemical and physical vapor deposition can fine metal coatings are applied. Most window and car glasses are provided with coatings that are impervious to infrared light in this way . The heat radiation is reflected and interior rooms are less heated by solar radiation. At the same time, heat losses in winter are reduced without significantly impairing transparency.
• The coating with dielectric material, which is itself transparent, but has a refractive index that differs from that of the glass substrate , enables both reflective and anti-reflective coatings . This is used in the manufacture of spectacle lenses and lenses for cameras to reduce annoying reflections . For scientific purposes, layers are made that reflect more than 99.9999% of the incident light of a certain wavelength. Conversely, 99.999% of the light can also pass through the surface.
• The surface can be roughened by sandblasting or with hydrofluoric acid so that the light is strongly scattered . It then appears milky and no longer transparent, but very little light is still absorbed. Therefore this technique is often used for lampshades or opaque windows (see also satin finishing ).
• The surface of a glass can also be acid-polished with hydrofluoric acid. The damaged surface layers are removed, which leads to a defect-free surface and thus an increased strength of the glass object.
• Another frequently used surface finishing method is the dealkalization of the glass surface. The reaction of the hot glass surface with aggressive gases (e.g. HCl or SO 2 ) forms salts with the alkalis from the glass, which are deposited on the glass surface. As a result, the alkaline-depleted glass shows increased chemical resistance.
• During the hollow glass production, a so-called hot and cold end coating is applied to the glass in two steps . These two types of remuneration are intended to prevent the glass bottles from damaging each other during production and later filling by reducing their coefficient of friction so that they slide past each other in the event of contact instead of scratching each other. Various tin and titanium compounds are used as layers for this.

### Quality control

In order to ensure the quality of the glass, extensive tests must be carried out regularly, including:

• Online control in the glassworks (optical checks of all individual glass products for dimensional accuracy, cracks, relics, contamination, etc.)
• daily or weekly chemical glass analysis with the ICP-OES to a. also monitor the heavy metals in the packaging glass (requirement of the packaging ordinance)
• Weekly or monthly Fe 2+ analysis and analysis of the redox state in order to assess the melting unit and the quality of the recycled glass grades used
• Daily voltage tests with 1st order red light under the microscope to reduce breakage problems
• If necessary, fracture analyzes with the SEM-EDX

### Glass staining and discoloration

Ordinary float glass is green in thick layers because of Fe 2+ impurities

#### Principles

Most types of glass are produced with other additives to influence certain properties, such as their color. Basically, there are three different coloring mechanisms for glasses: ion coloring, colloidal coloring and tarnishing. While the first option is mainly based on the interaction of the light with the electron shells of the coloring elements, the last two show a wide variety of diffraction, reflection and refraction phenomena of light, which are strongly dependent on the dispersed phases. In the case of tarnishing, it is an electron excitation in the crystal lattice of the chromophore.

#### Ion staining

Metal oxides, very often 3D elements, are used as coloring substances in glasses. The creation of the color effect is based on the interaction of the external electrons with electromagnetic waves. This can lead to the absorption of certain wavelengths and the emission of other wavelengths. If wavelengths of visible light are absorbed, a color effect is created because the remaining wavelength spectrum no longer produces white light. The coloration can thus be viewed as a selective transmission. The actual color of a glass depends on a large number of parameters. In addition to the concentration of the coloring ions, their coordination and the surrounding glass structure are also of decisive importance. For example, cobalt (II) oxide results in a different shade of blue in a silicate glass than in a phosphate glass. In order to obtain a special color tone, the various coloring oxides can be combined with one another, but any interactions that may occur must be taken into account.

#### Tarnishing

The tempered glasses include the chalcogenide- colored glasses, which are mainly used in silicate glasses with high zinc and potassium oxide contents . Most often, cadmium sulfide or cadmium selenide are added in low percentage ranges, but other metal chalcogenides are also conceivable. The glass is melted under reducing conditions, initially producing colorless glass. Only subsequent tempering causes the glasses to become colored - they tarnish . With increasing duration, the UV edge of the glass moves more and more into the visible area. Targeted tempering can thus achieve different color effects. The cause of this behavior are microscopic (cadmium) chalcogenide crystals that form during tempering and continue to grow with the prolonged tempering time. So it is a controlled devitrification . Investigations showed that with increasing crystallization of the chalcogenide, the forbidden zone between the valence and conduction band increases, which is the cause of the shift of the UV edge into the visible range. Due to their sharp color edge, these glasses are often used as filter glasses.

#### Colloidal staining

A gold ruby ​​colored glass bowl

Colloid-colored glasses are often referred to as (real) ruby ​​glasses . In these glasses, metal salts are added to the melt. At first there is also a colorless glass. A subsequent heat treatment separates metal droplets from the glass matrix and grows them. The color effect of the colloids is based both on the absorption of light by the particles and on the Rayleigh scattering of the light on them. The larger the colloids produced, the more their extinction increases. At the same time, the wavelength of its maximum absorption shifts towards longer-wave light. In addition, the effect of scattering increases with increasing colloid size, but for this the size of the colloid must be very much smaller than the wavelength of the light to be scattered.

#### Color effect of individual components (selection)

Bottles browned by iron and sulfur (
charcoal yellow )
Different color shades of ancient Roman glass bottles

The following list contains some of the more common raw materials used for coloring, regardless of their coloring mechanism.

#### Discoloration of glasses

The discoloration of a glass is necessary if there are large amounts of coloring components in the glass due to impurities in the raw materials ( unwanted color effect ), or if a product of a different color is to be manufactured in regular glass production. The discoloration of a glass can be done chemically as well as physically. Under the chemical discoloration changes are understood in the chemistry of the glass, which mean that the coloring is reduced. In the simplest case, this can be done by changing the glass composition. If polyvalent elements are present in the melt, not only their concentration but also their oxidation state determine the color effect. In this case, a changed redox state of a glass melt can also influence the color effect of the finished product. If the glass is colored by chalcogenides (tarnishing), oxidizing agents can be added to the melt . These cause the chalcogenides to decompose in the glass melt. Another way of compensating for incorrect colors in a glass is physical decolorization . For this purpose, the smallest amounts of coloring components are added to the melt. Basically, the complementary color is used to remove color casts. This creates the effect of a colorless glass. With increasing intensity of the original discoloration, higher amounts of decolorizing agents are necessary, which means that the glass appears colorless, but increasingly darker. Decolorizers are called glassmaker soaps (also known as glass soaps ).

#### Phototropy and electrotropy

These are discoloration and discoloration that occur under the influence of more or less sunlight; they are suitable for glasses that automatically darken in strong sunlight.

A similar effect can be achieved with a variable electric field; it is used, for example, for blackout windshields.

### Adjustment of the glass properties in general

Influences of the addition of selected glass components on the chemical resistance of a special base glass to corrosion by water (corrosion test ISO 719)

Glass properties can be determined and optimized by means of statistical analysis of glass databases. If the desired glass property is not related to crystallization (e.g. liquidus temperature ) or phase separation, simple linear regression analysis can be used with the aid of algebraic equations of the first to third order. Many methods for predicting glass properties are primarily empirical.

The second order equation below is an example, where C represents the concentrations of glass components such as Na 2 O or CaO. The b values ​​are variable coefficients and n is the number of all glass components. The main glass component SiO 2 is excluded in the equation shown and is taken into account with the constant b o . Most of the terms in the example equation can be neglected due to correlation and significance analysis. For more details and applications see.

${\ displaystyle {\ text {Glass property}} = b_ {0} + \ sum _ {i = 1} ^ {n} \ left (b _ {\ mathrm {i}} C _ {\ mathrm {i}} + \ sum _ {k = i} ^ {n} b _ {\ mathrm {ik}} C_ {i} C _ {\ mathrm {k}} \ right)}$

It is often necessary to optimize several glass properties and production costs at the same time. This is done using the least squares method , which minimizes the distance between the desired properties and the predicted properties of a fictitious type of glass by varying the composition. It is possible to weight the desired properties differently.

Composition of important glasses (data in percent by weight )
Glass type SiO 2 Al 2 O 3 Na 2 O K 2 O MgO CaO B 2 O 3 PbO TiO 2 F. As Se Ge Te SO 3
Quartz glass ≥99 - - - - - - - - - - - - - -
Borosilicate glass 70-79 1-4.5 2-6 0-4.4 0-0.3 0-0.5 14-17 - - - - - - - -
Crown glass 73 2 5 17th - 3 - - - - - - - - -
Soda lime glass 71-73 1-2.4 14-17 0.2-1.6 2.6-3.8 4.2-6.6 - - - - - - - - -
Float glass 72-72.6 0.1-1.1 13.5-14 ≤ 0.7 4-4.1 8.4-8.8 - - ≤ 0.2 - - - - - 0.2
Flint glass 62 - 6th 8th - - - 24 - - - - - - -
Lead crystal glass 58 - 4th 9 - - 2 24 - - - - - - -
glass fiber 54 14th - - 4.5 17.5 10 - - - - - - - -
E-mail 40 1.5 9 6th 1 - 10 4th 15th 13 - - - - -
Chalcogenide glass 1 - - - - - - - - - - 12 55 33 - -
Chalcogenide glass 2 - - - - - - - - - - 13 32 30th 25th -
Compositions of historical, modern and natural glasses (data in percent by weight )
Glass type SiO 2 Al 2 O 3 Fe 2 O 3 Na 2 O K 2 O MgO CaO H 2 O
Soda lime silicate glasses
Container glass (white) 73 2 - 13 1 - 11 -
Float glass 72 0.5 - 14th - 4.5 9 -
Historical glasses
mesopotamian glass 68 1 0.5 14.5 3 5 8th -
roman glass 70 2 0.5 18th 1 1 7.5 -
medieval church window glass 49 2 0.5 +
1 P 2 O 5
0.5 20th 4th 23 -
Natural glasses
Obsidian 75 14th 0.5 4th 5 - 1 0.5
Tektite 79 10 3 0.5 2.5 2 3 -
Libyan desert glass 98 +
0.2 TiO 2
1.3 0.3 - - - - 0.2

Glass aggregates include:

• other fluxes to lower the melting point

## History of glass production

### Early days

Glass chalice Thutmose 'III. , oldest glass vessel in the world that can be definitely dated ( State Museum of Egyptian Art , Munich )
Roman dropper bottle in the shape of a gladiator helmet , 1st century AD ( Römisch-Germanisches Museum , Cologne )
Location Thermengasse in the Roman Vicus Turicum ( Zurich ): Remains of window glass from the thermal baths
Salbölfläschchen from the Roman Vicus Turicum

Natural glass such as obsidian has been used for tools such as wedges, blades, scrapers and drills from the earliest times because of its great hardness and sharp breakage. However, unlike artificially produced glass, obsidian cannot be melted or colored with ancient means.

The naturally occurring translucent and fissile minerals mica and Marienglas were used as window glass before it was possible to artificially manufacture panes of the same size and thickness. The Romans called Marienglas Lapis specularis . The Roman historian Pliny the Elder (23–79) described in his encyclopedia Naturalis historia the mining and processing of lapis specularis into window panes and lamps.

Whether glass production was invented in Mesopotamia , ancient Egypt or on the Levant coast cannot be said with absolute certainty. The oldest glass finds come from Mesopotamia; Ancient Egyptian sources point to an import from the east for the initial phase of glass use in Egypt. The oldest textual mention comes from Ugarit and is dated around 1600 BC. Dated. The Nuzi pearls are the oldest finds . The oldest glass vessel that can be definitely dated is a chalice that bears the throne name of Pharaoh Thutmose III. carries and around 1450 BC BC originated. The chalice has been in the State Museum of Egyptian Art in Munich since the 20th century .

Glass has been in use in Egypt since around 1450 BC. Chr. Processed into vessels ( see below ). The place of manufacture of this earliest glass is unknown, it is believed to be in Thebes , opposite today's Luxor . The best-known processing technique is based on the production of hollow vessels by wrapping softened glass rods around a porous ceramic core, which was then scraped out. The best finds for this are from the excavations of Flinders Petrie from Amarna . The only known Bronze Age glassworks to date in which glass was made from its raw materials dates back to the Ramesside period and was discovered in the late 1990s during excavations of the Roemer and Pelizaeus Museum (Hildesheim) under the direction of Edgar Pusch in the eastern Nile delta in Qantir Piramesse found. Investigations provided information about the melting process. Thus quartz rock crushed, mixed with vegetable ash sodahaltiger filled in a jug and perhaps 800 ° C to a frit melted. After cooling, this frit was presumably crushed and melted in a second melt in specially manufactured crucibles at 900 to 1100 ° C. to form an 8 to 10 cm high bar with a 10 to 14 cm diameter. The glass was colored black, violet, blue, green, red, yellow or white by adding metal oxides. A concrete connection between glass production and metal extraction cannot be proven despite the similar temperatures. The colored raw glass was delivered in bar form to the further processing workshops, which made monochrome and polychrome objects from it. Such glass bars were found in the shipwreck of Uluburun near Bodrum , Turkey , which dates back to the 14th century BC. Is dated. The first known recipe comes from the library of the Assyrian King Ashurbanipal , which dates back to approx. 650 BC. Is dated: Take 60 parts of sand, 180 parts of ash from seaweed and 5 parts of chalk and you get glass. At that time, a lot more glass was being processed, and a new glass melting technique was developing.

Roman perfume bottle made of glass, 1st – 3rd centuries Century AD, 8.2 cm high

### Antiquity

Pliny the Elder describes the production of glass in the Historia naturalis . Chemical analyzes and findings of experimental archeology have confirmed Pliny in many questions. In Roman times , glass was melted with river sand and soda from Egypt. The Egyptian soda was mined at Wadi Natrun , a natural soda lake in northern Egypt, and exported by the Phoenicians to the Mediterranean via Alexandria . This was relatively pure and contained more than 40 percent sodium oxide (the information was based on the oxide , as is customary in petrology , but in fact sodium carbonate is present) and up to 4 percent lime . The composition made it an ideal flux. Pliny continues to write about glass sand deposits in Italy, Hispania and Gaul , but none of these places developed such significant glass production as on the Palestinian coast between Acre and Tire and in the Egyptian glassworks around Wadi Natrun near Alexandria.

In 301 Emperor Diocletian set the prices for a whole range of products, including raw glass. A distinction was made between judaicum and alexandrium , the latter being more expensive and probably discolored glass. At that time, glass production was essentially still divided into primary and secondary workshops. In the primary workshops, raw glass was melted in large melting tanks, which was then delivered to the secondary workshops, where it was melted down in crucibles and processed. In Bet Eli'ezer in what is now Israel, 17 glass melting tanks, each 2 × 4 m in size, were uncovered. After the mixture had been placed in the tub, the furnace was bricked up and fired for 10 to 15 days. Eight to nine tons of blue or green raw glass were melted in just one operation. After the furnace had stopped firing and had cooled down, the vault of the furnace was removed, the glass block pried out and the raw glass sent for further processing. A third-century shipwreck found on the southern French coast had more than three tons of raw glass loaded. In Egypt, raw glassworks were found that extended into the 10th century. The Egyptians used antimony to discolor, so they were able to produce colorless, transparent glass.

The secondary glass works were common throughout the Roman Empire and produced hollow glass, flat glass and mosaic stones. The raw glass was melted down in a crucible and taken out of the furnace with the pipe in its viscous state and processed. The glass on the pipe could be inflated, which enabled the production of larger vessels and new shapes. Until then, glass was used for pearls, perfume bottles and drinking bowls, but container glass in particular spread in the Roman Empire - in contrast to the usual clay, wood, metal or leather containers, glass is tasteless - as well as carafes for serving and in late antiquity also drinking glasses . The first window glasses were found in Aix-en-Provence and Herculaneum . The finds have sizes of up to 80 cm × 80 cm. However, no written record mentions the manufacturing process. For the early, thick-walled and one-sided matt window glass, there are different views in the professional world on its production. On the one hand, a manual stretching technique is being considered; on the other hand, a casting process is assumed for its manufacture. For the thin-walled and double-sided clear window glass, which appeared from the 2nd century AD, the cylinder blowing process is likely.

Glass arm rings are a typical form of jewelry that, along with glass finger rings and ring pearls, appeared as women's jewelry during the Middle La Tène period in Celtic Central Europe and was found as grave goods.

### Middle Ages and Modern Times

Glassmaker . From: Hrabanus Maurus , De universo , illustrated manuscript (1023), Montecassino Monastery (cod. 132)

In the early Middle Ages , the Teutons produced glass wherever the Romans had withdrawn, which seamlessly connects to the Germanized late antique design language. Today it is assumed that Roman glasses that were still in existence were recycled for Franconian glass.

#### Forest glass

With De diversis artibus by the Benedictine monk Theophilus Presbyter , a longer written source is available for the first time, describing glass production, the blowing of flat glass and hollow glass, and furnace technology. Theophilus, who was probably in Constantinople , mixed ashes from dried beech wood with sifted river sand in a ratio of 2: 1 and dried this mixture in the oven with constant stirring so that it could not melt or stick together for a day and night. This frit was then placed in a crucible and melted into glass in one night under high heat.

This text, which was probably written in Cologne at the beginning of the 12th century, probably forms the basis for the Gothic church windows and also for the forest glass. The vegetable ash with all its impurities also provided part of the lime that was necessary for the production of good glass. In order not to have to transport the enormous amount of wood that was necessary for firing the stoves and for ash extraction over long distances, the glassworks were built in remote forest areas. These forest glassworks mainly produced glass that was colored green by iron oxide (from contaminated sand).

There is a brief description of glass art in Georgius Agricola's De re metallica . He lived in Venice from 1524 to 1527 and was probably allowed to visit the island of Murano , as the detailed descriptions of the ovens suggest.

Transparent stones are named as raw materials , ie rock crystal and "white stones", ie marble , which are burned in the fire, crushed into coarse semolina in the stamping mill and then sieved. He also mentions table salt , magnetic stone and soda . Table salt and magnetic stone are rejected as useless by later authors. There was marble and soda in Altars and in Milan ; they are not available in Germany. Only a hint “salt that is represented from lye” points to a Venetian secret.

The glass melting furnaces of the Waldglashütten and Venice were egg-shaped constructions with a diameter of 3 meters and a height of up to 3 meters, made of clay bricks mixed with fired chamotte . The firing room was on the lower floor with one or two semicircular openings for wood to be thrown in. In the middle the flames broke through a large round opening into the second floor, where the harbor ovens were located. This approximately 1.20 meter high room was provided with oven doors measuring 20 × 20 cm all around, through which the mixture could be inserted and the glass removed. The cooling furnace, which was only 400 ° C, was located on the upper floor, which was connected to the melting room through a small opening. The cooling furnace was provided with a small opening through which finished workpieces were inserted. In the evening the hole between the melting room and the cooling room was closed with a stone so that the glass could cool down overnight.

#### Venice

The beginning of the Venetian glass tradition was probably the trade in Byzantine glass products, which were imported and exported to all of Europe as early as the 10th century. The first glassmakers can be found in the registers of the 11th century. They are called phiolarius ("Flaschner"). A merchant ship wrecked on the south coast of Turkey, which sank around 1025, transported no less than three tons of raw glass that came from Caesarea in Palestine. Whether it was intended for Venice cannot be said with certainty, but it is obvious. By 1295, all glassmakers were settled on the island of Murano and their freedom of travel was restricted by law. On this island cut off from the world, Angelo Barovier was able to unravel the secret of discoloration in the middle of the 15th century and for the first time produce unclouded, clear glass in Europe. The crystallo , a soda-lime glass that was decolorized with manganese oxide , was to establish the world fame of the Venetian glass. The soda was imported from the Levant or Alexandria , leached and soaked until a pure salt was created. A pure glass sand from Ticino or burnt marble was used as the sand . The manganese ores were probably procured from traveling ore prospectors from Germany who were known there as whales or Venediger. Another Venetian rediscovery is the lattimo ( milk glass ), an opaque white glass clouded with tin dioxide and bone ash that mimicked Chinese porcelain .

Many new techniques were developed, especially in the 19th and 20th centuries. The industry peaked in the 1950s and 1960s. Famous techniques from this period include: Anse Volante, Battuto , Canna, Colorazione a caldo senza fusione, Fenicio, Incamiciato, Murrina, Oriente, Pezzato, Pulegoso, Scavo , Siderale, Sommerso, Tessuto. Murano glass is now a sought-after collector's item. Sometimes very high sums are paid for rare and special pieces. Famous historical glass manufacturers are for example Venini & C., Pauly & C., Barovier & Toso, Seguso Vetri d'Arte. Some of these factories still exist today.

The glass beads became a sought-after commodity and quickly spread across Europe. For centuries, glass beads were a popular means of payment in the barter trade with gold, ivory, silk and spices. The colorful works of art have been sought-after objects for collectors for several years.

Glass beads from Venice are the most famous and sought-after pearls in the world. Venetian glass artists have influenced bead makers around the world for several centuries. There the glass beads are made over an open flame. It is a very time-consuming process as each bead is made individually.

A glass rod is heated until it melts using a blowtorch and wrapped around a metal rod until the desired bead shape is achieved. More glass colors can be gradually melted onto this basic bead and various decorative elements, such as thin glass threads or wafer-thin glass plates (confetti), can be applied. Then the pearl is cooled very slowly and removed from the rod, creating a hole through which the pearl can later be threaded. These pearls are called winding pearls .

#### Window glass

Moon glass production in the 18th century; the tablet comes from the Encyclopédie. The worker on the left carries wood for the fire. A glass drop is removed from the center or the workpiece is heated up. In the right foreground, a drop of glass is preformed by
marbles , in the background a pane is thrown out.
Rolled glass production in 1908: the same process as in 1688

Finds of window glass in Pompeii show that the Romans knew window glass as early as the 1st century, which was used in thermal baths or villas, for example. There are even isolated reports of glass greenhouses. Most of the time, it was about 20 cm × 30 cm to 80 cm × 80 cm in size and 3 to 5 mm thick, with a smooth side and a rough side. From the 2nd century AD onwards, smooth, thin-walled window glass on both sides seems to replace the thick-walled and, due to its rough side, only moderately transparent window glass, which is often difficult to distinguish from vessel glass and recent glass in archaeological findings. This thin-walled window glass was probably created using the cylinder blowing process. It came to a wider use with the emerging Gothic in the 12th century.

In the moon glass process, which was documented in the Near East as early as the fourth century and which was later widely used in France, a glass drop is blown into a ball with a glassmaker's pipe. The hot glass ball is attached to a metal rod on the opposite side and the glassmaker's pipe is blown off. The ball now has a hole, the edges of which are turned outwards. The ball was brought back to temperature for further processing. At around 1000 ° C, the glass was soft enough to be thrown into the shape of a plate by means of centrifugal force. The ball opened around the hole where the pipe was previously attached. This technique produced glass plates with a diameter of approx. 1.20 m. Then the outer edge was cut into rectangles. These were used as e.g. B. Church glass with lead frames. The middle piece with the connection point of the throwing rod is called Butze and was used for slug discs with a diameter of 10 to 15 cm.

The rolled glass process was documented for the first time in 1688 in Saint-Gobain , the nucleus of today 's global corporation of the same name . Molten glass is poured onto the roller table, distributed, and finally rolled. In contrast to the previously mentioned methods, a uniform thickness was achieved here. For the first time, pane sizes of 40 × 60 inches were also possible, which was used for the production of mirrors . However, problems are caused by the uneven surface. Window glass made by this manufacturing process is often blind and mirror glass can only be achieved by laborious cold polishing.

Drawing of the regenerative furnace by Friedrich Siemens

### Industrialization and automation

Cast iron mold for manual shaping of hollow glass

The industrialization and automation of glass production began gradually in the 19th century. First, individual process sections were optimized. In 1847, Joseph Magoun introduced metal molds to hollow glass production, which replaced the wooden molds that had been mainly used until then. In 1856, Friedrich Siemens developed the first glass furnace with regenerative firing , which in 1867 led to the first continuous tank furnace, also by Friedrich Siemens. The regenerative firing enabled considerable energy savings and, at the same time, improved temperature control in the glass melting tank. A little later, in 1884, Ernst Abbe and Otto Schott founded a glass factory for special optical glasses in Jena.

#### Flat glass

Manufacture of flat glass using the Fourcault process. The glass sheet is drawn vertically out of the melt through a nozzle.

In 1905, the American John H. Lubbers developed a method for manufacturing flat glass, attempting to implement the manual process of cylinder blowing on an industrial scale. Cylinders were drawn directly from the enamel, they could reach a diameter of 80 cm and were up to 12 m high. The cylinder was then cut open and flattened. However, the process was very cumbersome, especially the turning of the cylinder into the horizontal caused difficulties.

A patent for improved flat glass production was to follow from Émile Fourcault in 1902 . The Fourcault process named after him for the production of drawn glass . The glass is continuously pulled vertically upwards from the melt as a sheet of glass through a nozzle. The flat glass was thus produced without going through a cylinder. After pulling it up through a vertical cooling channel to a height of approx. 8 m, cooled flat glass can be cut to size at the top. The thickness of the glass could be adjusted by varying the pulling speed. The Fourcault method came into use from 1913 and was a great improvement.

The American Irving Wightman Colburn patented a similar process in 1904. The glass ribbon was also drawn vertically out of the enamel, but diverted into a horizontal cooling channel via a pulley for better handling. With its own factory, attempts were made to master the process until 1912, but ultimately remained unsuccessful, so that bankruptcy was filed. The patent went to the Toledo Glass Company . In 1917 the so-called Libbeys-Owens process came into industrial use. The advantages over the Fourcault process lay in the simpler cooling. On the other hand, several drawing machines were able to work on a glass melting tank. Since the cooling furnace could be of any length, this process achieved about twice the production speed. In the following years, both procedures existed in parallel. In 1925 the Plate Glass Company improved the advantages of the Fourcault and Colburn processes; With the Pittsburg process, she achieved a significant increase in production speed.

The German Max Bicheroux took the decisive step in the manufacture of cast glass in 1919 . In contrast to the processes mentioned so far, no glass sheet was drawn from the melt, but the liquid glass mass was formed into a glass ribbon between cooled rollers. While it was still heated, the ribbon of glass was cut into sheets and cooled in ovens. With this process, panes up to 4.5 m wide could be produced. A similar process was developed in 1921 by Pilkington and the vehicle manufacturer Ford for the continuous production of automotive glass as rolled glass . However, this method yielded narrower widths than that of Bicheroux.

The Pilkington company was the first to overcome the technical problems of float glass production in the 1960s , whereby the glass melt was poured onto a bath of liquid tin. This principle revolutionized flat glass production , as it had a very high productivity and made mirror glass production possible without further post-processing steps. In the 1970s this procedure became the general standard and almost completely replaced the others. The process is based on an idea by Henry Bessemer for which William E. Heal had already applied for a patent in 1902.

#### Hollow glass

Hollow glass production around 1910: The drop is blown into a bottle towards the bottle.
The Owens carousel-shaped machine for fully automatic bottle production (1912)
Heat-resistant glass ( Jenaer Glas ), here for tea sets

In the early 19th century new mechanical tools were used to blow glasses. Shapes were used which had a relief to be produced as a negative. The glass is pressed against the mold by the blowing pressure and the workpiece is given its shape. However, the glassmaker's lung power is not high enough for deeper reliefs, so mechanical aids were introduced. Sufficient pressure was achieved through the use of air pumps.

Another innovation in the mid-19th century was the introduction of metal molds. For the first time in 1847, the forms developed by Joseph Magoun replaced the old ones made of wood, which considerably increased their durability.

The British Alexander Mein and Howard M. Ashley developed the first semi-automatic bottle blowing machine in Pittsburg in 1859. However, manual work steps were still required.

A milestone was the Owens machine introduced by Michael Joseph Owens in 1903 as the first fully automatic glass machine. A vacuum is created in a feeder immersed in the melt and the required amount of molten glass is precisely absorbed. The feeder's arm swings back and presses the drop into the mold. The drop is blown into the metal mold with compressed air and the workpiece is given its final shape. This technique is called the suction-blow process . This made it possible to produce the enormous amount of four bottles per minute at the time.

Despite this achievement, machine-blown bottles remained heavier than hand-blown ones for many years. In order to surpass the glassmakers, the machines had to work much more precisely. This also explains why the various production processes were operated in parallel for a long time.

Substantial improvements in gob removal by Karl E. Pfeiffer's gob feeder in 1911 also led to an increase in productivity. The portioning of the glass mass was no longer done by skimming or sucking a quantity of glass from the bare enamel surface, but by a drop running off through an opening at the end of the feeder (feeder channel). The more precisely possible dosage of the amount of glass enabled more uniform bottles to be produced.

In 1924, the IS machine was patented by the namesake Ingle and Smith, and the first industrial application followed a few years later. This machine, which really uses the advantages of the drop process, works according to the blow-and-blow process. A drop is fed into a metal mold and pre-blown. The pre-formed gob is swiveled into a second mold in which the workpiece is blown to completion.

The first applications of the new process followed a few years later. The first machine from 1927 had four stations: A feeder fed a machine and it could produce four bottles in parallel. The principle of the blow-and-blow process is still valid today in mass production.

#### Tubular glass

Danner pipe hoist in the VEB Glaswerk Weißwasser

Up until the 19th century, glass tubes were also produced (hand-blown) exclusively discontinuously from one batch or one batch of glass. The industrial processes for glass tube production are divided into processes with a rotating pipe and drawing processes with nozzles. The latter can be further subdivided into variants in which the glass tube is pulled vertically down or up out of the melt. In 1912 E. Danner ( Libbey Glass Company ) developed the first continuous tube drawing process in the USA, for which a patent was granted in 1917.

In the Danner process, molten glass flows as a ribbon onto a rotating hollow ceramic cylinder that is inclined downwards - the Danner pipe . After the supply of compressed air via the inside of the pipe, the glass pipe that is forming is pulled off in the direction of the pipe axis. The drawing speed of the pipe and the pressure of the air supplied determine the pipe dimensions.

In France in 1929 L. Sanches-Vello worked out a vertical drawing process. This is a vertical pipe drawing process. The melt is drawn down through a nozzle in the bottom of the melting tank and shortly thereafter diverted to the horizontal.

A number of other processes exist for the production of tubular glass, but they all work on very similar principles.

## Markets for glass

Glass is a versatile material that is used in many areas of daily life. Glass plays an important role in research and science, in modern architecture and in industries of the future. Core areas in which glass is used are: construction industry, food and beverage industry, automotive industry, electrical (electronic) industry, household and gastronomy, medicine, research and science, chemistry, pharmacy, cosmetics, furniture industry and interior design, plastics and textile industry.

## Crafts and glass art

Old glasses. From: Meyers Großes Konversationslexikon. 6th edition. Volume 8, 1907, keyword: glass art industry
Methods for mouth / hand-made glasses (from left to right): lead glass, sandblasting, fusing / lamination, lighting, painting, bending, etching

### Egypt

The glass trade in Pharaonic Egypt can be traced back to the beginning of the 18th dynasty; Initially, these are small finds such as pearls, amulets or chain links as well as colored inlays in the typical Egyptian jewelry objects (e.g. pectorals). These are usually in turquoise or dark blue, as they should imitate such objects made of lapis lazuli or turquoise; this was not regarded as cheap jewelry , but the imitation of these noble, highly powerful stones was considered a special art . The process was very complex for the time and such small finds were made from pieces of raw glass, completely comparable to those made from stone. This is also supported by the fact that an Egyptian word for “glass” did not exist; it was called artificial lapis lazuli or artificial turquoise in contrast to true / genuine turquoise or lapis lazuli. In the first Egyptian glass art bloom (18th to 20th Dynasty), rod-shaped vessels appeared (which are also called core- shaped, based on the sand core technique ). They go back to models of contemporary vessels, especially those made of stone. Typical forms of Egyptian glass vessels are lotus goblets , pomegranate vessels, krateriskoi and make-up vessels such as cabbage pots and cabbage palm columns (for black eyeshadow, pronounced "kochel"). Since Thutmose III. from whose reign the oldest hollow glass finds also originate, there are also imported vessel forms from the Mediterranean region (e.g. amphoriskoi, lentil bottle, handle bottle, bilbils and other special forms); these are generally introduced into the spectrum of vessel shapes and thus also apply to vessel shapes made of ceramic and faience, for example. The older core-shaped vessels (around the time of Thutmose III to Amenhotep III) are mostly turquoise to deep blue (like real turquoise and lapis lazuli, because glass was considered an imitation of these precious stones). Later, especially in the Ramesside period, glasses in bright, strong colors such as yellow and green, white but also brown became popular. As decoration, thread decorations in zigzag or garland form in yellow, white and light blue as well as twisted threads in a light-dark contrast were created, sometimes they were left monochrome and only the handles or shoulder breaks were emphasized by thread decoration. The Egyptian glass vessels were used to store cosmetics such as ointments, oils, perfumes and eye make-up. The strongly colored, opaque glass had a preservative effect.

In the late period (from the 3rd interim period up to the Greek epoch), hollow glass craft remained underrepresented, hollow glass only occasionally appeared, still in the form of small, mostly undecorated ointment vessels. In contrast, glass inlays in jewelry or figurines were not uncommon and, as before, were treated equally with gemstones. In the Hellenistic period, glass production regained importance, also in Egypt. Together with new manufacturing techniques, a completely new world of shapes appeared, but it is not typical of Egypt, but rather typical of the time. Already in the 5th century BC BC Rhodes had established itself as an important center for glass production. In addition to inlays and pearls, there were now multicolored mosaic bowls and the vessels of the Canossa group .

### Roman Empire

The Lycurgus beaker, Roman glass from the 4th century.

In the 1st century, glass production increased to such an extent that the previously rare and expensive material became affordable for large numbers of people. An extensive production of drinking vessels, jugs, bowls and plates began, initially mostly manually shaped or lowered, then increasingly hand-blown. A large number of high-quality special glasses demonstrate craftsmanship, such as the mosaic thread glasses, cameo glasses, gold foil glasses, glasses with enamel painting and especially the diatret glasses , mostly bell-shaped, splendid light vessels in mesh glass technology, which are still admired today for their artistic quality. One of the most famous Roman glasses is owned by the British Museum located Lykurgosbecher from the 4th century, where a three-dimensional figurative representation is appropriate to the opaque green turn red on the back light and in reflected light.

### Venetian glass

From the first half of the 16th century, Venice became known for its colorless, thin-walled and finely elaborated cristallo . Nothing has survived from before that, and only a little from the 16th and 17th centuries. Dutch and Flemish still lifes in particular provide information about the breadth of variation in Venetian Renaissance glasses, their shapes and decors . Most of these are cups, bowls, jugs and bottles that had shafts with flat feet made of blown hollow balusters . These shafts became more and more sophisticated in the following years, wings were attached in imaginative ornaments and figurative decorations, sometimes the shaft was also made in a figurative, for example animal form.

There were special finishing techniques for the wall . With ice glass , made by quenching it in ice-cold water or by rolling it over small splinters, an effect is achieved on the surface like a window glass covered by ice flowers . In the case of thread or net glass ( Italian latticinio / vetro a filigrano / reticella ) - milk glass threads were melted into the clear glass mass and woven by turning so that a thread or net-like pattern was created. This technique was already known to some extent in antiquity.

As a glass à la façon de Venise , the Venetian style found access to the countries north of the Alps, despite all attempts by the Republic of Venice to keep its art secret.

### Jewelry techniques in the Baroque and Rococo

Baroque chopped glass (and rococo glass) primarily from Bohemia and Silesia, but also Nuremberg, Brandenburg and Saxony, more rarely Thuringia, Hesse, Northern Germany and the Netherlands overtook Venetian glass from the 18th century, as their glass was used for cutting and Ground glass was not suitable due to its thin walls.

The shapes with a foot, a baluster shaft and a thin-walled cupa were similar to the colorless Venetian glass, but without wings and had thicker walls. In Potsdam, Silesia, Bohemia, Kassel and other areas, people experimented with recipes for glass in order to produce a mass that would allow it to be ground and cut. The themes of the cut were varied. Hunting scenes were common, landscapes, but also allegorical figures with inscriptions, floral and leaf ornaments as well as contemporary personalities and battle scenes.

As early as the 17th century, glass cutters occasionally signed their works and glass cutters are also known from the 18th century, for example: Christian Gottfried Schneider and Friedrich Winter shaped the glass cutting of Silesia like Martin Winter and Gottfried Spiller that of Potsdam, Johann Christoph Kießling worked for August Strong , Franz Gondelach was in the service of Landgrave Carl von Hessen and David Wolff worked in the Netherlands.

Occasionally, the baroque cut glasses have gilding on the base, shaft or lip edge. The intermediate gold glasses were also popular in the 18th century. Two glasses were used for their production, one of which fitted exactly into the second, therefore larger glass. A gold foil was placed on the outer wall of the inner glass and motifs were scratched into it with an eraser. Then it was fitted into the second glass and processed further.

From porcelain painting came the technique of black solder painting , which in another context was already known in the Middle Ages. Johann Schaper and Ignaz Preissler shaped this art in Nuremberg and Silesia, Bohemia and Saxony.

A rural refinement technique for baroque glass is enamel painting . It can be found mainly on glass for use in rural areas (e.g. beer bottles from rifle clubs and schnapps bottles). The motifs match the provenance: farmer with cattle and farm implements, inn scenes, playing cards, sayings. In Bohemia enamel painting is also made on opaque milk glass , which brings this technique close to porcelain painting.

### Biedermeier glass

Friendship mug, mid 19th century

The English took over the types and shapes of Bohemian glasses in the 18th century and, with the help of the purity of their lead crystal , whose excellent light-refracting properties were effectively demonstrated by the brilliant cut, at the beginning of the 19th century they finally dominated the market, which at the time was characterized by classical taste ideas . In order to make up for the advantage of the English, the Bohemian glass manufacturers tried to improve the purity of their lead-free crystal glass . At the same time, they used all the possibilities of pattern sanding for varied decors and, above all, tried to produce cheaper. The result of these efforts can be seen in the masterfully cut Biedermeier glasses, which are considered to be admirable examples of handcrafted glass cutting.

The Biedermeier style reached its peak in the 1830s . In order to expand production and sales, the glassworks enriched their range with the newly developed colored glass after 1840, thus increasingly displacing colorless glass from the market. The North Bohemian glassworks in particular designed their glasses in ever more effective colors. In the course of this development, however, the glass cut became less important than the colorfulness of the decors, and the shape and cut became increasingly simpler, not least for cost reasons.

The variety of products made of colored glass and overlaid or stained (see red stain ) crystal glass with cut decoration as well as stone glass ( lithyalin glass and hyalith glass , which was painted with gold, enamel and transparent colors) finally reached a previously unknown extent. For example, drinking glasses and carafes made of colored glass, whole liqueur and dessert services, sets for chests of drawers and washstands, writing implements and perfume bottles, bowls, plates, centerpieces and, above all, vases were popular. In addition, there were the countless souvenir and friendship glasses, decorative and honor goblets, as well as export items such as water pipes and detonators for rose water.

### Art Nouveau glass

Long neck vase with etched decor, similar to Gallé

Around 1900 the designers of the younger generation were unanimous in their departure from traditional historicism . The term Art Nouveau has become established in German-speaking countries, the Netherlands and the Nordic countries for the resulting pursuit of new, fresh, original forms of expression based on old craft techniques , while the term Art Nouveau is commonly used elsewhere . The Art Nouveau artists' imagination was primarily inspired by the world of colors and shapes of the Far East. The essential parts or elements of Art Nouveau are characterized by decoratively curved lines as well as flat floral ornaments and asymmetry .

Glass played a central role in the development of Art Nouveau. The reason for this is to be found in the creative possibilities that met the desired organic nature of the design. The collaboration between designers and craftsmen resulted in imaginative, limited edition hand-made studio glass, which impresses with the variety of color effects. French glassmakers such as Emile Gallé and the Daum Frères created cut and etched flashed glass in strong colors. The Bohemian Art Nouveau glass owes its good reputation above all to Max Ritter von Spaun, owner of the Joh. Loetz Witwe company in Klostermühle in Bohemia. The iridescent glass and the famous creations by Louis Comfort Tiffany , which are regarded as exemplary in Europe, came from across the pond, from New York .

The constructive style, which endeavored to design all forms with the help of the simplest structures such as square, rectangle, circle and ellipse and to use strong contrasts of colors, was most consistently pursued by the Vienna School. Your leading representatives were Josef Hoffmann and Koloman Moser .

With the growing economic difficulties in the time of the First World War, the era of Art Nouveau came to an end. It lasted almost twenty years, but its effects are still felt.

### Fusing

In fusing or fusing (new German for glass melting), different (white or colored, possibly painted with glass melt) pieces of glass are melted together at 780–900 ° C. The melting temperature depends on the composition and thickness of the glasses. Temperature-resistant objects such as metals can also be melted down.

According to current archaeological knowledge, fusing is basically a glass processing method that is at least 2200 years old. In the last few decades it has been developed into one of the most versatile and technically sophisticated glass processing techniques. Many glaziers and artistic glass studios can process glass using the fusing technique. The process is used in a wide variety of ways: from costume jewelery and the decoration of objects to art objects (e.g. using the Murrine and Millefiori technique), large artistically designed windows and other glass elements in architecture and interior design.

A distinction is made between the following basic variants of fusing:

1. Relief ( tack fuse )
2. Full merger (Engl. Full fuse )
3. Glass flow (French: pâte de verre ), glass paste is melted into shape.

Conventionally, fusing can proceed as follows: from different colored glass plates, suitable parts are pinched off with special pliers or cut off with a glass cutter. The glass artist assembles the pieces of glass according to the design, for example as a pattern for the frame of a mirror or for the production of a glass bowl. Gaps are often filled with powdered glass from crushed glass plates. Now the pieces are fused in a glass fusing furnace . The temperatures are chosen in such a way that the glass does not yet run as a liquid, but that all glass parts and particles form a permanent bond. With the appropriate temperature control, a completely closed and hard glass body can be produced. This firing process takes about 18 to 22 hours, depending on the thickness and diameter of the glass.

The glass body is first fused to form a flat plate which, if necessary, is further shaped in a glass melting furnace in a second step, e.g. B. if it is to be made into a glass bowl. For this purpose, support forms or models are used, which are often made of clay or unglazed ceramic. In concave models, the heated glass plate can lower itself and over convex models it can bend up. The shape must be slightly larger than the glass plate, as glass expands when heated and contracts when cooled. After cooling, glass finishing techniques can be applied to the resulting objects: engraving , glass painting , grinding, sandblasting or etching.

Glass object in fusion technique

An advanced application of the process is the production of large self-supporting glass panes or glass objects, which can be designed in an artistically controlled manner, for example as contemporary art or church art. Industrially produced glass fragments (frits) and glass powder from colorless and colored glasses are also used for this. The artist Ulrike Umlauf-Orrom creates examples of fusing technology in glass art .

The production of such fusing pieces requires artistic talent and knowledge of the process tricks. The melted glasses must have the same coefficient of expansion (AKW) and the heating and cooling of the glass must follow precisely controlled temperature curves. Otherwise, mechanical stresses can arise in the glass, which can tear or shatter. Large fusing pieces can therefore only be produced in a flat bed in digitally controlled kilns.

Particularly advanced glass artists use Glory Hole type glass ovens because they allow smaller glass masses to be artistically processed directly in various melted or almost liquid states. Glass is repeatedly held through the hole in the furnace wall for a new work step and heated up so that it can then be processed outside the furnace.

Ovens with a pull-out flat bed are used for direct processing. The glass lying in the flat bed is brought to processing temperature and then pulled out of the oven for a short time. Using the correct procedures and precautionary measures, chemicals, metal dust or colored glass powder, for example, are then applied to the melted or melted glass. Special knowledge is required to intervene directly with tools in this glass mass.

Another new variant is the pàte de verre production of large-format glass sculptures.

## literature

### Glass manufacture and technology

• Joachim Lange: Raw materials in the glass industry . 3rd, revised. Edition. Wiley-VCH, Leipzig 1993, ISBN 3-342-00663-3 .
• Günther Nölle: Technology of glass production . German publishing house for basic industry, Stuttgart 1997, ISBN 3-342-00539-4 .
• Wolfgang Trier: Glass melting furnaces, construction and operating behavior (reprint) . 1st edition. Springer, Berlin 1984, ISBN 3-642-82068-9 .
• Günther, Rudolf: Glass melting furnace . Publishing house of the German Glastechnische Gesellschaft, Frankfurt am Main. 1954.
• Alexis G. Pincus: Combustion Melting in the Glass Industry (compilation of articles from Magazines for Industry Inc.) . 1980.
• II Kitaigorodski: Technology of Glass . 2., verb. and exp. Edition. VEB Verlag Technik, Berlin 1957.
• Hans Jebsen-Marwedel (Ed.): Glass-technical manufacturing defects . 4th edition. Springer, Berlin 2011, ISBN 978-3-642-16432-3 .
• W. Giegerich, W. Trier: Glass machines, construction and operation of the machines for shaping hot glass . Springer, Berlin 1964.
• Siegfried Rech: Glass technology 1 . 1st edition. VEB German publishing house for basic industry, Leipzig 1978.
• Jürgen Dispan: Glass industry in Germany. Industry report 2013 . Stuttgart (= IMU Information Service No. 3-2013). Link to the industry study

### History of glass production

• Birgit Nolte: The glass vessels in ancient Egypt . Hessling, Berlin 1968.
• Daniele Foy, Marie-Dominique Nenna: Tout feu tout sable . Aix-en-Provence 2001, ISBN 2-7449-0264-0 .
• Rita Hannig: Glass chronology of Northeast Bavaria from the 14th to the early 17th century . Greiner, Remshalden 2009, ISBN 978-3-86705-027-2 .
• Anton Kisa : The glass in antiquity . 3 volumes. Hiersemann, Leipzig 1908 ( digitized volume 1 , volume 2 , volume 3 ).
• Heinrich Maurach: Glass as a word and a concept. In: Glass technical reports. Volume 25, 1952, pp. 1-12.
• Frank Schweizer: Glass of the 2nd millennium BC In the eastern Mediterranean region . Greiner, Remshalden 2003, ISBN 3-935383-08-8 .
• Heike Wilde: Technological innovations in the second millennium BC. For the use and distribution of new materials in the Eastern Mediterranean . Harrassowitz, Wiesbaden 2003, ISBN 3-447-04781-X .
• Lukas Clemens, Peter Steppuhn (ed.): Glass production. Archeology and history. Contribution to the 4th International Symposium on Research into Medieval and Early Modern Glassworks in Europe . Kliomedia, Trier 2012, ISBN 978-3-89890-162-8 .
• Heidi Amrein: L'atelier de verriers d'Avenches. L'artisanat du verre au milieu du Ier siècle après J.-C. In: Cahiers d'archéologie romande . tape 87 . Lausanne 2001, ISBN 2-88028-087-7 (French).
• Axel von Saldern: Antique glass . Beck, Munich 2004, ISBN 3-406-51994-6 .
• Helmut A. Schaeffer (Ed.): Glass technology . tape 1 : Material glass . Deutsches Museum Verlag, 2012, ISBN 978-3-940396-35-8 .
• Helmut A. Schaeffer (Ed.): Glass technology . tape 2 : Hollow glass , 2010, ISBN 978-3-940396-16-7 .
• Margareta Benz-Zauner, Helmut A. Schaeffer (Ed.): Glass technology . tape 3 : Flachglas , 2007, ISBN 978-3-940396-01-3 .
• Margareta Benz-Zauner, Helmut A. Schaeffer (Ed.): Glass technology . tape 4 : special glass , 2003, ISBN 3-940396-07-9 .

### Crafts and glass art

• Walter Spiegl: Glass . Battenberg Verlag, Munich 1979, ISBN 3-87045-155-6 .
• Judith Miller: Art nouveau. The world of Art Nouveau . Dorling Kindersley Verlag, Starnberg 2005, ISBN 3-8310-0767-5 .

### Historical glass restorations

Commons : Glass  - collection of images, videos, and audio files
Wiktionary: Glass  - explanations of meanings, word origins, synonyms, translations
Wikisource: Glass  - Sources and Full Texts

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