Pottery furnace
The kiln is a furnace for burning of ceramics - earthenware , Fast stoneware , stoneware , faience , stoneware and porcelain .
Definition of a pottery furnace
"Enclosed or semi-enclosed chamber (s), usually relatively permanent or re-usable and non-portable, to hold ceramic materials during firing and to generate, contain, and channel heat for that purpose"
“Closed or semi-closed combustion chamber, usually permanent and reusable, not portable. The function of the combustion chamber is to hold the ceramic material during the fire and to hold together and channel the heat for the fire "
Demarcation
A variety of ceramic products can be fired in ovens. Nevertheless, in the classic sense it is not always a wood-fired, artisanal "pottery furnace". For this reason, the following furnace types should be dealt with in separate Wikipedia chapters:
- Clay pipe production ovens,
- Furnaces of the ceramic factories, ceramic industry or porcelain industry, such as B. lying Viennese porcelain ovens, round ovens with rising or overturning flame, rectangular ovens with side firing, muffle or modern tunnel ovens.
- Auxiliary ovens, e.g. B. for melting or processing or fritting of glazes or glaze ingredients.
- Furnaces of the artisanal brick production or brick industry, such. B. open top shaft furnaces, chamber furnaces, furnaces with side firing, ring furnaces (Kassel furnace) or tunnel furnaces.
Types of ceramic fire
The firing of ceramic vessels made of clay took place depending on the time, region of the world or the type and scope or specialization of production in very different, sometimes very simple ways:
- Open field fire (Meilerbrand, above ground),
- Pit fire,
- Fire in the single-chamber furnace,
- Fire in a standing or lying two-chamber furnace.
When firing in a single-chamber furnace, the furnace and the material to be fired are in a common chamber or under a furnace dome. The combustion chamber and combustion chamber can be arranged at an angle one behind the other or separated by a small step. However, there are no additional separating elements (fire grids or stands) between the furnace and the material to be fired.
In the case of the standing or lying two-chamber furnace, the furnace and the combustion chamber or the material to be fired are separated by a horizontal perforated or slot bar or a vertical stud wall or a fire grate.
In principle, it is sufficient if the chemically bound water evaporates completely at 650 ° C at the latest when the ceramic is fired. From this point on, the thermally induced change is irreversible. The clay has become “broken”. Which type of fire or which type of furnace is chosen depends at least in part on the type of ceramic to be produced, e.g. B .:
- Unsintered, slightly fired earthenware, approx. 650–800 ° C,
- Classic earthenware with beginning sintering, approx. 800-1050 ° C,
- Stoneware and porcelain with complete sintering, approx. 1100–1350 ° C.
Function of a pottery furnace
According to the above definition, the following can be regarded as the functions of a pottery furnace:
- Pick up the material to be fired, create a stable base for the ceramics during firing - furnace base or threshing floor. The design of the threshing floor limits the load-bearing capacity. This also limits the stack height of the ceramic used and thus the height of the furnace and the amount of ceramic that can be burned at the same time.
- Heat generation, usually by burning.
- Heat retention or storage and conduction,
- Heat transfer to the goods to be burned.
- Control of the firing atmosphere (reducing, oxidizing).
The fundamental problem of all historical pottery kiln types in this context is the uneven distribution and transfer or maintenance of the peak temperature to the entire kiln in all areas of the kiln. The temperature gradient within a furnace can often be several hundred degrees and depends on various factors: the way the furnace is used, the fire is controlled with the help of fixed or temporary drafts in the combustion chamber or draft holes in the furnace dome or the ability to target specific parts of the furnace to fire. Due to the temperature differences, parts of the furnace charge may be over- or under-fired ( false fires ). Variations in furnace construction are u. a. trying this problem e.g. B. to eliminate in porcelain or stoneware ovens or to use targeted in pottery ovens. This can be done on the one hand by narrowing the longitudinal and cross-section of the furnace, on the other hand by using a special furnace construction (desk furnace, grate furnace). The simultaneous production of stoneware and earthenware in ovens in Bürgel, Waldenburg, Crinitz or Duingen makes use of the temperature differences in the oven. The way the stove is used is at least as important for the success of a fire as the "design" of the stove itself. This is often underestimated.
Components of pottery kilns
In order to be able to fulfill the above-mentioned functions, a two-chamber pottery furnace consists of different parts, which do not always have to be present together.
- Work room, also known as a work pit, heater pit or control room. Can also act as an ash pit. For stoneware stoves in the Westerwald, this area is called "Schlondes". The furnace is fired from here in the event of a fire.
- Firing duct: Area that connects the work / heater pit with the firing room.
- Firing or combustion chamber: place of combustion and heat generation, usually in front of or under the combustion chamber, with or without a grate construction .
- Separating elements between the furnace and combustion chamber: Fixed or mobile trowel stones or clay rollers made of perforated or slotted stoves for upright stoves; Fire grids made of clay or pot pillars, vertical stud walls, mobile trowel stones or clay rolls or clay bars or fixed arms over the pulls for lying ovens.
- Combustion chamber , with or without coupling: area behind or above the separating internals. If the combustion chamber is not firmly closed at the top, it is a shaft furnace, if the combustion chamber vault is closed it is a dome furnace. Shaft furnaces can have a temporary, mobile covering of the material to be fired from broken fragments or other material.
- Trains , chimneys / fireplaces, ventilation or drainage holes: Escape the hot combustion gases for the production and management of the train, to pull the heat through the furnace. The chimneys / chimneys can usually be closed with a slide.
- Salt holes: Openings in the furnace vault for salting the goods in stoneware stoves. But it can also be salted through the furnace opening.
Furnace types
In the ceramic technology literature, pottery furnaces and furnaces in the ceramic industry have been differentiated since the late 19th century on the basis of various criteria.
According to the type of furnace operation:
- Furnaces with intermittent or intermittent fire. This includes all prehistoric and medieval-modern ovens.
- Continuous fire ovens. Apart from a few forerunners in England, France and Germany (Brandenburg), these have only existed since the development (before 1856) and patenting (May 27, 1858) of the Hoffmann ring furnace . A distinction can be made between furnaces with progressive fire (ring furnaces) and fixed fire (tunnel furnaces).
According to the type of flame guidance:
- Furnace with rising flame (vertical pull from bottom to top). These are all "standing" pottery ovens, but also the oldest types of round ovens or rectangular ovens with side firing.
- Stove with diagonal / horizontal flame, often, but not necessarily, in connection with a chimney (horizontal draft). These are all “lying” one or two-chamber furnaces.
- Stove with downward so-called "flashing flame" in connection with different chimney or draft constructions (vertical draft from top to bottom). These are on the one hand round ovens, but also ovens with side firing.
- Muffle furnace in which the flames and the heating gases only flow around the combustion chamber or pass through tubes (Roman terra sigillata production). These can be built with a rising or a flashing flame.
There are additional classifications in European ethnology / ethno-archeology, prehistory and early history or medieval and modern archeology.
The number of the oven parts or "chambers", which are arranged one behind the other or one above the other and which are separated or not separated by built-in components, results in a division into:
- Single chamber ovens. The fuel and the ceramic are in the same chamber, sometimes separated by a small step.
- Two- or multi-chamber furnaces. Usually there are two chambers, that is, the furnace is separated into a furnace and a combustion chamber. There are also more chambers, especially in Asian ovens.
Furthermore, a distinction can be made according to the way in which the horizontal perforated or slotted tenon is constructed (stationary or mobile) or carried (for example with a central pillar, single or double, axially arranged central tongue or central stand, belt arches, barrel vault, etc.), whether the furnace is slightly advanced (built in front) or built under (pushed under) or whether it is a two-sided furnace or a radially arranged furnace.
According to the type of arrangement of the furnace and combustion chamber or the position of the separating element in multi-chamber furnaces, a distinction is made between:
- Standing ovens with different layouts. The combustion chamber and the combustion chamber are arranged on top of each other, separated by a perforated or slot fencing of the most varied of permanent or mobile construction methods, the train of the flames runs vertically.
- Horizontal ovens. The combustion chamber and combustion chamber are arranged one behind the other, horizontally or at an incline, with or without a permanently installed separating element in the form of clay pillars, pot pillars, fire grids or stud walls. Depending on whether there are separating elements or not, one should actually speak of horizontal single or double chamber furnaces. The ovens have a diagonal or horizontal draft. Modern stoneware stoves with firing slid beneath them, which are generally struck next to the lying stoves, are in reality a mixed form in which part of the combustion chamber should actually be classified as standing, while the rear part with the puffs is clearly located.
In summary, it must be stated that in view of the enormous technological variability of the pottery kilns, from 5 to 20 In the 19th century, none of the above-mentioned classification systems were consistently applied. It is best to rely on the separation of "standing" and "lying" ovens introduced in the literature in connection with the specification of the draft system of the respective oven, although this does not lead to a satisfactory or unambiguous classification for all oven types observed.
history
Originally clay was burned in the open air beneath the pile of fuel, later this was also done in ovens. Commonly used in the manufacture of faience , earthenware , and china were an upright, intermittent furnace, such as a deck furnace for coal firing. This has three floors separated by strong vaults, of which the two below are used for the actual burning of the pottery, whereas these in the upper room are only set to burn out. The heating gases enter the vaulted rooms through ducts, where they flow around the stacked pottery and then rise through the opening in the vaulted ceiling into the cooling room and finally withdraw through the chimney. The doors on each floor for entering the material are bricked up when the stove is to be heated. More useful are furnaces in which the flames first hit upwards in the first combustion chamber, then sucked off through the furnace base and guided in channels in the outer wall to the upper floors, which they then only pass through from bottom to top. While larger pottery items are placed directly on top of one another on the floor of the oven chamber, finer items are placed on top of one another in capsules (muffles) made from a fireclay- like material to protect them from contamination and to keep them from changing shape.
construction
At least since the 4th millennium BC Chr. The kilns with perforated floor, and dome in the Middle East are known. In 1977, during excavations of the ancient city of Mumbaqat in Syria, a pottery kiln was found and documented outside the fortifications on the bank of the Euphrates . The furnace is of a type in which the fire and combustion chambers are separated from one another by means of a perforated fence. The pottery stands on the perforated tennis court above the approximately 80 cm high fire chamber (hell) and is only exposed to the hot smoke gases, but not to the immediate fire. After each firing process, the masonry dome over the firing chamber had to be destroyed to remove the pottery. The fire chamber with dimensions of 1.00 × 2.60 m, with 60 cm protruding lengthways at the firing opening, was formed by a vault made of clay bricks over the floor slabs of fired bricks in the combustion chamber in the size of 53 × 35 × 10 cm have been laid (perforated tennis). The fire chamber located in the middle under the combustion chamber in the longitudinal axis had the dimensions 1.00 × 2.60 m, with 60 cm protruding from the combustion chamber and the walls tapering conically to the oval furnace hole with the dimensions around 30 cm wide and 60 cm high. The flue gases flowed from the fire chamber through side channels through openings on the edges of the perforated antenna into the combustion chamber. The hot air channels and openings were arranged symmetrically, originally 10 channels with a total of 16 openings.
The approximately 1.60 m high Greek pottery kiln was built from bricks and was dome-shaped with an air hole at the top. Above the furnace there was a perforated barn on which the pottery was placed during the firing process. Pottery shards were inserted which, as “test pieces”, allowed conclusions to be drawn about the progress of the firing process. The temperature was read by the potter from the appearance of the fire.
See also
literature
- G. Delcroix, JL Huot: Les fours dits "de Portier" dans l'orient ancien. In: Syria. Volume 49, 1972, p. 35 ff.
- Winfried Orthmann : Pottery oven with a perforated tennis court and dome . Halawa 1977–1979 (= Saarbrücker contributions to antiquity. Volume 31) Bonn 1981, pp. 61–62.
- Erich Kretz: A pottery furnace with a perforated tennis court and dome in Mumbaqat . Festschrift Martin Graßnick. Kaiserslautern 1987, pp. 267-270.
- Alfred Werner Maurer : Mumbaqat city complex on the Syrian Euphrates, results of the excavations in 1977. Philologus, Basel 2007.
- Prudence M. Rice: The prehistory and history of ceramic kilns. In: Proceedings of the Prehistoric and Historic Ceramic Kilns. Presented at the 98th Annual Meeting of the American Ceramic Society, 1996 in Indianapolis, Indiana, April 14-17. April 1996 (= Ceramics and civilization. Volume 7), Westerville, OH 1997.
For Central Europe and with literature on chapters 1–5 cf. now:
- Andreas Heege: Pottery kilns - Pottery kilns - Four de potiers. Research into early medieval to modern pottery kilns (6th-20th centuries) in Belgium, the Netherlands, Germany, Austria and Switzerland . In: Basler Hefte zur Aräologie. Volume 4, Basel 2007 (2008).