The traditional craft of gilding consists in applying metal leaf to workpieces, in contrast to coating them with metallic effect pigment ( shell gold, "gold bronze"), which is part of the painter's craft . The chemical processes, with the exception of fire gilding , only developed in modern times.
Gold is not only noble in appearance, but also one of the most corrosion-resistant metals. In its pure form it is unsuitable for components and everyday objects because it is rare, expensive and has poor strength. The coating of readily available materials that are suitable for everyday use with a layer of gold has been valued since ancient times .
The most important group of materials that are particularly suitable for gold plating are metals and metal alloys, especially steel, stainless steel, zinc, brass, bronze, copper, silver and many more. Porcelain, glass, ceramics, wood, paper, leather, plastic and in rare cases even textiles are gilded on non-metallic materials.
Thanks to the latest technology, almost all organic and inorganic materials can now be permanently gilded. This is done through new processes in electroplating .
Historically, the most important functions of gold layers and coatings are:
- Decorative appearance
- Valuable and prestigious appearance
- Significance for cult activities and religion
- Corrosion resistance
In the modern era, the following functions were added:
- With gold-plated electrical contacts and plugs, a small foreign layer resistance
- Special properties for semiconductor technology , for example bondability
Gold coatings can consist of pure gold, but in most cases a gold alloy suitable for the respective purpose is chosen as the coating material. The gold alloys differ in color: rose gold, yellow gold, white gold and rose. Green and blue colors are also possible. The color tones are achieved by the following alloy elements: copper for red gold, silver , cadmium and zinc for yellow to white gold, nickel for white gold and indium for blue gold.
For technical applications, the hardness and wear resistance of the coating are more important than the color. These properties are mainly improved by the alloying elements iron , cobalt and nickel. Such a gold plating is called hard gold . The alloy elements are said to improve hardness and wear resistance. The amount of gold saved, on the other hand, is negligible.
The two basic techniques of gold plating are mechanical and chemical . The mechanical gilding types are the oldest processes; sheet gold was flattened. A distinction is made between gloss and matt gold plating, poliment gold plating, oil gold plating, mordent gold plating and gold plating behind glass. With the exception of fire gilding, chemical gilding came about much later. With electroplating technology , chemical gold plating reached its peak.
When gilding a craft business is called, the primary is engaged in the processing of wood, metal or plastic surfaces.
In painting , especially in panel painting , the application of gold leaf to a surface is also known as gilding. This subsurface can be of different types. In illumination , the gold can have been applied directly to the parchment or to a gold background ; this gold ground is usually found in panel painting. It is a primer (poliment gold plating) , which always consists of a bolus and a binding agent.
In the blacksmith's art , high-quality work such as lattices , church furnishings , grave crosses, etc. have been accentuated with gold leaf or even gilded over the entire surface , mostly by the blacksmith himself, for many centuries . The weather-resistant oil gilding with mixture or application oil is used.
The cementation is based on the fact that when immersing z. B. a copper sheet in a gold (III) chloride solution, the nobler gold ions are reduced and deposit on the surface of the copper, with copper being oxidized to copper (II) ions .
The copper surface must be cleaned or etched well beforehand and the solution must be made slightly alkaline . After the gold has been deposited in a layer of a few micrometers, the adhesion can be increased if the object is heated at temperatures around 700 ° C, a diffusion zone being created between the copper and the gold layer.
The method of cementation is already known from the metallurgy of the pre-Columbian Andean culture. To produce the gold chloride solution, gold foils were probably dissolved in a hot solution of potassium aluminum sulfate , potassium nitrate and sodium chloride , which, however, took several days.
Most gold coatings are applied by electroplating processes. This method has completely replaced many older methods. The usually metallic objects are immersed in a gold electrolyte and a gold coating is deposited by applying an electrical direct voltage . The first patent for the deposition of gold from cyanide-containing baths was granted in 1840 to George Richards Elkington and Henry Elkington.
During the electroplating process Gold (I) - or gold (III) ions usually from cyanide electrolytes at acidic, neutral or alkaline pH values by electron acceptance cathodically to elemental gold reduced . By varying the temperature, voltage or current strength and electrolysis time, layer thicknesses of 0.1 µm to 200 µm can be created. These processes are used in electrical engineering for gilding electrical contacts or in the surface treatment of electrical soldering surfaces on printed circuit boards . In these cases, the gold plating serves to prevent the contact surfaces from corroding .
In the field of electronics four different types of gold plating are applied:
- Soft gold plating in semiconductor technology . It is used to gold-plate the connection surfaces on semiconductor chips . The so-called bond wires , which are usually made of gold and which represent the electrical connection between the semiconductor chip and the connection pins located on the outside of the chip housing, are contacted at these connection surfaces . The Knoop hardness of the soft gold coating is in the range of 60-85.
- Hard gold plating of electrical contacts. This has a Knoop hardness of 120–300 and a purity of just over 99%. The remaining components are small amounts of nickel or cobalt . For chemical reasons, this form of hard gold plating cannot be used in the field of semiconductor technology for contacting semiconductor chips.
- Hard gold plating of electrical contacts on circuit boards , as is common with circuit board connectors . Hard gold plating is necessary because the plug contacts of the circuit board connectors are subject to higher mechanical loads than other areas of a circuit board.
- Soft gold plating of the soldering surfaces on electrical circuit boards. This gold plating serves to protect the copper soldering surfaces from oxidation during storage with the aim of being able to use less aggressive flux during the soldering process . Only those areas of the circuit board that contact electronic components in the further production process are gold-plated, the rest of the circuit board is covered by a passive solder mask. By equipping the circuit board with electrical components and the subsequent soldering process, the gold layer dissolves with the solder and loses its importance.
With gold plating, only certain base materials can be gold plated. For example, the copper used in electrical engineering can not be permanently gold-plated because of its good electrical conductivity , since copper has a tendency to diffuse through the thin gold layer, to accumulate on the gold surface and to oxidize there. This can be remedied by multi-layer electroplating processes, in which a thin layer of nickel is first electroplated onto the copper carrier and only the nickel layer is then gold-plated. However, the additional nickel layer on the outside results in poorer high-frequency properties of the line due to the skin effect .
The conductor material aluminum , which is common in electrical engineering in addition to copper, tends to form the undesired intermetallic compound AuAl 2 when it comes into contact with gold, for example with gold-plated switching contacts , which is also known as purple plague because of its typical purple color .
Ceramic burn-on gold plating
Using specially prepared solutions of gold salts and adhesion promoters such as rhodium (III) oxide , a metallic gold application can be achieved on glass and ceramics . Depending on the parameters, the metal appears matt or shiny after firing. The process is used in dental preparation , but also for gilding ceramics and glasses.
For gilding porcelain , one uses either gold (III) chloride precipitated by oxalic acid or iron (II) sulfate and mixed with basic bismuth (III) nitrate as a flux; it has to be polished after being burned on and is therefore called polishing gold . Such items should not be used in microwave ovens as the gold plating will be damaged.
The gloss gold plating (also gloss oil gold plating and Meissen gold plating ) directly provides a shiny surface. It is obtained by burning in a solution of sulfur gold or molten gold in sulfur balm , but it is much less durable; if it is brought a few times against the hair of the head, it will be removed as if by a fine file.
Roll gold plating
The gold plating (doublé) is based on the mechanical union of foreign metal and gold sheet through strong pressure. The process is used to manufacture semi-finished products from which inexpensive gold jewelry is made. The rolled gold plating is indissoluble and very durable. A disadvantage of rolling gold plating is that the objects are not covered on all sides. Rolled gold doublé of a certain thickness is also a quality mark.
Evaporation in vacuum
Rubbing gold plating
This group of processes should rather be viewed as historical and has been largely replaced by electroplating processes. In particular, all processes that work with mercury or amalgam are very harmful to health and represent a major environmental burden.
Red gold plating is produced by dipping the hot piece in glow wax , green with silver-containing gold amalgam. To mattify the gold-plated objects, they are heated with a molten mixture of saltpeter , alum and table salt , which develops chlorine and dissolves gold. In order to avoid the disadvantages of fire gilding, the objects are also electroformed with mercury, then abundantly with gold and again with mercury and smoke. Copper consumes more gold than tombac , and the gilding does not appear vivid on silver. Gold-plated silver is called vermeil .
Iron and steel are amalgamated by boiling them with mercury, zinc, iron (II) sulphate , water and hydrochloric acid and then treated like tombac. With cold gold plating on copper, brass , nickel silver and silver, gold tinder ( canvas soaked and burned with gold (III) chloride ) is rubbed onto the bare metal with a finger or a slightly charred cork dipped in salt water and polished with blood stone. It is much less durable than fire gilding on silver, but more beautiful than this and is therefore often used on very weak fire gilding. In wet gold plating, copper, brass, tombac, nickel silver, copper-plated steel, tinplate are dipped in a dilute gold chloride solution or in a boiling mixture of such with potassium carbonate , then rinsed, dried and polished. But you always get only light gold plating.
For green gold plating , silver nitrate is added to the gold (III) chloride . For wet gold plating of silver (also: Greek gold plating ) it is immersed in a solution of mercury (II) chloride and gold in nitric acid . Iron and steel are first copper-plated or, after etching with nitric acid, they are immersed in ethereal gold (III) chloride solution . This gold plating, which has to be strengthened by repeated immersion, adheres even more firmly to steel that has been etched matt with nitric acid. The ether gold plating is never permanent. Iron and steel are therefore copper-plated and then the hot solution with potassium carbonate is used. The steel is also connected to zinc with a wire and immersed in a solution of gold cyanide in potassium cyanide (cyanide) and potassium thiocyanate .
This very old method was already used by the Egyptians. Despite the name analogy, it has very little in common with hot-dip galvanizing . In fire gilding , a metal object, usually steel, non-ferrous metals or silver, is covered with gold amalgam and then heated. The mercury from the amalgam evaporates and the pure gold remains. Then the surface can be polished with blood stone , also known as hematite . For the use of this procedure, strict requirements according to the Emissions and Occupational Health and Safety Act must be met.
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- The J. Paul Getty Museum: Film about the technique of gilding