Gilding is the process of coating a material with a thin layer of pure or alloyed gold so that it gives the impression of solid gold: Wood, metal, stone, parchment, in rare cases even textile.
Various techniques are used, of which only the traditional ones, in which metal leaf is applied to a substrate, belong to the gilder's field of activity.
History of gilding
Gilding with leaf metals was already known in antiquity and is documented for Greece and Rome in written sources, and in Egypt even by grave finds - the Roman records suggest that it was already poliment gilding at that time. Gilding was possibly developed in India. The technique was also used in China and Japan around the turn of the century. In Europe, the profession of gilding originated from the painter's trade and has existed in an organized form since the Middle Ages; since then, the center for production and processing in Germany has been in Schwabach (Bavaria).
Production of gold leaf
For the production of gold leaf, the so-called zain is cast from an alloy, an ingot a few millimeters thick. This is rolled under constant reheating to form a strip about 10 cm wide, which is cut into squares and hammered by hand in two separate stages: once the leaves lie between parchment paper, then between oxen blanks floured with fibrous gypsum. Ducat gold, with a copper content of almost 2 percent, is a popular alloy for gold leaf. Most suppliers of gold leaf carry it as 23-karat ducat double gold, meaning that the leaf has a thickness of 200-220 nanometers (as opposed to single gold at 100-120 nanometers and triple gold at 300-320 nanometers). Other leaf metals, such as silver, copper and the like, are produced similarly and are called impact metals.
The traditional types of gilding: poliment gilding and oil gilding.
A poliment gilding is high-gloss and, because of its sensitivity to moisture, is only suitable for interior use, such as picture frames. It consists of several layers:
The filling base, a simple glue solution, opens the pores of the wood and prepares it for the next layers. The stone base (a mixture of glue and stone chalk) and the subsequent chalk base, which consists of glue with champagne chalk, Bolognese chalk and Chinese chalk. It is applied in many thin layers and can even be so thick that it is possible to engrave it, punch it or even mold it like clay. An inexpensive alternative to carving directly into the wood. The chalk base is then sanded down to smoothen it. Any irregularities or scratches would be accentuated by the gold. Finally, the gilder applies the quench, a diluted glue solution.
The next layer, the poliment, consists of glue with bolus or egg white with bolus; in each case, the egg white acts as a binder.
Applying the so-called nets, a mixture of alcohol and water, again slightly dissolves the glue - enough to make the gold adhere, but not so much that it would sink. The gilder picks up the leaves with a brush and places them on the substrate where, due to the capillary forces of the water, they immediately adhere completely to the substrate ("shooting on" or "shooting up"). Finally, after drying, the surface of the gold leaf is polished with a hard tool, in recent times mainly with an agate stone. This process compacts the slightly elastic substrate and produces a mirror-like shine to the gold leaf. The finished piece is now visually indistinguishable from solid gold.
Oil gilding is suitable for exterior use - the water-soluble components of the poliment are extremely sensitive to moisture and would therefore be unsuitable. The so-called Mixtion is applied as a base, a mixture of linseed oil, turpentine and lead litharge. It has a very long drying time. The gold leaf used for oil gilding is called transfer gold. It is embedded in a booklet between layers of parchment paper, so it can be easily applied even in difficult weather conditions. Since the hard substrate cannot be polished, the gloss remains matte.
Fire gilding is the process of applying an alloy of gold and mercury, called amalgam, to a substrate of metal and then evaporating the mercury at high heat.
Bronzing refers to the use of gold or other metal dust as a pigment that is rubbed into linseed oil or another binder and painted onto the substrate in question.
Electroplating is the process of applying electrical voltage to a gold salt solution so that the gold precipitates on a surface. This technique is more recent and is used for electronic components, for example.