Casting

Large-scale bronze statuary was widely produced by Roman sculptors. However only a handful have survived because, owing to the high demand for bronze, the great majority of such statues must have ended up in the melting pot. Most of these large statues featured emperors or adorned the temples of particular deities. Perhaps the best known surviving examples are the gilt bronze statue of the emperor Marcus Aurelius on horseback (once placed in the Capitoline Square in Rome) or the Horses of San Marco in Venice. Perhaps the best known surviving examples are the Horses of San Marco in Venice and the gilt bronze statue of the emperor Marcus Aurelius on horseback (once placed in the Capitoline Square in Rome and shown on a gold coin recently found in London).

The Romans used a range of copper and copper alloys. Copper is a soft metal which can be hardened by combining it with other metals - primarily tin and zinc - in order to produce alloys: bronze and a form of yellow brass. Large ungilded bronze sculpture was cast in an alloy called leaded bronze. The lead was added to help produce a high-quality cast.

Large statues were hollow-cast in piece moulds. Bronze workers must have used a variety of methods that allowed separate moulds to be made of the component parts and then reassembled.

One method was to prepare a clay sculpture. Moulds of the outside of each element would then be made in several pieces, using a medium like fast-setting plaster. The model would be removed and the mould reassembled, leaving a central hollow. Next, the inside of the plaster mould would be coated with a layer of wax of the intended thickness of bronze, and the remaining hollow filled with a clay and sand filling. The outer mould would then be removed and the wax model covered with a layer of clay. Finally, the wax would be melted out and bronze
cast into the subsequent spaces.

Normally the pieces of the statue were fusion-welded together. This involved placing a clay mould around the two edges to be joined. The molten metal was poured through the mould in order to heat up the joints. When the metal of the sections had reached the required temperature, the molten metal was prevented from escaping and the mould was filled. On cooling, the two edges had become fused together. Outside scars
caused during this process were removed during the finishing process.


The surface of the cast figure would have needed cleaning and polishing after the mould had been removed. Casting flaws had to be removed and patched. Flaws were caused either by small
obstructions in the molten metal or trapped air. Such flaws were cut away to a rectangular shape and replaced with a patch where the edges were hammered in. After final polishing such repairs would have been invisible.




Detail of the head of Hadrian found in London. Small rectangular patches were used to cover casting flaws. (Note. This object is in the collections of the British Museum; the photo is of an exact facsimile.)

Gilding

Several methods are known to have been used to cover a bronze surface with gold. From the 1st to 3rd centuries, a technique known as leaf-gilding was used. This is the method used on the Gresham Street arm. Thin sheets of gold leaf were applied to the finished cast surface using a binder or adhesive such as albumen (egg white). When laid on in this way, the gold is not in contact with the bronze surface, making it more likely for the gold leaf to flake off and be lost after time.

From the 2nd and 3rd centuries onwards, a superior method, mercury-gilding (sometimes known as fire-gilding) was introduced. There were two possible techniques of applying the
gold. Either it might be made into an amalgam with mercury and then be applied to the copper or bronze surface of the object; or, conversely, mercury might be applied to the surface of the object, so forming a copper amalgam to which gold leaf might be applied. In both techniques, the metal was then heated to expel the mercury, leaving a more resilient gilded surface.

The introduction of mercury gilding had consequences for the process of bronze-casting. Whereas previously it had been usual to include a percentage of lead in the bronze to produce more fluidity in the molten metal at a lower melting point, and so to make casting easier, this was no longer possible. The copper or bronze must have no significant lead content if mercury gilding is to be successful.



It has been possible to analyse the metal content of some of the earlier finds from London, so indicating how they may have been cast and gilded. Those pieces with a higher lead content are likely to date to the 1st and 3rd centuries AD and to have been leaf-gilded. Those with little or no lead may date to the later Roman period and been mercury gilded.



Detail of the newly-discovered statue fragment from London.
Gold was laid onto the bronze casting in thin sheets.
 
From: http://www.museum-london.org.uk/MOLsite/news/romanarm/statuesmade.html