In relation to the mechanical properties of metals, the term ‘malleability’ (from the Latin ‘malleus’ – a hammer) is seldom encountered. The reason is that malleability is important only in one specific process, namely the production of metal foil by hammering.
There is no doubt that gold takes first prize in the malleability stakes, which allows the production of gold leaf so thin that light can shine through it. No other metal is capable of being beaten to such an extreme thinness. The reason for gold’s virtuosity in this regard was explained in a 1977 paper by Nutting and Nuttall1. It is not so much the metal’s softness but rather its inert nature, which means that its surface is completely free of any oxide film (which in other metals results in foils disintegrating when they reach a certain thickness).
No other metal is capable of being beaten to such an extreme thinness.
Remarkable reduction in thickness
It is a remarkable fact that a sheet of gold of thickness 2 mm can be rolled then hammered (or beaten) to a foil of thickness 100 nm (that is 0.1 μm or 0.0001 mm or 500 atoms) without any intermediate annealing. This is equivalent to a reduction in thickness of 99.996%. The process is shown in a video published by Wrights of Lymm, the last British manufacturer of gold leaf.
Gold leaf is produced in a wide range of finenesses, from 24 carat to as low as 6 carat, and colours, which depend on the alloying elements.
It is available in three thicknesses – standard, double (somewhat misleading as ‘double’ is only 20% thicker than standard) or triple. Gold leaf is sold in books, typically consisting of 25 80x80 mm sheets interleaved with tissue paper. Gold leaf is applied by first coating the substrate, usually stone, wood, glass or leather, with an oil-based size containing white of egg or other suitable ingredients. See, for example the re-gilding of the Royal Exchange Grasshopper. Many examples of beaten gold from antiquity are preserved in museums around the world, emphasizing the durability of this form of decoration.
Gresham grasshopper. The grasshopper looking south-west with St Paul’s Cathedral in the distance and the Bank of England on the right. Image provided courtesy of DorotheaRestorations.com
Although the method of producing gold leaf remains, in principle, the same as that used in ancient Egypt, nowadays machines are used for the initial stages, but the final beating may still be performed by hand with a wooden-shafted cast-iron hammer. The process that was used at Wrights of Lymm involved as the first stage, known as the ‘cutch’, two hundred 30-mm squares of gold cut from a rolled ribbon with a thickness of around 25 μm (similar to domestic aluminium foil), which were placed between sheets of Montgolfier paper and beaten to around 3 μm.
The sheets were then quartered and placed between sheets of polyester PET film for the second stage of beating known as the ‘shoder’, which reduced the thickness to under 1 μm. The squares were then quartered again and placed between sheets of goldbeater’s skin (made from deer or ox intestines) for the final beating known as the ‘mould’. The goldbeater’s skill lies largely in their control of the hammer and the pack of foils during this stage, which lasts some three-and-a-half hours. The gold leaf is then taken from the skins, leaf by leaf, and deftly cut by hand into 80-mm squares before being placed in books holding 25 sheets of gold leaf interleaved between tissue paper that has been ‘rouged’ (treated with ferrous oxide) to keep the gold from adhering to the paper.
If gold leaf is to be used for outdoor application, where it would be carried away by the slightest current of air without a backing, transfer or patent, leaf is used. Here, sheets of non-rouged tissue paper, somewhat larger than the gold leaf, are inserted into the pages of the already filled books. The leaf adheres to them, thus enabling the gilder to apply them with ease in the open air.
Silver, platinum and palladium can also be beaten into leaf but are less malleable and cannot therefore be beaten as thin as can gold.
1Gold Bulletin, Volume 10 (1977), pages 2-8
Shop Sign. The sign of an arm and a hammer was traditionally associated with the goldbeating trade. The gilded sign shown here (made of wood, iron, and gold plate) was formerly hung outside the premises of the goldbeaters George M. Whiley Ltd on Whitfield Street in the city of London. The firm was originally established in the eighteenth century and removed the sign when refurbishing its premises in 1914. Image credit: Museum of London