Originally posted by John Robinson
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I don't have any specific knowledge of the technique of application used by badge manufacturers, but I would guess a piece of cloth may not be so practical for mercury amalgam gilding considering how dangerous (and expensive) the material is and you wouldn't want to waste any by having it soaked into a cloth.
Here's an excerpted description of fire gilding from “The Grove Encyclopedia of Materials and Techniques in Art” by Gerald W. R. Ward, 2008, in which brushing is described.
"Fire Gilding [Mercury, amalgam or parcel gilding]
(i) Technique. This method of gilding made use of the property of mercury readily to form an amalgam with gold and was usually applied to copper or silver. The technique as practiced in the medieval period was described in detail by Theophilus (first half of the 12th century) in De diversis artibus, and many 19th-century workshop manuals (e.g. those by Spon and Thorpe) contain detailed accounts of this process. There were two basic methods applying either gold leaf or a gold-mercury amalgam to a surface that had already been treated with mercury.
In both methods the metal had to be carefully cleaned free of all grease and scratch-brushed to key the surface; then mercury was applied. In the more remote past metallic mercury was vigorously worked into the surface with a short, stiff brush until the surface was uniformly covered. From the 18th century the usual practice was to dip the cleaned metal into a solution of mercuric nitrate, which precipitated mercury on to the surface in a thin, continuous layer. This process was known as quicking.
In the first method of fire gilding then sheets of gold were laid on to the prepared surface, vigorously burnished and then heated to drive off the mercury. This seems to be the process described by Pliny the elder (AD 23/4-79) in his Natural History (xxxiii.32; Rackham, 1968, p.77). In the second process an amalgam of mercury and gold was first prepared by heating mercury to about the temperature of boiling water and then adding about half the weight of pure gold filings, stirring with an iron rod until the amalgam had the consistency of butter (hence the term ‘butter of gold’) The health hazards of handling mercury were recognized even in medieval times, and the prepared amalgam was stored under water. The amalgam was then applied to the freshly quicked surface by brushing. When the piece was covered it was gently warmed to evaporate the mercury, while the gilding continued to be worked with a stiff brush to ensure even coverage. Alternatively the amalgam could be carefully applied and brushed in with a wire brush that had itself been quicked to promote even coverage, then heated in a closed furnace. In the latter process, however, the piece had to be kept under constant observation, periodically moved and the amalgam layer worked if necessary to keep the gilding even as it formed. The freshly gilded surface had a rather matt, granular appearance, which could then be further treated by scratch brushing, burnishing or by the use of chemicals to produce the desired colour and texture."
Again, this is only for fire gilding. Later gold finishes used on zinc badges would have been a different story.
---Norm
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