Fire Gilding – The Butter of Gold
Fire gilding has been known since ancient times, with the Greek, Roman, and Chinese civilizations using fire gilding to enhance their metalwork. The process was extensively used during medieval times and Theophilus, in his 12th century treatise De diversis artibus, describes the process of preparing a mixture of gold and mercury into an amalgam paste. The paste was applied to the metal surface and with the addition of heat created a durable gold gilt surface. The amalgam paste prepared by the metalsmiths had the consistency of butter and was often described as the “butter of gold”. Theophilus describes the application of the gold-mercury amalgam (approximately 1 part gold to 8 parts mercury) “using brushes of hog bristles” to the surface of copper alloy castings. The castings were then placed on a clay tray and slowly heated over charcoals to evaporate the mercury, which has an evaporation point of 357°C, leaving the gold strongly bonded to the surface of the object. The layer of gold was burnished using agate and steel polishing tools creating a bright, high-purity gold finish that was both beautiful and durable.
Gilding was a commonly employed technique in Ireland and occurs on many pieces of Church metalwork from the late 11th and early 12th century including The Cross of Cong and St Manchan’s shrine. Scientific tests carried out on St Manchan’s shrine found the gilding to contain mercury indicating that the fire gilding technique was used to cover the copper alloy (Bourke 1988, 122-125). Although the gilding is badly worn on many of the metal castings found on the shrine, we can still imagine what it looked like in the 12th century, newly gilded and polished. The deterioration of the gilding was not simply due to age, in fact the James Graves publication The Church and Shrine of Saint Manchan (1875, 135-50) sheds some light on the care of the Shrine in the late eighteenth century: “I am informed by the Rev. Mr. Dardis that the former’s priest’s servants maid, of course without instructions, industriously set to work to clean the Shrine, and succeeded but too well in scouring off most of its gilding.” Fortunately, we are left with some excellent examples of the original gilding on some of the figures and animal motifs.
The use of mercury in the fire gilding technique was associated with health problems, well documented as far back as the 12th century. This was highlighted by Theophilus in reference to the preparation and application of the gold/mercury amalgam:
“Be very careful that you do not mill or apply gilding when you are hungry, because the fumes of mercury are very dangerous to an empty stomach and give rise to various sicknesses against which you must use zedoary and bayberry, pepper, garlic and wine.”
During the late 1800’s fire gilding was phased out as a new electroplating method of gilding was much cheaper, easier and safer. The gold/mercury gilding process can be found in research and restoration workshops to this day.