This portrait is painted in enamel on metal. The advantage of enamel over traditional miniature painting (watercolour painted on vellum or, from about 1700, on ivory) is that it does not fade when exposed to light. The process of painting with enamels is, however, less free than the miniature technique and is fraught with danger. The first colours to be laid on the metal support have to be those needing the highest temperature when firing. More colour is added and the enamel refired, the process ending with the colours needing the lowest temperature. Such labour meant that it was an expensive option.

Enamel was first practised in England in the 1630s by the Swiss goldsmith Jean Petitot at the court of Charles I. It was reintroduced around 1680 by a Swede, Charles Boit. Boit fell into debt and left Britain in 1714, going to work at the French court and becoming an ‘Agree’ of the Academie Royale in 1717. In that year Peter the Great employed him in Paris and Boit is recorded as having copied Jean-Marc Nattier’s painting of Peter’s second wife, Empress Catherine I, in enamel. The original three-quarter-length portrait was painted by Nattier at the Hague in 1717, and Boit worked either from the original or a copy. This enamel is signed faintly ‘C. Boit’, the ‘C’ and ‘B’ forming a monogram.