This portrait is painted in enamel on metal. The advantage of enamel over traditional miniature painting (watercolour painted on vellum or on ivory) is that it does not fade when exposed to light. But the process of painting enamels is less free than miniature and is fraught with danger. The first colours to be laid on the metal support have to be the ones that need to be fired at the highest temperature. Then more colours are added and the enamel is re-fired. The process ends with the colours that need the lowest temperature. Such labour meant that it was an expensive option.

Enamel was first practised in England by a French goldsmith, Petitot, at the court of Charles I (ruled 1625–49). It was reintroduced about 1680 by a Swede, Charles Boit, and achieved wide popularity with the work of a German, Christian Friedrich Zincke. Both Boit and Zincke were goldsmiths by training.

This enamel is by Willaim Prewett, who was a pupil of Zincke and worked between about 1735 and 1750. In the early 18th century a number of miniaturists also took up enamel so as to offer their clients a choice. But as the market for all portraiture grew and as miniature painters worked on ivory with increasing confidence and bravura, enamel painters such as Jeremiah Meyer decided to learn their rivals’ art. In the 19th century the rich colour of enamel made it popular as a copyists’ art.