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 risk. 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.

This portrait is possibly a copy after a portrait miniature by the celebrated English miniaturist Samuel Cooper, and probably dates from the late 17th century. It depicts John Thurloe, a supporter of Oliver Cromwell during the English Civil Wars (1642–51). After the overthrow of the monarchy, Thurloe was secretary of state, heading a vast intelligence service that aimed to foil royalist plots. When the monarchy was restored in 1660, Thurloe was arrested for high treason, but his life was spared in return for supplying important government documents to Charles II.