Short answers, pulled from the story.
A portrait miniature is a small portrait painting, usually executed in gouache, watercolour, or enamel, originating in Renaissance art. Early examples were painted on vellum, chicken skin, or the backs of playing cards; during the 18th century, watercolour on ivory became the standard medium. Some were as small as 40 mm by 30 mm.
Samuel Cooper, born in 1609 and died in 1672, is considered the greatest English portrait miniaturist. He was a nephew and student of John Hoskins the Elder, and he painted on card, chicken skin, vellum, and on two occasions on thin pieces of mutton bone.
Portrait miniatures remained highly popular until the development of daguerreotypes and photography in the mid-19th century. The arrival of photography offered a faster and cheaper way to produce the same kind of intimate likeness.
James I of England gave large numbers of portrait miniatures as diplomatic and political gifts, using the form in a way that went beyond the typical private or family exchange. While most miniatures were intimate gifts given within families or by men in courtship, rulers used them as tools of statecraft.
Goya shaped lines in his ivory miniatures using water rather than dotting colour as most artists did. He claimed this technique was unlike the accidental ink wash method developed in England by Alexander Cozens around 1800. Between 1824 and 1825, Goya recorded over 40 miniature commissions on ivory.
On the 28th of April 1733, a fire destroyed the entire collection held by Sir Andrew Fountaine at White's Chocolate and Coffee House in London. The collection included works by Hilliard, the Olivers, and Samuel Cooper. The ashes were carefully sifted afterward to recover the gold from the incinerated mountings.