Unlike fragile portrait miniatures painted in watercolor on vellum or ivory, which are prone to cracking, fading, and flaking, enamels are resilient, impervious to the effects of light, and retain their striking original colors over time. Partly for this reason enamel was considered ideal for reproducing famous paintings and treasured portraits in a reduced and luminous form. The complicated and labor-intensive process of enameling required the artist to fire numerous layers of colored metal oxide at different temperatures. This process made it difficult to produce a faithful portrait likeness, though masters of the medium were able create portraits of remarkable subtlety imbued with the sitter's personality. The heyday of enamel painting was the late-seventeenth and early eighteenth centuries. Among the enamel specialists was the German Christian Friedrich Zincke, who worked in England where he was patronized by Queen Anne, King George I, and King George II, the latter a great lover of enamels.