A Claue glass (or Black Mirror) is a small mirror, slightly convex in shape, with its surface tinted a dark colour. Bound up like a pocket-book or in a carrying case, black mirrors were used by artists, travellers and connoisseurs of landscape and landscape painting. Black Mirrors have the effect of abstracting the subject reflected in it from its surroundings, reducing and simplifying the colour and tonal range of scenes and scenery to give them a painterly quality.