A Claude 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.
They were famously used by picturesque artists in England in the late 18th and early 19th centuries as a frame for drawing sketches of picturesque landscapes.[1] The user would turn his back on the scene to observe the framed view through the tinted mirror—in a sort of pre-photographic lens—which added the picturesque aesthetic of a subtle gradation of tones. Father Thomas West in his A Guide to the Lakes (1778) explained “The person using it ought always to turn his back to the object that he views. It should be suspended by the upper part of the case…holding it a little to the right or the left (as the position of the parts to be viewed require) and the face screened from the sun.”