A Claude glass paintings

A Claude glass (or Black Mirror) is a small mirror, slightly convex in shape, with its surface painted in a dark color.

Bound up like a pocket-book or in a transport case, black mirrors were used by artists, travellers and connoisseurs of landscape painting.

Black Mirrors have the effect of abstracting the subject reflected in it from its surroundings, reducing and simplifying the color and tonal range of scenes and scenery to give them a painterly quality.

They were famously used by artists in England in the late 18th and early 19th centuries as a frame for drawing sketches of picturesque landscapes.
The user would turn his back on the scene to observe the framed view through the painted mirror—in a sort of pre-photographic lens—which added the aesthetic of a subtle gradation of tones.
A 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 and the face screened from the sun."

The Claude glass is named for, a 17th-century landscape painter, whose name in the late 18th century became identical with the picturesque visual.

The Claude glass was supposed to help artists produce works of art similar to those of Claude. Reverend, the inventor of the picturesque ideal, advocated the use of a Claude glass saying, "they give the object of nature a soft, mellow shade like the coloring of that Master".

Black Mirrors were widely used by tourists and amateur artists, who quickly became the targets of satire.

The Davis observed their facing away from the object they wished to paint, commenting: "It is very typical of their attitude to Nature that such a position should be desirable".