Sheet Metal

¶ But protection from mechanical injury is not ensured by a thin coating of tinfoil. If that further protection be desired the back of the painting may be lined with sheet metal. According to the size of the picture, sheet copper or brass, or even sheet iron coated with tin or zinc, may be employed. Paintings executed on canvas may be fastened to the sheet of metal by the shellac¹ cement before named or with thick amber varnish. Thus the back becomes protected both chemically and mechanically by the same contrivance, and a high degree of durability is thus assured for the picture.

¶ When it is a question of producing new pictures the painting may be executed directly on metal. Such a procedure applied to sheet copper was frequent with Dutch miniaturists: the flawless condition of their works justifies this method from the present point of view. Yet, as this use of copper involves some risk that its green and blue oxidation-products may give rise to discoloration, a more appropriate painting-ground is offered by sheet aluminium, which yields only colourless compounds. Moreover, when aluminium is exposed to the atmosphere there is formed on the surface a transparent and imperceptible film of oxide which retains oil-colours very firmly. Even on unprepared surfaces of aluminium it is possible to paint very easily, for the metal possesses a peculiar 'tooth' in relation to the paint so that one can readily lay on successive coats of paint, stroke by stroke. Some years ago I covered a piece of sheet aluminium with a coating of oil-paint and exposed it in the laboratory to all the accidents of the place. The paint remains sound to this day and shows no tendency to crack or peel.

A sketch executed on strong sheet aluminium stands in a greenhouse, where it receives all available sunshine and is exposed to great variations of temperature: after exposure during six months of spring and summer it showed no signs of change for the worse. There is therefore good reason for contending that in sheet aluminium we possess an ideal painting - ground, especially for work in oils.

1	Perhaps marouflage is preferable (see Spirit-Fresco).