I have heard that you have found the secret of engraving on copper on white ground, as Elsheimer did. To bite the plate with acid, he covered the copper with a white paste. He then drew with the point down to the metal, which is of reddish color, and it looked as if he were drawing with red crayon on white paper. I cannot remember the composition of this write paste, although he communicated it to me.

Rubens in a letter to Pieter van Veen, 19 June 1622 - Quotes of Rubens, 1605 - 1625