Originally Posted by
Brontosaurus
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With larger surface areas, for example vegetable-tanned leather cut to a standard American barber's size of 2-1/2" x 24" and sanded to ~180x to give it some tooth, a series of Xs (more like stray marks in this case) are marked out with the crayon (red or black), followed by charging the surface with pigment (red or black ferric oxide). The X's create areas where the pigment can bond immediately, followed by a more even distribution in the areas where there is no paste. (Wear a dust mask, goggles, and nitrile gloves in doing this.)
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