The paper
used must be the best drawing paper, thoroughly sized, and on this the
solution, consisting of 25 parts of gum arabic dissolved in 100 parts of
water, to which are added 7 parts of potassium bichromate and I part of
alcohol, is spread with a broad, flat brush. It is then dried, and if
placed in a cool, dark place will keep good for a long time. When used,
it is placed under the plan to be reproduced, and exposed to diffused
light for from five to ten minutes--that is to say, to about 14 deg. of
Vogel's photometer; it is then removed and placed for twenty minutes in
cold water, in order to wash out all the chromated gum which has not
been affected by light. By pressing between two sheets of blotting-paper
the water is then got rid of, and if the exposure has been correctly
judged the drawing will appear as dull lines on a shiny ground. After
the paper has been completely dried it is ready for the black color.
This consists of 5 parts of shellac, 100 parts of alcohol, and 15 parts
of finely-powdered vine-black. A sponge is used to distribute the color
over the paper, and the latter is then laid in a 2 to 3 per cent. bath
of sulphuric acid, where it must remain until the black color can be
easily removed by means of a stiff brush. All the lines of the drawing
will then appear in black on a white ground. These nigrographic tracings
are very fine, but they only appear in complete perfection when the
original drawings are perfectly opaque.
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