The following process for making photographic copies of drawings in blue
lines on white background was invented by H. Pellet, and is based on the
property of perchloride of iron of being converted into protochloride on
exposure to light. Prussiate of potash when brought into contact with
the perchloride of iron immediately turns the latter blue, but it does
not affect the protochloride.
A bath is first prepared consisting of ten parts perchloride of iron,
five parts oxalic or some other vegetable acid, and one hundred parts
water. Should the paper to be used not be sufficiently sized, dextrine,
gelatine, isinglass, or some similar substance must be added to the
solution. The paper is sensitized by dipping in this solution and then
dried in the dark, and may be kept for some length of time. To take a
copy of a drawing made on cloth or transparent paper, it is laid on a
sheet of the sensitive paper, and exposed to light in a printing frame
or under a sheet of glass. The length of exposure varies with the state
of the weather from 15 to 30 seconds in summer to from 40 to 70 seconds
in winter, in full sunlight. In the shade, in clear weather, 2 to 6
minutes, and in cloudy weather, 15 to 40 minutes may be necessary. The
printing may also be done by electric light. The print is now immersed
in a bath consisting of 15 to 18 parts of prussiate of potash per 100
parts of water. Those parts protected from the light by the lines of the
drawing immediately turn blue, while the rest of the paper, where the
coating has been converted into protochloride by the effects of light,
will remain white. Next, the image is freely washed in water, and then
passed through a bath consisting of 8 to 10 parts of hydrochloric acid
to 100 parts of water, for the purpose of removing protoxide of iron
salt.
It is now again washed well in clean water and finally dried, when the
drawing will appear in blue on a white background.--_Proc. Eng. Club,
Phila._
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[PROCEEDINGS OF THE ENGINEERS' CLUB OF PHILADELPHIA.]