permanence; but
perhaps the best process for black, or generally useful neutral tint,
without silver, that has yet been offered to the public, I believe to be
the process alluded to with the bichromate of potash and sulphate of
copper, toned by an iron salt. * * * This process, the cuprotype (as
also the uranotype and manganotype) is applicable perfectly to films of
_albumen_ or gelatine on glass or porcelain, textile fabrics, parchment,
paper, tiles and many other substances besides paper."

THE DESIGNS.

HOW TO MAKE A NEGATIVE DRAWING

The drawing paper for designs to be reproduced by the cyantotype and the
other processes described in this book should be of a fine texture, free
from opacities and very white; and, as the design must serve as a cliche
it is a sine qua non that it be drawn with a very black ink and with
well-fed lines, especially those which are very fine. To obtain a
complete opacity, and, at the same time, to keep the ink quite fluid,
which gives great facility to the designer, one adds some gamboge (or
burnt sienna) to the India ink. The ink of Bourgeois, which is compounded
with yellow and can be diluted as easily as India ink, is excellent, so is
also the American ink of Higgins.(3)

As much as possible it is desirable to replace the colored lines
indicating the constructions, the axis, projections, etc., by differently
punctuated lines made with India ink. However, if the use of colors be
obligatory on the original design, one should trace the red lines with
very thick vermilion or sienna, the yellow lines with gamboge, and the
blue and green lines with a thick mixture of Prussian blue and chrome
yellow in different proportions.

One must abstain from applying washes of any tints on the original. If
necessary they should be brushed over when the reproductions are made;
moreover they can be often replaced by cross-lines more or less open, and
the shadowing represented by thicker but not closer lines.

Tracing paper is recommended instead of linen, which latter, on account of
its thickness and granulation, gives less satisfactory results in regard
to the transparency of the ground and the continuity of the lines.

To reproduce a design on ordinary paper--not too thick--or an engraving,
etc., the paper is rendered transparent by rubbing over on the back of the
original a solution of 3 parts in volume of castor oil in 10 parts of
alcohol, by means of