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The Primuline Or Diazotype Process

Primuline, discovered in 1887 by Mr. A. G. Green, an English chemist, is a
dye of a primrose color, possessing a great affinity for cotton fibers, to
which it is readily fixed by simply immersing the material for a few
moments in a hot solution of the dye. If the material so dyed be placed
in an acidified solution of nitrous oxide, the primuline is diazotized,
forming a derivative compound of a deeper color, which fades in the light,
and which in presence of amines and phenols gives rise to a variety of
dyes whose color depends on the reagent employed, while, when acted on by
light, the resulting compound is entirely deprived of this property. In
other words, the diazotized primuline acts as a mordant only when not
altered by the luminous action.

The chemical change light effects in the diazotized primuline is not well
known. It is pretty certain, however, that nitrogen is set free, for if
gelatine imbued with primuline be immersed in water after insulation,
nitrogen is set free and can be collected as usual in a tub filled with
water and inverted on the substance.

By itself diazotized primuline is slowly influenced by light, but quickly
acted on in presence of organic substances. It is more sensitive when
applied on cotton or paper than on wool, silk, linen, and such organic
compounds as gelatine, albumen, caseine, starch, etc. Its sensitiveness
is about one-tenth less with gelatine than with cotton.

The sensitiveness of diazotized primuline to light, when united to organic
substances and the different colors which can be obtained with the
unaltered compound, have given rise to an interesting printing method, the
invention of Messrs. A. G. Green, C. F. Cross, and E. J. Bevan, which
yields positive impressions from positive cliches. The manipulations of
the process are simple:

In a certain quantity of rain water, kept at nearly the boiling
temperature by an alcohol lamp placed under the vessel, dissolve per cent.
2 parts of commercial primuline, and in this immerse, by means of a glass
rod, some pieces of calico--free from dressing--turning them over several
times during the immersion. When the fibers are well imbued, which
requires from four to five minutes, remove the calico with the glass rod
and rinse it thoroughly in water. This done, wring out the superfluous
liquid as much as possible, and, finally, immerse each piece separately in
a solution of

Sodium nitrite, 7 parts
Hydrochloric acid, 16 parts
Water 100 parts

After turning the pieces of calico two or three times over, they are
rinsed to eliminate the acid, then drained and placed between sheets of
blotting paper to dry. All this, except the impregnation with primuline,
should be done in the dark room.

As said above, primuline is transformed by nitrous oxide into a diazotized
compound, and consequently the material is now susceptible of being acted
on by light. It does not keep, and should be exposed, etc., soon after
its preparation.

Paper is impregnated with primuline either by floating or brushing. The
best results are obtained with paper previously sized with arrowroot or
gelatine in order to keep the image entirely on the surface of the paper.

Linen, silk and wool are treated as calico.

The cliches should be positive to obtain positive expressions and somewhat
more opaque than those employed in the processes before described, else
vigor and intensity could not be obtained. Here we must state that the
primuline process seems to be better adapted for the reproductions of
drawings, such as made for the black process, and of opaque photo-cliches
in lines, or white and black, than for printing in half tone.

When the material to print upon is thick and wholly impregnated with
diazotized primuline, it is advisable, since the insulation could not be
prolonged to effect the change through, to expose the back of the material
for a certain but short period in order to clear it. This is especially
advantageous when the cliche is not of good intensity.

During the exposure, which varies from 30 seconds to 10 minutes and more
by a dull light, the progresses of the luminous action is seen by the
bleaching of the material which assumes a dingy coloration. But in order
to ascertain when the decomposition is complete on the ground of the
image, it is well to use tests as in the cyanofer process, dipping one
of them in the developer from time to time.

The developers are compounded as follows:

Beta-naphthol 4 parts
Caustic potassa 6 parts
Water 500 parts

Rub the alkali and the naphthol with a little water in a mortar and add
the remainder of the water.

Resorcin 3 parts
Water 500 parts

When dissolved add

Caustic potassa 5 parts

Carbolic acid, cryst 5 parts
Water 500 parts

Naphthylamine 6 parts
Hydrochloric acid, in 6 parts

Mix in a mortar, then add

Water 500 parts

Eikonogen, white crystals 6 parts
Water 500 parts

Pulverize the eikonogen, add the water and, at the same time, the material
on its removal from the printing frame, and keep in motion until the
development is effected.

Pyrogallol 5 parts
Water 500 parts

After the development, which requires but a few moments, it suffices to
wash the material to fix the image by eliminating the soluble compounds.
However, for purple the material should be passed in a dilute solution of
tartaric acid and not washed afterwards; it should remain acid.

When it is desirable to obtain an impression in several colors, the
various developers are thickened with starch, then locally applied with a
brush on the image, which is always visible after exposure.

For printing on wood, glass and porcelain, see further on.

Next: Printing On Wood Canvas Opal And Transparencies

Previous: The Aniline Process

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