Photographic Reproduction Processes Part 4

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, alb.u.men, 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 quant.i.ty 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 gla.s.s 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 gla.s.s 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 commercial Hydrochloric acid, 16 parts commercial 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 brus.h.i.+ng. 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 a.s.sumes 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:

FOR RED.

Beta-naphthol 4 parts Caustic pota.s.sa 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.

FOR ORANGE.

Resorcin 3 parts Water 500 parts

When dissolved add

Caustic pota.s.sa 5 parts

FOR YELLOW.

Carbolic acid, cryst 5 parts Water 500 parts

FOR PURPLE.

Naphthylamine 6 parts Hydrochloric acid, in 6 parts volume

Mix in a mortar, then add

Water 500 parts

FOR BLACK.

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.

FOR BROWN.

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 pa.s.sed 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, gla.s.s and porcelain, see further on.

PRINTING ON WOOD, CANVAS, OPAL, AND TRANSPARENCIES

_Printing on Wood._-To print on a wood block a design to be engraved on the same presents certain difficulties. In the first place, the sensitizing solution must not be absorbed by the wood, but remain wholly on its surface; then the photo film, although thick enough to produce an image sufficiently intense to be distinctly visible in all its details, should not scale or clip away under the graver, and not interfere in any way with the work of the artist; the least touch of the graver must reach the wood and make its impression. Lastly, the design should be permanent.

These difficulties will be avoided by adhering to the instructions given in the lines following.

The solution to render impervious the surface of the wood consists of

Common gelatine 5 parts Gum arabic 3 parts Castile soap 3 parts Water 100 parts

Dissolve by heat on a water bath.

To apply it, the wood is rubbed with fine sandpaper, then heated over a spirit lamp to about 86 deg. Fahr. (30 deg. C.) and upon it is poured in excess the liquefied and quite warm solution, which must be allowed to penetrate in the pores of the wood by letting it gelatinize, when it is wiped off clean. Nothing must remain on the surface of the wood. This done, and while still damp, the preparation is rendered insoluble by pouring over a solution of alum at 5 per 100 of water. The object of this preliminary operation is to render the wood impervious, and therefore to prevent the sensitizing solution to penetrate its texture. The wood is then heated again and its surface whitened with a little silver white or sulphate of barium, diffused in a small quant.i.ty of the following warm solution:

Gelatine 1 parts Alum 0.1 part Water 100 parts

While wet, this is smoothed with a jeweler's brush, taking care to leave on the wood, a very thin layer of the mixture, only sufficient to obtain a white surface which, by contrasting with color of the wood a.s.sists the engraver in his work. The wood should now be allowed to dry thoroughly, when it is coated with a tepid solution of

Isingla.s.s 3 parts Water 100 parts

and dried.

Now the sensitizing process differs according as whether the cliche is positive or negative. In the former case the preparation is sensitized with the solution employed in the black process, proceeding afterwards as usual; in the latter, that is, when the cliche is negative, the best process is the cuprotype.(12)

For printing, special frames are employed to permit one to examine the progress of the impression from time to time without the possibility of either the wood block or the cliche moving. These frames open in two.