Collotype and Photo-lithography

by Julius Schnauss

The gelatine is first soaked in the water for one hour, and then complete solution effected at a moderate temperature, and the solution filtered through best filter paper. Boiling heat is never required. Japanese filter paper is the very best. Heat will be necessary during the operation, which may be conducted either in the drying box or a warm oven. When filtered, add, with constant stirring, 8 ccm. of a solution of bichromate of ammonium in water (1 in 8, so that 8 ccm. solution contains 1 gramme of the ammonium salt), which should also be thoroughly filtered before addition to the gelatine.

The combined solutions being well incorporated, clear and free from air bubbles, should, as previously described, be heated to 40° or 50° C., poured upon the warmed plate, and at once conveyed to the drying box, the temperature of which has been slightly raised. The box should be closed, and gradually increased to 50° C., at which heat it should be steadily maintained until the drying is complete, which usually occupies one to one and a-half hours, varying somewhat according to the thickness of the coating and the strength of air current, which is necessary to the proper and rapid drying of the films. It may, under some circumstances, be necessary to augment the admission of air; this may be done by slightly raising the lid of the drying box. The current must freely circulate in all parts of the box, and remain as constant as possible. If subjected to sudden fluctuations in the air current or temperature, the plates will dry unevenly, and endless troubles ensue.

The Grain.—The so-called grain which is necessary for the printing first shows itself on the completion of the washing of the plates after printing in the copying frame. It is a peculiar, net-like formation, varying in fineness, and similar in character to the unwelcome appearance known to carbon printers as reticulation. It arises in consequence of the uneven distribution of the soluble layer in comparison to the substratum. In Collotype, its production is promoted by thicker layers and slower drying at a temperature of 45° to 50° C. The addition of isinglass also seems to aid the production of grain. Certain varieties of gelatine which possess great power of absorption give a smaller grain. Creutz’s gelatine may be mentioned as an example of this class. An addition in small quantities of dilute acid, particularly those of organic origin, as acetic or tartaric, tend likewise to the production of grain. Insoluble powders, as of glass or flint stone, may be added, but only in the very finest state of division, and in very small quantities.

Formula with Isinglass. Two grammes of the best Russian isinglass are cut up into the smallest possible pieces with a pair of sharp scissors, soaked in 25 cc. of water for 12 hours, heated nearly to boiling point until solution is effected, and then filtered while hot through close flannel. Cover six grammes of good Collotype gelatine with 50 ccm. of water and allow to stand, heat nearly to boiling point and thoroughly incorporate the two solutions, again, while hot, filter through paper or close flannel. To this is added 11 cc. of the filtered solution (1·8) of bichromate of ammonium, as previously referred to. The complete solution is then heated to 50° C., or should it exceed that temperature it is allowed to cool to it, and the plates warmed, coated, and dried in the manner previously described. It is not advisable to overcrowd the drying-box by attempting to dry too many plates at the same time: it necessitates a too frequent examination of the contents and so interferes with the evenness of the drying, and as this is a point of even vital importance no risk should be incurred.

The plates being properly dried allow them to remain in the box for some time longer, the source of heat being either removed or extinguished, and the whole allowed to cool gradually. If the plates are removed too quickly in their fully-heated condition—particularly when thickly coated—the too rapid contraction of the glass will crack and destroy the film; by gradual cooling such accidents are avoided. When finished the plates may be kept in a cool, dry place for two or three days. Many operators keep them this length of time purposely, believing they thereby obtain finer results. The plates so kept must, of course, be thoroughly shielded from actinic light.

Negatives, with regard to their suitability for Collotype, may here be considered, as the after result naturally depends considerably on their adaptability to the process. They should be quite free from fog or yellow stain, and for half-tone subjects not too intense. The greatest difficulties will present themselves in the production of portraits or such work from nature by the Collotype process. Photographer and printer ought, so to speak, to work hand-in-hand. The negative must show the finest gradation from clear glass in the shadows to the necessary density in the high lights, quite free from veil or fog, and intensified very little, if at all. They should be produced in a studio where the light is thoroughly under control. If not to be stripped all negatives should be taken on patent plate and protected by a coat of varnish, sufficiently thin to offer no obstacle to the production of sharp prints, and yet sufficient to obviate in a great measure the risk of damage by the pressure it is subjected to in the pressure frame in contact with the unyielding surface of the Collotype plate. The risk of breakage at this stage always renders stripped films preferable, their elasticity better enabling them to withstand the pressure. The gelatine layer applied to the negative before stripping must be as thin and even as possible, as from thick or uneven films it is almost impossible to produce a sharp print. If the negative is of a line subject, and it is desirable to have clear whites that the impressions may resemble as much as possible the original, the lines in the negative must be clear, but the remaining portions of great density. The ink for such work should consist only of pure chalk printing ink, which will be generally found to give stronger contrasts than the usual mixed Collotype inks.

Retouching Negatives and Plates.—The negatives must have all defects made good by the retoucher, all blemishes being moderated as far as possible. Retouching on the Collotype plate or print should not be required; but this is seldom so, as during the printing the negatives frequently sustain slight damages. Retouching the printing surface is seldom very successful. The alteration easiest effected is the removal of small light spots which refuse to take the ink, and such may be removed by touching with a fine brush dipped in a strong aqueous solution of tannin, this alteration being made upon the finished plate after it has been washed and dried. With this tannin solution inscriptions may be inserted which have to appear black in the print. Or a solution of gum arabic, to which has been added a solution of bichromate of potassium, may be used, and after such addition the plate is once again exposed to light. Writing which has received too much exposure, and which should appear in the finished print black on a white or light ground, often appears white. This defect arises from the gelatine on which the light has had little or no action swelling to such an extent that the letters are buried or sunk into the plate too deep to be touched by the roller when the plate is inked. It may be rectified by further thinning the ink with varnish. The removal of dark spots from the print layer can never be completely effected. Ammonia, cyanide of potassium, oxalic and other acids have from time to time been recommended, but this treatment will prove effective for only a few impressions.

Negatives should therefore be very carefully and systematically retouched and carefully preserved from damage during the printing. Another method of applying writing, such as the names of firms, &c., is to get them printed from small type on thin gelatine films and attach them to a clear or erased portion of the negative during the printing in the pressure frame. Writing so added will appear as white letters on a black ground. It will often be found practicable to attach the written or printed matter to the original while photographing, and so include it in the negative at one operation.

Printing under the Negative.—The printing of half-tone plates is best performed in diffused light; to hit the exact exposure is as important as it is difficult, and depends not only on the actinic power of the prevailing light, but also on the quality of the negative and the sensitiveness of the layer. The greater the amount of bichromate the latter contains, the greater its sensitiveness, and consequently the exposure is shorter than for one containing less of the sensitising salt. A thinner layer must not be printed so long as a thicker one, or dark prints, lacking in contrast, will result. On the other hand, a thicker layer may be submitted to a comparatively longer exposure without harm; the exact time must not be greatly exceeded or it will happen that the blackest portions, written titles for instance, will not take the ink. During printing, one may with advantage shade such portions a little. The examination of the back of the Collotype plate is the safest method of judging the correct exposure during the copying, but every precaution must be observed during such examinations that neither the plate nor negative be moved from their original position. Only when the negative is very clear may the picture be seen by reflected light as a positive; it will in such cases appear on a dark ground. It will be found difficult, particularly for a beginner, when making an examination in the copying frame, to distinguish between the dark original picture and the pale brown image of the Collotype plate, as they naturally completely cover each other. By the dark brown tone of the uncovered margin of the print layer a certain indication is afforded of the completion of the copying.

An Actinometer may be used as an exposure gauge, as in carbon printing, and as a slight indication of the necessary exposure. When using the small cube[H] photometer with chloride of silver paper, five to seven tints may be printed, but the different modifying circumstances previously referred to must all be duly considered. Under a clouded sky in winter, using a dense negative, it may take a whole day to complete the copying, while in clear direct sunlight, perhaps, from the same negative, a quarter of an hour would suffice.