Julius Schnauss

Investigations on Collotype.

AUGUST ALBERT[L] is of opinion that it is by no means a matter of indifference whether bichromate of potassium or bichromate of ammonia is used in the preparation of the Collotype plates; on the contrary, he believes that the latter salt materially conduces to interruptions in printing, especially in unsettled weather. The results of his experiments with the two salts in the Collotype process, and of their behaviour in combination with different sorts of gelatine, are as follows:—

To 1000 cc. of water are added ten drops of a saturated solution of chrome alum; this mixture, after having stood for a quarter of an hour, must, if shaken up, show an opalescent appearance, and a slight milky turbidity; if too much chrome alum has been added, the water acquires a greenish colour. The gelatine is now added, and, for about fifteen minutes, kept at a temperature of 134°, when the chrome salt is added, stirring all the while, and the temperature of the water bath raised to 150° to 160°. The temperature and time must strictly be adhered to. The jelly thus prepared should be kept at least ten hours in a solidified state previous to using.

Hard Gelatine.—Plates prepared with this, show after drying, a shiny, transparent film, more or less according to the quality. Such plates give hard prints, and are soon used up in printing, the finest and even the middle tints becoming weaker, and the shadows gradually coalescing, specially in the case of short exposure and very dense negatives, and they are generally useless after a few impressions have been taken. If ammonium bichromate is used, together with hard gelatine, then, as a general rule, it is impossible to obtain more than one or two prints from each plate. By washing the gelatine, these inconveniences are not at all removed, no matter which of the two salts has been used.

Soft Gelatine.—If this is prepared with potassium bichromate, then the plates will have a dull but rough and untransparent film, which will be found very unserviceable for printing, as the prints will lack vigour; the high lights will tone, and the whole picture appear blurred. If, however, ammonium bichromate is used in conjunction with soft gelatine, the plates will also have a dull appearance, but though they work somewhat better than those prepared with potassium bichromate, they will not be of sufficient durability, the finer tones soon disappearing during printing. Therefore, neither hard nor soft gelatine are suitable for Collotype work, as the printing operations are rendered more difficult. If both sorts are mixed, the results will be somewhat better, but not like those obtainable with middle hard gelatine, which must be accepted as the most suitable kind for Collotype work.

Middle Hard Gelatine, manufactured by Ferd. Friedr. Creutz, absorbs five times its own weight of water at 66°, without signs of disintegration after standing therein for thirty-six hours. If prepared with it and with chemically pure potassium bichromate, the plates after drying have a beautiful matt surface, from which long numbers can be printed without any falling off of the half tints, and, if the printing is done carefully, even the details in the shadows will be maintained to the last. This bichromated gelatine requires also only a very small amount of re-damping, reproduces faithfully the character of the negative, and the jelly can be used even after a period of four days. If, however, the work in hand requires a harder bichromated gelatine—as, for instance, reproductions of black and white—then the addition of a small quantity of ammonium bichromate will have the desired effect; in some cases equal quantities of the bichromates of potassium and ammonium. If ammonium bichromate is used instead of potassium in conjunction with middle hard gelatine, then the plates will acquire a shining, transparent coating, the manipulation of which is very difficult in printing, and which work too hard, being serviceable, at any rate, for reproductions without half-tint. If, however, the middle hard gelatine is washed before use, it will give, when mixed with the chromic salt, transparent films of insufficient durability. Middle hard gelatine should therefore be used unwashed for Collotype work, and mixed with chemically pure potassium bichromate.

The Production of Grain in Collotype plates takes place in the last instant of drying the plates in the drying-cupboard; it depends entirely upon the larger or smaller quantity, and the consistency of the bichromated gelatine used. If the latter is of good quality, all artificial means for the production of grain are not only superfluous, but also disadvantageous. A larger addition of chrome salt generally gives rise to the crystallisation of the salt; chloride of sodium produces plates highly sensitive to moisture, but no grain; a larger addition of chrome alum gives plates which will lack vigour, and the manipulation of which, during printing, will be difficult, as they often will require an “after-etching,” so that even printing is made impossible.

To test the suitability of gelatine for Collotype work, two small portions of the gelatine to be tested may be made up, adding chemically pure potassium bichromate to the one, and ammonia bichromate to the other, and a plate prepared with each solution. If both plates show a dull and untransparent coating, this indicates that it is a soft gelatine; if, on the contrary, both plates appear glossy and transparent, then the gelatine is hard. Should it, however, be middle hard, then the dried film prepared with potassium will be dull, but not rough; that with ammonia, however, shiny and transparent.