(4) For Prints
Heat a piece of glass, and rub a little wax over it with a bit of cotton-wool. Pour water over the plate, and press the picture down upon it with a piece of filtering paper. When dry, the picture is removed, and will be found to possess a brilliant surface.
(5) For Gelatine Negatives
One of the minor difficulties in connection with the working of gelatine plates is that of varnishing. In the abstract it should, no doubt, be the simplest of all operations; and with collodion, given a moderate amount of care, such was the case. But with gelatine, for various reasons, the apparently easy process of giving the final protective coating to the otherwise finished negative is found to be, to say the least of it, a somewhat unsatisfactory one; and many operators refuse to varnish their negatives at all, prefering to risk the dangers of printing from bare gelatine films as being the safer plan.
The difficulty arises partly from the physical and partly from the chemical properties of the gelatine. Its powerful affinity for water and its great absorptive powers, and consequent swelling under the action of moisture, render the gelatine film anything but a suitable companion, at close acquaintance, with the hard, brittle, and inflexible layer of resinous matter which is laid upon it, as its protector. This latter, no matter of what materials it may be composed, has hitherto proved wholly inadequate to perform the role expected of it, and, though supposedly waterproof, appears only to intensify the danger which moisture may threaten to the film of a gelatine negative. A single drop of water on a varnished gelatine negative will, after a very brief contact, leave a mark which, if not indelible, requires the entire removal of the varnish before it can be obliterated, penetrating, as it does, through the film of varnish to the gelatine underneath, and causing the latter to expand and crack the inflexible layer of resin above it.
If the moisture be presented in the form of silver solution, or if silver from damp sensitised paper gain access to the film, the result, though less immediately visible, is far more fatal. Combination takes place between the silver and the gelatine, and brown stains result, which, in the majority of cases, it is quite impossible to remove. Dissolving off the varnish is useless, for the action of the silver will be found to have passed right through that and to have attacked the under layer of gelatine, forming a compound which is amenable only to those reagents which attack the developed image itself.
It appears useless to hope for any way out of these difficulties by merely altering the constituents of the varnish. The fact appears to be that none of the resins usually employed (not even shellac) are really impervious to moisture; and, this being the case, the absorptive powers of the gelatine and its consequent swelling render it hopeless to expect to do more than ameliorate matters. A preliminary coating of collodion has been recommended, and certainly is a great improvement upon varnish alone; but, as applied in the ordinary way, a still greater improvement is to employ the collodion alone, and apply the varnish to some other purpose. The intervention of the collodion greatly retards the absorption of moisture by the gelatine film, though it does not arrest it entirely, consequently the ultimate effect is the same as with varnish alone; but, if collodion by itself be used, it is sufficiently elastic to accommodate itself to the expansion of the gelatine without splitting or cracking, and when the plate is dried, no trace of any injury remains.
The hardness of a collodionised gelatine film is very surprising to those who are not accustomed to this mode of "varnishing," but something more is wanted. The author tried many different kinds of varnish, some of them specially prepared for "dry-plate" work; but whether with or without the preliminary film of collodion, he found nothing that would absolutely resist moisture. He, however, succeeded in attaining with some trouble a degree of protection which practically answers for all the dangers that any ordinary negative is likely to be exposed to, though it may be a question as to whether it is worth while to adopt it in all cases.
His experience with all varnishes, as supplied commercially, is that they are too thick for gelatine films, though they may answer for collodion. The latter, it must be remembered, is spongy and porous, and absorbs some of the alcoholic Varnish; while gelatine, on the contrary, rejects it entirely. Consequently, Varnish of ordinary thickness flows badly on a gelatine plate, dries slowly, and is a very long time before it loses its "tackiness." He therefore dilutes the varnish with at least an equal volume of methylated alcohol, and filters carefully through filter-paper. The negative, after drying, is heated and polished with an old silk handkerchief and a little powdered talc; it is then collodionised, dried, and varnished. After the latter operation, strong heat is applied for at least 10 minutes; the plate is then allowed to cool, and is again polished with talc. If extra protection be required, the operations of collodionising and varnishing are repeated, the result being a surface which is far harder and more impervious to moisture than if a single coating of collodion and thick varnish had been used.
A good, cheap varnish for this purpose consists of hard white spirit varnish diluted with 4-5 times its volume of methylated alcohol. The collodion may be ordinary "enamel" collodion, diluted with an equal quantity of a mixture of ether and alcohol. (H. Y. E. Cotesworth.)
(6) Water Varnish
Take 1/4 lb. of shellac (in thin flakes), and 1 pint water; place them in a tin saucepan or other suitable vessel on the fire or over a gas stove, and raise to boiling point. When this is reached, add a few drops of a hot saturated solution of borax, stirring vigorously with a glass rod or clean stick until this shellac is all dissolved, which will be in a few seconds. Do not use too much borax, but add slowly, and stop short of complete solution rather than the other way. After this, the solution is filtered through charcoal, and the water varnish is ready for use.