A long article on this subject appeared in the 1st Series of' Workshop Receipts,' but since that time many new processes have come into use, and will be discussed here.
Dry-plate makers are aware that the quality of gelatine varies as regards tendency to frill, and their idea that this tendency is governed by the amount of water different specimens will take , up is generally but not altogether true. Different gelatines dry in different times, and throw those stains upon the plate which are the cause of the tendency to frill. Abney prepared a solution of gelatine of such a strength that 5 gr, of the colloid were made to form a film upon each quarter-plate; after being dried, the films were stripped from the glass and accurately measured. Some were then placed in water to soak; others were treated with ammonia of the strength usually employed in developing, while more were soaked respectively in solutions of monocarbonates of soda and potash, in which they were allowed to swell for one hour. The results showed that ammonia promotes swelling, whilst the monocarbonates of potash and soda have less tendency so to do; consequently they are in this respect better for developing.
An easy method of getting emulsion to flow freely over a glass plate is to cover a squeegee with swans-down, and to rub it over the plate just before coating with emulsion. Abney's experiences with silicates as a substratum to promote the flow of gelatine have not been good. Warnerke is in favour of the use of monocarbonate of potash, as it gives less tendency to frilling. Gelatine is firmer the quicker it is developed. When you want to avoid frilling, add alum (up to 5 per cent.) to the first washing water in which the emulsion is squeezed out in fine threads; the succeeding washings clear out the alum again. Use ordinary alum - not chrome alum. The threads become crisp under the treatment, and do not increase in bulk. The plates develop quicker than when there has been no treatment with alum at all, but the increase in rapidity of development is small.
Sebastian Davis states that, if gelatine be moist for a long time, decomposition has a tendency to set in near the centre of the plate. He made one batch of plates with Heinrich's very hard gelatine, and another batch with Nelson's No. 1 - a very soft gelatine. The latter were 4 or 5 days in drying without artificial appliances for the purpose, whilst the former dried in 48 hours under similar circumstances. The hard gelatine has no direct tendency to blister.
The effect of metallic salts upon gelatine ought to be tried by experiment, for probably some of them will bo found to give the plate much less tendency to frill.
Capt. Abney states that his experience with soft gelatine artificially hardened is that its expansive power is materially lessened. If plates were 48 hours in drying he should be astonished if they did not frill and blister. He should get perfect fog under the circumstances. The use of a substratum prevents the mark round the edge of the plate in drying. Not less than 8 hours or more than 24 hours is the best time for drying, and quick drying by alcohol tends to blistering. Altogether, 16 hours is a good time for the drying of plates. England's experience in relation to the effect of slow drying on the centre of the plate shows that his emulsion was not sensitive enough at the time of coating if he desired the maximum of rapidity. Zinc nitrate has a hardening effect upon gelatine, and with the use of zinc bromide for decomposing the silver nitrate, there is no risk of getting slowness; but the zinc salt is not so "certain" as the potassium salt.