WHETHER it is feasible for the photographer, amateur or professional, to prepare his own dry plates is a question hardly worth discussing. The processes are attended with precariousness and intricacy at every stage. Each batch of that very uncertain substance, gelatine, must first undergo special testing. The apparatus is expensive. Provision has to be made against changes of temperature; space set apart in workshops where scrupulous cleanliness can be observed, and, above all, freedom secured from that ubiquitous enemy, dust. Moreover, the greater part of the work has to be performed in non-actinic light. Even under the best conditions, it is not unusual to see in any great commercial establishment whole tubs full of emulsion, which has had to be condemned for one reason or another. Still it is expedient that the photographer should have some insight into the manner in which the emulsions are prepared, if only to assist him to unravel any problem arising during exposure and development.
There are several different methods of preparing the emulsion; but we shall here only describe two typical ones - the boiling process with hydrochloric acid, and the Monokhoven, or ammonia process. In the first of these, to take a typical formula, 60 parts Nelson's No. 1 photographic gelatine is dissolved at moderate heat in 900 parts of water, and removed into non-actinic light, when 350 parts nitrate of silver dissolved in 480 parts of water, at about 1200, is stirred in, and the mixture well shaken up; after which the solution:
Potassium Bromide..... 280 parts.
Potassium Iodide..... 10 ,,
Hydrochloric Acid..... I minim to every 2 oz.
Water........ 1,200 parts is slowly poured in, with much shaking of the vessel to ensure perfect incorporation. All is then digested in a water bath raised to boiling point, the emulsion being occasionally shaken, for 30 to 45 minutes.
Meanwhile 300 parts of hard gelatine have been soaked and then dissolved at 110°, or less, in 1,960 parts of water; this is added to the emulsion when it has been allowed to cool down to 8o° or 900, together with ammonia in 10 per cent. solution, 10 minims to each ounce of emulsion.
As this is the more attractive process for experimental purposes, we give alternative formulas. Both of these are for slow plates, such as the amateur might possibly succeed in carrying to successful conclusion.
(1) Ammonium Bromide . . . . . . 15 gr.
Solution of Potassium Iodide (10 per cent.) . . 3 min.
Hard Gelatine . . . . . . . 30 gr.
Nitrate of Silver . . . . . . . 25 ,,
Distilled Water....... 1 oz.
(2) A. Potassium Bromide (Dr. Eder) . . . . 120 gr.
Solution of Potassium Iodide ..... 20 min.
Hard Gelatine ........ 100 gr.
Water........ . 2 oz. 5 dr.
B. Nitrate of Silver...... 150 gr.
Water......... 2 oz. 5 dr.
The gelatine is first dissolved in the water, by application of moderate heat; after soaking for about half an hour the bromide and iodide may be added, at a temperature not exceeding 110°. The nitrate of silver solution (to which sufficient ammonia has been added, drop by drop, to cause the precipitate at first formed to redissolve) is usually poured into the emulsion warm and by degrees, with vigorous shaking. According to the speed required the emulsion may then be poured out into a flat dish to set, or digested over a water bath at 100° for 45 minutes. The proportion of iodide is variable. For landscape work the iodide is recommended as decreasing the liability of halation, and, according to Dr. Eder, plates are rendered thereby somewhat more sensitive.
If an emulsion after mixing is digested in the water-bath at 100° to 1400, according to circumstances, for a time varying from 30 minutes to 2 hours, and then cooled rapidly in running water and allowed to stand for 48 hours, the sensitiveness is enormously increased. The time must not exceed 48 hours, or fog will probably occur in the plates. No doubt the gelatine itself plays an important part in the formation and development of the latent image; but, although some theories exist, none are capable of proof, in view of the difficulties involved in the chemistry of gelatine. The ripening process may also be set going by the addition of ammonia to the emulsion. One alteration brought about by emulsion is an increase in the size of the halide particles.
The difference between a slow and rapid emulsion is, therefore, not merely in the proportions of the amount of bromide and iodide salts to the nitrate of silver; although in the latter it is usual to employ a larger quantity of iodide, and also much more bromide than would be required according to Ackland's Tables. A slow emulsion is usually set at once, immediately after mixing. Digestion in the water bath for half an hour at 1100 may increase the speed from 4 to 5 times on the Hurter & Driffield Scale.
An excess of bromides and iodides with prolonged digestion at higher temperatures may raise it to 30 times. For instance, in formula (1), by increasing the bromide to 32 gr. and the iodide solution to 12 minims, stewing for an hour after addition of silver nitrate at 1200, then cooling rapidly and allowing to stand for 32 hours, the emulsion would possibly show a speed of 200 H. & D.
Most silver chloride emulsions are slow and suitable only for contact lantern places, gaslight papers, etc., etc. Luppo-Cramer's formula represents an endeavour to produce a sensitivity in the chloride comparable in some degree with that of bromide:
Common Salt........ 7 „
Hydrochloric Acid (sp. gr. 1.9) . . . . 10 c.c.
Water ............................................ 100 c.c.
Heat to 1760 Fahr. and add:
Silver Nitrate ...... . . 10 grams.
Water........ . 100 c.c.
At same temperature. Digest in water bath for a quarter of an hour, and then treat as usual. The ammonia method tends to produce fogged emulsion.
The hard, stiff jelly must now be forced, either by machinery through a sieve, or squeezed through canvas netting in cold water, and then collected once more into the canvas bag for from half an hour under running water to a soaking of several hours in still water; the squeezing process is then repeated, exposing fresh surfaces of gelatine to the action of the water. By this time the excess of nitrates and bromides will have been removed, and the emulsion is very carefully drained over a hair sieve, then collected and melted once more at a temperature of about 1200, when it is filtered, and is finally ready for coating the glass plates. If the gelatine shows signs of degeneration small quantities of formaline or chrome alum are introduced; but the latter will tend to lessen the sensitiveness of the plate. Special machinery is of course employed in coating plates for commerce. For our trial plates we may take the glass from which old negatives have been stripped, and immerse them, first in some acid bath, and then in caustic potash, after which they are rinsed in warm water and rubbed thoroughly dry with a clean dishcloth. Beware of dust at all stages, but at this moment every speck of dust is a source of destruction and ruin.