Sensitiveness of a gelatine emulsion depends upon the fineness of the silver bromide in the gelatine; the finer it is, the more rapid. Weak solutions give finer precipitates than strong solutions. Acid solutions give finer precipitates than alkaline solutions. Some have said that the sensitiveness of an emulsion depends upon its being more or less alkaline - the more alkaline, the more rapid - therefore they use ammonia to make it alkaline. Now, ammonia partially dissolves the silver bromide, making it finer, therefore more rapid. It is not because it is alkaline. Anything that will dissolve the silver bromide and not decompose the gelatine, whether it is acid or alkaline, will do the same thing. By boiling an emulsion, the heat partially dissolves the silver bromide. The more bromide that is used in excess of what it actually takes -to precipitate the silver used, the less boiling it will take, because the excess of bromide will dissolve the silver bromide, and the heat will hurry it along.
The more acid an emulsion, the longer you can boil; the more alkaline, the shorter time, as the ammonia aids in dissolving the silver bromide.
All know how to make ammonia-nitrate of silver, by adding ammonia to a silver solution until the silver oxide formed has redissolved. Hartley tried this with silver bromide, and found by adding ammonium bromide or potassium bromide to a precipitate of silver bromide, that the excess would redissolve the silver bromide, making a clear solution, same as excess of ammonia redissolves the silver oxide in making silver ammonia-nitrate.
When the silver bromide is all re-dissolved, it is in the finest possible state of division. He added this to the gelatine, which makes a clear, transparent mixture, and when he began to wash it, it began to emulsify. The longer he washed it the quicker it would work. A trial proved that it was too rapid, and the plates would have to be made and handled in absolute darkness in order to work them successfully. Besides, the silver bromide added to the gelatine in this state decomposed it, so that it was difficult to get it to set. He therefore got his fineness of silver bromide by the same theory, but in a different way.
The intensity of a negative depends upon the. proportion of silver to the gelatine. The more silver, the less intensity and more detail. 1/2 gr. silver to each grain of gelatine will make a very intense negative, while 2 gr. silver to every grain of gelatine will make a very thin but full detail negative. When gelatine is boiled, it refuses to set without a cooling slab; and when not heated above 140°-15O° F., and then only long enough to flow the plates, they set and dry without any trouble.
It is not necessary to use the gelatine so thick as recommended in published formulas (namely 30 gr. to the ounce), as it takes longer to set and dry, and is a waste of emulsion, as intensity can be regulated by the proportion of silver to the gelatine, and not by the thickness of the film. A gelatine solution, when acid and in a jelly state, will hold 200 per cent. of silver, and when it is alkaline it will hold sometimes not more than 1/2 gr. of silver to each grain of gelatine; this depends upon how alkaline the gelatine is. Driffield states that gelatine in a jelly state will hold but a small amount of silver, and then only as a sponge holds water; that if you attempt to cut or squeeze it, the silver comes out in solution. This is only the case when the gelatine is alkaline and never when acid. Acid gelatine will hold 200 per cent. of silver, and can be cut or squeezed without danger of losing a drop of silver.
Silver bromide formed from alkaline solutions is not as fine as when formed from acid solutions; and the weaker the solution of bromide, the finer the precipitates of silver bromide. Pure silver bromide is not sensitive to light, but only becomes so when in combination with organic matter, such as gelatine gives to it.
Take any hard gelatine - Swiss or Heinrich's - soak it for 12 hours in water, changing the water 3 times during the 12 hours. Do not cut it up. Take it out of the water and lay it on clean paper to dry. No matter how much of this you fix in this way, as when dry it will keep as before.
Take 75 gr. soda carbonate (not bicarbonate) and 60 gr. citric acid, and put into 3 oz. warm water in a quart pitcher; when the citric acid and soda carbonate are dissolved, and all effervescence ceases and carbonic acid gas has passed off, add 16 oz. cold water and 720 gr. of the soaked and dried gelatine and let it stand 30 minutes; now dissolve 720 gr. silver (prepared according to formula given) in 6 oz. water. Place the pitcher containing the gelatine in hot water - do not let the temperature exceed 110°.
When the gelatine is all dissolved, which will take about 15 minutes, add the silver solution to the warm gelatine; rinse out the vessel you dissolve the silver in with 2 oz. water, and add it to the gelatine solution; when the silver is added, it will immediately turn white (if it turns brown, the gelatine is not good, and must be discarded). Stir well. Now take a bowl large enough to hold the emulsion, wax it inside with beeswax, being careful not to leave an excess of wax; this is to keep the emulsion from sticking to the bowl. Pour the silvered gelatine into the bowl, and set away in a temperature of 40°-60° F. to set.
When this becomes a jelly, which will take about 4 hours, cut it into strips about 1/4 in. square, using a silver knife or horn spatula. Put strips of silver gelatine into a half-gallon pitcher, put pitcher into the washing box (described hereafter), put on cover and pour the following solution through the pipe into the pitcher on to the gelatine:
Soda carbonate .. .. 150 gr.
Ammonium bromide .. 720 gr.
Alcohol........ 4 02.
Water (about 60°-70° F.) 12 oz.
Stir this every 1/2 hour for 2 hours. When you stir it, take washing box into dark room, shut the door, and by red light only take off the cover; give it a good stirring with a glass rod and replace the cover. To tell when the silver is all formed into silver bromide, take a small piece of the jelly to the light and cut it in two; if it is all alike, the action has taken place, if it is not the same colour all the way through, there is still free silver present. Do not put the piece back into the pitcher, as it would fog the balance. When the silver has all been formed into silver bromide, the emulsion could be washed, melted, and plates prepared, but they would be slow. Now leave the emulsion standing in this brine of ammonium bromide, etc, from 1-5 days or more, according to rapidity you want: 3 days makes a rapid plate, 5-6 days makes a very rapid plate. The reason of this is the excess of ammonium bromide partially dissolves the silver bromide. The longer it remains in the brine, the finer it becomes; also, the weaker the brine, the finer the silver bromide.
But by making it weaker than is recommended here takes too long for the silver bromide to form.