SENSITISING PAPER.
The process of sensitising paper, although it presents no grave difficulties, is one which is somewhat trouble some and is rather messy. It is almost impossible to perform it unless some special apartment can be put aside for photographic work, at least for the time being.
The apparatus, materials, and chemicals necessary are as follows : A flat dish, somewhat larger in each direction than the largest picee of paper to be sensitised.
A number of American clips.
Albumenised paper—as much as may be required. Nitrate of silver—enough to make up the solution to be afterwards described.
An argentometer.
A small quantity of kaolin.
A funnel and filtering paper.
The flat dish differs in no way from the ones which we have been already using in the manipulation of prints. It should, however, be kept for the one purpose of sensitising paper alone.
American clips are after the nature of diminutive letter clips, and are used to suspend a piece of paper from a string stretched across a room.
The albumenised paper is bought in rolls like the sensitised paper already described, and does not differ from it in appearance. It will keep for a very long time.
The argentometer is an instrument used to discover the strength of the silver solution to be afterwards described. It is simply a cheap form of hydrometer,—that is to say, it consists of a bulb of glass with a stem, as shown in the cut. The bulb is so weighted that when the whole apparatus is dipped into clean water it will sink till the stem is nearly sub merged. When, however, the specific gravity of the water is increased by dissolving nitrate of silver in it, the stem is floated up to a certain extent, and by observing to what degree this takes place we can tell the strength of the solution. The stem is graduated, and by reading off the figure at the surface of the liquid we can find, with a sufficient degree of correctness, how many grains of silver nitrate there are to each ounce of solution. An argentometer is usually provided with a tall glass vessel, which will hold just enough solution to float it.
The funnel used for filtering is a well-known piece of apparatus. It is best to have a good large one. It is usual to support a funnel on a retort-stand whilst filter ing is being performed, and if the photographer happen to have such a piece of apparatus he may by all means use it, but it is scarcely advisable to purchase one for the special purpose. If a fairly wide-mouthed bottle be used to hold the silver nitrate solution, the funnel may be caused to stand in the neck of the bottle.
Preparation is made for sensitising paper in the fol lowing manner : The room is either lighted through a yellow calico blind or else all outside light is shut out, as much lamp light as is desired being used. If the weather is cold, a fire is lighted, or there may be used a gas or other stove of any description, so long as there are provided means of getting rid of the products of combustion. Strings are stretched across the room in any direction convenient, to be used to attach pieces of paper to for drying.
The paper is cut up into pieces of the size which is desired. If very small prints are to be used, it is best to sensitise the paper in such sizes as will cut up to make several prints. Of course the size of the pieces of paper to be sensitised regulates the size of the bath.
Enough solution to cover the bottom of the dish to a depth of a quarter of an inch is prepared as follows :— Silver nitrate . GO grains.
Distilled water . 1 ounce.
The solution is poured into the dish.
A piece of the paper is taken by two opposite corners ; it is allowed to drop in the middle between these, so that when it is lowered on to the solution the first part which will touch the liquid will be a line between the two corners not held in the hand. When the paper has been so lowered on to the solution, the corner held first in one hand, then that in the other, is gently lowered, till the whole is floating on the surface of the silver bath.