The object of this particular method of laying down the paper is to avoid any air-bubbles being imprisoned under it, as such would leave white circular spots when printing was performed. If the process be carefully performed there will probably be no bubbles, but it is always best to make sure, for which reason, after the paper has remained floating for one minute or so, it is taken in the fingers by two adjacent corners and is gently lifted till all except a narrow portion of the other end is lifted from the solution. If we see any bubbles, they must be broken. This can generally be done by moving about the piece of paper, whilst one portion is kept floating on the solution and the rest is held above it. If the bubble will not break with this treat ment it may be touched with the end of a glass rod or with a clean quill.
During the whole of this process great care must be taken not to allow any solution to get on to the back of the paper.
After we have assured ourselves that there are no air-bubbles under the paper it is left floating for two or three minutes. It is probable that, whilst the paper is so floating, the edges may be noticed to curl up and rise from the solution. If we breathe gently upon these, they will be lowered.
At the end of the time mentioned the paper is to be removed from the solution. To do this, it is taken by two adjacent corners and is lifted very slowly from the solution. By this means all superfluous liquid is drained from the surface by capillary attraction.
The paper is fixed by one corner to one of the pieces of string by an American clip. A fragment of blotting paper about an inch square is brought into contact with the lower corner. It will stick to the albumenised paper by capillary attraction, and will absorb a drop or two of silver solution which otherwise would fall.
Another piece of paper is meantime floated on the bath. It has to be borne in mind that as paper con tinues to be sensitised the bath loses strength. After a time it will require some silver nitrate added. Each sheet of paper, measuring 17 x 22 inches, will extract something like 40 or 50 grains of the silver salt from the solution. The solution is reduced both in quantity and in strength. For this reason we should add to it, after every three sheets of paper 17 x 22 inches have been sensitised, one ounce of a solution made up as follows :— Silver nitrate . 90 grains.
Water, up to . • 1 ounce.
This will keep the bath approximately constant both as regards quantity and strength. At the end of a day's work we must test the strength of the solution by means of the argentometer, and correct any slight error which may have arisen even in spite of the additions of strong solution made. It will probably be found that the bath
is slightly reduced in bulk, because, except in the most careful working, every three sheets of paper take up a little more than an ounce of liquid. In the first place, then, we make up the bulk to the original amount. We now pour a little of the solution into the argentometer tube and place the argentometer floating in the liquid.
We will suppose that we find that the surface of the liquid crosses the stem at 55. This indicates that we have lost 5 grains of silver nitrate for each ounce of solution. We therefore add this amount to each ounce of " bath " that we have made up. If, as is quite pos sible, the solution indicates about 60, we may reduce the strength by the addition of a little water.
The bath which we have given is a very strong one, and no ill will result if it become considerably weaker during use. It will certainly work well when reduced to 45 grains of silver per ounce of solution.
After we have done using the bath for the clay we must filter it before replacing it in its bottle. We may then shake it up with a little of the kaolin, which is a white chalky-looking powder. It takes some time to subside, and as it does so carries with it such organic matter as was in too fine a state of division to be re tained in the filter.
One of the greatest troubles in connection with sensi tising paper is that, if it be allowed to hang up till it is dry it will curl up so tightly as to be almost immanage able. The writer has found the following plan an ex cellent one to prevent such curling up : The paper is allowed to hang up only till, if the piece of blotting-paper be removed from the corner, no solution will drop from it. It is then laid, albumenised side up wards, on one of the "drying boards "—already described —a second drying board is placed over the paper, and the greater portion of the solution is blotted off. The paper is now laid face down on a second drying board, and other pieces are treated similarly. As soon as the first piece of paper has so far dried that the edges begin to turn stiffly inwards, it is laid face upwards on a piece of drying board a little larger each way than itself. A second piece of drying board is placed over it, and above this a flat wooden board with a weight over it. When the next piece of paper is ready it is placed over the second piece of drying board, and a third piece of drying board is placed over it. In this way all the paper is piled, and it will dry quite flat.