The silver nitrate which is taken up from the sensitising bath when albumenised paper is sensitised, serves 3 distinct purposes. In the first place, double decomposition takes place between it and the soluble chloride in the albumen, so as to form silver chloride in the film of albumen, and a soluble nitrate, which remains in the solution. The silver chloride is, as all know, the principal sensitive substance in the paper. There is, besides this, an organic compound formed by the decomposition of the albumen itself by silver nitrate. The formation of this second compound is a second function of the silver nitrate. The third is that of acting as a "sensi-tiser." The silver chloride darkens only slowly if there be no silver nitrate present, nor any other substance which will take the place of the silver nitrate to absorb the chlorine given off when the silver chloride is reduced by light.
To estimate the quantity of silver nitrate necessary to convert the soluble chloride in the albumen into silver chloride is easy enough, if one knows how the albumen was salted, and how much each sheet takes up.
According to a formula given in Hard-wich, each sheet of paper of the usual size, 17 X 22 in., takes up with the albumen about 7 gr. ammonium chloride. To convert 7 gr. ammonium chloride into silver chloride requires, as nearly as possible, 22 1/4 gr. silver nitrate.
We know of no reliable data for the amount of silver nitrate used to produce the organic compound referred to, but from experiments we have made with albumenised paper, estimating the amount of silver nitrate necessary to convert the soluble chloride, noticing the quantity of silver nitrate taken from the bath, and the quantity afterward recovered by thoroughly washing the paper - without exposing it to light - we conclude that the quantity is very small, probably not more than 2-3 gr. to the sheet.
The quantity of free silver nitrate remaining in the paper after sensitising may very easily be ascertained. It is only necessary, after sensiting a sheet, to wash the free silver nitrate out of it with distilled water, and to estimate the quantity of it in the usual volumetric manner. From estimations made by ourselves on the lines indicated, we may say that the amount of silver nitrate remaining in the paper in excess after sensitising is fairly represented by the actual diminution in bulk of the printing bath. We mean that, for example, if 3 sheets of paper take up 1 oz. of a 60-gr. bath, there will remain in each sheet of paper 20 gr. free silver nitrate, the further nitrate required to produce the silver chloride and organic compound serving only to weaken the remaining solution.
Taking Hardwich once more as an authority, we find that an average quantity of bath to be used up by a quire of paper is 8 oz., that is to say, each sheet of paper removes 1/3 oz. of fluid from the bath.
Our own experience would incline us to put this figure a little higher in ordinary circumstances; the quantity absorbed varies greatly with the paper. Thick paper naturally absorbs much more of the bath than thin paper does. Making an average, we should say that a quire of paper reduces the bath by about 9-10 oz. Taking this quantity as correct, and supposing the bath to be a 60-gr. one, we find that almost precisely the same quantity of silver nitrate is taken up merely to form a chlorine absorber as is used to form the important sensitive salt. This appears to be somewhat extravagant, and has been felt to be so for a long time. More than 20 years have elapsed since it was pointed out that the silver bath might be greatly reduced in strength if part of the silver nitrate were replaced by sodium nitrate, which would serve as well as the silver nitrate to coagulate the albumen of the film. Doubtless economy resulted; but somehow, so far as we know, the method was never very popular.
There are other methods of economising, however, which ought to be taken notice of. It is well known to practical printers that it is. a great advantage to have the albumenised paper damp before it is floated. The edges do not curl away from the bath in the aggravating manner that they do when the paper is dry, and air-bubbles are less likely to be formed. These advantages are fairly well known, but we do not think that it is known how great an absolute economy there is in using the paper as damp as is practical, this economy arising from the mere fact that much less of the silver nitrate bath is absorbed by paper when it is damp than when it is dry. We were astonished to find how great an economy really results from the use of damp paper.
Some time ago we were working with a very highly albumenised and very thick paper, which reduced the bulk of the bath very greatly. We noticed that the amount of bath absorbed varied with the dampness of the paper, and determined to find out the extent of the variation.
We first used the paper very dry - as dry as it was practicable to handle it. We discovered that each sheet reduced the bath by more than 1/2 fl. oz.
We next made the paper as damp as was possible without danger of softening the albumen. We now found that each sheet absorbed barely 1/3 oz. of bath. The prints given by the paper sensitised when damp were of quite as good quality as those got on that sensitised dry. The bath which we were using was a 60-gr. one, and we found the damping of the paper to result in the saving of quite 12 gr. silver nitrate per sheet, or not far short of 380 gr. per quire.
Concerning the manner of damping the paper, a few words may be said.
It is a common plan to keep albu-menised paper in a damp cellar. In fact, this course is often recommended by dealers in albumenised paper. Many have not a cellar damp enough at their command. Moreover, it is objectionable to store albumenised paper in a damp place. It should merely be placed in such, if such be available, a few days before it is to be used, which often involves some trouble.
The following is the plan we have ourselves adopted: On the shelf of a cupboard which has but one, the albumenised paper is laid flat, the albumen sides of the sheets upward; 1/2 hour before commencing to sensitise, a vessel of boiling water is placed on the bottom of the cupboard. There is room for the steam to pass up between the front edge of the shelf and the door of the cupboard. The water is replenished from time to time during working. The paper is thus kept very damp, and apart from the saving in silver effected, the comfort in working is greatly increased. (Photo. News.)