Having been informed that the paper-makers are in the habit of bleaching their papers with sulphur and sulphites, I have submitted a considerable quantity of the browned papers to careful examination. In all cases where the paper, covered with chloride of silver, has suddenly blackened, I have detected the presence of sulphur. Consequently, when the darkening goes on rapidly, and terminates in blackness, we may, I think, correctly attribute it to the formation of a sulphuret of silver.
It is, however, certain that the slow action of organic matter is sufficient, under certain circumstances, to set up a chemical change, which, once started, progresses slowly, but certainly, until the compound is reduced to its most simple form.
China clay—kaolin—has of late years been much used by the paper manufacturers, for the double purpose of giving weight to the paper, and of enabling them to produce a smooth surface upon all the finer varieties of paper; such as the enamelled satin post. This compound of alumina and silica would not, if the finest varieties of clay were employed, be likely to do much mischief in the papers used for photography; but the less pure varieties of the Cornish clay are employed, and this commonly contains the oxides of iron and other metals in a state of very fine division; and these, where they come to the surface, form little centres of action, from which dark circles spread in rather a curious manner. Thin papers have been tried, and many varieties would answer exceedingly well, but that nearly all kinds are found penetrated with small holes, which, though of minute dimensions, suffer light to pass freely, and consequently produce a spottiness on the resulting picture. Sir John Herschel found that this evil could be remedied by fastening two pieces of such paper together; but this method is troublesome and uncertain.
Returning to the consideration of size in the paper, the above-named authority—who employed the lead salts in some of his photographic processes—has the following remarks :—
" The paper with a basis of lead turns yellow by keeping in the dark, and the tint goes on gradually deepening to a dark brown. But what is very singular, this change is not equally rapid upon all kinds of paper—a difference depending, no doubt, on the size employed; which, it may be observed here once for all, is of the utmost influence in all photographic processes. In one sort of paper (known by the name of blue wove post) it is instantaneous, taking place the moment the nitrate (if abundant) is applied. And yet I find this paper to resist discolouration, by keeping, better than any other, when the mordant base is silver instead of lead. On the other hand, a paper of that kind called smooth demy, rendered sensitive by a combination of lead and silver, was found to acquire, by long keeping, a lead or slate colour, which increases to such a degree as might be supposed to render it useless. Yet, in this state, when it is impressed with a photographic image, the process of fixing with hyposulphite of soda destroys this colour completely, leaving the ground as white as when first prepared. This fortunate restoration, however, does not take place when the paper has been browned as above described. Some of the muriatic salts also are more apt to induce this discolouration than others, especially those with the earthy bases".
It will be evident from these remarks that it is of the utmost importance to secure a paper which shall be as chemically pure as possible. Experience has proved that recently-manufactured paper does not answer equally well with that which has been made for a year or two. It has been thought by many that this was an unfounded statement, but it is not so ; and the causes operating to the improvement of paper by age are evident. The organic matter of the size is liable to a spontaneous change : this goes on for a considerable time, but at length the process becomes so exceedingly slow that it may, for all practical purposes, be said virtually to rest. Paper changes its colour by keeping from this cause, and I have found that such as I have selected from the shop-worn stocks of stationers has been generally superior to that which has been more recently manufactured.
Select, therefore, paper of a uniform texture, free from spots, and of equal transparency, choosing the oldest rather than the newest varieties.
Where the process is highly sensitive for which the paper is desired, it is important to treat it in the following manner :— Having a shallow dish sufficiently large to receive the sheets of paper without in any way crumpling them, it is to be filled with very clear filtered water, to which a sufficient quantity of nitric acid has been added to make it slightly sour to the taste. Taking a sheet of paper, it should be laid on a porcelain slab, and sponged with clean water on both sides, after which it should be placed in the acidulated water, and allowed to remain in it for several hours. Too many sheets should not be placed in the vessel at the same time. After a time they should be removed in mass, placed on the slab, and left for half an hour under gently flowing water,—this removes all the acid, and all those metallic and earthy matters which it has removed from the paper. After this it is to be dried, and it is then fit for photographic use.
This chapter has been deemed unnecessarily long; but, upon renewing many of the experiments, I have become so convinced of its importance in all particulars, that I could not induce myself to curtail it. All who aim at excellence in photography should repeat the experiments and closely observe the results.