Читать книгу The Art and Practice of Silver Printing - H. P. Robinson - Страница 5

Table of Contents

Into a glass beaker put a couple of pinches of common salt, which must be dissolved in a little water.

In a test-tube[1] dissolve about an equal amount of silver nitrate (AgNO₃), and add it to the salt solution. We shall find that we have an immediate precipitate, for chloride of silver will be formed by what is called double[2] decomposition, and there will remain in solution a soluble salt known as sodium nitrate. When the silver chloride has settled down, decant off the liquid, and add water to it once or twice, draining off each time. Divide the chloride into four parts, placing each part on a strip of glass. On two of them pour a little common salt solution, and on the other two pour a little solution of silver nitrate; take one of each pair, and place it in a dark cupboard (if warmed, the quicker will be the operation) to dry. Take the other two moist portions of chloride into the open air, and expose them to daylight, and note the results. It will be seen that one of these will darken very rapidly to a violet colour, whilst the other will remain much lighter, though perceptibly blackening. After a time the latter will appear to grow deeper, whilst the former will become a deep black. The one that blackens most rapidly will be found to be that one on which the silver nitrate was poured. Divide the slightly blackened chloride on the strip of glass into two portions, and over one pour a little beer, and over the other a weak solution of potassium nitrite, and again note the difference. It will be found that here the blackening commences anew, but proceeds much more rapidly on that portion over which the nitrite was poured. Here are the experiments. What do they teach?

Potassium nitrite, and silver nitrate, are both inorganic salts, and they both have an affinity for—that is, tend to combine with—any of the halogens (by which are meant such bodies as chlorine, iodine, bromine, and fluorine). In the former case we have silver chloride formed with a little hypo-chlorous acid; in the latter we have a more difficult decomposition: the potassium nitrite is decomposed into hydrochloric acid and potassium nitrate.[3]

We can tell that chlorine is liberated by the action of light on silver chloride, since if we prepare some as above, well wash it, and expose it to light in pure water, we shall find that the latter contains chlorine, since a few drops of silver nitrate poured into it after exposure give a white precipitate.

If we make the same experiments with the dried portions of silver chloride as we did with the moist, we shall obtain the same results, with the exception that with the dried, in which there is excess of salt, there will be hardly any discolouration. The experimentalist should also note that if the darkened chloride be broken up, the interior retains its white colour in all its purity. This tells us that the discolouration is almost confined to the surface, hence it is useless, for printing purposes, to have such a mass of chloride as would be opaque, since all but a very thin film would be unacted upon.

If the darkened chloride be examined closely, it will be seen that the colour varies, being bluer in the case of that which has silver nitrate in contact with it (either moist or dry) as compared with that which is darkened in contact with the potassium nitrite. We have the best of reasons for believing that the blue colour is really due to a combination between the sub-chloride and the oxygen contained in the water or in the air. The true colour of the sub-chloride is that which is exposed beneath an oxygen absorbent such as the nitrite.

Practical printers are aware that albumenized paper containing a chloride is employed for producing silver prints, and the probability is that the albumen must exercise some kind of influence on the resulting picture. Let us examine this, and see what effect it can have. Carefully break an egg, and separate the yolk from the white, pouring the latter into a beaker. Beat up the white with a bundle of quill pens, allow the froth to subside, and then filter it. Pour a little of the filtered albumen (the white of egg) into a test-tube, and add a little silver nitrate solution to it, and expose the precipitate which falls to light. It will be seen that it darkens rapidly, assuming a foxy red colour. Take a couple of glass plates and coat them with plain collodion, wash under the tap, and whilst still moist flow albumen over them two or three times, and set them up to dry. When thoroughly dry, plunge them for a few seconds into a weak solution of silver nitrate (30 grains to the ounce of water will suffice), wash one under the tap, and then allow both to dry again. Take both plates out into the light, and note the results. The one from which the silver nitrate has not been washed will darken very rapidly, the other will take some time to start; but if the exposure be sufficiently prolonged, it will gradually assume a hue equally as dark as the other.

If we repeat these experiments with gelatine, which is used as a sizing in some papers, we shall find very much the same nature of things taking place, the differences being so slight, however, as not to require detailed notice.

So far, then, we have considered the darkening properties of the silver compounds which are to be used by the printer, but it remains to be seen what permanency of darkening they possess. If we treat the darkened silver chloride solution exposed with the silver nitrate or the potassium nitrite to a solution of hyposulphite of soda or ammonia, both of which are solvents of the white chloride, we shall find that a residue of metallic silver is left behind. If we treat the darkened albuminate of silver with the same agents, we shall find that very little change is effected by them.

From this we may gather that the action of light on them is of a totally different nature.[4] This is also most marked if we treat the two with hydrosulphuric acid solution (sulphuretted hydrogen[5]). It will be found that the colour of the darkened silver chloride becomes more intense, while the other is bleached, or, rather, becomes of a yellow tint. This last effect has an important bearing on the permanency of silver prints, as will be more fully explained when considering the subject of fixing the print.