The first insoluble double hyposulphite is formed when there is only a small quantity of sodium hyposulphite present; the soluble kind when the sodium hyposulphite is in excess. Since it is the soluble kind which we wish to form, it is manifest that the presence of a sufficiency of hyposulphite in the fixing bath is necessary. If not, we have left the insoluble form on the paper. If either of these two kinds of hyposulphite be made in a test-tube, we can readily simulate the effect of atmospheric exposure. If slightly acid water be added to the hyposulphite, it will be seen, when chloride of silver has been dissolved by the hyposulphite, that the precipitate or solution commences to blacken, sulphide of silver being formed. On the other hand, if we take albumenate of silver, and dissolve it in hyposulphite of soda, we shall find that the addition of acid gradually causes a yellow-looking compound to separate out, and it is probably this body formed in the paper which causes the gradual yellowing of the whites of silver prints.
What is taught us, then, by this observation is, that by thorough washing we must try and eliminate all traces of hyposulphite of silver, and, indeed, of the hyposulphite of soda, since the latter decomposes as rapidly in the presence of acid as does the silver compound.
The formula for the fixing bath which we recommend is:—
| Sodium hyposulphite | 4 | ounces |
| Water | 1 | pint |
| Ammonia | 1/2 | drachm |
The addition of the ammonia prevents any possibility of an acid reaction arising, and otherwise softens the film of albumen and the size of the paper, causing more rapid fixation and more thorough washing. Another thing the ammonia does is, that it prevents, in a great measure, blistering of the film of albumen, which is common in some highly-albumenized paper.
Experience has shown that one ounce of solid hyposulphite will fix with safety three sheets of paper, so that an idea can be formed of how much must be used for a day's printing. The hyposulphite bath which has been used one day should never be used the next, since it invariably contains the germs of decomposition in it from some cause or another. Indeed, the appearance of the solution indicates this is so, since it is usually of a yellow or brownish appearance.
The time required for fixing a print varies with the thickness of the paper used. As a rule, prints on the medium-sized paper require ten minutes' soaking in the bath, whilst thick-size requires fifteen minutes. Whilst toning, the dish containing the hyposulphite should be kept in a gentle rocking motion, as in toning, and for the same reasons. Prints may be examined from time to time, to see how the fixing progresses. When a print is not quite fixed, small spots of dark appearance will be seen when it is examined by transmitted light. The operation of fixing should be continued after these disappear for at least three or four minutes, in order that the hyposulphite of soda in the dish may get impregnated with the double silver and sodium salt which is in the print, and thus render washing more effectual. It should be noted that the dish for fixing should be at least as long and wide as the dish used for toning; that it should be deeper when, as a rule, all the prints are fixed at one time. Care should be taken that dishes which are used for sensitizing, toning, or fixing, should not be used for anything else. The glaze of porcelain dishes is often soft, and frequently absorbs a certain amount of the solutions used. Thus, if a porcelain dish be used for a solution of any aniline dye, it will often be found that it is permanently stained. Colour in this last is merely indication of what happens with any other solution. It will thus be seen that it is a mistake to use a dish for fixing when the glaze is cracked, since old hyposulphite must find its way into the body of the fresh solution that may be used, and thus institute a spontaneous decomposition, and a consequent want of permanence in the print. For our own part, we believe that a gutta-percha dish is a safer dish to use than any other, since it is impervious to any solution, and can be well scoured after fixing, and before being again brought into use. We believe that much of the fading of prints may be traced to the use of unsuitable dishes for fixing.
[CHAPTER XVII.]
WASHING THE PRINT.
There are very many apparatus designed for washing prints; but we believe that, where few prints have to be treated, careful hand-washing is as superior to machine-washing, as hand-made paper is to machine-made. In our own practice we take the prints from the fixing-dish, and immerse them in a large puncheon of water, and allow them to soak for five minutes, after which we carefully pour off all the water, and replenish with fresh, in which we leave them for a quarter-of-an-hour. After that we take the prints and place them on a glass slab, and, with a squeegee, squeeze as much water as possible out of each separately; this we repeat twice. After two more washings of a quarter-of-an-hour, we then wash for half-an-hour, and, with a sponge, dab them as dry as possible, and again immerse for half-an-hour. After repeating this operation twice, we allow a stream of running water to pour into the puncheon for a couple of hours, carrying the stream through an india-rubber pipe, at the end of which is a glass tube, to the bottom of the puncheon, and so that the pour of water goes against the side. By this means there is a constant stir in the water, and the water flows over the edge of the puncheon. It is convenient to cut a notch in the top rim of the puncheon, so that the water may find an exit before reaching the level of the rim. The prints are then taken out, sponged once more, and dried. By this arrangement we have got prints which are perfectly unfaded, though they have been in existence eighteen years, and have been to the tropics, and in the dampest climates. This method of washing, though tedious, should be applied to all prints; but, in the present day, it can hardly be hoped that it can be immediately adopted, on account of the attention it requires; we therefore describe an apparatus which can be used. It was designed by Mr. England, and consists of a working trough, as shown in the figure, which is automatically worked by an overshot wheel. We need not enter into the details of the invention, as they are self-evident.