The Art and Practice of Silver Printing. H. P. Robinson

Water 1 ounce Modified Solution. 2.— Silver nitrate 50 grains Ammonium nitrate or sodium nitrate 50 " Water 1 ounce

      The reason of the addition of the ammonium or sodium nitrate is that prints are better obtained on paper which is not absolutely free from water. When very dry, the liberated chlorine (see page 32) is apt to attack the albuminate, whereas it is deprived of much of its activity when it is able to be absorbed by water, which, in the presence of light, is decomposed into hydrochloric acid and oxygen.[10]

      Hydrochloric acid can attack the silver nitrate present in the pores of the paper, and produce fresh silver chloride. If the paper were quite dry, the liberated chlorine would scarcely be able to attack even the silver. Moisture, though very little, is desirable. In the excessively dry climate of India, &c., in the summer, one or other of these deliquescent salts should be invariably present for the purpose indicated, unless fuming be resorted to.

      The sensitizing bath should also never be allowed to be acid with nitric acid, since the resulting prints would invariably be poor.

      The best way of securing this neutral state is by keeping a little carbonate of silver at the bottom of the bottle in which the solution is kept. A few drops of a solution of sodium carbonate added to the bottle over-night will secure this. The reason why nitric acid is to be avoided is shown by placing a print in dilute nitric acid. It is well known that darkened silver chloride is unaffected by it; but the print will be found to change colour, and to become duller and redder than if washed in water alone. The nitric acid evidently attacks the albumen. Nitric acid decomposes the carbonate of silver (which, be it remembered, is an insoluble body), forming silver nitrate, and liberating carbonic acid.[11]

      Alum in the printing bath has also been recommended for preventing the bath from discolouring, and it is effective in that it hardens the surface of the albumen; but the ordinary explanation of its effect is defective. If a solution of common alum be added to the silver nitrate we get silver sulphate (which is best out of the bath, and it is slightly soluble in the solution), and aluminium nitrate is formed.[12]

      The same effect would be produced if aluminium nitrate were added to the bath solution. We, however, give a means of adding it as recommended by some writers. When filtering the solution, put a small lump of alum in the filter paper, and pour the solution over it, or add one grain of alum to every ounce of solution, and then filter.

CHAPTER IV.

       HOW TO KEEP THE SENSITIZING BATH IN ORDER.

       Table of Contents

      Experience tells us, however strong we may make the bath solution to coagulate the albumen on the paper, that a certain amount of organic matter will always be carried into it. At first this is not apparent, since it remains colourless in the solution; but after a time, after floating a few sheets of paper, the organic silver compound gradually decomposes, and the solution becomes of a brown or red tint, and if paper were floated on it in this condition there would be a dark surface and uneven sensitizing. It is, therefore, necessary to indicate the various means that may be employed to get rid of this impurity. The earliest, if not one of the best, is by the addition of white China clay, which is known in commerce as kaolin. A teaspoonful is placed in the bottle containing the solution, and well shaken up; the organic matter adheres to it, and precipitates to the bottom, and the liquid can be filtered through filter-paper or washed cotton-wool, when it will be found decolourized. Another mode of getting the liquid out of the bottle is to syphon it off by any syphon arrangement, and this prevents a waste in the solution from the absorption of the filtering medium. The accompanying arrangement (fig. 4) will be found useful for the purpose, and can be applied to other solutions where decantation is necessary. A is a wide-mouthed bottle holding the solution. B is a cork fitting the mouth, in which two holes have been bored to fit the two tubes, D and C, which are bent to the form shown. When the kaolin has subsided to the bottom, air is forced by the mouth into the bottle through C, the liquid rises over the bend of the tube D, and syphons off to the level of the bottom of the tube inserted into the liquid, provided the end of D, outside the bottle, comes below it.