The Art and Practice of Silver Printing

by H. P. Robinson; Sir William de Wiveleslie Abney

It must be remembered that albumen is soluble in water: it is coagulated or insoluble in water when combined with silver nitrate. The fact is that the quantity of silver nitrate in the solution we have been experimenting with is too small. The water dissolves the albumen first, and then the silver has time to act upon it to form the insoluble albuminate. If we soak paper in common salt, and treat it in the same way with the same strength of solution, we shall find that this is not the case: the silver chloride will remain on the paper. From this we learn two facts.

1st. That the silver solution has a greater affinity for the chloride than for the albuminate, and that in an equal mixture of the two more chloride would be formed than albuminate; in other words, that the ammonium chloride would be totally converted into silver chloride long before the silver albuminate was formed.

2nd. That a certain strength of silver nitrate is necessary to prevent the albumen dissolving from off the paper.

This last fact has fixed the lowest strength of any sensitizing solution to be thirty grains to the ounce, and even if this be taken as a limit, it is necessary that the water should be rendered less active by holding some other soluble matter in its embraces. This is usually effected by adding some other neutral and inactive nitrates. There does not seem to be any theoretical limit to the amount of silver nitrate in solution, but practically it rarely contains more than 80 grains to the ounce, though occasionally we have heard of it being used of a strength of 100 grains to the ounce.

The important point now presents itself. How are we to fix the strength of the bath? What principles must we follow?

To answer these questions we extract a passage from another work of this series.[9]

"If a paper be coated with albumen (say) in which has been dissolved a certain quantity of a soluble chloride, and floated on a silver solution, both chloride and albuminate of silver are formed. It depends, however, on the strength of the solution as to what proportions of each are present, owing to the fact that the organic compound is much slower in formation than the chloride, and has less affinity for the silver. If the silver solution be not sufficiently strong, the chloride may rob that portion of it with which it is in contact of all the silver before any (or, at all events, sufficient) albuminate has been formed, the molecule being composed almost entirely of silver chloride. The stronger the silver solution the more 'organate' will it contain; whilst if it be very weak, very little will be present. Hence it is with albumenized paper which is weakly salted with a silver chloride a weak sensitizing bath may be used, whilst if it be rich in the chloride it must be of proportionate strength."

It will now be seen that the proportion of chloride to albumen has to settle the point. We next have to consider the time during which the silver should be in contact with the paper when the floating is commenced. Let us take the case of a strong silver solution, and consider the action that will follow. Immediately the paper is placed in contact with the solution, silver chloride is formed, and the amount of the silver nitrate in the layer of fluid in immediate contact with the surface being scarcely diminished by the formation of silver chloride, the albuminate is formed almost simultaneously, forming a film which is to a great extent impermeable to the liquid. But even before this layer is coagulated, the next layer of chloride will have been formed, so that we may say we have one layer of albuminate and chloride of silver, and one layer of chloride of silver alone.

The further penetration of the silver solution will be very slow; hence, for fully saturating both the albumen and the salt with silver, the time of flotation must be prolonged. For some purposes, however, this is not necessary, as will be seen presently.

Next let us trace the action of a weak solution, not weak enough to dissolve the albumen off the paper, but of the minimum strength. The solution, as before, would immediately form the silver chloride, but before the albumen had coagulated at the surface, the solution would penetrate to the interior of the film, and then the formation of the albuminate would proceed nearly equally throughout the whole of the interior. Evidently, then, in this case, the contact of the silver solution would be less prolonged than in the former case. If the floating be prolonged the silver solution in the interior will become weakened, and partially dissolve the albumen and be carried by the water into the interior of the paper; it will also partially dissolve off the surface, and a negative printed on such a paper would have all the appearance of being dead in lustre, and existing in the paper itself instead of on the surface.