Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9
|Sulphate of the protoxide of iron,||20 grains.|
|Bichloride of mercury,||40 grains.|
|Chloride of sodium, (salt,)||15 grains.|
Select for this operation a vigorous good positive; a faint and thin film does not answer well. One that has been rather under-exposed is most suitable. Then, whilst tire collodion film is still moist from fixing, pour upon it a quantity of the above solution, and keep it in motion. At first the picture assumes a dead and gray appearance; but this soon chases, and becomes continually more and more brilliant.
It is sometimes necessary to add a little more of the fresh solution, and to retain this solution on the surface until the whites are perfectly clear. The time required for this operation varies according to the temperature and the thickness of the film. Heat promotes the effect; the plate is therefore frequently supported on the ring of a retort-stand, with the fluid on its surface, whilst a small flame is kept in motion beneath it. Unless this precaution be observed, there will be a liability to break the plate. It happens sometimes that a few minutes are sufficient; but generally more time is required. If no heat is applied, the operation may require in some cases as much as an hour. As soon as the whites have attained their utmost purity, the operation is complete. It is better to be quite certain that the whites have attained the purity required, than to shorten the time, and have the effect underdone. 'there is no danger in giving too much time; but it is a disadvantage to remove the fluid from the plate too soon; because in drying, the whites in such a case arc apt to grow darker again, and the picture assumes then the cold blue tone, which arises from treatment with corrosive sublimate alone.
As soon as the effect has been reached, the plate is thoroughly washed in several waters, and then dried over the spirit-lamp. The plate is now ready for the first coating of varnish, which communicates transparency to the shadows, without at all impairing the whites.
The next operation is to lay on the colors carefully and artistically on those parts that require them. It is unnecessary to apply any to the Shades. Where much color is desired on a given surface, it is better to apply it by repetition, and not in one thick blotch. Colors thus tastefully laid on produce a very brilliant effect, by reason of the purity of the whites; and this effect is again increased by the softness communicated to the whole picture by the application of the penetrative varnish, which causes the color to permeate into the pores of the film, or to be seen at least in full beauty from the opposite side. This varnish is nothing more than a very pure strong-bodied protective varnish. The picture so far finished is backed up with a piece of black velvet, but never with black Japan, which would injure the film.
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