This section is from the American Hand Book of the Daguerreotype, by Samuel D. Humphrey. Published S. D. Humphrey, 37 Lispenard Street 1858.
This subject is worthy the attention of every operator. The following process is so plain and easy of trial that any Daguerreotypist can try it. This is as given by Mr. James Campbell, and was published in Humphrey's Journal of the Daguerreotype and Photographic Arts, vol. 5, page 11. Mr. Campbell has done much to further the process announced by M. Neipce, and his experiments have proved highly successful.
The following is submitted as worthy of trial:
"The proper preparation of the chloridated plate, to enable it to receive colored impressions is an object of the first importance to those wishing to experiment on it, and consequently requires particular notice. The plate may be prepared by making it the positive pole of a battery, and letting it at the same time be immersed in chlorine water. The negative pole should be a slip of platinum. All the colors may be produced from a plate so prepared if the chlorine and water are in the right proportions; but generally one color or the other predominates, according to the amount of chlorine in the liquid. By adding the chlorides of strontian, uranium, potassium, sodium, iron, or copper to the liquid, various effects may be produced, and these bodies will be found to produce the same color on the plate that their flame gives to alcohol.
"The honor of this discovery is due to M. Neipce. Copper gives a variegated flame; hence many colors may be impressed on a plate prepared with a solution of its chloride.
"M. Neipce recommends a solution of the mixed chlorides of copper and iron, and it is with these, that I have been most successful. As the chlorides of copper and iron are not much used in the arts, they are not generally found for sale in the shops; and it may be well to furnish those not much versed in chemistry with an easy method of preparing them.
"They may be made directly from either metal by dissolving it in hydrochloric acid; but they may be formed by a cheaper method, and by which also the acid fumes are avoided.
"Sulphate of iron or copper, or both together, may be dissolved in water and then neutralized with common crude potash, or its carbonate or bicarbonate--known commonly as pearlash and saleratus. If either of the latter be used, there will be formed sulphate of potash and a carbonate of the metal used, and there will also be a considerable effervescence of carbonic acid, which will, if care is not taken, cause the mixture to run over the vessel. After the copper or iron salt is neutralized, which is known by its ceasing to effervesce, the carbonate of the metal will settle slowly, and will at first nearly fill the vessel. The supernatant fluid, which is sulphate of potash in solution, may now be carefully poured off, and its place filled with water; this operation should be repeated several times until the water which passes off is tasteless. The carbonate of the metal rapidly changes to an oxide by contact with the air, and it will generally be found, when it is sufficiently washed, that it is at least half oxide. On adding hydrochloric acid cautiously to the mixture, a chloric of the metal will be formed, and carbonic acid will be evolved from the remaining carbonate. The chloride formed is soluble; but as there are two chlorides of these metals, and we wish to produce the one which contains the most chlorine, it is best to add the acid cautiously until the solution is decidedly acid. After filtering the solution, it is fit for use; and it should be preserved in well-stoppered bottles. The water used should be rain or distilled water.
"About one part of the mixed chlorides should be used to three or four of water.
"The battery may be either Smee's, Daniell's, or Grove's; if of either of the former, it should be of two series; if of the latter, one cup is sufficient.
"The plate on being immersed in the liquid, almost instantly takes a violet color. It should be allowed to remain from two to five minutes, according to the strength of the battery, and until it becomes nearly black. It should now be carefully washed, and afterwards heated over a spirit lamp until it takes a cherry-red color, and it is then ready for exposure in the camera. Before speaking of exposing the plate, it may be well to speak of some difficulties which the inexperienced operator may find in preparing it. If the battery is not in good order, and a sufficient current is not passed through the solution, the plate will become coated-- and apparently almost as well as when the battery is working well-- but on exposure it will give a negative picture, and but little colored; while if the battery is in good order, the impression is invariably positive.
"Sometimes on heating the plate after washing, the surface is covered with spots or assumes a variegated appearance. This indicates that the solution is impure, or that the plate have not been thoroughly washed and are still contaminated with the soluble chlorides which are contained in the solution.
"From the fact that the plate if prepared with positive electricity gives a positive picture, while it prepared otherwise it gives a negative, it is evident that electricity plays an important part in this process. The same is true to some extent with the compounds formed with iodine, bromine. and fluorine.
"On heating the plate, the brown coating of chloride melts into a translucent enamel, and the heat should be withdrawn when a cherry-red color is produced. It the heat is continued longer, the plate assumes a lighter color, and becomes less sensitive; and the enamel will finally scale off. To produce a picture by the ordinary process of M. Neipce, unaccelerated, it should be exposed for from three to five hours to sunlight in the camera, though pictures may be procured by contact, in from fifteen to thirty minutes."