had a sorely trying journey, wintry and wearisome indeed;，
The question is often asked, why is it that the plates receive the coating so unevenly? I will answer by saying that it may arise from two causes: the first and most general cause is that those parts of the plate's surface which will receive the heaviest coating have been more thoroughly polished, and the consequence is that it is more sensitive to the chemical operation. second. and might perhaps be considered a part of the first, the heat of the plate may not be equal in all its parts; this may arise from the heat caused by the friction in buffing. It is a well known fact, with which every observing practitioner is familiar, that a silver plate at a temperature of 45 deg. or less, exposed to the vapors of iodine, is less sensitive and takes a longer time to coat, than when it is at a temperature of 60 deg. or more.
Whenever a view is to be taken, or any impression which requires the plate to be turned on the side, it should be buffed in the other direction, so that the marks will always be horizontal, when the picture is in position. With the finest possible polish, the plate is ready for the coating box.
The question is often asked by operators, what is the state of the plate when polished and allowed to stand for a time before using? To meet this point we hare only to consider the silver and the power acting upon it. Pure atmosphere does not act upon silver; but we do not have this about in our operating rooms, as it is more or less charged with sulphurated hydrogen, which soon tarnishes the surface of the plate with a film of brown sulphurate. It is this that sometimes causes the specks which appear on finishing the impression, and are a great annoyance. Hence we see that the plate should be buffed just before receiving the vapor of iodine. Mr Hunt gives his opinion of the use of diluted nitric acid as the best solution for freeing, the surface of the plate; he says:
"Numerous experiments on plated copper, pure silver plates, and on silvered glass and paper, have convinced me that the first operation of polishing with nitric acid, etc., is essential to the production of the most sensitive surface. All who will take the trouble to examine the subject, will soon be convinced that the acid softens the silver, bringing it to a state in which it is extremely susceptible of being either oxydized or iodized, according as the circumstances may occur of its exposure to the atmosphere or the iodine."
I cannot see the objection to this solution; not, however, in general use. Our operators do not find it of sufficient importance to the success of their pictures to accept it, the alcoholic solution being in its nature less objectionable.
I will say here, that a plate submitted to only an ordinary polish is found to contain numberless minute particles of the powder made use of. Should the same plate be buffed for a long time, the polish will nearly all disappear, leaving the cavities in the surface free for the action of agents employed in subsequent operation. For this reason, I find that great amount of polishing powder should not be applied to the last buff, and it is obvious that three buffs can be employed to adventure; the two last should not receive any polishing materials. I have examined a plate that was considered to possess a fine finish, and similar had produced good impressions; these same plates, when subjected to a long and light buffing, would present a surface no finer in appearance to the naked eye; but upon exposure to the solar radiation, would produce a well-defined image in one fourth less time than the plate without the extra buffing.
Coating the Plate.--For this purpose our mechanics and artists have provided a simple apparatus called a coating-box, which is so arranged as to be perfectly tight, retaining the vapor of the iodine or accelerators, and at the same time allowing, by means of a slide, the exposure of the plate to these vapors. They can readily be obtained by application to any dealer, all of whom can furnish them.
The principal difficulty in coating the plate, is that of preserving the exact proportion between the quantity of iodine and bromine, or quick. It is here necessary to say, that hardly any two persons see alike the same degree of color, so as to be enabled to judge correctly the exact tint, i. e. what one might describe as light rose red, might appear to another as bright or cherry red; consequently, the only rule for the student in Daguerreotype, is to study what appears to him to be the particular tint or shade required to aid him to produce the desired result. Practise has proved that but a slight variation in the chemical coating, of the Daguerreotype plate will very materially affect the final result.
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