DAGUERREOTYPE PROCESS The earliest commercial photographic process, the invention of Louis Jacques Mande Daguerre. By its means a photographic positive image is produced on a polished silver surface. It was published in France, in July, 1839, and during the next twelve years attained great popularity, but the introduction of Frederic Scott Archer's wet collodion process, in 1848, soon had the effect of rendering the older process obsolete, and it is now not practised except experi mentally. Being the first in the field, extreme interest naturally attaches to it, and it has been thought desirable to explain its working in Daguerre's process, as slightly modified in details by the inventor in the course of its com mercial practice, is the one here described, it being impossible in the space at command to discuss the many modifications introduced by other experimenters during the ten or twelve years following 1839. Briefly, a daguerreotype photograph is an image formed by mercury vapour upon a silver-coated copper plate. The process comprises five operations, namely, clean ing and polishing the silvered plate, sensitising, ex posing in the camera, developing, fixing and finish ing. The sheet copper was silvered either elec trically or mechanically. In England, as a rule, Sheffield plate was employed, this being made by soldering silver to copper to form an ingot and then rolling to the required thickness. The most perfect polish upon the silvered surface was necessary, and to obtain this it was cleaned with weak nitric acid and polished with pumice powder, tripoli, and olive oil, the final polish being applied with buffs made of velvet, the plate having been previously heated to drive off the oil. For the purpose of heating during polishing, the plate was supported upon an iron wire frame A, and heated with a spirit lamp. It was essential to obtain a high polish, and dozens of methods of securing this were sug gested.
The second operation, sensitising, was modified quite a number of times. In Daguerre's original plan the plate was subjected to fumes of iodine, until it assumed a definite golden yellow. If the action of the iodine was prolonged, a violet colour was produced, and this was much less sensitive to light. Daguerre's iodising box B
had double walls. The vessel of iodine H had over it a ring supporting a piece of wire gauze. The small lid j was in position only when the box was not in use. The plate to be iodised was attached to the underside of the proper lid x ; the lid 1, enclosed the whole. The sensitising took a long time, as the crystals of iodine had to remain in their natural state, and must not be heated because of the possibility of moisture condensing upon the plate. The vapour caused silver iodide to be formed on the silver plate, which was then sensitive to light. Although Daguerre appeared to be satisfied with plates prepared in this way, many other photo graphers tried to increase the sensitiveness. God dard, for example, in 184° exposed the iodised plate to the action of bromine vapour, thereby forming silver bromide upon the plate in addition to the iodide ; and in 1841 Claudet used chlorine vapour in the same way ; either of these modi fications reduced the exposure by about four fifths. Bingham followed with bromide of lime, which for a time was widely used. These accelerators caused the yellow film to assume other hues, and in each case the plate was put back again to the iodine fumes until it assumed a rosy hue.
The third operation was that of exposing the plate in a camera (for Daguerre's instrument, see " Camera "), and, as in the case of modern dry plates, the time of exposure depended upon the actinic value of the light, etc. Daguerre's times of exposure in Paris, and with plates prepared simply with iodine, are said to have been from five to thirty minutes ; objects in shadow, even in the brightest weather, required twenty min utes at least. The daguerreotype plate, how ever, in its modified form needed only from five to thirty seconds, according to subject, light, etc. It was not an uncommon practice, when Daguerre's original process was used, to whiten the face of the sitter by means of powder in order to shorten the exposure for the face ; then, in order to bring out the details of the dark objects, such as the dress, 6 piece of black cloth attached to a long stick was held in front of the sitter's face during the time the extra exposure was given to the dress.