PROCESS BLOCKS.—Printing blocks obtained by the aid of photographic processes. The following excellent instructions for the production of half-tone or process blocks by the modern methods are given by Mr. Robert Whittet : Initiatory process in its entirety consists of three distinct operations : r. Making the negative. 2. The preparation of the metal plate, zinc or copper, to be engraved. 3. The etching; to which might be added a fourth, the mounting of a plate on a wood block prepar atory to printing, but which is so entirely dissimilar from the process of engraving, and so purely mechanical, that it may scarcely be considered as belonging to it further than necessary to make available to its ultimate purpose the plate which has been already completed so far as engraving is concerned. These several operations we will endeavor to explain in as minute and practical a manner as possible, both as to formulae and method of manipulation, so that the tyro may attain to a degree of excellence proportionate to his intelligent apprehension and diligent and persever ing prosecution of the methods described.
The dry plates are now specially manufactured with a view to suiting the process, and no doubt when the light employed is of a character that will secure steadiness and uniformity—as when the electric arc light can be made available—these will be found very convenient; but as the novice will probably be dependent on the ordinary light from the luminary of day, which is so very variable and liable to cloud and shadow, the dry plate would be found too uncertain in result, and the consequent waste too expensive for the beginner. He will hence find it most economical to adopt the wet-plate process of the old time photographer. In prosecution of this his first intention will be to the choice of glass, for which he will find crystal plate, about one-eighth of an inch in thickness, to be the most suitable, by reason of its being uniformly clear and free of bubbles and scratches. There are other makes, however, of varying excellence, and it will be well to see that what he uses is of good clear color, flat, and as free of imperfections as he can procure it.
Cleaning the procured the glass, the next operation is to have it thoroughly cleaned; and in this, as, indeed, in every subsequent part, scrupulous cleanliness is a sine qua non to success. This is best effected by allowing it to steep in a strong lye for several hours, say over night, and after a thorough rubbing on both sides with a piece of clean coarse cloth under the tan put it into a dilute solution of nitric acid—of. sav three ounces to the gallon of water—and _ _• _ 1_ _It let ror at least a nair a clay, ana none Inc worse for a longer time. From this it should be again washed and rubbed with the canvas cloth under the tap, until it is seen to be abso lutely clean by examining it toward the light. It is sometimes found that the water from the public reservoirs, from impurities held in solu tion, leaves a stain or cloud on the plate after drying ; in such case it will be needful after washing to give a last rinsing in pure water, either distilled or melted ice, filtered, which may be contained in a dish laid conveniently for the plate to be laved in it.
Albumenizing the next oper ation is the albumenizing of the glass. This is necessary as a substratum for the collodion to be afterward flowed over it. This is done immediately after rinsing from the pure water last used in washing. The proportions of albumen to water are very varying among operators in practice, some using the white of one egg to twenty-five ounces of water, while others give to the same quantity of albumen eighty ounces of water. Any proportions between these will work, the main idea governing being the injury to the silver bath, which may result by the gradual communication to it of organic matter from this source. It will readily be understood, however, that a very thin or delicate solution is all that is necessary, and the white of one egg to forty-eight ounces of water will be a desirable medium to adopt.