. The book of photography; practical, theoretical and applied. y or so before printing from. The Etch. The next proceeding is to treat theplate with a deliquescent or moisture-retaining mixture known as the etch,with the object of keeping it in a dampcondition. The term is altogether a mis-nomer, since no etching takes place, theplate being simply rendered moist invarying degrees, according to the differeniaction of the light on each part, which PHOTO-MECHANICAL PROCESSES. 681 has rendered some portions comparativelynon-absorbent, while others are scarcelyaffected. It is, of course, this great
. The book of photography; practical, theoretical and applied. y or so before printing from. The Etch. The next proceeding is to treat theplate with a deliquescent or moisture-retaining mixture known as the etch,with the object of keeping it in a dampcondition. The term is altogether a mis-nomer, since no etching takes place, theplate being simply rendered moist invarying degrees, according to the differeniaction of the light on each part, which PHOTO-MECHANICAL PROCESSES. 681 has rendered some portions comparativelynon-absorbent, while others are scarcelyaffected. It is, of course, this greater orlesser moisture of various portions of theplates vrhich causes the greasy ink to beattracted or repelled. A suitable formulafor this purpose is :— Glycerine ... ... ... ... 24 ozs. Water ... ... ... ... IG ozs. Sodium chloride (common suit)... 40 grs- The plate is placed on a level table andthe solution poured on, care being takenthat it is equally distributed. It is lefton for a time varying with the correctnessof the exposure. An over-exposed plate,. Fig. 887.—Wooden COLLOTyPE Pkess. being more insoluble, will require muchlonger treatment. If left long, however,the plate will absorb too much moisture,yielding a grey, flat print. It is best,therefore, to allow the solution to remainfor, say, half an hour, and then to mop itoff with a sponge. The plate is now verygently dabbed with a soft, dry rag until itis perfectly clean, and it is then inked the etching is seen to be insufficient,the ink can be washed off with turpentine,and another etch given. The etchingsolution may be used repeatedly. Machines and Presses. Before treating of the inking process itwill be advisable to devote some atten-tion to the different patterns of printingpresses and machines specially made forthis class of work. For experimentalattempts a letter-copying press will givegood results; but for any quantity ofwork a proper collotype printing machineis indispensable. Of these presses ormachi
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