ENGRAVINGS, COPYING In copying engravings and drawings the only difference in working arises from the fact that the subject consists of black lines on a white ground, a type of subject that is usually con sidered difficult on a gelatine plate, as it is far from easy to obtain sufficient contrast with ordinary plates. By adopting the following method of working little difficulty should be experienced in obtaining a negative that will give all the contrast required in the print. A slow plate of the kind specially prepared for this work should be chosen, and it should be backed. A fine-grain ordinary, or a process plate, will answer well. Correct exposure is very important. The following method of timing will ensure correct exposure. Place the exposure meter flat against the drawing to be copied, and note the time required to match the standard tint. Using f/ i6 and a plate of the speed of 4o H. and D., one half of the time that a Wynne meter requires, or one fourth of the time required by a Watkins meter to reach the tint, will be the correct exposure for copying the same size as the original. For other plates or for different
scales, the proportionate exposures can easily be found from these data. Should the lines show distinct signs of veiling, development should be stopped, and the negative afterwards intensi fied, if necessary. (See also " Copying," etc.) In process work, an engraving on thin, dean paper, and with good black lines, without printing on the back, may be copied by using it as a diapositive. It is placed in contact with a sensitised zinc plate and exposed to light for a sufficient time. A negative image will be developed, but this may be converted into a positive one by flowing with a shellac varnish containing some colouring matter to make it visible. The latter will attach itself to the bare zinc lines, whilst that which rests on the sensitive colloid film will be dissolved away by treating the plate in a bath of weak add.
Playertype (which see) is also an easy process for copying engravings on to sensitive paper.