Photomechanical Processes 
The wet collodion process, collodion emulsion, lead, copper, 
and acid silver intensifiers, and reducers are dealt with in 
other sections. See index. } 
CLEANING ZINC AND CoppEer.—For line work very fine 
pumice powder should be applied with a felt pad or soft 
scrubbing brush and water and applied in the same direction 
as the original lines of the polish. For half-tone work on 
zinc or copper well washed whiting made into a paste with 
water and a little ammonia should be used with a swab of 
absorbent cotton. Or, for copper, American polishing char- 
coal, with rounded end, may be used in straight lines with 
plenty of water. 
PASSING OR GRAINING BATH FoR ZINC.— 
Powdered alum 31.25 ¢ Y, oz. 
Nitric acid 3.125 ccm 24 min. 
Water 1000 ccm 16 oz. 
GRAINING BATH FOR COPPER.— 
Chromic acid 23 g 175 gr. 
Water 1000 ccm 16 oz. 
ALBUMEN SENSITISER FOR LINE WorK ON ZINC.— 
Albumen (white of egg) 23 ccm 175 min. 
(Or dried albumen) 6.5 ¢ 50 gr. 
Potassium bichromate Se 40 er. 
Water 1000 ccm 16 oz. 
FisH-GLuE SENSITISER.— 
Fish glue 47.5 ccm 365 min. 
Ammonium bichromate 0.132 ¢ 1.04 gr. 
Water 1000 ccm 16 oz. 
Add enough ammonia to give a bright yellow colour. This 
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