354 PHOTOGRAPHIC FACTS AND FORMULAS 
The exposed image is not burnt in, only heated to dry and 
harden. 
Scum REMOVER FOR COPPER.— 
Acetic acid 100 ccm 698 min. 
Common salt 187.5 ¢ 3 0z. 
Water 1000 ccm 16 oz. 
The same quantity of hydrochloric acid may be used instead 
Or the acetic. 
MacHINE EtcH1ING.—Levy machine, first etch, 10 per cent 
acid for 30 to 40 seconds; second etch, 15 per cent for 1 to 
114 minutes; third etch, 20 per cent for 4 to 5 minutes. Air 
pressure for first etch, 44 to 34 lb.; second and third etches, 
1 1b. A single etch with 10 per cent acid for 30 to 40 seconds 
is generally enough except for extra deep work. For Holt 
machine and line work on zinc, 7.5 per cent with three bites 
and rolling up between. Total time for full depth, about 18 
minutes. For half-tone on zinc, use the same solution as for 
line work, for 114 minutes; then roll up and powder, and etch 
for 2 minutes. 
For half-tone on copper use perchloride of iron solution 
35° Bé. or a 35 per cent solution of anhydrous ferric chloride. 
Air pressure 14 to % |b., time for first bite 30 seconds and 
total time 314 to 4 minutes after stopping out. If the iron 
solution is very acid, add a little ammonia until a slight pre- 
cipitate is formed, then boil and filter. 
CoLLotyPE.—Grinding New Plates. Sift over the surface 
finely sifted emery powder, wet well with water and grind 
with another plate with circular motion. Wash well after 
grinding. 
Cleaning Old Plates. Soak in 5 per cent caustic potash 
solution for 4 days or place in hot 10 per cent solution of 
sodium carbonate. Scrape off old film with hard brush. Re- 
grind before use. : 
