8 PHOTOGRAPHIC FACTS AND FORMULAS 
tion of iodine in potassium iodide, or a few drops of tincture 
of iodine in water, until the grey fog turns yellow; then 
rinse, immerse in weak ammonia water until it turns white, 
wash and dry. The longer the plate is developed in the first 
place, the more pronounced becomes the mattness. As a rule, 
this gives a surface that is only useful with a magnifier, the 
grain being too fine for ordinary visual work. Or the follow- 
ing may be used: 
Rice starch 20 parts 
Water glass 100 parts 
Water 200 parts 
The water glass is the ordinary commercial solution of 
sodium silicate of 1.3 sp. gr. Mix the above by first rubbing 
the starch up with the water and then adding the silicate. 
Level the glass and pour enough of the solution on to cover 
it, allow to dry, then coat with celluloid varnish. 
To CLEAN GLass.—Immerse the glass in the following: 
Potassium bichromate 50 g 1 oz. 
Sulphuric acid 25 ccm Y4 oz. 
Water 1000 ccm 20 oz. 
Immerse the glass and scrub with an old toothbrush or mop 
made by tying some rags on a stick, turning the glasses over 
with a stick, as the solution bites the fingers. Rinse in cold 
water, then in hot water and dry. Or make a thin cream of: 
Tripoli 500 g 8 oz. 
Denatured alcohol or 
methylated spirit 500 ccm 8 oz. 
Ammonia 500 ccm 8 oz. 
Rub well with a swab of cloth and polish with a clean cloth, 
finishing with filter paper. 
CEMENTS.—Melt rubber and shellac in equal parts on a 
sand bath and apply to the warm glass or metal. This is 
useful for cementing glass to glass, or glass to metal. Or 
