STUDIO AND WORKROOM 13 
and first the silver and then the Rochelle salts added, both 
being dissolved in as small a quantity of boiling distilled 
water as possible. Boiling should be continued for about 25 
minutes, until a grey powder has collected at the bottom of 
the flask. The liquid should then be at once filtered until 
quite clear, and it may be necessary to repeat this three or 
four times. The longer the boiling the more rapid the 
deposition of the silver, and the more uniform it will be. 
Perfectly prepared solutions will keep for about a month. 
Equal volumes of the two solutions are mixed just before use. 
The Formaldehyde Process (Lumiére)—Clean the glass 
thoroughly with nitric acid, and wash well with distilled water. 
Pour on the surface a 25 per cent solution of alcohol and 
leave for a few minutes. Prepare the following solution, 
which must not be made until required for use: 
Formaldehyde, 40% solution 24 drops 24 drops 
Alcohol, 90% 24 ccm 407 min. 
Distilled water 24 ccm 407 min. 
Silver solution 48 ccm 812 min. 
This is sufficient for 1000 qcm (155 sq. in.), and the tempera- 
ture should be from 16° to 20° C. (60° to 68° F.). The 
silver solution is prepared as follows: 
Silver nitrate 10¢ 77 gr. 
_ Distilled water 100 ccm 2 oz. 
Add solution of ammonia drop by drop until the brown 
precipitate first formed is redissolved; but extreme care must 
be taken not to add too much ammonia. Then add: 
Silver nitrate Hey 505 15.4 gr. 
Distilled water 100 ccm 2. OZ. 
Then add sufficient water to make the total bulk 1000 ccm 
(16 oz.), and filter several times until quite clear. The glass 
should be drained from the alcohol bath and immediately 
covered with the silvering solution. The deposition begins in 
