Copper sulphate 1 lb. 11 ozs. 800 grams
Ammonium citrate (neutral) 5 lbs. 3 ozs. 2500 ”
Potassium ferricyanide 1 lb. 11 ozs. 800 ”
Ammonium carbonate 13 ozs. 400 ”
Water to 50 gals. 200 liters
Dissolve each ingredient separately in as little water as possible,
mix the filtered solutions so obtained in the order given, and dilute
to the required volume. The solution obtained should be light green
in color and perfectly clear. The ammonium carbonate should be almost
transparent and free from white powder, and should be dissolved in
cold water, while the ammonium citrate employed should be neutral.
Most samples contain acid, which acid would neutralize the ammonium
carbonate and affect the resulting tone. The free acid is usually
citric acid, and should be neutralized with a solution of ammonia
using litmus as indicator.
_Temperature of Toning_—To obtain the best results the temperature of
the bath should be from 65 degrees to 70 degrees F.
_Time of Toning_—Five to ten minutes.
_Time of Washing_—10 minutes.
_Life of Bath_—With use the bath precipitates a brown sludge of
copper ferrocyanide, and in consequence becomes weaker by virtue of
the loss of copper, though 50 gallons of the solution will tone about
10,000 feet of film.
It is necessary to keep the film entirely submerged in the tank,
or constantly in motion when on the drum, otherwise if the film is
frequently examined, especially during the first few minutes of
toning, streaks are liable to occur.