All toning solutions containing potassium ferricyanide are sensitive
to light, the ferricyanide being reduced to ferrocyanide. Therefore
when not in use the tanks should be carefully covered so as to screen
the solutions from sunlight.
It is also very important that no metallic surface however small
should come in contact with the solutions; any metal pegs on the
racks or pins used to hold the film together will gather the tone in
preference to the film, causing the formation of sludge. Plain wooden
racks with or without wooden pegs should be used, and metal faucets
replaced by wooden plugs.
If any considerable amount of sludge forms in the tank at any time,
it is due either to incorrect mixing, the action of light, or to the
presence of foreign metallic substances.