In response to Bob Schramns question about the effect of time on the
(loss of) sensitivity of ammonium dichromate solutions, you wrote:
Ammonium dichromate will lose from 10 to 20% speed in a year's time, so
> that is not the cause, either.
This may/may not be accurate. The light sensitivity of dichromate
solutions are greatly influenced by the pH of the solution. The highier
the pH, the lower the sensitivity. Over time, all dichromate solutions
will show an increase in pH (whtether stored in a dark bottle or not)
until they get to the point where they are virtually insensitive to
light. Also, if the stored dichromate solution has turned a dark brown
over time, it will act a a light-filtering dye, preventing actinic light
from reaching the sensitized gel/gum layer.
I have found that the best way to maintian the speed of dichromate
solutions is to regularly "top-off" old solutions with freshly made
ones and to carefully monitor the pH of the solutions.
Best regards,
Charles