Dichromate browning - shall I be concerned?
I recently started experimenting with carbon transfer printing. I like brush sensitizing my tissue, for a number of reasons including the fact that I can keep a set of pre-mixed dichromate solutions of different strengths around.
So I mixed a 10% stock solution of potassium dichromate, and from that I made a 6% solution, a 4%, a 2% and a 1% one. I mixed them with distilled water and 90% rubbing alcohol, and kept them in amber glass bottles inside a cabinet at room temperature.
After 3 weeks, the dilute solutions are not orange as they started, but brown.
I pulled a few prints with the brown dichromate solutions, and they look pretty good (for as good a carbon printer I can be), but I don't have a real term of comparison from when the solutions were fresh.
I have read somewhere on APUG that dichromates have a long shelf life, so I was wondering whether the color change is related to a change in sensitivity, curves etc. Does anyone have experience with this?
Dichromate solution should not turn brown - it is probably contaminated. "Rubbing alcohol" can sometime contain colourant, fragrance, aloe vera, and other unfriendly chemicals - read the label very carefully! Be sure to use pure isopropyl alcohol or acetone for carbon work.
I've had dichro/water/alcohol solutions stay "fresh" for several years - no colour change, and as effective as the day they were mixed.
Should you be concerned? As carbon printing can be tricky enough, I'd be inclined to mix fresh chemicals and eliminate the worry
I used 91% isopropyl alcohol from Target, no added ingredients on the label.
The stock solution was made with distilled water only, and that is still bright orange. So the alcohol must be the culprit. Or the fact that I mixed the dilute dichromates on a full moon...
I can't find it in my notes or on my books, was it potassium dichromate which can only be mixed with acetone and not alcohol, or was that ammonium dichromate?
I could mix fresh dichromates every time, but unfortunately my printing sessions are relatively short these days and dealing with powdered dichromates is nasty and requires extra care.
More weeks passed...
Alcohol is definitely the bad guy here. In an experiment I added some pure alcohol to a well mixed 10% solution, and dichromate crystals started dropping to the bottom of the container after few seconds.
After switching to acetone, my solutions are still bright orange after a couple of months.