Any mild reducing agent will do to coax the slightly-dissociated halide anions (chloride, bromide, iodide, etc.) away from the silver, leaving atomic silver in the gelatin emulsion.
Including, BTW, vitamin C.
Son2 tried coffee as a developer and it worked, with only moderately satisfying results. Images resulted, but they were kind of muddy. It was a good question as to whether this was due to the coffee "dyeing" the gelatin brown,
or because the film he used was ridiculously outdated,
or because the time-temperature parameters were wrong,
or because the moon was in the wrong phase
or all three.
But it did act as a developer.
He knows I'm a coffee
hound addict, and he resented my remark about it being a waste of good coffee.
It also seems to be a (lousy) indicator, since the dried remains of it tend to turn greenish in the presence of alkaline ions. I remember noticing that once, but I'm not about to go testing it again.
Terry, 230RN
REF:
https://en.wikipedia.org/wiki/PH_indicatorIOW, like Litmus paper.
