> George,
>
> You wrote, "I keep the Van Dyke sensitizer in the refrigerator
> in a blackened bottle."
>
> I take it that means in one bottle (singular). The Van Dyke formula I
> use calls for three seperate solutions which are combined in equal
> parts just before use unlike Kallitype where everything is mixed in
> one solution and allowed to "ripen" for 3 days before use. I am wondering
> if something is happening as a result of premixing the Van Dyke sensitizer,
> that is, I wonder how long you can keep it even in the fridge. There is
> nothing wrong with your coating technique. Any other opinions?
This is the first I ever heard of either keeping VDB formula in
refrigerator or storing it as 3 separate solutions (which I suspect would
keep worse separately than combined). I have for 15 years used formula of
fe am cit, tartaric acid & silver nitrate solutions combined, stored in
brown bottle at room temp. The emulsion keeps until it's used up.
Sometimes it plates out on inside of bottle, but that doesn't affect
effect. I've kept it from one to two years without a problem. Sometimes
at very end black precipitate is visible in the dropper, but as soon as
you start to spread it on paper, it goes back into solution.
Then I've taught VDB to several hundred students, some of them air babies,
and NEVER seen the blotches described. Do you let it come to room
temperature before coating? I suspect something about the fridge. What's
your formula?
And, Bob, I never heard of letting kallitype "ripen" for 3 days either...
Where did you find that? Did you test it both ways? I never tried the
ripening act, but never saw need to -- mixed ferric oxalate and silver
nitrate solutions at time of use. What's the 3rd ingredient?
Judy