Sandy King (sanking@hubcap.clemson.edu)
Wed, 13 Jan 1999 17:23:31 -0400
FotoDave wrote:
>I wrote:
>>Dichromate will lose strength with age.
>
>sanking@hubcap.clemson.edu wrote:
>
>> I believe your statment below to the effect that dichromate will lose
>> strength with age is wrong.
>
>Hi Sandy,
>
>I believe your statement above is right.
>
>Sorry that I was not very accurate in my description. I didn't think the
>actual dichromate itself was losing strength (at least I couldn't find any
>technical literature that explained that). That's why I said I wasn't sure it
>was because of chemical change of the dichromate itself or whether it was
>simply precipitation. What I meant was a bottle of dichromate stored for a
>long time could lose its working strength. In fact, even in my own example, I
>don't think the dichromate itself has changed, but the concentration has due
>to precipitation.
Dave, I don't understand how this happens. Can dichromate precipitate out
of a solution mixed with distilled water? If so I am unaware of this having
happened in my own work.
>
>What did you use the dichromate for? Did you use it mainly for carbon? For
>carbon, the hardening starts from the top, the unhardened part will be washed
>away, and since it only took a little amount of dichromate to harden colloid,
>there probably won't be any effect on carbon even if the dichromate is losing
>concentration a little.
Yes, I use the dichromate almost always for carbon. However, the difference
between a 1% dichromate solution and one of 3% is a very big deal in carbon
printing, and the loss of concentration of the dichromate in solution would
be immediatley noticed.
>
>Then I suddenly realized (and understood) the statement in some books that
>while it took only a little dichromate to harden the colloid, one usually
>needs more so that the hardening will reach the base. I can't remember where I
>read it from. I think it was from Kosar although I have also read it somewhere
>as (probably from that Web site that sells fish glue).
But I don't think Kosar made this point with reference to the strength of
the dichromate solution, or did he? If he did, please tell me where.
>
>So if you are doing carbon, you might not have problem either because
>hardening is from top and then the layer gets transferred. It doesn't need
>much dichromate to harden the top layer.
But again, the strength of the dichromate solution would play a big role in
carbon, as noted earlier.
Sandy King
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