From: Sandy King (sanking@clemson.edu)
Date: 07/06/03-09:47:33 AM Z
Chris,
I don't know where the extra "no" came in, but the fact is this.
Adding ammonia to potassium dichromate to the point where it becomes
lemon yellow changes the dichromate to a chromate, and the pH is
increased. Carbon tissue sensitized in the chromate has less
sensitivity but greater contrast. More importantly, tissue sensitized
in the chromate has better keeping qualities than tissue sensitized
in potassium dichromate.
Regular grocery store ammonia is fine.
BTW, tissue sensitized in ammonium dichromate also has better keeping
qualities than tissue sensitized in potassium dichromate. For
example, tissue sensitized in potassium dichromate will start to fog
within several hours of drying, while tissue sensitized in ammonium
dichromate usually takes five to ten times as long to show the same
level of fog. This may also apply to gum printing, though that is
just speculation.
Sandy
>Sandy said:>
>> Acidity is an important issue. Again, in carbon printing the practice
>> in former days was to add ammonium to potassium dichromate, which
>> converted the solution to a chromate of much higher pH. This would
>> result in less sensitivity but greater contrast. no
>>
>Sandy, just to clarify what you say above, because I'm not sure of the
>punctuation: adding ammonia results in less sensitivity and NOT greater
>contrast? Is the no for the greater contrast or just a typo?
> One other question: is just the normal ammonia at the grocery store OK
>to use or do I have to get some special scientific kind of ammonia--in other
>words is ammonia ammonia? My Tri-Ess catalog is packed...
>Chris
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