From: Sandy King (sanking@hubcap.clemson.edu)
Date: 03/29/01-07:04:09 PM Z
Katharine Thayer wrote:
>I think it's probably best not to try to compare differing results of
>ammonium and potassion dichromate across different printing processes.
>In gum printing, my experience is that ammonium dichromate is FASTER,
>meaning shorter exposure times. In fact, I suspect that the reported
>observation of some that they get a heavy green-brown dichromate "stain"
>with ammonium dichromate is because they don't reduce the exposure time
>enough, relative to their exposure time with potassium dichromate.
>Katharine Thayer
If the comparison were between radically different processes such as
gum and platinum I would agree. However, there is a rather formidable
amount of research that suggests one can make useful comparisons
among all of the dichromated colloid processes, specifically
including carbon(gelatin based) and gum.
As for color, in carbon printing there is no difference in results
between the ammonium and potassium dichromate, irrespective of
strength of sensitizer or time of exposure, with the same pigment
tissue.
Sandy King
>
>
>
>Sandy King wrote:
>>
>
>>
>> Also, I believe some gum printers (Dave Rose at the start of this thread)
>> are using the term "faster" in comparing differences between ammonium and
>> potassium dichromate (or in comparing different strenghs of these
>> chemicals), when in fact what they are describing is sensitivity. In my
>> work with carbon the speed of tissue (as determined by how long it takes to
>> print the first maximum black) is very similar regardless of which
>> bichromate I use. However, sensitivity is slightly greater with ammonium
>> dichromate than potassium, and of course increases significantly with
>> stronger sensitizers, the practical range going from about 1/2% up to 6-8%.
>> Sensitivy controls how many steps of the step wedge you can print,
>> measuring from the first maximum black to the last visible step, and is
>> there related to contrast.
>>
>> Sandy King
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