Ammonium Dichromate vs Potassium Dichromate in Gum Printing

I have been using Potassium Dichromate for gum printing and due to circumstances my exposures are between 20-25 minutes. I understand that Ammonium Dichromate is faster and I may be able to lower my exposure times. Is it a good rule of thumb to half your exposure times with Ammonium Dichromate? Also are there other variables to take into account between the two chemicals?

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Ammonium dichromate may be harder to obtain as it is flammable and therefore there may be an additional HAZMAT charge.

Gerald,
I already have the ammonium dichromate. Just interested in others experiences


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It is not faster except by weight, AD has more dichromate ions (that do the heavy lifting) than PD since the Potassium part weighs more than Ammonium part relative to the amount of Chromium. Add a touch more PD and you'll get the same results...but the difference is not by a factor of two.

PD K2Cr2O7
AD (NH4)2Cr2O7

There are some other differences such as AD can go into a stronger concentration in water than PD before its starts to precipitate out.

Hope I have that all right.

At equal concentrations, AD will be more sensitive due to more chromium(IV), other than that I have not observed any difference.

Although the information is not directly related to gum printing, it seems ammonium dichromate produces better images compared to potassium dichromate in holography. The principles are almost the same colloid + dichromate exposed under UVA light. There are people who use PVA(polyvinyl alcohol) as gum arabic substitute, so I think it is fair to say technically AD could be better than PD (I am not sure if that is observable in gum prints). http://holoforum.org/data/doc/Lessard-PVA_dichromate.pdf

I tried doubling the KDc in a printing session the other evening and was getting much better exposure times in the range of 8-10 minutes. I mixed the gum and dichromate at a 1:2 ratio gum to dichromate.


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