U of S | Mailing List Archive | alt-photo-process-l | Ammonium and Pottasium Dichromate

Ammonium and Pottasium Dichromate

Hi everyone,

I am interested in knowing the difference of a tonal rage by using ammonium and potassium dichromate.

It seems to be that ammonium dichromate produces a longer tonal range than potassium dichromate, say, in gum printing. Also, dilution of dichromate also affects the contrast rage, say, in carbon printing. So the kind and dilution affects the contrast range.

I was in Kyoto for a last couple of days working with Benrido, who makes collotype prints. There were a few technical points that interested me. It seems to me that there is a room for an improvement. One of them is the use of dichromate and the dilution of it when they prepare plates.

I was told that they mix gelatin and both potassium and ammonium dichromate at a certain dilution to pour on to 1/2" plate of glass. Also, they told me that they have not changed the way they make plates for a long time. The company has been around since late 1800's.

As you can see in the video clip on Youtube, collotype is not purely chemicals process, and they are many variables during the process especially at printing stage. Once the plate is ready, a printer quickly assesses the condition of the plate based on their experience to determine how hard and how much of ink they use, including relative humidity and "wetness" of the plates. All these factors affect the tonal range a quite bit.

So, I am interested to know if possible to gain a wider tonal range by changing the way they prepare the gelatin solution. Also, are there other effects we can see if they were to change the formula i.e. speed. Why people prefers one over the other in a given process in the first place?

Any suggestions and insights would be appreciated.