Hello again Tsuyoshi,
I don’t know if anyone replied to your posting – I didn’t see anything on the list.
I too went to Benrido and was intrigued to hear they used a Pot / Amm Dichromate mix.
I have no experience of the collotypye process myself but I can tell you my experience of different dichromate mixes with gum and PVAL.
The higher the concentration of dichromate the shorter the printing time – it seems to go roughly in proportion – 10% dichromate is twice as fast as 5%; 20% is 4x as fast. Of course you can only go to about 13% with Potassium but 27% (?) with Ammonium dichromate. Other than the strength I know of no difference between Pot and Amm.
One other impact of changing the strength of the dichromate is the tonal range that prints. This is also affected to a lesser extent by the light source and also by the pigment being used. Using PVAL and 5% dichromate I find a narrow tonal range gets printed. This is useful with B&W prints because it allows – by changing the exposure time – me to print different blocks of tones in different colours or shades. 13% dichromate and gum is generally sufficient to print a wide range of tones – more than 10 stops on the negative – so I rarely bother with stronger Ammonium.
Hope that helps
Geoff Chaplin
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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.
Warmly,
Tsuyoshi