[alt-photo] Re: Trying to understand how ferric chloride reacts with gum arabic

[Kees Brandenburg]

On 8 dec 2010, at 15:55, Mike and Jan Andrews wrote:

> I am intrigued when you refer to
> "... carbon techniques where a coat of dichromate is exposed and pigmented gum is brushed over it. Here the hardening compound is allso diffusing in the colloïd."

I spoke a little bit to soon, because the technique I meant is a slightly different but  also works by diffusion from the bottom up. But exposure is at a different moment.
I was refering to this book:
It is the first book about gum printing in the Netherlands and was written by W. H. Idzerda: De Gomdruk, published by Hansma, Apeldoorn in 1899. In his method a gelatin sized and hardened paper is coated (chromated he calls it) with a 10% solution of Potassium or Ammonium dichromate. Then it is dried. A pigmented layer of gum is brushed over the paper, dried and the exposed. He describes how the dichromate diffuses in the gumlayer starting at the paper surface. He states: 'the dichromate concentration is at the contact point between paper surface and gum layer at its highest. Thus ensuring a better bond between paper and gum". The paper is developed in water with the use of several manipulations (friction with wet cotton, sawdust etc..

He gives the credits for this method to James Packham, F.R.P.S. I found a copy of an original text: http://www.photographyhistory.net/category/gum-bichromate/
In later publications Idzerda changed to the 'normal' gum technique.

Kees