From: Sil Horwitz (silh@earthlink.net)
Date: 04/26/00-10:26:09 PM Z
At 2000/04/27 03:21 AM +0000, Emily wrote:
>here's a question.. i heard that i could tone a cyanotype brown/black by
>immersing it in an ammonium hydroxide solution for a few minutes, then
>immerse it in water+ tannic acid.
>
>can any other acidic solution substitute the tannic? maybe nitric?
First of all: "tannic acid" isn't an acid in the sense that nitric acid is
- it's an organic compound that can form complex salts with positive ionic
material (such as metals). When you tone a cyanotype with tannic acid, you
form a very complex ferric tannate, which is normally brown. The hydroxide
is supposed to (note: "supposed to") create ferric oxide which also
supposedly reacts with the tannic acid to form the above complex compound.
In practice, there are many pitfalls. Also, ferric tannate is not very
permanent.
Using nitric acid not only will destroy the image, but could possibly
release cyanic acid, which is a gas commonly called "cyanide." You don't
want that around. (Nitric acid + ferric oxide = ferric nitrate, which is a
very soluble compound, which will completely dissolve in the wash water. I
stated "could possibly" release the cyanide, because there could be traces
of the ferrocyanide remaining in the image.)
Sil Horwitz, FPSA
Technical Editor, PSA Journal
teched@psa-photo.org
silh@earthlink.net
Visit http://www.psa-photo.org/
Personal page: http://home.earthlink.net/~silh/
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