[alt-photo] Re: coffee and cyanotype

[Ryuji Suzuki]

One thing to keep in mind when talking about tannic acid:

Tannic acid is NOT a well defined compound of uniform quality. Instead, it is a 
mixed bag of many similar compounds, and composition of the stuff is highly 
variable, unless it is controlled and specified by the supplier.

Some "tannic acid" is very pale yellow, some orange, some very dark, when dry, 
and when dissolved in water. The color doesn't mean much by itself, but it 
varies. Many stocks also leave insoluble residue but some don't.

Companies selling chemicals in small volumes do not know or appreciate these 
things and they usually repackage whatever most available and cheapest in small 
quantities and sell at the highest price. What's worse, they don't tell you what 
it is and who made it (they think it is all the same). If the label gives you 
"formula weight" don't believe it unless you are given a specific analytical 
data or something that is more substantial. Those numbers are just copied and 
pasted from standard database.

To avoid staining and maximize toning, it's probably best to use smaller 
variants, tetra-, penta- or hexa-galloylglucose, not bigger molecules. In more 
common in low quality stocks, you'll find mostly octa- nona- and 
decagalloylglucose than smaller stuff. And then it is probably most effective if 
you use rather dilute solution in alkaline bath (pH of 8 to 10).

I'm not a cyanotypist but I have deep interest in gelatin, cellulose and other 
polymers and iron chemistry so I looked into this years ago. It wasn't easy to 
get a decent quality tannic acid (mostly comprising hexagalloglucose or smaller) 
from a source I could trust. But if I didn't get that stock and compare with 
stuff from Sigma Aldrich and others, I would've never learned these stuff.

--
Ryuji Suzuki
"Don't play what's there, play what's not there." (Miles Davis)


Francesco Fragomeni wrote:
> Toning in coffee is indeed different then tea toning or pure tannic acid
> toning. There are a number of different components in coffee which act
> differently on the paper and iron in a cyanotype. Depending on the coffee
> and duration of toning time, coffee toning can actually yield a more grey
> to black tonality with less stain to the paper base then can be achieved
> with tea toning and tannic acid which are notorious for bringing down the
> paper base significantly.