Although I haven't yet made any cyanotypes myself, I do like to read up on all kinds of stuff photography and alternative process related.
One thing that struck me was the possibility of "(tea-)toning" cyanotypes. At first, my thought was that the process involved here, was nothing more than simply "dye-ing" the prints to lay a brown tea color on top of the blue. So not actually a chemical reaction transforming the cyanotype, but simply applying a kind of transparent "paint" layer. However, after reading (there was a url link here which no longer exists) on APUG (archived article, was part of Article system on APUG before it was renewed), I was highly surprised and confused to see a "bleaching" step involved in the process, suggesting a chemical transformation of the Prussian blue pigment that forms the cyanotypes blue color, a much more complex process than I expected and similar to toning for example a silver image with sepia or selenium toner.
So than, through some completely different search, I hit on this page about historically used "Iron gall ink", known for its corrosive and destructive properties on historical papers, sometimes completely eating through it:
The ink corrosion website
and especially:
Iron gall ink - Chemical reactions
Ink corrosion
and I began to realize there may be a connection between the two. The iron gall pigment that colors the ink is made by the reaction of Fe2+ cation derived from Ironsulphate, with gallic acid or gallotannic acid (two different but related chemical substances, see links above), also called tannin, derived from galls.
In this process, the iron cation binds in a chemical reaction with the tannin / gallic acid and / or gallotannic acid, to form the dark black/blue ferrogallotannate pigment, which gives the ink its black color.
Now the process Jason described involved "bleaching" the Prussian blue of the cyanotype with sodium carbonate, an alkaline solution. Prussian blue is known to be broken down in alkaline conditions, probably releasing free Fe2+ cations (Cyanotypes consist of Prussian blue, which is a pigment with the chemical formula K+Fe3+[Fe2+(CN)6]-, also called ferricferrocyanide).
The next step Jason described was using pure tannin (or tannin derived from strong tea) to tone the print, suggesting a similar reaction is involved as in the formation of the pigment that forms the color of "iron gall ink". The prussian blue of the cyanotype image is than (at least partly, depending on amount of bleaching), transformed to the same ferrogallotannate pigment created when making iron gall ink.
I am surprised to see this process being a true chemical toning procedure... Are my conclusions right that the chemistry involved in producing "iron gall ink" and the "tea toning" of cyanotype are related?? Can someone confirm this?
There is one difference though, for those who fear their "tannin/tea" toned cyanotypes may be as self-destructive as "iron gall ink" writing on paper. In the process of iron gall ink formation, excess ironsulphate can form sulphuric acid, and it is these excess remains and the acid formed from them that is thought to be the main culprit in the self-destructive properties of "iron gall ink" on paper, although the reactive and catalytic properties of free Fe cations seem to play a role as well. But no sulphate is present in the process of tannin/tea-toning cyanotypes...
Marco
One thing that struck me was the possibility of "(tea-)toning" cyanotypes. At first, my thought was that the process involved here, was nothing more than simply "dye-ing" the prints to lay a brown tea color on top of the blue. So not actually a chemical reaction transforming the cyanotype, but simply applying a kind of transparent "paint" layer. However, after reading (there was a url link here which no longer exists) on APUG (archived article, was part of Article system on APUG before it was renewed), I was highly surprised and confused to see a "bleaching" step involved in the process, suggesting a chemical transformation of the Prussian blue pigment that forms the cyanotypes blue color, a much more complex process than I expected and similar to toning for example a silver image with sepia or selenium toner.
So than, through some completely different search, I hit on this page about historically used "Iron gall ink", known for its corrosive and destructive properties on historical papers, sometimes completely eating through it:
The ink corrosion website
and especially:
Iron gall ink - Chemical reactions
Ink corrosion
and I began to realize there may be a connection between the two. The iron gall pigment that colors the ink is made by the reaction of Fe2+ cation derived from Ironsulphate, with gallic acid or gallotannic acid (two different but related chemical substances, see links above), also called tannin, derived from galls.
In this process, the iron cation binds in a chemical reaction with the tannin / gallic acid and / or gallotannic acid, to form the dark black/blue ferrogallotannate pigment, which gives the ink its black color.
Now the process Jason described involved "bleaching" the Prussian blue of the cyanotype with sodium carbonate, an alkaline solution. Prussian blue is known to be broken down in alkaline conditions, probably releasing free Fe2+ cations (Cyanotypes consist of Prussian blue, which is a pigment with the chemical formula K+Fe3+[Fe2+(CN)6]-, also called ferricferrocyanide).
The next step Jason described was using pure tannin (or tannin derived from strong tea) to tone the print, suggesting a similar reaction is involved as in the formation of the pigment that forms the color of "iron gall ink". The prussian blue of the cyanotype image is than (at least partly, depending on amount of bleaching), transformed to the same ferrogallotannate pigment created when making iron gall ink.
I am surprised to see this process being a true chemical toning procedure... Are my conclusions right that the chemistry involved in producing "iron gall ink" and the "tea toning" of cyanotype are related?? Can someone confirm this?
There is one difference though, for those who fear their "tannin/tea" toned cyanotypes may be as self-destructive as "iron gall ink" writing on paper. In the process of iron gall ink formation, excess ironsulphate can form sulphuric acid, and it is these excess remains and the acid formed from them that is thought to be the main culprit in the self-destructive properties of "iron gall ink" on paper, although the reactive and catalytic properties of free Fe cations seem to play a role as well. But no sulphate is present in the process of tannin/tea-toning cyanotypes...
Marco
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