The Effect of Acid in the Washing of Traditional Cyanotypes
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Hi, Ced:
It is indeed a tall order to get neutral tone and no staining in the paper simultaneously. I am beginning to think that it is the stain that makes the color go neutral or at least it helps significantly. I have some thoughts on how that may be so.
I know you have done quite a bit of experimenting on this subject. I am personally not interested in a neutral tone – I generally prefer warm tones which a fully bleached and toned cyanotype does well. So my experimenting is strictly aimed at getting stain-free process.
The subject is obviously way off topic for this thread. Perhaps we can open a new thread about this elusive stainless neutral-toned cyanotype at some point.
:Niranjan.
Back to the topic of difference between with plain water and acidified water in “development” I thought I will demonstrate the role of equilibria as elucidated by Mike Ware here. In case of plain water (neutral to slightly alkaline) development, the idea is that some of the ferrous ions produced from photo-reduction of ferric ammonium citrate do not react with the ferricyanide either in the paper or in the solution because of thermodynamic impediment and are lost to the developing effluent as a result. In this case, the effluent should still contain these ferrous ions and given the right condition should form Prussian blue.
Figure 1 is the plain water developing solution as it came after ½ minute with an exposed strip of sensitized paper – it is clear, slightly yellow as would be expected from unreacted compounds from the sensitizer.
However, as soon as I added a little bit of citric acid and gave it a stir – the solution turned blue instantly (Figure 2.)
So there was still a lot of ferrous ions in the solution and since it did not get a chance to form Prussian blue on the paper, that much density is lost. And that’s the reason we get lower Dmax (as well as clearer whites and higher contrast) in a cyanotype developed in plain water as opposed to one developed in acidic water, even though the former looks clear and the latter is blue post-development.
Figure 1 is the plain water developing solution as it came after ½ minute with an exposed strip of sensitized paper – it is clear, slightly yellow as would be expected from unreacted compounds from the sensitizer.
However, as soon as I added a little bit of citric acid and gave it a stir – the solution turned blue instantly (Figure 2.)
So there was still a lot of ferrous ions in the solution and since it did not get a chance to form Prussian blue on the paper, that much density is lost. And that’s the reason we get lower Dmax (as well as clearer whites and higher contrast) in a cyanotype developed in plain water as opposed to one developed in acidic water, even though the former looks clear and the latter is blue post-development.
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