The Effect of Acid in the Washing of Traditional Cyanotypes

Recently, while making cyanotypes, I have had a couple of sessions where the prints I made came out much lighter than I was expecting. The prints looked correct immediately after exposure and immediately after 'developing' in 25% (v/v) vinegar. However, they faded much more than usual upon washing and drying.

The first time this happened, I just chalked it up to the vagaries of life and moved on. After the second occurrence, I decided this was a problem.

Before I proceed further, there are two things about my working methods which I must relate.

The first is that I live in the woods and thus rely on a private well for water. This water is untreated, untested and subject to variation over time, particularly when we have drought.

The second is that I have no running water in my work space. I transport water from a tap to my "dim room" by filling five gallon buckets. Sometimes I use water which has been freshly drawn from the tap. At other times, I use water that has been standing in the bucket for days. Until, now had had not thought about controlling this variable in my methods.

Not having running water means that I wash my cyanotypes (after an initial development in 25% vinegar) in multiple trays (six, to be exact) of static water.

In thinking about my situation and the chemistry of cyanotypes, I hypothesized that my 'problem' with light prints might be related to the acidity of my water. Cyanotypes are know to be unstable in alkaline conditions. Thus too high a pH during washing would be expected to lead to excessive fading.

Furthermore, it is worth remembering that pure water, when exposed to air, becomes more acidic. This is because the CO2 in the air dissolves in and reacts with the water to form carbonic acid. Thus, water straight from the tap (will little or no exposure to the air) tends to become mildly acidic upon standing.

I hypothesized that my occasional problems with fading cyanotype prints is due to that fact that the pH of the water
in my wash trays was uncontrolled and varied substantially over time.

In order to test this hypothesis, I made four identical exposures from the same negative. Each of these prints was initially developed for one minute in 25% (v/v) vinegar and then washed. One print was washed in water freshly drawn from the tap. A second print was washed in the same water to which vinegar (at 1% v/v) was added. A third print was developed in water that had been "aged" by standing in a bucket for a couple of days. And finally, a fourth print was developed in the aged water plus 1% (v/v) vinegar. The wash volumes used and the timing of the washes was the same in all cases.

The results (see below) clearly show that both the "aging" of the wash water and the addition of 1% (v/v) vinegar to the wash water are useful for keeping cyanotypes from fading during the washing stage. The aged water gave a sightly better print that the freshly draw water. Adding the vinegar to either type of water gave a better print that the plain water.



Based on this experiment, I will be trying to remember to keep my water buckets full (so that I do not have to use freshly drawn water). Additionally, I plan to routinely add a small amount of vinegar to my wash water going forward.

I'm not sure how useful this information will be for others, but here it is!

It looks very much as if your well water may be a bit alkaline, and becomes less so on standing (dissolved calcium will react with air and precipitate out as chalk). As you note, the simple solution is to ensure you fill your buckets at the end of a printing session, rather than at the beginning. Another option might be to use a deionizing filter setup to ensure consistent water quality, though this introduces additional annoyance an expense (replacement DI cartridges aren't cheap, never mind free, and have a limited life measured in, at most, tens of buckets per cartridge for common ones). And on-faucet or under-sink drinking water filter might do well enough, and costs a good bit less per gallon filtered than a DI column. You might also be able to use Calgon (sodium hexametaphosphate) economically to remove dissolve calcium bicarbonate.

I recall Patrick Gainer mentioning that his well water changed pH enough over the course of the year (due to seasonal water table changes) to see the results in his negatives -- if you're getting this much change in your cyanotypes, that's something you may also need to watch for.

Fascinating!

Donald,

Be careful... you are 'talking' to a retired chemistry professor!!!!

That said, I agree with all that you wrote.

I omitted the influence of hardness in my post for simplicity. But you are correct that changes in hardness which are often seasonal are the likely culprit here. Especially since we are in the midst of a particularly dry summer here in southwestern NH.

For now, I am going to (as you suggest) fill my buckets at the end of session so that it ages. However, I think that I'll just add some vinegar to my washes rather than mess with the complication and expense of treating the water (especially since our water is not hard enough to cause the typical household problems).

Regards,

On another note, did you acidify the Stonehenge paper? For me, it never was a good candidate for Kallitype or Cyanotype, unless I acidified it first. Much better darks, and smoother tones. Cheers! It's a lovely composition!

fgorga said:

'developing' in 25% (v/v) vinegar

Click to expand...

Holy crap! I just used to add a tiny dash of vinegar to the wash water, like 1% or thereabouts. The main point about the acid "development" isn't really development at all, but just ensuring that the wash water has a pH slightly below 7 so that no insoluble iron salts are created that won't wash out.

But I preferred a half gram or so of citric acid to a liter of water. Citric acid makes the blue shift very subtly towards magenta; you get more of an indigo tint, as opposed to acetic acid which gives a more cyan hue.

You really only need a tiny bit of acid. Especially with classic cyanotype, which easily bleaches out as you've found. Give it a try; you might see some improvement in dmax and overall contrast.

Another concise and useful study, Frank. Thanks for sharing.

In my experiments, what I found is that some minimum concentration of acid and length of time in "development" are required to maximize the density and at the same time make it relatively unaffected from subsequent plain tap-water washing. I was not looking for variations in the quality of wash water (may be I should have as I too have no running water in my dim room so use tray processing with a gallon jug at a time of mostly but not always freshly filled tap water when I start a session, although luckily in my testing the city water seems to be of relatively constant pH in the 6-7 range.)

Basically I varied the citric acid concentration from 0 % to 5% and time of development from 1/2 min to 5 minutes followed with tray-wash with tap-water for different amounts of times up to 10 minutes. The conclusion was that density improvement plateaued around 1% CA and it required at least 3 minutes of development to see no observable loss of density during washing. Based on these findings, my initial process was 3 minutes in 1% CA followed by 4 washes of 1 minute each in tap-water. This gave me the highest Dmax for a given set of other conditions, but unfortunately since there is no free lunch in cyanotypes, it was also accompanied by higher Dmin caused by fogging or blue staining at zero exposure. So I made a compromise and added a 1/2 min development in tap water as a first step followed by the 3 min 1% CA treatment and final 4 tap-water washes of 1 minute each. It does bring down the Dmax a bit as a price to pay for getting zero-exposure step much closer to paper white.

Relevant question to the your study here, I wonder if you increase the development time with or without acetic acid concentration, is it possible to make the outcome independent of whether or not aged water is used for washing.

:Niranjan.

Andrew O'Neill said:

On another note, did you acidify the Stonehenge paper? For me, it never was a good candidate for Kallitype or Cyanotype, unless I acidified it first. Much better darks, and smoother tones. Cheers! It's a lovely composition!

Click to expand...

Andrew,

I did not pretreat the paper with acid. I'm pretty happy with the tonality I'm getting. I wonder if this is because I use a relatively high concentration of vinegar to develop y cyanotypes.

Regards,

koraks said:

Holy crap! I just used to add a tiny dash of vinegar to the wash water, like 1% or thereabouts. The main point about the acid "development" isn't really development at all, but just ensuring that the wash water has a pH slightly below 7 so that no insoluble iron salts are created that won't wash out.

But I preferred a half gram or so of citric acid to a liter of water. Citric acid makes the blue shift very subtly towards magenta; you get more of an indigo tint, as opposed to acetic acid which gives a more cyan hue.

You really only need a tiny bit of acid. Especially with classic cyanotype, which easily bleaches out as you've found. Give it a try; you might see some improvement in dmax and overall contrast.

Click to expand...

Hmmm... to each his own. My method seems to work well for me. I have no complaints about either Dmax or contrast. And, I suspect that the relatively high concentration of vinegar might extend the range of papers I can use without acidification.

I am not sure that I agree with your idea that the only thing the acid is doing is preventing the formation if insoluble iron salts. The Prussian blue itself, is clearly affected by pH. One gets drastic bleaching of cyanotypes upon exposure to alkali.

Your observation that the hue of the cyanotype subtly shifts in hue is interesting. In my view, this also argues against the idea that all the acid is doing is keeping the iron(III) in solution. Citric acid, being a dicarboxcylic acid can be expected to bind to iron more tightly than acetic acid (the monocarboxcylic acid in vinegar). Thus, it is plausible that citrate is incorporated into the pigment of the cyanotype causing the shift in tone. The chemistry here would be similar to that seen in the toning of cyanotypes using polyphenols (i.e tannic acid, etc.)

I'll have to get some citric acid!

Regards,

nmp said:

Another concise and useful study, Frank. Thanks for sharing.

In my experiments, what I found is that some minimum concentration of acid and length of time in "development" are required to maximize the density and at the same time make it relatively unaffected from subsequent plain tap-water washing. I was not looking for variations in the quality of wash water (may be I should have as I too have no running water in my dim room so use tray processing with a gallon jug at a time of mostly but not always freshly filled tap water when I start a session, although luckily in my testing the city water seems to be of relatively constant pH in the 6-7 range.)

Basically I varied the citric acid concentration from 0 % to 5% and time of development from 1/2 min to 5 minutes followed with tray-wash with tap-water for different amounts of times up to 10 minutes. The conclusion was that density improvement plateaued around 1% CA and it required at least 3 minutes of development to see no observable loss of density during washing. Based on these findings, my initial process was 3 minutes in 1% CA followed by 4 washes of 1 minute each in tap-water. This gave me the highest Dmax for a given set of other conditions, but unfortunately since there is no free lunch in cyanotypes, it was also accompanied by higher Dmin caused by fogging or blue staining at zero exposure. So I made a compromise and added a 1/2 min development in tap water as a first step followed by the 3 min 1% CA treatment and final 4 tap-water washes of 1 minute each. It does bring down the Dmax a bit as a price to pay for getting zero-exposure step much closer to paper white.

Relevant question to the your study here, I wonder if you increase the development time with or without acetic acid concentration, is it possible to make the outcome independent of whether or not aged water is used for washing.

:Niranjan.

Click to expand...

Niranjan,

Thanks for your response.

Municipal water is generally quite constant in its composition (hardness, pH, etc.) compared to water from a private well. The municipal water system is constantly monitoring the water and adjusting it , if needed, at the treatment plant.

You observation about the time for 'development ' is interesting. I had not considered investigating different times for developing, I simply left it long enough so that the print was not visually changing anymore. Yet another variable!

I'm going to have to get some citric acid to try. It should be a better chealator of iron than the acetic acid in vinegar. I like vinegar because it is cheap and available in any grocery store.

Regards,

fgorga said:

I have no complaints about either Dmax or contrast

Click to expand...

It might just be the scans, but the dark blue borders look very pale in comparison to what I got with classic cyanotype.

Acid wash vs plain water issue to me is a paradox. When an exposed paper coated with classic formula is dunked in either tap water or even distilled water, the resulting effluent is yellowish clear solution (more recently I have found exceptions to this but I will leave this for a future discussion) whereas the same when placed into a tray containing acidified water, 1% CA for example, the result is an immediate blue solution. When I saw this for the first time, I thought obviously the latter where the Prussian blue seems to be leaching out from the paper will have lower density and the former should have higher density. In reality though, the opposite is true. The Dmin of the water-developed print is very close to the paper white and Dmax is much lower while the acid developed print has good bit of fogging in the whites and Dmax is off the charts, comparatively.

So what is happening?

I have some theories - still under formulation, though. Any guesses?

Niranjan - Buried in the Appendix III.8 of my Cyanomicon (see my website) is an attempt to answer the questions that you ask above.
No-one is likely to read it, so here is a rough transcript (with apologies for unformatted sub- and superscripts!)

In the usual cyanotype process, the sensitizer consists of a mixture of potassium hexacyanoferrate(III) with a photosensitive iron(III) salt - ammonium iron(III) citrate in Herschel’s Classic recipe, which is replaced by ammonium trisoxalatoferrate(III) in later commercial blueprint papers and my New Cyanotype. Upon exposure to ultraviolet light, the iron(II) salt is formed photochemically; for the latter case, we can write an equation:

UV + 2[Fe(C2O4)3]3– = 2[Fe(C2O4)2]2– + (C2O4)2– + 2CO2

(In the case of the citrate system the precise identity of the iron(II) photoproduct is not known, but it is certain to be a citrato- complex.) Prussian blue is then supposed to be formed by an overall reaction such as:

[Fe(C2O4)2]2– + [Fe(CN)6]3– = Fe[Fe(CN)6]– + 2(C2O4)2–

However, this complex displacement reaction cannot occur in one concerted step, because the carboxylate ligands must be stripped off the iron(II) before it can be incorporated into a Prussian blue lattice by ferricyanide.
Iron(II) does not bind oxalate very strongly, as may be seen from the overall dissociation constant:

[Fe(C2O4)2]2– = Fe2+ + 2(C2O4)2– β2 = 4 x 10^–7

Removal of oxalate will be promoted by the presence of hydrogen ions, which compete with the metal to form undissociated oxalic acid, for which the overall equilibrium constant is:

(C2O4)2– + 2H+ = H2C2O4 Ka = 3.24 x 10^5

Accordingly, the equilibrium:

[Fe(C2O4)2]2– + 4H+ = Fe2+ + 2H2C2O4

lies to the right hand side, with a calculated equilibrium constant of:

K = Ka^2 β2 = 4.2 x 10^4

The Fe2+ is then free to be incorporated, so the development of the Prussian blue image from such a sensitizer is strongly promoted by initial treatment in a bath of dilute mineral acid, (1% v/v nitric, or hydrochloric) rather than water, as may be seen from the D/logH curves for New Cyanotype. In the Classic process, with citrate as ligand, the iron-citrato complexes are weaker and will therefore only require a more dilute acid for development, such as acetic or citric; mineral acids may cause fogging.
I don’t have any numbers - or indeed, formulae - for the complexes in the Classic citrate system - do you?

Mike Ware
https://www.mikeware.co.uk

Mike Ware said:

Niranjan - Buried in the Appendix III.8 of my Cyanomicon (see my website) is an attempt to answer the questions that you ask above.
No-one is likely to read it, so here is a rough transcript (with apologies for unformatted sub- and superscripts!)

In the usual cyanotype process, the sensitizer consists of a mixture of potassium hexacyanoferrate(III) with a photosensitive iron(III) salt - ammonium iron(III) citrate in Herschel’s Classic recipe, which is replaced by ammonium trisoxalatoferrate(III) in later commercial blueprint papers and my New Cyanotype. Upon exposure to ultraviolet light, the iron(II) salt is formed photochemically; for the latter case, we can write an equation:

UV + 2[Fe(C2O4)3]3– = 2[Fe(C2O4)2]2– + (C2O4)2– + 2CO2

(In the case of the citrate system the precise identity of the iron(II) photoproduct is not known, but it is certain to be a citrato- complex.) Prussian blue is then supposed to be formed by an overall reaction such as:

[Fe(C2O4)2]2– + [Fe(CN)6]3– = Fe[Fe(CN)6]– + 2(C2O4)2–

However, this complex displacement reaction cannot occur in one concerted step, because the carboxylate ligands must be stripped off the iron(II) before it can be incorporated into a Prussian blue lattice by ferricyanide.
Iron(II) does not bind oxalate very strongly, as may be seen from the overall dissociation constant:

[Fe(C2O4)2]2– = Fe2+ + 2(C2O4)2– β2 = 4 x 10^–7

Removal of oxalate will be promoted by the presence of hydrogen ions, which compete with the metal to form undissociated oxalic acid, for which the overall equilibrium constant is:

(C2O4)2– + 2H+ = H2C2O4 Ka = 3.24 x 10^5

Accordingly, the equilibrium:

[Fe(C2O4)2]2– + 4H+ = Fe2+ + 2H2C2O4

lies to the right hand side, with a calculated equilibrium constant of:

K = Ka^2 β2 = 4.2 x 10^4

The Fe2+ is then free to be incorporated, so the development of the Prussian blue image from such a sensitizer is strongly promoted by initial treatment in a bath of dilute mineral acid, (1% v/v nitric, or hydrochloric) rather than water, as may be seen from the D/logH curves for New Cyanotype. In the Classic process, with citrate as ligand, the iron-citrato complexes are weaker and will therefore only require a more dilute acid for development, such as acetic or citric; mineral acids may cause fogging.
I don’t have any numbers - or indeed, formulae - for the complexes in the Classic citrate system - do you?

Mike Ware
https://www.mikeware.co.uk

Click to expand...

Thanks you Mike for your input. I do not have any data on the structures or the equilibria involved in the classic cyanotype chemistry. If it is not in your treatise, I wouldn't know where to look.

So if I understand correctly, H+ from an acid acts as an intermediary to replace the still bound oxalate from the Fe(ii) ion, which then is free to form Prussian blue with the potassium ferricyanide. Would I be correct in making a corollary that in the case of the classic cyanotype with FAC, citrate, also being a ligand (stronger? or weaker?) for iron, will also hang on similarly after photo-reduction. Hence, an acidic environment would facilitate faster Prussian blue formation.

Based on the above, let's see if my thought process makes sense. There is some Prussian blue formation during exposure in the solid state (perhaps aided by residual moisture) as is formation of Prussian white - evidenced by the color changes in the paper as exposure progresses. However most of it may be restricted to clusters of nearest neighbors. So after exposure, majority of the Fe(ii) ions formed by photo-reduction are still unused. When water is added, acting as a solvent, it allows both reactants to move around freely and find each other more efficiently. However, the reaction is still slow because of the need to displace the ligands away from Fe(ii), increasing the probability of them being just get carried away outside the paper as a result. On the other hand, if there was excess H+ around then it will act as a catalyst/intermediary and spontaneous formation of ferric ferrocyanide ensues. In case of plain water, in absence of excess H+, a good percentage of the Fe(ii) goes out in the wash. This would explain the overall lower density in a water developed print as compared to the acid developed print where more of the precipitated Prussian blue takes place within the paper fibers. It can also perhaps explain why the acidic developer turns blue instantly - I suppose some of the unpaired Fe(ii) that did manage to escape the paper surface still was able to find potassium ferricyanide in the solution.

I think a part of my earlier question still remains - why does the effluent in the case of plain water stay clear. Is Prussian blue simply unstable in neutral or even mildly acidic water (distilled water and tap water, mine is ph 6-7, behave identically in that respect in my observation.) In my earlier thinking I was looking more at the phenomenon of peptization which is also elucidated in depth in Cyanomicon. Where does that fit in this overall scenario?

Feel free to mark it up.

So much more to learn....Thanks!

:Niranjan.

nmp said:

Acid wash vs plain water issue to me is a paradox. When an exposed paper coated with classic formula is dunked in either tap water or even distilled water, the resulting effluent is yellowish clear solution (more recently I have found exceptions to this but I will leave this for a future discussion) whereas the same when placed into a tray containing acidified water, 1% CA for example, the result is an immediate blue solution. When I saw this for the first time, I thought obviously the latter where the Prussian blue seems to be leaching out from the paper will have lower density and the former should have higher density. In reality though, the opposite is true. The Dmin of the water-developed print is very close to the paper white and Dmax is much lower while the acid developed print has good bit of fogging in the whites and Dmax is off the charts, comparatively.

So what is happening?

I have some theories - still under formulation, though. Any guesses?

Click to expand...

Niranjan,

My experience is similar. When I 'develop' with 25% vinegar and wash with water, the developer turns blue, the first wash turns green, and the second wash yellow. When I wash with the 1% vinegar both of these washes are a blue-green. In both cases, subsequent washes are pale yellow getting to colorless by the last two.

No idea as to why... I'll need to think more, but I'm glad to hear your ideas.

@Mike Ware

Thanks for your reply.

To summarize... it's all about equilibrium constants!

There is another current thread (see https://www.photrio.com/forum/threads/van-dyke-over-cyanotype.177290/) that began with an experiment on combining van dyke brown with cyanotype that has evolved to show that adding tartaric to the cyanotype mixture might be useful in controlling contrast.

The mechanism here would, in general, be similar... different ligand, different equilibrium constants affecting the outcome.

nmp said:

Thanks you Mike for your input. I do not have any data on the structures or the equilibria involved in the classic cyanotype chemistry.
...........
:Niranjan.

Click to expand...

Niranjan - Thanks for your response - your thinking is along the right lines - except that I believe it is not a question of the rates of reactions (which are all very fast), but the positions of equilibria (i.e. the relative concentrations of reactants and products) - as Frank says: it's all about equilibrium constants!

I think the 'Classic' chemistry parallels the 'New', but both Fe(II) and Fe(III) form less strong complexes with citrate than with oxalate. I don't see any remaining questions or paradoxes: processing of the exposed print with a neutral aqueous bath washes out much of the Fe(II)citrate photoproduct, which is colourless, as you observe, along with the pale yellow original Fe(III)citrate sensitizer. Processing with an acidic bath immediately strips off the citrate ligand from some of the photoproduct, leaving some Fe(II) free to form Prussian blue with the ferricyanide, both in the paper image and in the aqueous bath, which instantly becomes blue.

There is an added complication if the acid is too strong: it can also strip the citrate ligand from some of the original Fe(III)citrate sensitizer which is lying around in the highlights, and the consequent release of free Fe(III) will also lead to the formation of Prussian blue - i.e. fogging the highlights. (This comes about because ferric ferricyanide - Prussian yellow/brown - is a very powerful oxidising agent, and reacts with almost anything - like cellulose or citrate - to form Prussian blue.) That is the reason you only use weak acids like acetic or citric for developing 'Classic Cy'. If you also appreciate that oxalate complexes of Fe(III) and Fe(II) are stronger (more tightly bound) than their citrate complexes, then you have the reason why 'New Cy' can get away with using stronger (mineral) acids for development.
Loss of Prussian blue from the image by peptization is a completely separate issue.

I hope this helps resolve your remaining questions.

Mike

Mike Ware said:

Niranjan - Thanks for your response - your thinking is along the right lines - except that I believe it is not a question of the rates of reactions (which are all very fast), but the positions of equilibria (i.e. the relative concentrations of reactants and products) - as Frank says: it's all about equilibrium constants!
................

Mike

Click to expand...

Thanks. That does it. It's nice to understand what is going on while you are rocking that tray thinking.

:Niranjan.

To remove some variables from the equation, I would make one large print, cut it into four equal pieces and treat those differently. The experiment you did leave me thinking about the possibility of the coating or exposure of the four prints have influenced the result more than the development of water.

Don't take this the wrong way, Kudos for experimenting and sharing!

Glenn bech said:

To remove some variables from the equation, I would make one large print, cut it into four equal pieces and treat those differently. The experiment you did leave me thinking about the possibility of the coating or exposure of the four prints have influenced the result more than the development of water.

Don't take this the wrong way, Kudos for experimenting and sharing!

Click to expand...

Glenn,

Thanks for the comment. I have never liked the approach of splitting an image as you suggest in order to make comparisons, because you then necessarily have to compare different parts of the image to one another.

In my experiments, the difference in coating and exposure are likely very small.

The same paper was coated with the same batch of sensitizer at the same time and left to dry naturally. It only takes me 15 or 20 minutes to coat the entire batch of a dozen or so sheets of paper. I let the paper dry for 2 hours minimum, but depending on the day it might be longer. Thus, the sheet to sheet differences in drying time are relatively small.

Likewise with the exposures, the light source is on a timer and all exposures were made one right after the other. In this case all four exposures were completed within about a half hour.

I am quite confident that the differences I see are due to the differences in processing and not differences in coating or exposure.

fgorga said:

Glenn,

Thanks for the comment. I have never liked the approach of splitting an image as you suggest in order to make comparisons, because you then necessarily have to compare different parts of the image to one another.

In my experiments, the difference in coating and exposure are likely very small.

The same paper was coated with the same batch of sensitizer at the same time and left to dry naturally. It only takes me 15 or 20 minutes to coat the entire batch of a dozen or so sheets of paper. I let the paper dry for 2 hours minimum, but depending on the day it might be longer. Thus, the sheet to sheet differences in drying time are relatively small.

Likewise with the exposures, the light source is on a timer and all exposures were made one right after the other. In this case all four exposures were completed within about a half hour.

I am quite confident that the differences I see are due to the differences in processing and not differences in coating or exposure.

Click to expand...

I see, and agree on the comparing different parts.
My UV unit is a facial tanner and I suspect variations in output on startup, and that it's different on a cold and hot start. so I really should find a way to measure Uv dose...

Glenn bech said:

so I really should find a way to measure Uv dose...

Click to expand...

https://www.photrio.com/forum/threads/solar-uv-exposure-meter-sharing-a-diy-project.176686/
This may be of interest to you; check the link in the first post. There's a commercial product available as well that does the same thing, but it costs an order of magnitude more than your UV light source...

Don't mean to hijack from the debate here but seeing that Mike is about I would like to pose a question on how to get the best near neutral black in a print when toning cyanos without too much influence on paper colour?
Thanks & hope others find it useful.

ced said:

Don't mean to hijack from the debate here but seeing that Mike is about I would like to pose a question on how to get the best near neutral black in a print when toning cyanos without too much influence on paper colour?
Thanks & hope others find it useful.

Click to expand...

I've little to add to what I've already written on Cyanotype Toning in Chapter 8 of my e-book Cyanomicon - freely downloadable from my website:
https://www.mikeware.co.uk/mikeware/downloads.html
Gallic acid gave fairly neutral, acceptable grey/black images at first, but after a few years the specimens I made became perceptibly faded and stained, and after 15 years they are now worthless.
Perhaps I'm missing something in the processing, but I'm not an enthusiast for toning cyanotypes...except with lead(II) to give violet tones, which are permanent.
The elaborate black toners specially developed by Reg Heron in the 1970s are reported by him to have faded to brown after 25 years.
Mike

Man, I love this website !
Thanks for visiting Dr Ware, what a treat
John