If you follow my blog, you will know that back about a month ago I started making cyanotypes again after a hiatus of roughly twelve years. I began by adapting my previous knowledge to my current situation.
Since pretty much everything (printer for digital negatives, UV light source, physical space, etc.) had changed I began more-or-less from scratch. The learning curve was relatively short since I was not starting anew in terms of experience. Once I had the basics figured out I moved on to toning cyanotypes.
Earlier this week, I decided to do a bit more systematic experimenting with the toning of cyanotypes. I began the process by accumulating multiple copies of more-or-less the same cyanotype prints to use as starting material for toning. I won’t bore you with the details, but it took me two evenings of work to accumulate enough small prints for a toning trial.
Toning cyanotypes requires two components, a polyphoenol and a base (or alkali). There are three commonly used pure polyphenols: tannic acid, gallic acid and pyrogallic acid. Natural mixtures of polyphenols in the form of coffee and various teas are also sometime used. They are not considered here.
Most of my previous work was with tannic acid. I did have a stock of gallic acid which I had tried only randomly before. I had never tried pyrogallic acid before so I ordered some for this set of trials.
The commonly used bases for toning are sodium carbonate (washing soda) and ammonium hydroxide (household ammonia). I also wanted to include (because of some very preliminary tests a week or so ago) sodium bicarbonate (baking soda) in my trial.
The general procedure I used for these experiment was as I described previously. I pre-wet the dried prints in water for two or thee minutes minimum and then transferred the print to a bath of the 2% (w/v) polyphenol in water (i.e. 2 g / 100 mL). After soaking in the polyphenol solution for 5 minutes, the print was drained and dipped briefly (10-15 seconds) in water and then transferred to the base solution (7.5% w/v for sodium carbonate and sodium bicarbonate or 5% v/v of household ammonia) for another brief interval (15-60 seconds; tone judged by the eye of the beholder). Finally the print was washed thoroughly in water before drying.
In general this procedure causes, in addition to the desired shift in hue, a very mild bleaching of the color . Thus is is best to start with a print that is on the dense side of acceptable rather than one on the light side.
There is also a lose of contrast due to staining of the highlights. This staining seems least with gallic acid and worse with pyrogallic acid. I have not investigated the staining of papers in any systematic way, but expect that there will be some variation among papers.
My first series of tests investigated the effect of changing the polyphenol. All the toned prints in this series were treated with ammonia as the base. These prints were on Rives Heavyweight cream paper (175 gsm). Here are the results:
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For the second series, I used prints made on Arches Hot Press (300 gsm) paper and varied the polyphenol again. The base was ammonia as it was in the first series.
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It is interesting to note that the paper seems to affect the final tone achieved by each combination; compare the corresponding images in the first two series. Given that I have only tested two papers, this will need further testing to confirm the generality of this observation.
For the third series, I again used prints made on Arches Hot Press paper (300 gsm). Tannic acid was the polyphenol and I varied the base. (Note that first two prints in this series are the same as the first two in the second series.)
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When toning with sodium carbonate, the color, which ends up as a chocolaty or reddish brown at completion, passes through a warm purple phase on the way from the native blue. In the past, I have tried to stop the process at the intermediate stage without much success and without any reproduciblity.
Since bicarbonate is a weaker base than carbonate, my thought was that it might let me “trap” the intermediate tone more reliably.
The result of this trial shows that bicarbonate certainly gives a different tone that is less reddish-more purplish than does carbonate; compare the last two prints in this series.