As attractive as that color is, I usually don't regard it as good news when a cyanotype turns that color. As explained above, it most likely signifies an alkaline environment, which will reduce longevity of the image. The acrylic coating probably won't matter, because the chemical reaction is going on inside the emulsion itself. If you want to use the acrylic but you're not insistent on the color, you might consider impregnating the paper with acid before drying (i.e. a dilute vinegar bath or something). That way it's buffered against the alkali in the acrylic.
Variably. It is much more stable in acid than alkaline environments.
Originally Posted by Marco B
Also be aware that the iron blue toners are chemically identical to Prussian blue, i.e. they cause the image silver to be replaced with ferric ferrocyanide, and images toned in this way are known to degenerate faster than plain silver images.
Our atmosphere is acidic due to the carbon dioxide and sulfur dioxide present. This results in the fact that any object that sits around which is capable of absorbing things from the air (such as paper) will gradually become mildly acidic. Ammonia or amines will gradually be neutralized.
Paints or overcoats using the type of resin noted in the OP use this effect to assist in curing the polymer. They are mildly alkaline and are cured by the acid in the air.
What made you think that I'm saying Prussian Blue formed with Cyanotype is non-archival? My comment about protecting the image from bleaching was suggested as a protective measure for the case where you want to overcoat it with something alkaline... DrPablo too explained / suggested something similar later.
From the book "Cyanotype" by Mike Ware:
"...Chapter 6 recommends development in dilute acid, which will destroy any unwanted alkaline buffer that may be present in paper and ensure that the print is left in a condition favorable to the Prussian blue, possibly by the incorporation of acid cations into its lattice..." (page 131)
Therefore my (and DrPablo's) suggestion of treating (or developing) Cyanotypes in a mild acidic solution definitely have grounds.
Unfortunately Prussian blue is absolutely not stable in alkaline environment. Also, not all Prussian Blues are created equal -> see Mike Ware's book for detailed / scientific information...
Originally Posted by Marco B
What am I missing here? The OP said that when he treated another cyanotype in a straight 10% bisulfite solution, he found a similar shift in hue. Bisulfite solutions are acidic and reducing. Could you possibly have uncleared iron in your print?
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Bisulfite anions are the conjugate base of sulfuric acid (H2O3-), and like the conjugate bases of most strong acids its pKa is close to neutral. With a pKa of 6.97 the acidity or alkalinity of a 10% solution is likely affected by other unrelated solutes in the bath (or paper) than the bisulfite itself.
Prussian blue pigment and ammonia
Originally Posted by Marco B
I can't speak to Prussian blue pigment used in an oil medium; my experience and knowledge of pigments is limited to pigments in a gum arabic medium (watercolor paints and dichromated gum photosensitive coatings).
Prussian blue has become one of my favorite pigments for gum printing; I use it a lot. As far as stability: the lightfastness of Prussian blue watercolor paint is variable and depends on the brand more than most other pigments; I use a brand (M. Graham) which has been shown in tests, both the manufacturer's tests and independent tests, to be very lightfast. As to susceptibility to fading in an alkaline environment: Bruce MacEvoy mentions on handprint.com that Prussian blue (PB 27) can fade on contact with alkalis such as calcium carbonate, ammonia, or bleach, "but in my experience the alkali must be fairly concentrated to affect the color in a good quality pigment."
I've never seen any fading or color-shifting with Prussian blue, and tonight I did a quick test: I applied household strength ammonia to (1) straight Prussian blue watercolor paint brushed on paper and dried, and (2) the border of an old gum print made with Prussian blue pigment. I applied the ammonia to half of each swatch so that the two halves could be compared side by side for any change in color. The results are below. I let the ammonia sit on the color for several minutes in both cases and watched closely; I could see no change in hue on the ammonia side, nor could I see any fading. I attach scans of the two samples below. I drew a white line on each sample to show approximately how the samples were divided; in each case the ammonia was applied on the right side of the line.
This shows us nothing related to our subject (Cyanotypes). You should repeat this with a Cyanotype (or with pure Prussian Blue pigment). Your test may show us either that
a) Your ammonia is off (Somehow! I think this is unlikely - unless you have anosmia and can't tell if it smells or not...)
b) There are compounds [in binder / plasticiser / additive ect. in water color paint and/or hardened gum] which are protecting the pigment from alkaline hydrolysis.
All: see http://www.mikeware.co.uk/downloads/...Cyanotypes.doc for an article about Conserving Cyanotypes (and the methods / causes of deterioration and how to protect them)...
Originally Posted by Katharine Thayer
Last edited by Loris Medici; 08-17-2007 at 02:43 AM. Click to view previous post history.
I agree, but here is also a bit of text about Prussian Blue as a pigment in oil paintings, taken from the National Gallery, London, website, which confirms its (un)stable nature in different environments and paints:
Originally Posted by Loris Medici
"Fading and Colour Change of Prussian Blue: Methods of Manufacture and the Influence of Extender
Jo Kirby and David Saunders
The tendency of Prussian blue to change colour or to fade was widely known from the mid-eighteenth century onwards. This study investigates the influence of the method of preparation on the properties and stability of various Prussian blues, as well as reversibility of the colour change in painted samples of the pigment. A comparison is made between characteristics observed in examples of Prussian blue in paintings in the National Gallery Collection and those found in laboratory-prepared and collected historical samples of the pigment."
Still makes me wonder how they achieve the highly stable nature in modern oil paints, because the artist quality paint that I have lying around ("Rembrand 508" which does use the official PB27 Prussian Blue pigment) is marked as highly stable (three pluss's), which should guarantee centuries of life time if properly used...
And two other strange articles regarding Prussian Blue. It seems there is even a "darker site" to Prussian Blue and cyanotype, the first article will certainly make me look different to cyanotypes the next time
* "Expert Report About the Formation and Detectablility
of Cyanide Compounds in the ‘Gas Chambers’ of Auschwitz"
Chapter two is called "Formation and Stability of Prussian Blue"...
* From http://www.riotinto.com/documents/Re..._of_Metals.pdf:
"Blue grass and red faces
Prussian Blue, discovered accidentally in 1704,
is the oldest of the chemically synthesized
colours. It is made by complex reactions between
dissolved iron and potassium ferrocyanide, which
happens to be one of the few chemicals that
Some years ago an imaginative mining
company had the idea of using weak cyanide
solutions to dissolve gold from buried gravels in
an historically important Australian goldfield. The
gold was certainly there, in a permeable gravel
layer with impermeable rocks above and below.
The plan was to drill a pattern of boreholes
that would allow cyanide solution to be pumped
down some holes, percolate through the gravel
layer, and be pumped up through other holes
carrying dissolved gold. It was a variation of a
standard technique, and standard technology
would turn the dissolved gold back into metal.
The whole idea seemed very attractive. For
one thing, there would be no mining as such, and
very little occupation of the land. The approach
was similar to harvesting a crop, and once any
piece of land had been harvested it could quickly
be returned to its normal use as high quality
The fly in the ointment was a mineral called
marcasite, an iron sulphide with the same
formula as pyrite but a much greater tendency to
decompose in moist air or slightly acidic
groundwater. The gravel layer was impregnated
with marcasite, and when cyanide solution was
pumped in through trial boreholes the reactions
that produce Prussian Blue started up.
Luckily they could be damped down and
stopped, as soon as it was realized what was
happening. Blue grass and red faces were
avoided, but it was a reminder that pioneers are
always at the mercy of the unexpected."
Last edited by Marco B; 08-17-2007 at 04:11 AM. Click to view previous post history.