Sensitiser gone cloudy in the bottle

Owned by Break/Fix — the iron-silver print that would not clear, whose sixth case names this entry as the general treatment and separates four different clouds, of which only two condemn a bottle: a precipitate in a fresh Van Dyke sensitiser and one of silver oxalate in a mixed kallitype sensitiser are expected and are decanted from, while a ferric oxalate solution that has left yellow-green is decomposing, and the ferricyanide test in the kallitype lab finds it in a minute. That page also records what no source states, which is what a decomposed Van Dyke sensitiser looks like. This entry was written earlier from the ammonium iron(III) citrate page and Part II’s storage work.

What you see

A bottle that was clear and is not. The specific appearance matters more than the fact, and it is the diagnosis:

What is in the bottle	What it is
Fine crystals, settling, gritty between finger and thumb	A sparingly soluble salt come out of solution
A furry growth on the surface, often within a week or two	Mould on a citrate solution
A fine even turbidity or a coloured haze, with no solid	Decomposition, or an expected precipitate
A dense settled layer under a clear supernatant	Often an expected precipitate that the maker’s own sheet describes — check the instructions

Likely causes

1. A sparingly soluble salt crystallising out. Ware’s case is potassium ferric oxalate, which comes out when ammonium ferric oxalate is mixed with potassium ferricyanide at concentrations high enough to give a pictorial maximum density, rendering the mixture useless as a sensitiser.
2. Mould. Ware is specific: the ferric ammonium citrate solution is an excellent nutrient medium for the growth of moulds, which will usually cover the surface within a week or two, and a fungicide such as thymol will prevent it — with the note on the chemical page that thymol is itself harmful.
3. An expected precipitate. Photographers’ Formulary say of their kallitype sensitiser that it is very common for a precipitate of silver oxalate to form, that it does no harm, and that the requirement is simply not to transfer it to the paper. Kodak say the same of two of their own toner formulas.
4. Decomposition. The serious one, and the one that costs prints rather than time.

What is happening: the chemistry and physics

Three quite different chemistries produce a cloudy bottle and they have different consequences.

Crystallisation is a solubility limit being crossed. Two concentrated solutions mixed together can produce a product less soluble than either, and it comes out. It is a physical separation rather than a chemical loss — but the sensitiser left behind is no longer the sensitiser you meant to make, because part of its iron is now in the bottom of the bottle. And the crystals themselves are a coating fault waiting to happen: Ware records that “a gritty sensitizer produces small white spots on the print, typical of surface crystals”, which is one of the four faults in blotchy and streaked hand coating.

Mould is biology in a bottle of nutrient. Citrate is a carbon source; a dark cupboard at room temperature is a culture. It does not change the iron chemistry directly, but it consumes the ligand and it fouls everything downstream.

Decomposition is the sensitiser doing in the dark what it is supposed to do in the light. Ware’s fogging algorithm places it first for a reason: fog apparent on coating means “the sensitizer is already decomposed”, and a decomposed sensitiser cannot be filtered back into usefulness. It is also why the classic cyanotype is stored as two stocks and mixed at the point of use — Jacquard put the shelf-lives side by side: “the stock solutions are stable long term” while “the mixed sensitizer is stable just 2-4 hours”.

!The confirming test: filter it, coat it, and look before exposing

Filter a portion through paper, coat a test strip, dry it in the dark, and look at the dry coating before any exposure.

• The filtered solution coats cleanly and the dry coating is uncoloured: it was a precipitate, and the solution is usable with the precipitate left behind — which is exactly what Photographers’ Formulary instruct for their kallitype sensitiser.
• The dry coating already carries tone: the sensitiser has decomposed, and Ware’s algorithm says so at its first branch. Filtering will not help. Discard it.
• It will not filter clear, or a growth returns within days: mould. Discard it and start again with thymol in the new stock.

And check whether the cloud was ever a fault. Read the process’s own instructions first: Kodak state that a black precipitate forming when the hypo alum solutions are combined will not adversely affect the toning action with proper technique, and that a fine white precipitate in a stored toner will not interfere with it. A cloudy bottle that the manufacturer describes is not a fault at all.

Diagnostic questions

• What does the instruction sheet say about a precipitate? The first question, and it settles a good proportion of cases.
• Is it crystalline, furry, or an even haze? The three appearances map onto the three chemistries.
• Is this a stock or a mixed sensitiser? Jacquard’s contrast — long term against two to four hours — is the whole storage discipline of the classic cyanotype.
• How long since it was made, and is that written on it? The unlabelled or undated container entry is the discipline that makes this answerable.
• Did the powders ever fully dissolve? Jacquard allow 24 hours for each powder to dissolve in making the stocks, and an undissolved solid is not a precipitate.
• Has it been cold? Stock crystallised in cold storage is the same appearance from a different cause, and it often redissolves on warming.

Corrective action

Filter and use, if the maker’s own instructions say the precipitate does no harm to the sensitiser — and make sure none of it reaches the paper. That judgement is the manufacturer’s, about that product, and it does not transfer to a precipitate you cannot account for.

Discard, if it is mould or decomposition. Neither is recoverable and both will cost more in wasted paper than the chemicals are worth.

Warm gently and re-dissolve, if the cause was cold. That test is quick and it is the one case with a happy ending.

Never re-mix a crystallised mixed sensitiser back to full strength by adding water. The composition has changed; adding solvent changes it again in an unknown direction.

Prevention

Store the components separately and mix at the point of use, which is the classic cyanotype’s own arrangement and the reason for it.

Add a fungicide to a citrate stock — Ware’s thymol — and keep it in a brown bottle in a cupboard.

Label and date every bottle, per the labelling SOP.

Allow full dissolution when mixing. Twenty-four hours is Jacquard’s figure for their own powders, and a solution finished in a hurry looks exactly like a solution that has gone off.