Calcium carbonate

Most entries in this encyclopaedia describe something a photographer buys in order to use it. This one mostly describes something a photographer has to get rid of — because the paper industry, acting on the best archival advice available, puts it into every good sheet, and it is poison to the iron processes.

In photography

The problem: archival paper is buffered, and iron printing hates buffer. ISO 9706 for permanent paper and ISO 11108 for archival paper both require an alkaline reserve, and Ware works out what that means at the coating rod: for a 250 g/m² sheet and a typical siderotype coating weight, the reserve amounts to an eightfold molar excess of alkali over the sensitiser, area for area. Two things then go wrong. Chalk reacts with the sensitiser directly — Ware says it hydrolyses the iron(III) salt and precipitates insoluble calcium oxalate — and alkali destroys the image substance of a cyanotype outright.

⚗Why pH 9.4 is the number to know

Ware reports Holtzman’s investigation: a buffer at pH 9.4 completely decolourises Prussian blue, by irreversible hydrolysis, in one to ten minutes, depending on how the pigment was prepared. Ware then makes the connection that gives this page its point — pH 9.4 is not a strong alkali. It is the pH of a saturated solution of calcium carbonate, which is to say the pH of the buffer routinely incorporated in archival papers and boards.

So a cyanotype mounted on the best conservation board available is sitting on a slow, mild, thoroughly well-intentioned bleach. The course’s rule for Prussian blue follows from this one measurement: unbuffered mounts and unbuffered enclosures, for cyanotypes and for every iron-blue toned print.

The remedy, and the acid that does not work. Ware’s papers list names sheets that are free of alkaline buffer; where a buffered paper is unavoidable, the chalk is destroyed before coating in a bath of 5 per cent v/v hydrochloric acid or 5 to 10 per cent w/v sulfamic acid for about ten minutes, then washed. He calls the procedure tedious and undesirable, because it degrades the paper’s surface and strength.

He is also emphatic about a mistake many practitioners make: oxalic acid is not suitable, because calcium oxalate is just as insoluble as calcium carbonate and can leave a gritty texture. The numbers bear him out and go slightly further. From the OpenStax table, calcium carbonate has a solubility product of 8.7 × 10⁻⁹ and calcium oxalate monohydrate 1.96 × 10⁻⁹ — the oxalate is, if anything, the less soluble of the two. Treating chalk with oxalic acid exchanges one insoluble calcium salt for another.

The uses, which are real but small. Reilly records the neatest of them. A gold chloride stock solution is acidic, and one approach to alkaline gold toning neutralises it with calcium carbonate before the toner is compounded — and Reilly is careful about what that achieves: chalk will not make the gold chloride alkaline, but will neutralize any free acid. That is the whole virtue of a sparingly soluble carbonate as a buffer. It cannot overshoot, because it stops dissolving once the acid is gone.

Bostick & Sullivan’s wet-plate kit supplies 100 g of it for a different reason entirely: as a non-scratching abrasive for cleaning glass. Half a teaspoon on an 8 × 10 plate with a teaspoon of rubbing alcohol, worked in and rubbed for about thirty seconds on both sides, then rinsed with hot soapy water and cool water until streak-free. A mineral hard enough to cut a film of grease and soft enough not to scratch glass is a genuinely useful thing to have in a darkroom.

And it is in the tap. Water hardness is reported as milligrams per litre as calcium carbonate, whatever the dissolved salts actually are. Thames Water’s bands are the ones this course quotes: soft up to 100, slightly hard 100 to 150, moderately hard 150 to 200, hard 200 to 300, very hard above 300. Ware warns cyanotype printers away from running tap water containing calcium salts for exactly the reasons above.

Properties

CaCO₃, relative molecular mass 100.09, CAS 471-34-1, with ground natural limestone carrying its own number, 1317-65-3. White, odourless, tasteless, stable, and effectively insoluble in water — the two published solubility products differ by a factor of two and a half and both are around 10⁻⁹, which for practical purposes means the same thing.

The property that does all the work is the one every school chemistry lesson uses: it dissolves with effervescence in dilute acids. That single reaction is the paper pre-treatment, the buffering action in a gold toner, the reason it cannot share a shelf with hydrochloric acid, and the reason an alkaline reserve protects paper from acid attack in the first place. A carbonate mops up hydrogen ions and leaves carbon dioxide.

Handling

This is one of the few pages in the encyclopaedia where an absent classification rests on a large sample: two ECHA aggregations, one of them covering 3,535 company reports, both reporting that the substance does not meet GHS hazard criteria. The course still declines to call it harmless, and HSE supplies the reason without needing an argument — calcium carbonate is listed in EH40, at 10 mg/m³ inhalable dust and 4 mg/m³ respirable dust. Those are the general dust limits rather than a toxicity finding, and they exist because inert dust is still dust.

⚠Why Level A, and the one operation that is not

Against the rubric, calcium carbonate is Level A on every criterion: an unclassified solid, used in gram quantities, at room temperature, producing no vapour and no corrosive splash, with waste that needs no specialist neutralisation. The controls are dust discipline when the powder is worked dry and the ordinary gloves and eye protection of the bench.

The one operation that takes it out of Level A is destroying it. Stripping chalk from a buffered paper means immersing that paper in 5 to 10 per cent w/v sulfamic acid or dilute hydrochloric acid for ten minutes, and the level of that operation belongs to the acid, not to the carbonate — Level B, with splash goggles, an apron and eyewash within reach. It is worth being clear about which substance is the hazard there, because the one being removed is the innocent one.

Waste

Small quantities of the solid are an insoluble mineral. The stream that needs thought is the spent acid bath from a paper pre-treatment, which is an acid solution of calcium salts and is handled as an acid waste under the rules for the acid used. Neutralising it before disposal is the obvious move, and the obvious neutraliser is more of this compound. ILFORD’s advice to domestic users in the United Kingdom is to bottle used chemistry, label it and take it to a household waste and recycling centre. Check your local regulations; they govern, and they differ.

History

Chalk has been on the darkroom shelf since there were darkrooms: Towler’s 1864 list of what to stock in an operating room includes carbonate of lime, chalk, among the acids, the iodides and the nitrate of silver. For a century it was a minor reagent nobody thought about.

The modern problem is recent and was created by good intentions. Ware traces it: the growing awareness of paper conservation led to the ISO specifications for permanent and archival paper, both requiring an alkaline reserve, and fine-art papers followed. The result, he writes, is that over the last few decades it has become increasingly difficult to find papers that work well for siderotype printing — because a sheet that boasts of being “acid free” probably contains a chalk filler that will decompose an iron(III) sensitiser. A standard written to make paper last longer made one family of photographic processes harder to practise, and neither side was wrong.