Calcium nitrate tetrahydrate

This one never touches the paper. It is not a sensitiser, not a developer, not a toner and not a clearing agent, and no reaction in any process this course teaches involves it. What it does is sit in the bottom of a lidded tray as a saturated solution with a heap of undissolved solid under it, and hold the air above it at 55 per cent relative humidity, which is the variable that does most to decide whether a print-out palladium image comes out sepia or neutral black. It is an instrument, and it belongs in this encyclopaedia for the same reason a thermometer belongs in the equipment list.

In photography

It makes a box of known humidity. Mike Ware’s method, given in nearly identical terms across his workshop handouts and in his Chrysotype Manual, is a tray with a close-fitting lid in which the sensitised sheet is held face down over — but not touching — a saturated aqueous solution. Cat litter trays are ideal, he says, because photographic dishes are too shallow and their lip makes sealing difficult; the sheet is held on the underside of the lid by two strips of self-adhesive magnetic tape, which is quick, clean and does not mark the paper.

The salt sets the number. Ware’s fullest list is Table 2.3 of his Chrysotype Manual, “Substances for making constant RH enclosures (at 20 °C)”, and it gives the solubility of each alongside the humidity it holds — the salts, he says, chosen for their convenience, relative cheapness and low toxicity:

Saturated solution	Relative humidity at 20 °C	Solubility, g/L
Anhydrous calcium chloride (used as the solid, a desiccant rather than a solution)	9 per cent	use solid
Lithium chloride	15 per cent	800
Calcium chloride	32 per cent	750
Potassium acetate	35 per cent	2560
Potassium carbonate	44 per cent	1100
Calcium nitrate	56 per cent	1290
Sodium chloride	75 per cent	360
Ammonium chloride	80 per cent	372

His three workshop handouts give a shorter list and, for this salt, name the tetrahydrate and print 55 per cent — one point below the manual’s 56, and with common salt at 76 rather than 75. Nothing in practice turns on a point of relative humidity, and the discrepancy is recorded below rather than resolved.

The condition that makes any of it work is stated in every one of Ware’s accounts and it is not a quantity. There must be excess solid salt in continuous contact with its saturated solution. That is what makes the humidity constant: as long as undissolved salt remains, the solution cannot become more dilute, and the vapour pressure above it is fixed. Give it at least half an hour for evenness; the upper limit “is not critical and can be a few hours”.

⚗Why a saturated salt solution is a humidity standard, and why there is no weight to get wrong

Water above a solution is in equilibrium with it, and the vapour pressure over a solution is lower than over pure water — the more concentrated the solution, the lower it goes. A saturated solution is the most concentrated one that can exist at that temperature, so its vapour pressure is a fixed fraction of pure water’s, and that fraction, expressed as a percentage, is the relative humidity the box settles at.

The excess solid is the regulator. Let the box take up water from a damp sheet and some solid dissolves; let it give up water to a dry one and some crystallises out. Either way the solution stays saturated and the humidity stays put, until the last of the solid is gone.

So there is no dose to get wrong, and this is one of the few places in the course where that is true. You cannot use too much salt and you cannot weigh it inaccurately. What you can do is use too little, so that it all dissolves and the box quietly stops being a standard. The check is visual: solid in the bottom, every time you open the lid.

And it is nearly temperature-independent, which Ware states and explains. The relative humidity in such a system “varies very little with temperature provided a saturated solution is maintained by contact with excess solid”, because the tendency of water’s vapour pressure to rise with temperature is offset, in most cases, by the salt’s solubility rising too. The absolute humidity — the weight of water in a given volume of air — does climb with temperature, but that is not what the paper responds to. This is why a figure quoted “at 20 °C” is still usable in a room at 24.

What the 55 per cent box is actually for. Ware’s palladiotype handout puts it in one sentence under Regulation of Image Colour: drier conditions, at about 55 per cent relative humidity, yield Van Dyke brown or sepia tones and a softer image — that is, a longer exposure range, with great delicacy in the high values. Well-humidified paper, at 70 to 80 per cent, gives near-neutral tones instead, provided the exposures are not too short and no surfactants are used. He is explicit that print contrast is fine-tuned by regulating the humidity of the sensitised paper before exposure.

That makes this salt a contrast and colour control that lives outside the sensitiser, which is unusual. In almost every other process in this course, colour is changed by changing what is in the bottle. Here it is changed by changing what is in the air.

Where the course uses it. Two humidity boxes — sodium chloride at 76 per cent and calcium nitrate tetrahydrate at 55 per cent — are the apparatus of Part XXV’s palladium tonal-scale experiment, which measures what the humidity does to the curve and to the colour. The same box appears in the humidity ladder of the print-out platino-palladiotype, in the Ziatype kit’s humidity series, and in Part XXI’s account of papers and negatives, where the technique is introduced.

📓Ware's own figures for this salt differ by one point, and the course prints both

The platino-palladiotype and palladiotype handouts both name calcium nitrate tetrahydrate and give 55 per cent. The New Chrysotype handout’s Table 3 and the Chrysotype Manual’s Table 2.3 both name calcium nitrate, without a hydrate, and give 56 per cent; the same two tables give common salt as 76 and 75 respectively, so the offset is not peculiar to this entry. One point of relative humidity is well inside the uncertainty of a home hygrometer and nothing in practice turns on it, but the course records the difference rather than smoothing it, because the two sets of entries also differ in which substance they name — and on a balance those are two different articles, 236.15 against 164.09.

What to do about it in a notebook is simple: record the salt you actually put in the box, by name and hydrate, and record what your hygrometer read, rather than the figure from whichever table you copied.

Properties

Ca(NO₃)₂·4H₂O, relative molecular mass 236.15, CAS 13477-34-4. White crystals, flakes or granules; odourless; relative density about 1.84; a 10 per cent solution sits between pH 5 and pH 9. It is freely soluble in water, and hygroscopic enough that the anhydrous salt becomes this one on a damp shelf.

Its melting point is 43 °C, and that is the most surprising number on this page. What melts is not really the salt: a hydrate carrying four waters per formula unit dissolves in its own water of crystallisation a little above blood heat, which is low enough that a jar left in a sunny window or beside a radiator can find itself holding a liquid. Set that beside the anhydrous salt’s 560 °C and the point about hydrates makes itself — these are two different substances, not two grades of one.

!Which record you are reading, and why it is easy to read the wrong one

There are two ECHA records here and they overlap in a way that catches people out.

• The tetrahydrate, EC 603-865-8 — a list number rather than an EINECS one — CAS 13477-34-4. Its regulatory processes are a CLP notification, a 2008 pre-registration and nothing else. No tonnage band. No harmonised classification: the index number is empty.
• The anhydrous salt, EC 233-332-1, CAS 10124-37-5. REACH registered, tonnage band 100,000 to 1,000,000 tonnes, listed under all three annexes of the Inland Transport of Dangerous Goods Directive, and typed by ECHA as both a substance and a group. Its index number is empty too.

And the anhydrous record lists 13477-34-4 among its own CAS numbers, so a search on the hydrate’s number finds both. The manufacturer’s sheet cited on this page prints EC 233-332-1 against a product it correctly identifies as the tetrahydrate, which is the same conflation happening in the wild. None of this changes a hazard — ILO-WHO Card 1037 says in its own notes that its recommendations apply to the tetrahydrate as well — but all of it changes a mass, and the two masses are 236.15 and 164.09.

On heating it does not simply melt and stop. The manufacturer’s sheet records that it emits toxic acid fumes when heated to decomposition, giving nitrogen oxides and calcium oxide. That is the ordinary fate of a metal nitrate and it is the same chemistry that makes it an oxidiser at room temperature: the nitrate ion has oxygen to give, and heat is what persuades it.

Handling

It is an oxidiser, and that is the first thing about it rather than a footnote. ILO-WHO Chemical Safety Card 1037 — whose own notes say it applies to the tetrahydrate — states it in three places. Under chemical dangers: “The substance is a strong oxidant. It reacts with combustible and reducing materials.” Under fire and explosion: not combustible itself, but it enhances the combustion of other substances, with a risk of fire and explosion on contact with combustible substances, and the prevention line reads NO contact with flammables. Under storage: separated from combustible substances and reducing agents, dry. The transport regulations agree independently and in three jurisdictions: UN 1454, hazard class 5.1, packing group III, which is the class for oxidising substances.

Both safety data sheets the course could read classify it H272, may intensify fire; oxidiser, at signal word Warning, with acute oral toxicity category 5, skin irritation category 3 and an eye statement. As the hazards note above sets out, those two sheets are the same document under two letterheads rather than two independent opinions, and their eye classification contradicts itself between them. So the course treats the oxidiser statement as settled and the eye statement as the weak part of the evidence, and specifies eye protection anyway — nothing about putting a spoonful of salt in a tray requires the looser reading to be right.

The specific toxicology is on ingestion and it is a nitrate’s. Card 1037’s symptom list for swallowing is abdominal pain, blue lips, fingernails and skin, confusion, convulsions, dizziness, headache, nausea and unconsciousness, and the mechanism it names is methaemoglobin formation, with the warning that the effects may be delayed and medical observation is indicated. The oral LD50 in the rat is 3900 mg/kg, which is not a small dose; the reason the card gives that list anyway is that nitrate poisoning is not a dose-response you can judge by how you feel at the time. Hands washed, nothing eaten in the room, and the jar labelled and never a food container.

There is no exposure limit. HSE’s EH40 has no entry for calcium nitrate, and none for nitrates as a class; its calcium entries are the carbonate, cyanamide, hydroxide, oxide, silicate and sulphate. EH40’s own introduction says that absence from the list does not indicate that a substance is without risk. Card 1037 gives no occupational limit either, and describes the dust as reaching a nuisance-causing concentration quickly when dispersed. So the control is dust discipline rather than a number.

⚠The instruction that tells you what kind of hazard this is

Card 1037’s closing note is one sentence and it is the one to remember: rinse contaminated clothing with plenty of water because of the fire hazard.

Not because the salt will burn your skin — it will not, on any published classification — but because a sleeve that has been splashed with a saturated nitrate solution and then allowed to dry is a piece of cotton impregnated with an oxidiser. That is the whole character of the substance in one instruction, and it applies with equal force to the blotting paper under the tray, to a towel used to mop up a spill, and to the tray itself if the solution is left to evaporate.

Why Level B. The course rubric caps Level A at substances whose classification is “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. An oxidiser is outside that list, exactly as it is for silver nitrate and for iron(III) nitrate, both of which sit at Level B for the same reason. Level B’s own criterion of a procedure “whose failure mode is a splash, a burn or a spill of something corrosive rather than merely a spoiled negative” applies in its fire-risk form here. It does not reach Level C: no source read for this page names a fume cupboard, a licensed waste route or supervision as the recognised control, and the recognised controls are separation from combustibles, dust suppression, gloves, eye protection and hygiene.

What is not a hazard here, and why. It is not corrosive, not a sensitiser, not a carcinogen on any classification the course found, and its published aquatic toxicity is mild. It is not an explosive and not self-reactive: Card 1037 lists no physical dangers, and the manufacturer’s sheet calls it stable under ordinary conditions of use and storage. And in the use this page is about it is a dissolved salt under a lid, which is the least reactive state it can be in — the fire hazard belongs to the dry powder, the dried splash and the evaporated tray, not to the box while it is working.

Waste

Small volumes of a saturated solution, diluted well and not evaporated. The published aquatic figures are undramatic — an LC50 for rainbow trout above 98.9 mg/L over 96 hours, no persistent or bioaccumulative components, and the sheet’s own statement that the material is not considered an environmental hazard — and the anhydrous salt is spread on fields by the hundred thousand tonnes under the names Norwegian saltpeter and lime saltpeter. The course holds no source giving a discharge limit for nitrate and does not invent one, so what it says instead is the honest thing: this is a nutrient salt, dilution is the right treatment for the small volumes involved, and the local rules are the ones that govern.

The real waste discipline here is physical. Nothing oxidiser-bearing is allowed to dry out and then be binned: not the solution, not the blotting paper, not a spill cloth. Card 1037’s spillage instruction is to sweep the solid into covered plastic containers and to wash away the remainder with plenty of water. Keep it out of the silver and oxalate bottles as well — calcium oxalate is essentially insoluble, and the oxalate developer on the next bench is the one thing in the room a calcium salt visibly ruins. That reaction is written out on the papers-and-negatives lesson, where it is the reason an alkaline-buffered paper destroys an iron sensitiser.

Check your local regulations; they govern, and they differ.

History

Calcium nitrate has a claim on the first page of photography that almost nobody makes for it: it is the substance Johann Heinrich Schulze was trying to make in 1727 when he made a photograph instead.

Schulze was pursuing Balduin’s phosphor, a luminous stone made by dissolving chalk in nitric acid, and the salt that operation produces is calcium nitrate. The nitric acid he used happened to be an assayer’s parting acid with silver dissolved in it, and it was the silver, precipitated onto the excess chalk as a light-sensitive carbonate, that darkened where the sun struck it. Ware’s reading of the episode names both halves — the intended product was Balduin’s phosphor and the necessary salt calcium nitrate; the photograph was the by-product.

Part I records the control that settles it: calcium nitrate alone proved insensitive. Schulze tested it, and the effect belonged to the silver. So the salt’s first appearance in the history of this subject is as the negative result in the first photographic experiment ever performed, and its second — three hundred years later — is as a tray of saturated solution under a lid, deciding whether a palladium print comes out brown or black.