Kaolin

A silver sensitising bath is the most expensive liquid in a printing darkroom and it has exactly one maintenance treatment. It is not a filter, not a titration and not a replenisher: it is about fifteen grams of china clay per litre, left in the bottom of the storage bottle, shaken up after every session and allowed to settle overnight. The practice is in Hardwich in 1864 and in the instruction sheet of a kit sold today, and in a hundred and sixty years the only thing that has changed is the size of the spoon.

In photography

The bath goes brown, and then black, and the clay is what takes the colour out. Reilly’s account is the clearest and it is a mechanism rather than a remedy. Organic matter from the paper’s sizing or from the binder — albumen, gelatin, starch — always partly dissolves into the silver solution. Some of that organic matter reacts with some of the silver, and the reacted silver is eventually reduced spontaneously to the metallic state. Finely divided metallic silver is what colours the solution.

Ag⁺ + e⁻ → Ag

The reduction that discolours a silver bath, with the paper's own organic matter supplying the electron

Hardwich, writing a hundred and sixteen years earlier, had named the intermediate: a brown “Sub-Albuminate of Silver”, partially soluble in silver nitrate solution, which is what produces the colour before the metal appears. The same reaction on a coated sheet instead of in a tray is why sensitised paper yellows, then goes reddish brown, and finally bronzed black in storage.

⁇How dark a bath may be allowed to get, and what colours it

Two disagreements sit on top of one another here, and the course states both rather than choosing.

How far it may be used. Hardwich says the solution “becomes after a time discoloured by the Albumen, but may be used for sensitizing until it is nearly black.” Reilly is explicit that it may be used in a slightly discoloured state but not when it is nearly black. The course follows Reilly, because he is writing after a century of permanence research and because the failure his rule prevents — reduced silver carried onto the sheet — is invisible until the print is made.

What the colour is. Hardwich and Reilly both attribute it to silver, reduced by dissolved organic matter. Bostick & Sullivan, writing for their albumen kit, describe the bath as taking on a red colour after a session and attribute it “to a small amount of salt from the albumen solution contaminating it” — a chloride, that is, rather than a reduced metal. The observation is theirs and the difference is not resolved in anything the course has read. Both accounts prescribe the same treatment, which is a hint that the clay is not fussy about which it is removing.

The dose and the routine. Reilly: about 15 g of kaolin per litre is sufficient to clear one litre of silver solution repeatedly; leave it in the container the bath is stored in; shake bath and clay together after each use; it settles out overnight and the bath is clear and ready the next day. He builds a siphon from a two-hole rubber stopper and glass tubing so the clear liquid can be drawn off without disturbing the layer on the bottom. If there is no time to let it settle, the clay can be taken out by filtering through medium filter paper — and filtering stops being optional the moment a surface scum appears, because scum prints as a metallic marbled sheen.

Hardwich’s proportion is a quarter of an ounce of finely pulverised kaolin to twenty ounces of bath, shaken and then filtered through paper — about 12 g/L, close enough to Reilly’s figure that the two are the same practice. Bostick & Sullivan supply 100 g with the albumen kit, “for cleaning Silver Nitrate”, and ask for a half teaspoon into the storage bottle after each session, 12 to 16 hours of settling to a fine black precipitate, a decant that leaves the cloudy bottom behind, and a pass through a paper coffee filter before each session — in which, they note, you will see a small amount of black silver powder.

The alternatives, and why the clay won

The nineteenth century tried the obvious substitutes and wrote down how they compared.

• Animal charcoal. Hardwich mentions it first and dismisses it in the same sentence: the colour can be removed by animal charcoal, “but a better plan is to use the Kaolin”.
• Freshly precipitated silver chloride, curdy. Hardwich’s fallback when kaolin cannot be got. It “has an affinity for the brown Sub-Albuminate of Silver which produces the colour, and gradually carries it down” — the same adsorption, on a different surface — “but it is inferior to Kaolin as a decolorizer.”
• Sunlight on a neutral bath. Abney: neutralise the bath to litmus and stand it in the sun, and the organic matter is deposited together with silver oxide.
• A pinch of common salt. Abney again: add a little sodium chloride, shake, and the organic matter comes down with the silver chloride that forms and can be filtered off. This is Hardwich’s curdy chloride made in the bottle instead of beforehand — and it costs you silver.

Abney’s verdict on the kaolin method itself is the honest one and is worth having: it “answers perfectly, but is rather wasteful.” The clay carries silver-bearing liquid down with it, and what is left in the bottom of the bottle is not free.

What else it did

Cassell’s 1911 entry lists three uses beyond the bath, and two of them are inside the photographic material rather than the darkroom: kaolin is used in the production of matt-surface paper and is added to emulsions for the same purpose, as well as in the manufacture of crayons for working up prints. A separate entry adds a fourth — silicate fillings such as kaolin are loaded into paper to make it more opaque and to give it a greater glaze, “at the same time lessening somewhat its strength”.

Hunt, in 1854, gives the reason a photographer should care about that last one. China clay had recently come into wide use by paper manufacturers for weight and surface; the finest clays would do little mischief in a photographic paper, “but the less pure varieties of the Cornish clay are employed, and this commonly contains the oxides of iron and other metals in a state of very fine division; and these, where they come to the surface, form little centres of action, from which dark circles spread in rather a curious manner.” That is a foxing defect described sixty years before anyone had a word for it, and its cause is an impurity in a clay.

Properties

A rock, and the page will not pretend otherwise. ECHA’s own description is the one to quote: “A clay that is essentially kaolinite, a hydrated aluminum silicate. It has a high fusion point and is the most refractory of all clays.” NIOSH’s note says the same in three words — the main constituent of kaolin is kaolinite. Because it is a natural mixture there is no molecular formula and no molar mass: ECHA files seventeen CAS numbers against the single substance and puts nothing but an indicative formula in that field, and NIOSH prints the molecular weight as varies. It is in the same class as this encyclopaedia’s four natural binders — albumen, gelatin, gum arabic and arrowroot — with the difference that this one is mineral and they are biological.

Where it comes from. Hardwich: prepared by careful levigation from mouldering granite and other disintegrated felspathic rocks. Wall’s 1912 dictionary adds the number and the process — a very fine hydrous silicate of alumina containing about 14 per cent of water, a decomposition product from the natural decay of felspar. Levigation is grinding and settling in water: the fine fraction is carried off and dried, which is why the article is a powder of a very narrow particle size and why it settles in a bottle overnight rather than in a minute or a week.

A white to yellowish or greyish powder, insoluble in water, specific gravity 1.8 to 2.6, a noncombustible solid. NIOSH adds the detail that gives it away on a bench: when moistened it darkens and develops a clay-like odour.

🔬Why an insoluble powder takes colour out of a solution

Nothing here is a reaction. Hardwich, who was a chemist and knew the difference, is careful about it: kaolin is “perfectly insoluble in water and in acids, and produces no decomposition in solution of Nitrate of Silver”, and NIOSH’s entry reports its incompatibilities and reactivities as none.

What it has instead is surface. Kaolinite is a layered mineral and a levigated clay is a suspension of very small plates, so a spoonful presents an area out of all proportion to its weight. Dissolved organic matter — the sub-albuminate, the sizing, the protein fragments — adsorbs onto those plate faces and is carried down with them when they settle. The silver ions in solution are not adsorbed in any comparable way, which is why the bath comes back clear and still 12 per cent, and it is why the same treatment works whether the colour is Hardwich’s reduced silver or Bostick & Sullivan’s chloride haze.

The course marks this paragraph as its own reading of the mechanism. The sources establish the practice, the insolubility and the absence of reaction; the account of why an adsorbent works is standard surface chemistry applied to what they describe, and no document read for this page spells it out.

Handling

Safety Card 1144 says in so many words that there is no hazard classification according to GHS criteria, and ECHA’s record agrees to the extent that there is no harmonised CLP entry behind it. That is the strongest form of “not classified” this encyclopaedia ever gets to print — an international expert body having looked and found the criteria unmet, rather than a substance nobody has notified. Read the rest of this section anyway, because the card attaches a condition to it and because an unclassified dust is not the same thing as a dust you can raise.

The hazard is dust, and four authorities give numbers for it. NIOSH recommends 10 mg/m³ total and 5 mg/m³ respirable over eight hours; OSHA permits 15 and 5; HSE’s EH40, binding in Great Britain, lists Kaolin, respirable dust, CAS 1332-58-7, at 2 mg/m³ long-term with no short-term limit and none of the Carc, Sen or Sk notations; and Card 1144 gives a TLV for the respirable fraction of the same 2 mg/m³, with the A4 notation, not classifiable as a human carcinogen, and MAK carcinogen category 3. The course works to the strictest, which is 2 mg/m³ respirable.

What repeated exposure does has a name. Card 1144: the lungs may be affected by repeated or prolonged exposure to dust particles, and this may result in fibrosis — kaolinosis. NIOSH’s entry gives the same finding as chronic pulmonary fibrosis and adds stomach granuloma, with the respiratory system and the stomach as target organs, and its IDLH as not determined. Both are occupational findings from clay industries rather than anything a teaspoon into a bottle will produce, and the page says so plainly — but they are why “it is only clay” is not the whole answer, and why the instruction is to pour rather than to tip.

⚠The condition attached to that absence of classification, and it is the one that matters

Card 1144 ends with a note that changes the reading of everything above it: “This card applies to kaolin containing <1% crystalline silica”, and if the kaolin contains more than 1 per cent crystalline silica, ICSC 0808, Quartz, applies as well.

Card 0808 is a different document in every respect. Signal word Danger; may cause cancer if inhaled; causes damage to the lungs, the immune system and the kidneys through prolonged or repeated exposure if inhaled; silicosis under long-term exposure; a TLV for the respirable fraction of 0.025 mg/m³, which is eighty times stricter than kaolin’s, with the A2 suspected-human-carcinogen notation and MAK carcinogen category 1.

Kaolin is a mined clay, and quartz is the mineral it is most often mined beside. So the unclassified substance on this page is a purified clay, and the powder in an unlabelled bag is an unknown one. Buy a named grade, read the sheet that comes with it, and do not decant a clay of unknown origin over an open bottle.

Why Level A. The rubric puts a substance at Level A when its classification is at most irritant in the quantities of a home darkroom and the controls are ordinary darkroom ones. That is this material: an inert, noncombustible, insoluble mineral powder with no reported reactivities, handled a teaspoon at a time. The control is not to raise a cloud — pour at bench level, cap and shake, and never sweep a spill dry.

What is not a hazard here, and why. There is no reactivity to manage: NIOSH reports its incompatibilities and reactivities as none, Card 1144 records both its physical dangers and its chemical dangers as no data, and Hardwich’s test — that it produces no decomposition in a solution of nitrate of silver — is a nineteenth-century statement of the same thing. There is no fire risk; both sources call it noncombustible, and the layered silicate is what makes it, in ECHA’s words, the most refractory of all clays. The skin route is a contact-irritation one rather than an absorption one: NIOSH’s personal protection entry recommends nothing specific for skin, eyes or clothing, and the card asks only for gloves, spectacles and washing with soap and water. The session it belongs to is another matter entirely. The teaspoon goes into a bottle holding a litre of 12 to 15 per cent silver nitrate, which is corrosive, oxidising and very toxic to aquatic life, and the controls for that operation are the silver’s and are Level B. Kaolin is the one thing on that bench you do not need goggles for; you wear them anyway, because of what it is going into.

✕The impurity that spoils the cure

Hardwich’s caution is specific, testable and easy to skip. Commercial kaolin may contain chalk. Test it by dropping acid on a little of the powder: if it effervesces, calcium carbonate is present, and it must be washed in diluted vinegar and then in water before it goes anywhere near a silver bath. Left in, in the presence of ammonium salts, it makes the bath alkaline — and an alkaline printing bath dissolves the albumen off the paper. You would have cured a discoloration by stripping the coating.

Neither of the two suppliers whose sheets the course reads publishes a safety data sheet for kaolin, although one of them ships 100 g of it in a kit; the library search on 7 September 2026 returned nothing. Read whatever sheet comes with the powder you buy, because what varies between clays is the accessory minerals, and that is exactly what Hardwich and Hunt are both warning about.

Waste

The clay is inert; the sludge is silver-bearing. Everything the layer at the bottom of the bottle has adsorbed came out of a silver sensitising bath, and the fine black precipitate that settles with it in the modern account is reduced metallic silver. So the settled clay, the decanted dregs, the coffee filter and the filter paper all go wet into the labelled silver-bearing container, and none of it is rinsed to a drain or dropped dry into a bin.

Kodak’s J-214 sets the United States federal line at 5 ppm of silver in a liquid waste, EPA number D011, and a clay that has been shaken repeatedly with a 12 per cent silver solution is not a material to estimate. ILFORD’s advice to domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern.

History

Kaolin is one of a very small number of substances in this course whose photographic use has not changed at all since it was first written down.

Hardwich gives it a chemical entry of its own in 1864, between the iron salts and the acetate of lead, with its geology, its insolubility, its use and its impurity — the treatment of a working reagent rather than a curiosity. Abney in 1905 lists it first among three ways of purifying a printing bath and calls the method wasteful but perfect. Cassell in 1911 files it under both Kaolin and China clay and records that by then it had migrated into the materials themselves, into matt papers and emulsions. Wall, in 1912, puts it in the past tense — it “was used in the wet process as a purifier of the silver bath” — which was true of the wet plate and premature about everything else.

Because in 2026 it is still in the box. A hundred grams of it ships with an albumen printing kit, with an instruction sheet that says to shake a half teaspoon into the storage bottle after every session and let it settle overnight, and that is Reilly’s paragraph of 1980, which is Hardwich’s paragraph of 1864, with a teaspoon in place of a quarter of an ounce.