Titanium dioxide

Every resin-coated print you have ever made has titanium dioxide in it, a fraction of a millimetre under the image. It is there to be white, and it is there because the alternative — a mineral coating laid on paper — is what resin-coated paper was invented to replace.

In photography

It is the whiteness of an RC print. The Getty Conservation Institute’s account of the resin-coated support is exact: “the emulsion side of the paper is coated with PE filled with titanium dioxide (TiO₂). The bright-white pigmented layer simulates the visual effect of the baryta layer of fiber-based, baryta-coated photographic papers. The emulsion layer is coated onto the pigmented layer and overcoated by a topcoat or supercoat of hardened gelatin as protection against mechanical damage.” Two sheets of polyethylene sandwich the paper; the one under the emulsion carries the pigment. The paper base never gets wet, which is why an RC print washes in four minutes and a fibre-base print in an hour.

And it is the substitute for baryta, not an addition to it. The pigment does the job that barium sulfate does in a fibre-base paper: to put an opaque, brilliant white between the image and the fibres, so that the highlights are the paper’s whiteness and not the pulp’s. Two different insoluble white solids, chosen for the same optical property, in two different binders.

The two are hard to tell apart by instrument, which is a fact worth carrying. Getty’s analytical section records that in X-ray fluorescence “the main spectral peaks of Ti and barium (Ba) almost overlap perfectly”, and that distinguishing them needs the barium Kα and Kβ peaks at 32.19 and 36.38 keV and the other barium peaks below the Ba L peaks, “which are missing in XRF spectra of material containing only titanium”. A conservator identifying a print’s support is doing exactly this.

Where a printer must refuse it. Ware’s specification for a paper intended for the iron-based processes lists what the sheet must not contain: an alkaline buffer of chalk or another alkali, clay fillers and aluminosilicates, retention aids, wet-strength resins, optical brightening agents, residual bleach, and “dyes or white pigments such as titanium dioxide”. A hand-coating printer is buying paper rather than making it, so the pigment appears in this course twice — as the thing that makes a factory paper white, and as one of the things that makes a factory paper unusable for a cyanotype.

⚗Why an insoluble oxide is a pigment, and why that is also the safety argument

CAMEO records titanium dioxide as insoluble in water, NIOSH as insoluble, the ILO-WHO safety card as having no water solubility at all; it resists hydrochloric, nitric and dilute sulfuric acid and yields only to hydrofluoric acid and hot concentrated sulfuric acid. A pigment has to be insoluble — a soluble white would leach into the emulsion, migrate in the wash and be gone in a decade. The same property governs the hazard: an insoluble solid cannot be absorbed, so the only route by which titanium dioxide does anything to a person is as respirable dust in the lung.

That is why the harmonised classification is written the way it is. It applies, by its own wording, to the substance “in powder form containing 1 per cent or more of particles with” a stated small aerodynamic diameter — the record’s copy of that name is truncated at the figure, so this page does not quote the cut-off. The hazard is a form, not a composition. Rutile and anatase are the same formula, TiO₂, in two different crystal habits, and the argument about carcinogenicity is not about which crystal it is but about how small the particles are and whether they reach the deep lung.

Not sourced here. No document this course holds explains the mechanism proposed for the lung effect, and this page does not supply one.

Properties

An odourless white powder, pH about 7.5, occurring in three crystalline forms; TiO₂, relative molecular mass 79.87, CAS 13463-67-7. Rutile and anatase have registry numbers of their own — 1317-80-2 and 1317-70-0 — and the ECHA inventory files them as separate substances with separate classifications, which is why the numbers below differ from one another. NIOSH gives a specific gravity of 4.26, so a layer that looks like a whitewash is in fact denser than aluminium.

Handling

Three regulators, three positions, and this page reports all three. The harmonised CLP entry gives Warning, the health-hazard pictogram and H351, suspected of causing cancer, limited by its own wording to the fine-powder form. HSE’s EH40 lists titanium dioxide at CAS 13463-67-7 with long-term limits of 10 mg/m³ total inhalable and 4 mg/m³ respirable dust, with no short-term limit and, notably, no carcinogen notation — the treatment EH40 gives a nuisance dust. NIOSH goes the other way: its recommended exposure limit is simply “Ca”, its designation for a potential occupational carcinogen, with no number, an immediately-dangerous-to-life-or-health value given as “Ca [5000 mg/m³]” and a cross-reference to its Appendix A; the OSHA permissible limit beside it is 15 mg/m³.

The notified data are as divided. Of 11,189 company reports for titanium dioxide, 62.8 per cent say the substance does not meet GHS hazard criteria, and 36.1 per cent give H351. For rutile the proportions are 87.1 per cent unclassified against 12.6 per cent H351; for anatase, 90.1 per cent unclassified with no code reaching the reporting threshold. A separate entry covering seven reports gives Danger with H319, H335, H372 and H413. Japan’s NITE-CMC gives H351 and H372 for both nanoparticle and non-nanoparticle grades. The course does not reconcile these. What they have in common is that every classification that exists is about inhaling the powder.

Why Level A. The article a reader of this course meets is titanium dioxide bound in polyethylene, laminated to paper, under an emulsion and a supercoat. It is not a dust, it is not soluble, it cannot be inhaled, and handling a print is not a chemical exposure at all. Level A of the course rubric is the right classification for a substance encountered in that state, and it is the level of every printing page that uses RC paper. The operation that would take it out of Level A is handling the loose pigment powder — which no page in this course asks anyone to do, and which the harmonised classification exists to govern. If you ever do, Princeton University’s darkroom guidance for mixing powders is the applicable control: a glove box, local exhaust ventilation, or an approved dust respirator.

!Important

The absence of a carcinogen notation in EH40 is not a clean bill of health, and the presence of one in NIOSH’s REL is not a verdict on a print. Both statements are about airborne respirable particles in an industrial workplace. Neither is about the sheet in your dish, and this course would be misleading you in opposite directions if it quoted only one of them.

Waste

There is no titanium dioxide waste stream in a darkroom. The pigment never leaves the polyethylene it is dispersed in; it does not enter a developer, a fixer or a wash, and it does not appear in the silver stream. Print trimmings are ordinary solid waste — although a resin-coated print is a plastic laminate and is not recyclable as paper, which is a fact about the support rather than about the pigment. Loose pigment, if a reader holds any from another use, should not be raised as dust indoors and should not be washed down a drain, where an insoluble dense solid simply settles; take it to a licensed household hazardous waste route, in England and Wales through the government’s hazardous waste service finder. Check your local regulations; they decide.

History

Titanium dioxide arrived in photography as an engineering answer to a laundry problem. A fibre-base paper absorbs water through its edges and its back, which is why it needs a long wash and a long dry and why it curls; the resin-coated construction seals the base between two polyethylene skins so that only the emulsion gets wet. That construction needs its own whiteness, because the sealed base is no longer visible through the emulsion in the way a baryta-coated fibre base is, and the pigment had to be one that could be milled into molten polyethylene and stay put.

The Getty’s account records what the choice was measured against — the pigmented layer “simulates the visual effect of the baryta layer” — which is an unusually candid statement of a substitution’s brief. A century of prints had established what a photographic white looks like, and the new material’s job was to match it.