Barium sulfate

Hold a fibre-base print to the light at a damaged corner and you can see three layers: paper fibres, a white mineral coat, and the image. The white coat is baryta, and it is barium sulfate in gelatin. It is the reason a photographic white looks like nothing else in a room full of paper.

In photography

What the layer is for. A print made straight onto sized paper has its image partly inside the fibres. The AIC’s conservation catalogue puts the difference plainly: the image “can either be partially within the paper structure as with printing-out processes, or on top of a baryta layer as with developing-out processes”. Putting an opaque white coat between the two does three things at once — it stops the emulsion sinking into the fibres, it hides the pulp’s own colour and texture, and it gives the highlights a brightness the paper alone cannot reach. The Getty’s atlas records what it looks like under a microscope: at a damaged edge you can see “separated microplatelets of the white (more white than the paper fibers) baryta layer” still held in the fibres, and in an intact area the layer “looks flat, solid, and almost structureless”.

When it arrived. The Getty dates the development of the baryta layer to “its introduction by José Martinez-Sanchez and Jean Laurent in 1866”, before the gelatine emulsion existed. The AIC gives the commercial dates: 1894 for baryta layers added to commercial developing-out papers, 1900 for their introduction to Kodak papers; the Getty’s own timeline agrees, marking “Kodak introduces baryta coating c. 1900”. By the printing-out era it was near-universal — “with the exception of very early silver gelatin POP and some later types of technical and copy POP, almost all other POP papers were produced using a baryta-coated paper stock” — and some were tinted, the Getty noting POP papers with pink and blue baryta layers.

It is not a constant. The AIC’s warning is worth quoting for anyone coating their own paper: “baryta layers may be non-existent, very thinly or thickly applied in both home/boutique-made and commercial made photographic material”. There is no standard thickness, and the whiteness of a hand-coated print is whatever its maker put under the emulsion.

How a conservator finds it. Barium is heavy, so it answers to X-ray fluorescence: the Getty records that most printing-out silver gelatin photographs “show the presence of a baryta layer identified by the presence of barium (Ba) and strontium (Sr)”, the strontium riding along from the ore. The same section carries a trap that ties this page to its successor — in resin-coated paper, where the white is titanium dioxide in polyethylene, “the main spectral peaks of Ti and barium (Ba) almost overlap perfectly”, and telling the supports apart needs the barium peaks at 32.19 and 36.38 keV.

⚗Barium is a poison; barium sulfate is a contrast medium. The difference is solubility.

Soluble barium salts — the chloride, the nitrate, the carbonate — are acutely toxic, and HSE’s EH40 gives “Barium compounds, soluble (as Ba)” a long-term limit of 0.5 mg/m³. Barium sulfate has its own line in the same table, at 10 mg/m³ inhalable dust and 4 mg/m³ respirable, with no short-term limit and no notation at all: a factor of twenty, for the same element. NIOSH makes the separation explicit rather than by omission — its entry for soluble barium compounds carries the note that the limits “also apply to other soluble barium compounds (as Ba) except Barium sulfate”.

The reason is in the solubility. HSDB gives 0.00031 g per 100 g of water at 20 °C; NIOSH and CAMEO give 0.0002 per cent. Two to three milligrams in a litre. A barium ion that never enters solution never reaches a bloodstream, which is why ChEBI records barium sulfate’s role as a radioopaque medium — the substance is swallowed by the litre for X-ray examinations, precisely because it passes through unchanged.

Two things this course will not round off. HSDB’s own caveat is that solubility in water “increases considerably in presence of chloride & other anions”; that is a real qualification and the page keeps it, without extrapolating from it to any claim about fixers. And no source held here gives a solubility product for barium sulfate, so the course quotes measured solubilities and does not compute an equilibrium it has not been given.

Properties

A white or yellowish odourless powder or small crystals; BaSO₄, relative molecular mass 233.39, CAS 7727-43-7. It melts at 1580 °C with decomposition and has a density of 4.25 to 4.5 g/cm³ — heavier than titanium dioxide and nearly twice the density of glass, which is why a baryta coat has weight in the hand and why the mineral is called heavy spar.

Four names, one entry. EH40’s synonym index maps Barite, Barytes, Blanc fixe and Heavy spar all to “Barium sulphate”. The distinction that matters commercially is between the ground natural mineral and the precipitated synthetic grade — blanc fixe — which is finer and purer, and it is the synthetic grade that goes into a coating. ECHA files the mineral separately, under EC 236-664-5, and its classification differs from the manufactured salt’s; the record also carries a second CAS number, 13462-86-7.

Handling

Not classified, on a large base. For barium sulfate, EC 231-784-4, 1,648 of 1,811 company reports say the substance does not meet GHS hazard criteria — 91 per cent — though PubChem notes that only 9 per cent of companies supplied information at all. Two dissenting views belong on the page and this one carries them: the mineral barite, filed under its own EC number, draws Warning and GHS07 from one notification, with H302 and H332 at 16.6 per cent of its 331 reports; and Japan’s NITE-CMC gives Danger with H372, damage to organs through prolonged or repeated exposure, plus H402 and H412. Neither is echoed by any European notifier of the manufactured salt, and this course does not know why the ore and the precipitate are classified differently.

The exposure limit is the operative control, and it is a dust limit rather than a toxicity one. EH40 gives barium sulphate 10 mg/m³ inhalable and 4 mg/m³ respirable as eight-hour averages, with no notation; NIOSH recommends 10 mg/m³ total and 5 mg/m³ respirable, OSHA permits 15 and 5, and the immediately-dangerous-to-life-or-health value is not determined. Those are the numbers a regulator attaches to a nuisance dust: the hazard is the load on the lung, not the chemistry.

Why Level A. What a reader of this course meets is barium sulfate bound in gelatin, laminated under an emulsion, on a sheet of paper. It cannot be inhaled, it does not dissolve in any bath the course uses, and handling a print is not a chemical exposure. Level A of the course rubric covers that completely. The operation that would take it out of Level A is coating a baryta layer from the powder, which no page here asks anyone to do and which would put a respirable mineral dust in the air; that is Level B work with extraction, on the dust criterion alone. And the other way a person could come to harm through this entry is not an operation at all: it is buying the wrong barium salt. Barium chloride and barium nitrate are acutely toxic and carry a limit twenty times tighter, and the only thing they share with baryta is the first word on the label.

!Important

Read the whole name and the CAS number on any barium container. 7727-43-7 is the sulfate, insoluble and treated as a dust. Anything else with barium in the name should be assumed to be soluble and acutely toxic until its own entry says otherwise, and this course handles none of them.

Waste

There is no barium sulfate waste stream in a darkroom. The baryta stays in the paper; it is insoluble in developer, stop, fixer and wash alike, so it never enters the silver stream and never reaches a drain. Fibre-base trimmings are ordinary paper waste. Loose powder, if a reader holds any from another use, is an inert dense solid: do not raise it as dust indoors, and do not wash it into a drain where it will simply settle. Take it to a licensed household hazardous waste route — in England and Wales through the government’s hazardous waste service finder, which is the route ILFORD gives domestic users for photographic chemicals. Check your local regulations; they decide.

History

Baryta is older than the material it is famous for. The Getty dates its introduction to 1866 — Martinez Sanchez and Laurent, working with albumen and collodion papers — and notes that the development of the layer was “not directly related to silver gelatin photography” at all. It was waiting when the gelatine emulsion arrived, and the dates in the AIC catalogue show how fast it was adopted once developing-out paper existed: commercial DOPs from 1894, Kodak from 1900.

What it did to the look of a photograph is easy to see. Before baryta, a print’s whites were the paper’s whites and its surface the paper’s surface; after it the maker chose both, and the smooth brilliant highlight became the thing a photographic print does that no other printed sheet does. When resin-coated paper replaced the fibre base for speed of washing, the first thing its designers had to reproduce was this layer — the Getty describes the pigmented polyethylene as simulating “the visual effect of the baryta layer of fiber-based, baryta-coated photographic papers”. Baryta is still the standard its substitutes are measured against.