Making Lithopone — The Co-Precipitated White Pigment of the Industrial Age

Set up in a well-ventilated area — preferably outdoors or under a fume hood. Barium sulfide produces toxic hydrogen sulfide gas (H₂S) on contact with acids or moisture, recognisable by its rotten egg smell. H₂S is dangerous at concentrations above 20 ppm and immediately life-threatening above 100 ppm. Wear chemical splash goggles, nitrile gloves, and a lab coat. Keep a basin of water nearby for decontamination.

Dissolve 30 g of zinc sulfate heptahydrate (ZnSO₄·7H₂O) in 200 ml of warm distilled water in a glass beaker. Stir until fully dissolved — the solution should be clear and colourless. Zinc sulfate is the source of zinc ions that will form the zinc sulfide component of lithopone. It is mildly irritating but not acutely toxic.

In a separate beaker, dissolve 20 g of barium sulfide (BaS) in 200 ml of warm distilled water. Stir carefully — barium sulfide reacts with water to form a strongly alkaline solution and releases trace amounts of hydrogen sulfide gas. The solution will be yellowish and smell faintly of rotten eggs. Do this step under ventilation. Barium sulfide is toxic if ingested — never pipette by mouth.

Filter both solutions through filter paper to remove any undissolved particles or impurities. Clarity matters — any insoluble contaminants will end up trapped in the co-precipitate and discolour the final pigment. The zinc sulfate solution should be perfectly clear. The barium sulfide solution may be slightly cloudy due to trace barium carbonate — filter until clear.

Place the filtered zinc sulfate solution on the hot plate and heat to 70 °C. The precipitation works best when both solutions are warm — the higher temperature produces finer, more uniform co-precipitate particles, which translates to better opacity in the finished pigment. Monitor with a thermometer.

With vigorous stirring, slowly pour the warm barium sulfide solution into the warm zinc sulfate solution. The double decomposition reaction occurs instantly: ZnSO₄ + BaS → ZnS↓ + BaSO₄↓. A dense white precipitate forms immediately — this is the raw lithopone, an intimate mixture of zinc sulfide and barium sulfate. Add the barium sulfide slowly over five minutes to ensure thorough mixing. The smell of hydrogen sulfide will intensify during this step — stay upwind.

Continue stirring for ten minutes after all the barium sulfide has been added, then allow the white precipitate to settle for 30 minutes. The supernatant should be nearly clear — if it is still milky, the reaction may be incomplete. The settled precipitate is a soft, white, gelatinous mass. It has almost no opacity at this stage — raw lithopone is a poor pigment until calcined.

Carefully decant the clear supernatant. Add 400 ml of clean distilled water, stir thoroughly, and allow to settle again. Repeat this washing process three times to remove residual soluble salts — any remaining barium sulfide would decompose during calcination, and residual zinc sulfate would leave water-soluble impurities. After three washes, the supernatant should taste no different from pure water (test a tiny drop on the back of your gloved hand, not by mouth).

Pour the washed slurry through filter paper in a funnel. Allow to drain completely, then press the filter cake gently with a clean spatula to expel as much water as possible. The filter cake should be a uniform, chalky white mass — any grey or yellow discolouration indicates impurities.

Spread the filter cake on a clean ceramic tile or glass plate and dry at 100–110 °C in an oven for two hours, or air-dry overnight. The dried raw lithopone is a white powder with a slight yellowish tint. At this stage it has poor hiding power — the zinc sulfide crystals are amorphous and scatter light inefficiently. The critical calcination step transforms it into a useful pigment.

Break the dried cake into small pieces and grind to a coarse powder in a mortar. The powder should pass through a coarse sieve without difficulty. This pre-grinding ensures even heating during calcination — lumps would calcine unevenly, leaving uncalcined cores with poor opacity.

Transfer the ground powder into a clay refractory crucible, filling no more than two-thirds full. Place a lid on the crucible but do not seal — the calcination does not require an inert atmosphere, and gases from residual moisture need to escape. The crucible protects the powder from direct flame contact, which would cause local overheating and grey discolouration.

Place the crucible in a preheated kiln or furnace at 700–750 °C. Hold at this temperature for one hour. During calcination, the amorphous zinc sulfide crystallises into the hexagonal wurtzite structure, whose higher refractive index (2.37) gives lithopone its white opacity. The barium sulfate component is already crystalline and simply sinters slightly. Do not exceed 800 °C — the zinc sulfide begins to decompose above this temperature, releasing sulfur and darkening the pigment.

This is the step unique to lithopone manufacture. Using long-handled tongs, remove the hot crucible from the furnace and immediately pour the glowing contents into a large bucket of cold water. The thermal shock 'freezes' the wurtzite crystal structure in place and shatters the sintered mass into fine particles. Without this quench, the crystals would slowly revert to the less-opaque sphalerite form on cooling. The water hisses violently — stand back and wear a face shield.

Allow the quench water to cool, then decant. The white pigment settles at the bottom. Add fresh water, stir, and decant twice more to wash out any soluble material released during calcination. The quenched lithopone should be noticeably whiter and more opaque than the pre-calcined material — test by smearing a trace on dark paper.

Filter the washed pigment slurry through filter paper. Press gently to remove excess water. The filter cake should be a bright, clean white — significantly whiter than the pre-calcined material. Any yellow or grey tint means the calcination temperature was too high or too low.

Spread the filter cake on a clean surface and dry at room temperature or at low heat (below 100 °C) for 24 hours. Do not dry in direct sunlight — uncalcined trace zinc sulfide on the surface can darken under UV light (a phenomenon called 'lithopone disease' that plagued early manufacturers). Dry in shade or indoors.

Transfer the dried pigment to the mortar and grind to a fine, uniform powder. Lithopone grinds easily — it is softer than most mineral pigments. Pass through a 120-mesh sieve to remove any remaining coarse particles or sintered lumps. The finished powder should be a bright, clean white with good hiding power when rubbed out on dark paper.

Transfer the sieved lithopone to a clean, dry glass jar with a tight-fitting lid. Label with the pigment name (Lithopone, PW5), date, and composition (ZnS + BaSO₄). Store in a cool, dark place — away from UV light to prevent darkening. Expected yield from 30 g zinc sulfate and 20 g barium sulfide: approximately 25–30 g of finished pigment. Lithopone has excellent chemical stability and will not yellow, but should be protected from prolonged UV exposure in its pure powder form.