Making Soda Ash by the Leblanc Process — The Chemical That Made Soap and Glass Affordable

Set up inside a functioning fume hood — this process produces large volumes of hydrochloric acid gas, one of the most corrosive gases encountered in chemistry. Wear a respirator with acid gas cartridges, chemical splash goggles, heavy-duty nitrile gloves, and a full lab coat. Concentrated sulfuric acid causes immediate severe burns on skin contact. Have a large bucket of water available for acid spills. The furnace step later requires heat-resistant gloves. All three stages of the Leblanc Process involve serious hazards — maintain full PPE throughout.

Weigh 20 g of sodium chloride (table salt, NaCl). Use fine-grained salt, not rock salt — a larger surface area reacts faster with sulfuric acid. Transfer the salt to a round-bottom flask fitted with a side-arm adapter or a bent glass tube leading into a beaker of water. The tube will carry the hydrochloric acid gas away from you and absorb it in water (producing hydrochloric acid solution as a useful byproduct — exactly as Leblanc's successors learned to capture this former pollutant).

Carefully pour 10 ml of concentrated sulfuric acid (96% H₂SO₄) onto the salt in the flask. The reaction begins immediately at room temperature with visible fizzing and white fumes of HCl gas: NaCl + H₂SO₄ → NaHSO₄ + HCl↑. At this stage, only one chloride is displaced, producing sodium bisulfate. The white, choking fumes are hydrochloric acid gas — in early Leblanc works, this gas was simply vented into the atmosphere, destroying crops and corroding buildings for miles downwind until the Alkali Act of 1863 forced its capture.

Heat the flask gently, raising the temperature gradually to 150–200 °C. At higher temperature, the second chloride is displaced: NaCl + NaHSO₄ → Na₂SO₄ + HCl↑. Vigorous white fumes pour from the flask as the remaining HCl is driven off. Continue heating until the fuming stops completely — this indicates all the hydrochloric acid has been expelled and only sodium sulfate remains. The process takes 30–45 minutes. The residue in the flask is 'salt cake' — a white, powdery mass of anhydrous sodium sulfate (Na₂SO₄).

Allow the flask to cool to room temperature. The salt cake is a hard, white, crusty mass adhering to the flask walls. Scrape it out with a glass stirring rod or spatula. If it is firmly stuck, add a small amount of water to dissolve the surface layer and pour out — then dry the recovered salt cake at 100 °C. Expected yield from 20 g NaCl: approximately 22–24 g of sodium sulfate. The salt cake feels gritty and slightly chalky — quite different from the original table salt.

Transfer the salt cake to a porcelain mortar and grind to a fine powder. Thorough grinding ensures intimate mixing with the charcoal and limestone in the next stage — the solid-state reaction at high temperature proceeds faster when particle sizes are small and contact between the three components is maximised. The powdered salt cake is white and odourless.

Weigh 5 g of hardwood lump charcoal and crush to small pieces (3–5 mm) in the mortar. The charcoal serves as the reducing agent — at high temperature, carbon reduces sodium sulfate to sodium sulfide: Na₂SO₄ + 2C → Na₂S + 2CO₂. This is the critical chemical transformation — converting the stable, unreactive sulfate into the reactive sulfide that can then exchange partners with limestone. Use hardwood charcoal, not activated charcoal — the latter is too finely divided and burns away before the reaction completes.

Weigh 20 g of crushed limestone (calcium carbonate, CaCO₃). The limestone plays the decisive role in the final reaction: Na₂S + CaCO₃ → Na₂CO₃ + CaS. Sodium sulfide swaps its sulfur for the carbonate of the limestone — producing the desired sodium carbonate (soda ash) and calcium sulfide (the infamous 'tank waste' that plagued every Leblanc works). Use limestone crushed to pieces of 5–10 mm — finer if available.

Combine the ground salt cake, crushed charcoal, and crushed limestone in a mortar or bowl. Mix thoroughly — every grain of salt cake should be in contact with both charcoal and limestone. This three-component mixture is the charge for the 'black ash furnace' — the heart of the Leblanc Process. In the industrial process, these ingredients were mixed in a reverberatory furnace and raked continuously by workers in brutal heat.

Pack the mixture firmly into a deep clay crucible. Press down to ensure good contact between the components — the solid-state reactions require intimate mixing. The crucible must be refractory (fireclay or graphite-clay) rated to at least 1000 °C. Leave 1–2 cm of space at the top — the charge will sinter and fuse but does not expand significantly. Place a loose-fitting lid on the crucible to retain heat while allowing CO₂ to escape.

Place the crucible in a charcoal furnace and bring to bright orange heat — approximately 900–1000 °C. At this temperature, two reactions proceed simultaneously: the carbon reduces sodium sulfate to sodium sulfide, releasing carbon dioxide (Na₂SO₄ + 2C → Na₂S + 2CO₂), and the sodium sulfide immediately reacts with the limestone to form sodium carbonate and calcium sulfide (Na₂S + CaCO₃ → Na₂CO₃ + CaS). Maintain full heat for at least one hour. The charge fuses into a dark, sintered mass — the 'black ash' that gives this stage its name.

Remove the crucible from the furnace using long-handled tongs and set it on a heatproof surface to cool. The cooling takes 2–3 hours — do not quench with water, as thermal shock will crack the crucible. The black ash cools to a dark grey-black, hard, sintered mass with a characteristic sulfurous smell (from the calcium sulfide). This mass contains the sodium carbonate mixed with calcium sulfide — the task now is to separate them.

Break the black ash out of the crucible and crush to small pieces in the mortar. The material is hard and britite — it fractures rather than crumbles. Crush to pieces of 5–10 mm to maximise the surface area for leaching. The black colour comes from residual carbon; the grey from calcium sulfide and sodium carbonate. The smell of hydrogen sulfide (rotten eggs) may be noticeable — calcium sulfide reacts slowly with atmospheric moisture to release H₂S.

Transfer the crushed black ash to a glass beaker and add 200 ml of distilled water. Bring to a gentle boil and stir for 20 minutes. Sodium carbonate is highly soluble in hot water (about 450 g/L at 100 °C) and dissolves readily. Calcium sulfide is only sparingly soluble and remains as a grey-green sludge at the bottom — this is the 'tank waste' that the Leblanc alkali industry dumped in enormous heaps around their works, creating a characteristic stench of hydrogen sulfide that could be smelled for miles.

Filter the hot solution through filter paper in a glass funnel into a clean collection beaker. The pale yellow-green filtrate is a solution of sodium carbonate — crude soda liquor. The grey residue on the filter is calcium sulfide mixed with unreacted charcoal and limestone. Discard the residue carefully (it releases H₂S in contact with acids). The filtrate should be clear or only slightly coloured. If cloudy, filter a second time.

Pour the filtrate into a porcelain evaporating dish and heat gently to evaporate the water. Do not boil vigorously — gentle evaporation produces larger, purer crystals. As the volume reduces, white crystals of sodium carbonate decahydrate (washing soda, Na₂CO₃·10H₂O) begin to form around the edges. Continue until most of the water has evaporated and the dish is filled with a mass of white crystals. These can be heated further to 100 °C to drive off the water of crystallisation, converting them to the anhydrous soda ash (Na₂CO₃) — a white powder.

Scrape the crystals from the evaporating dish and spread on a clean surface to dry at 100 °C for 2 hours. The finished product is a white, slightly gritty powder — soda ash. Test by dissolving a pinch in water: the solution should be strongly alkaline (pH 11–12 on indicator paper) and feel slippery between the fingers — the characteristic 'soapy' feel of sodium carbonate that made it essential for laundering and cleaning for centuries. Expected yield from 20 g salt: approximately 12–16 g of soda ash.

Transfer the dried soda ash to a glass jar with a tight-fitting lid. Label with the product name (Sodium Carbonate / Soda Ash, Na₂CO₃), date, and source (Leblanc Process from NaCl + H₂SO₄). Store sealed — sodium carbonate absorbs moisture from the air and will cake if left open. The Leblanc Process made this essential chemical available in bulk for the first time, slashing the price of glass, soap, and paper — and in doing so launched the modern chemical industry. Its inventor Nicolas Leblanc saw none of the profit: the Revolution confiscated his factory in 1794, and he took his own life in 1806.