Where does the sodium carbonate Senku wants to extract from seaweed in Dr Stone come from?

Sodium carbonate (Na₂CO₃) is found in alkaline lakes (Egyptian natron) and in the ash of burned halophyte plants: glasswort, saltwort, kelp. Humanity long extracted it by leaching, making it, alongside potash, one of the two great bases of pre-industrial chemistry. Since 1900, the Solvay process has synthesized it from sea salt and limestone.

Substance chemistry

Sodium carbonate in Dr Stone S1E2

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While listing the uses of calcium carbonate, Senku mentions soapmaking: “Lime, mixed with sodium carbonate from seaweed and oil, gives you a fatty substance.” The reference flies by, but it introduces the second alkaline base essential to Senku’s chemistry. Sodium carbonate, commonly known as soda, is the ingredient that will let him break down fats to make hard soap, a cornerstone of hygiene and survival in a world without antibiotics.

The mention stays theoretical in S1E2. The actual synthesis of soap comes several episodes later, when Senku collects seaweed on the shore of Kohaku’s village.

The subject in depth

The molecule: two sodium cations for one carbonate anion

Sodium carbonate has the formula Na₂CO₃. It’s an ionic salt made of two sodium cations Na⁺ and one carbonate anion CO₃²⁻. Pure and anhydrous, it’s a white, odorless powder, strongly basic in aqueous solution ^[Wikipedia]. A 1% solution gives a pH of around 11.4, making it a moderately strong base, usable bare-handed with care, but already caustic enough to saponify fats and dissolve proteins.

Sodium carbonate also exists in several hydrated forms: the monohydrate Na₂CO₃·H₂O, the heptahydrate Na₂CO₃·7 H₂O, and above all the decahydrate Na₂CO₃·10 H₂O, better known as natron or washing soda crystals. It’s under this last form that it most commonly occurs in nature, as whitish crystals deposited along the shores of certain alkaline salt lakes.

Na₂CO₃’s basicity comes from the carbonate anion, which can capture a proton from water to form hydrogen carbonate ion and release a hydroxide ion:

CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻

It’s this release of OH⁻ that makes the solution basic and allows it, at sufficient concentration, to attack triglycerides and produce soap. But Na₂CO₃ isn’t strong enough on its own to fully saponify: classic chemistry proceeds in two steps, first converting sodium carbonate into sodium hydroxide (NaOH, caustic soda) by reacting it with slaked lime. This is causticization, and it’s exactly what Senku sets up in E02 when he pairs lime with sodium carbonate.

Three natural sources, three histories

Humanity knows three major sources of sodium carbonate before the industrial era.

Evaporite alkaline lakes. In arid climates, some salt lakes concentrate their waters through evaporation and precipitate natron crystals along their shores. The most famous is Wadi El Natrun, the “valley of natron,” northwest of Cairo, exploited since the 3rd millennium BCE. The Egyptians used it for mummification (absorbing moisture from tissue), glassmaking, ritual cleansing, and laundering ^[Wikipedia]. The alkaline lakes of the East African Rift (Magadi, Bogoria), Owens Lake and Searles Lake in California, and the massive trona deposits of Wyoming (the largest known reserve, 24 billion tonnes) are the other major examples.

Land-based halophyte plants. Plants growing along coastlines or on salty soils concentrate sodium in their tissues as osmotic regulation. Burned in the open, they leave an alkaline ash rich in sodium carbonate. Glasswort (Salicornia europaea), sea blite (Suaeda maritima), and saltwort (Salsola soda) supplied Southern Europe and the Levant with plant soda for centuries. Spain, from the 16th to 18th century, became the world’s leading exporter under the name barilla, grown to be burned and exported in blocks to France, Italy, and England. The industry collapsed with the arrival of the Leblanc process.

Kelp and marine algae. Along northern coastlines (Brittany, Scotland, Norway) seaweed gatherers burned large kelp fronds to produce kelp ash, an alkaline residue mixing sodium carbonate with potassium carbonate, plus iodine (the raw material that let Bernard Courtois discover iodine in 1811). These are precisely the algae Senku mentions in S1E2 when he talks about “sodium carbonate from seaweed.” The route is historically attested, but it was never dominant: land halophytes and evaporite natron offered far better yields ^[Livre].

From the Leblanc process to the Solvay process

Over the course of the 18th century, European demand for soda explodes, driven by glassmaking, soap, and papermaking. Natural sources become insufficient. Louis XVI launches an academic competition in 1775 to find an industrial route. Physician Nicolas Leblanc wins the prize in 1791 with a two-step process: sea salt + sulfuric acid → sodium sulfate + hydrochloric acid, then sodium sulfate + coal + limestone → sodium carbonate + calcium sulfide + carbon dioxide. The Leblanc process brings soda to large-scale production, but releases enormous quantities of hydrochloric acid and calcium sulfide, which become major industrial pollutants.

In 1864, Belgian engineer Ernest Solvay proposes an alternative process ^[Wikipedia]. His key reaction is elegant:

NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl

The precipitated sodium bicarbonate is then heated to give Na₂CO₃ + H₂O + CO₂. The ammonia is regenerated by reacting the ammonium chloride with slaked lime, which closes an almost complete loop. Yield is 75%, and the production is clean and economical. The Solvay process supplants the Leblanc process from 1900 on and remains, with only minor variants, the dominant industrial process today. World production stands at around 60 million tonnes a year, half of which goes into glassmaking, a quarter into soaps and detergents, and the rest into specialty chemistry ^[Livre].

In real life

The use of sodium carbonate long predates its chemical identification. The Egyptians of the 3rd millennium BCE extracted natron from Wadi El Natrun for mummification: this salt absorbs water from the corpse and inhibits bacterial decay, combined with a 40-day drying process. The Mesopotamians, from the 2nd millennium BCE on, master an alkaline glass melted around 1500 °C whose exact composition (silica + natron + lime) is documented in the cuneiform tablets of Ashurbanipal. Pliny the Elder, in his Natural History, recounts the Phoenician legend of glass’s accidental discovery by sailors who supposedly set their cooking pots on blocks of natron on a beach.

In China, the Han dynasty (2nd century BCE) exploits alkaline lakes in Tibet and Qinghai for glassmaking and dyeing. In India, sajji-mati (impure sodium carbonate gathered from the arid soils of Rajasthan and Gujarat) is used for washing textiles and tanning. In the Maghreb, Tuareg nomads exploit sebkhas, briny evaporation depressions, as a combined source of salt and soda.

Medieval Christian Mediterranean societies mainly used Spanish plant soda and Levantine soda (imported from Egypt and Lebanon). In Venice, the supply contract for Alexandrian soda was one of the most strategic held by the Republic, which depended on it for its Murano glass industry. Marseille soap, formalized by Louis XIV’s edict of 1688, requires 72% olive oil and the remainder in Mediterranean plant soda, until the Leblanc process industrializes production in the 19th century ^[Web].

Current world production, mostly from the Solvay process and direct trona mining in Wyoming, exceeds 60 million tonnes a year. The glass industry absorbs half of it, the chemical and detergent industry the rest. Sodium carbonate remains one of the ten most-produced industrial chemicals in the world.

The fact-checker’s eye

The fact-checker's eye

Senku shortens the chemistry slightly when he says “sodium carbonate from seaweed.” In the historical reality of artisanal soapmaking, two distinct sources of alkaline bases coexisted and competed: plant soda (sodium carbonate from halophytes or algae, giving hard soaps) and potash (potassium carbonate from ordinary wood ash, giving soft soaps). In the Mediterranean, soda dominates, because the arid climate favors coastal halophytes and limits wood supply. In Northern Europe, potash dominates, because forests are abundant there. Marseille soap follows the soda route; the liquid black soap of the North follows the potash route.

Marine algae, as Senku mentions, did supply soda in some coastal regions of Northern Europe (Scottish and Breton kelp from the 17th to 19th century), but they were never the world’s main source. For a Paleolithic team on a Japanese coastline, the approach is technically defensible: Laminaria and other large Japanese brown algae, when burned, can yield a usable alkaline ash. But the yield is far lower than land halophytes or natron. To recreate the experiment today in chemistry class, you’d more likely use wood ash (potash) or store-bought sodium carbonate.

Try it at home

⚠️ Caution: to be done outdoors, with an adult.

Extracting an alkaline ash by leaching

Materials: 200 g of cold ashes from a clean wood fire (do not use barbecue ashes with fat residue), 1 liter of distilled water, a plastic container, a coffee filter or fine cloth, a food thermometer, gloves and goggles.
Procedure: heat the water to 60 °C. Add the ashes while stirring. Let it steep for 24 h, covered. Filter the resulting yellow-brown solution: this is wood-ash lye, rich in potassium carbonate and potassium hydroxide. Its base concentration is modest but real.
pH test: dip a strip of pH paper into it. The solution should turn deep blue, a sign of pH above 11, enough to kick off artisanal saponification, the founding gesture of black soap.
Precautions: avoid prolonged skin contact (redness possible), rinse thoroughly if splashed. Do not heat the lye unattended. Store in a labeled plastic bottle, out of children’s reach.
Note: to reproduce the “plant soda” route rather than the “wood potash” route Senku uses in S1E2, you would need glasswort or seaweed ash, which is hard to source away from a coastline. The underlying physical chemistry is identical.

Going further

Calcium carbonate: the complementary alkaline base in saponification
Soap: a direct application with fats and bases
Discipline: Chemistry: all chemistry entries on the site
Coming soon: Glass: another major historical use of sodium carbonate
Coming soon: The Leblanc process: the industrial ancestor of the Solvay process

Glossary

Natron ↗: An evaporite mineral mainly composed of hydrated sodium carbonate decahydrate, mined from alkaline salt lakes.
Halophyte ↗: A plant adapted to salty soils or waters, which concentrates sodium in its tissues. When burned, it yields an ash rich in sodium carbonate.
Leaching ↗: An extraction technique by dissolving a solid solute in a solvent percolated through the mixture.
Solvay process ↗: An industrial process developed in 1864 by Ernest Solvay to produce sodium carbonate from sea salt, limestone, and ammonia.

Sources

Sodium Carbonate, French Wikipedia ^[Wikipedia]
Solvay Process, French Wikipedia ^[Wikipedia]
Natron, French Wikipedia ^[Wikipedia]
Robert P. Multhauf, The Origins of Chemistry, Oldbourne, 1966 ^[Livre]
Musée du Savon de Marseille, The Marseille process and its historical ingredients ^[Web]
Lewis Dartnell, The Knowledge, The Bodley Head, 2014 ^[Livre]

Published on May 12, 2026

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Glossary

NatronAn evaporite mineral mainly composed of hydrated sodium carbonate decahydrate (Na₂CO₃·10 H₂O), mined from alkaline salt lakes. Used since ancient Egypt for mummification, glassmaking, and cleaning.

HalophyteA plant adapted to salty soils or waters, able to concentrate sodium in its tissues. When burned, halophytes (glasswort, saltwort, kelp) leave an alkaline ash rich in sodium carbonate, historically known as plant soda.

LeachingAn extraction technique by dissolving a solid solute in a solvent percolated through the mixture. Poured over alkaline ashes, water dissolves the sodium carbonate and leaves the insoluble residue behind.

Solvay processAn industrial process developed in 1864 by Ernest Solvay to produce sodium carbonate from sea salt, limestone, and ammonia. Became the dominant process from 1900 on, supplying around 60 million tonnes a year today.