Alkalis

Leach hardwood ash as for soap, boil the lye down to dryness in iron pots — hence pot-ash — then calcine the residue at red heat to burn off organic matter, leaving white pearl ash, mostly potassium carbonate.

The yield is small: a great deal of wood gives little potash, which is exactly why it does not scale. Use ash from wood already burnt for fuel or clearing, and never cut timber for the ash alone.

You cannot control a process you cannot measure. Chop red cabbage, steep it in hot water, strain: the purple liquid turns red in acid, stays purple near neutral, and goes blue, green and then yellow as alkalinity rises.

Then titrate: dissolve a weighed sample of alkali in water with a few drops of indicator, and add an acid of known strength drop by drop — counted, or from a graduated tube — until the colour turns. The volume used tells you how much alkali the sample held. Keep a standard acid for this.

On a coast, barilla — ash of salt-tolerant plants such as saltwort and glasswort — and kelp give sodium carbonate rather than potassium. It is better than potash for glass.

Dry the plants, burn them slowly in a pit until the ash fuses into a hard grey mass, break it up and leach it. Test the product by titration; crude barilla varies widely and the good batches are worth keeping apart.

1. Salt + sulphuric acid → sodium sulphate (salt cake) and hydrogen chloride gas.
2. Salt cake + limestone + coal, roasted at around 1,000 °C → black ash.
3. Leach the black ash with water, and evaporate → soda ash.

It works, and it is achievable. It also produces large volumes of HCl gas — historically vented, killing vegetation for miles and driving the first industrial pollution laws. Absorb the gas in a tower packed with coke or stone with water trickling down, which captures most of it as useful hydrochloric acid. Plan this before the plant is built.

The leached black ash leaves calcium sulphide waste, historically dumped in heaps that released hydrogen sulphide — rotten-egg gas, deadly in enclosed or low-lying spaces, where it deadens the sense of smell before it kills — and leached foul liquors into streams.

Store the waste covered and away from water, in a lined, drained pit, and do not enter pits or low hollows where the smell has been strong. Later works recovered the sulphur from it. Sited and handled badly, the Leblanc works poisons its own valley.

Saturated brine + ammonia + carbon dioxide → sodium bicarbonate precipitates → heat it → soda ash. The ammonia is recovered with lime and recycled, and the process is cleaner, cheaper and less wasteful than Leblanc.

It needs tighter process control — gas absorption towers, temperature control, and ammonia recovery that works — and ammonia is a serious inhalation hazard in its own right. Reach for it once the Leblanc plant runs steadily and the measurement habit is established.

Boil a dilute solution of soda ash — around 10–15% — with slaked lime — see lime. Chalk precipitates, leaving sodium hydroxide in solution. Let it settle, decant the clear liquor, and concentrate it if needed.

Caustic soda is what makes hard bar soap — see soap — and chemical pulping for paper. It absorbs water and carbon dioxide from air, so store it sealed in glass, iron or thick plastic — never aluminium, which it dissolves while releasing hydrogen.

Strong alkali is at least as damaging to eyes as acid, and often worse: it keeps penetrating tissue after contact, and it does not hurt immediately, so exposures get ignored until the damage is done.

Eye protection whenever alkali is handled. Alkali in the eye: flush with running water for at least 30 minutes, holding the eyelids open, and keep going while help is fetched. On skin: flush until the soapy, slippery feeling is completely gone.