Lime

Limestone, chalk, marble, seashells and coral are all calcium carbonate. The test: a drop of vinegar fizzes on a freshly broken surface; any weak acid will do. Dolomite fizzes weakly, and only when powdered.

Break stone to fist size — about 10–15 cm — and keep the sizes even. Large pieces keep a raw, unburnt core, and small pieces and dust choke the airflow in the kiln. Shells can be burnt whole.

A lime kiln is the pottery kiln scaled up: a vertical stone or brick shaft with a draw hole at the base. Load alternating layers of fuel and stone, or build a fuel-fired arch at the bottom with stone stacked above it. A continuous shaft kiln is drawn from the bottom and recharged from the top, so it runs for weeks. See the lime kiln.

Build it into a bank where you can, so stone can be tipped in at the top from level ground.

The limestone gives off carbon dioxide and becomes quicklime — calcium oxide. It needs about 900–1,000 °C — a bright orange heat — and time, because the reaction works inward from the surface. Fist-sized stone needs a day or more at full heat.

Test a burnt lump: it should be noticeably lighter — pure limestone loses 44% of its weight as CO₂ — and break to show white or pale grey throughout. A darker, harder core that still fizzes is under-burnt and wasted. Burnt lime reacts with a hiss and heat when water is dripped on it.

Quicklime reacts with moisture in the air and slakes itself, crumbling to powder and losing its value. Store it dry, covered and off the ground, and slake it within weeks.

That same reaction makes heat: damp quicklime stored against timber, straw or sacks can set them alight. Keep it in stone, brick or metal, away from buildings with thatch — ships carrying lime have burned from rainwater getting into the cargo.

The most immediately dangerous routine operation in the tier. Quicklime and water release a violent amount of heat; the mix boils, spits and blinds.

Eye protection, gloves, covered arms, standing upwind. Put water in the tub first — about 3 parts water to 1 part quicklime by volume — and add the lime to the water, a shovel at a time, never water onto lime. Stir with a long hoe, keeping your face back. Add more water if it starts to dry out and steam hard. Sieve out unburnt lumps.

Lime in the eye: flush with running water for at least 20 minutes, and keep going while someone gets help. Lime burns are permanent to eyes.

Run the slaked lime into a pit or covered tub and keep a layer of water over it so air cannot reach it. Leave it at least 3 months — longer is better. It becomes smooth, buttery and more workable, and any slow-slaking particles finish hydrating, which prevents them from swelling and popping out of plaster later.

Lime putty kept under water keeps indefinitely. Label each pit with the burn date.

Mortar and plaster: about 1 part lime putty to 3 parts clean, sharp sand, beaten together until the mix holds on a vertical trowel. Whitewash: putty thinned with water to the consistency of milk, applied in 3–5 thin coats rather than one thick one.

Lime sets by absorbing CO₂ from the air — slowly, over months and years. It is not cement. Keep work damp and shaded for the first week or two, protect it from frost and rain, and do not load a wall early. Lime mortar is self-healing in small cracks and lets damp walls dry out, which is why it outperforms cement on earth and old stone.

• Whitewash interior walls — it is antibacterial and brightens dark rooms.
• Dehairing hides in tanning.
• Flux in smelting.
• Correcting acid soil: agricultural lime spread at a few tonnes per hectare raises pH over months and can raise yields sharply on strongly acid ground.
• Stabilising latrine waste and disinfecting after an outbreak.

Limestone containing 10–20% clay gives hydraulic lime, which sets underwater and is the precursor to cement. See the lime guide.