Continuous draw lime kiln

Burn limestone into quicklime — mortar, plaster, whitewash, flux, tannery liquor and soil correction, from one shaft.

Build Tier 2 · Bronze and Iron Age Materials Hard — and genuinely dangerous 20–30 person-days Crew 4–6 to run continuously

Home: Binders, Level 2 · Forge, level 1 of 2 in its track. Serves 3 steps.

Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.

**A stone draw kiln cut into a bank — mortar, plaster and clean water all start here.** Lime Kiln in Churston Woods, Brixham · 2017-03-11 11:58:59 · JLParton · CC0 · Commons ↗

What it is and why this shape

Lime is the highest-leverage substance available in this tier and it is nearly always built too late. One kiln gives you mortar that sets underwater, plaster, whitewash that genuinely disinfects, flux for smelting, the dehairing liquor for tanning, soil correction for acid fields, and the feedstock for cement later.

The chemistry is one line: heat calcium carbonate above about 900 °C and it gives off carbon dioxide, leaving calcium oxide — quicklime. Add water and it violently becomes calcium hydroxide, which slowly reabsorbs CO₂ from the air and turns back into stone. That last step is what makes lime mortar work, and it is why lime mortar takes months to gain strength rather than hours.

The difficulty is not the chemistry. It is that this must be held hot, for a long time, continuously — a batch kiln wastes most of its fuel heating the structure back up every cycle. A draw kiln is charged at the top and drawn at the bottom, running for weeks. That is a logistics commitment as much as a construction one.

Read the safety notes before building. Quicklime and the gas from this kiln both injure people badly and neither gives much warning.

Section through a mixed-feed draw kiln Fig 1

Earth / subsoilMasonry / brickTimberLoose fill / grainGravel / aggregateFire / hot zoneVoid / air

The three zones are not built in — they establish themselves in a shaft of the right proportions. The batter matters: the shaft must widen towards the base or the charge arches, hangs, and then collapses into the burning zone all at once.

Slaking — the dangerous step Fig 2

Loose fill / grainWaterFire / hot zone

Quicklime plus water releases a great deal of heat very fast: the mix boils, spits and can reach temperatures that ignite nearby wood. Add lime to water, never water to lime. Eye protection is not a suggestion here — alkali burns to the eye are not recoverable at this tier.

Keyed parts

1	Charging platform and mouth	Alternating measured charges of stone and fuel. Access must be safe — people work here in the dark.
2	Preheating zone	Incoming stone is dried and warmed by the exhaust before it reaches the fire. This is where the efficiency comes from.
3	Calcining zone, 900–1,100 °C	Where the reaction happens. Roughly the middle third of the shaft.
4	Cooling zone	Burnt lime gives its heat back to incoming combustion air on the way down. The second half of the efficiency.
5	Shaft lining, firebrick	Erodes fastest in the calcining zone. Design it to be relined without rebuilding the outer shell.
6	Outer shell and binding	Stone or brick with iron bands or buttresses. The shaft pushes outward hot.
7	Draw arch and eyes	Burnt lime is raked out here. Usually two or three eyes round the base so drawing is even.
8	Air inlet	Combustion air enters at the base and is preheated by the descending lime.
9	Batter — shaft widens downward	So the charge descends by gravity without jamming. A parallel shaft hangs and then drops.

Specifications

Shaft height	3.5 m	Tall enough for the three zones to separate. Shorter wastes fuel
Bore at top / base	0.90 m / 1.40 m	The batter is what keeps the charge moving down under gravity
Lining	Firebrick, 230 mm	Designed to be relined independently of the outer shell
Calcining temperature	900–1,100 °C	Below 900 the core stays raw; much above 1,200 the lime deadburns and will not slake
Fuel : stone ratio	≈ 1 : 4 to 1 : 6 by weight	Depends heavily on fuel. Coal, coke and charcoal all work; wood is bulky and marginal
Stone size	60–120 mm, graded	Uniform size is more important than absolute size — mixed sizes pack and stop the draught
Residence time	18–30 h top to bottom	Sets the draw rate
Output	0.5–1.0 t/day	Continuous once lit; the kiln runs for weeks
Yield	≈ 55% of stone weight	The rest leaves as carbon dioxide — that mass loss is the reaction
Draw eyes	2–3 round the base	Draw a little from each in turn, never all from one

Bill of materials

Material	Quantity	Notes and substitutes
Limestone, chalk or shell	continuous supply	Tests by fizzing in vinegar. Shell and coral work identically. Broken to a uniform 60–120 mm.
Firebrick	≈ 1,400	For the lining. Ordinary brick will not survive the calcining zone.
Stone or common brick	≈ 12 m³	Outer shell.
Clay mortar	≈ 1.5 m³	Clay and sand. Not lime mortar, and definitely not cement.
Iron binding bands	3–4	Or heavy stone buttresses. The shaft expands and pushes outward when hot.
Fuel	continuous	Coal, coke or charcoal. Wood works but its bulk disrupts the descending column.
Cast-iron or stone lintels	2–3	Over the draw arches, or turn proper arches.

Tools: Long draw rake · Barrow · Screen for grading stone · Long-handled shovel · Eye protection and long gloves — mandatory

Construction

Site it into a bank if you possibly can

The ideal site lets you charge at the top from ground level and draw at the bottom from ground level — a bank, a quarry face or a hillside. Otherwise you need a ramp or a hoist and every tonne is carried up. Site it downwind of the settlement: this kiln produces carbon dioxide and, if it runs rich, carbon monoxide.

✓ Check: Charging and drawing are both at working height without a ladder.

Foundation and the base structure

A kiln full of stone is very heavy and it runs for weeks. Build a substantial foundation and get the draw arches right, because everything above is supported on them and they are the hardest part to repair later.

✓ Check: Arches turned true, and rake access clear at every draw eye.

Build the lining with the batter

Raise the firebrick lining, widening steadily from 0.9 m at the top to 1.4 m at the base. Work to a template — the batter is easy to lose by eye and it is the single feature that keeps the charge descending freely.

✓ Check: Template fits at every course; no parallel or waisted sections.

Outer shell and binding

Build the outer shell with a gap or a rubble/ash layer between it and the lining, so the lining can expand and be replaced later without demolishing the shell. Fit the iron bands or buttresses as you go up.

✓ Check: Lining is not bonded to the shell anywhere.

Dry it thoroughly, then warm it for days

This is a large mass of masonry holding a lot of water. Air-dry for weeks, then run small fires for two or three days, raising the temperature slowly. A structure this size heated quickly will crack right through.

✓ Check: No steam anywhere on the outer shell at moderate heat.

Light it and build the zones

Charge with fuel and stone and light from the base. Over the first day or so, build the fire until the calcining zone establishes itself in the middle of the shaft. You will know it has by what comes out of the draw eyes.

✓ Check: First lime drawn is friable, light for its size, and slakes vigorously in water.

Charge and draw on a rhythm

Now run continuously: a measured charge of stone and fuel at the top on a fixed interval, and a measured draw at the bottom on the same interval. The rhythm is the control system. Draw a little from each eye in turn so the charge descends evenly rather than channelling down one side. Record every charge and draw.

✓ Check: Level at the top falls consistently between charges — no hanging, no sudden drops.

Test the product continuously

Take a lump from every draw and drop it in water. Good quicklime cracks, hisses, steams and crumbles within a minute or two. If it just sits there, the stone is either underburnt — a raw core, so increase residence time or fuel — or deadburnt from too much heat, which is unslakeable and cannot be fixed. Break lumps open and look: a discoloured core means underburnt.

✓ Check: A representative lump slakes vigorously and breaks with a uniform texture right through.

Slake safely

Add quicklime to a deep bath of water, never water to a heap of quicklime. Stand upwind. Wear eye protection and long gloves. The reaction boils, spits caustic slurry and gets hot enough to ignite adjacent timber. Then run the putty off into a pit and keep it under water.

✓ Check: Nobody has been splashed, and the slaking pit is well clear of anything that burns.

Mature and store

Lime putty improves for months under water and becomes far better mortar for the wait. Store quicklime that you are not slaking yet in a sealed, dry container — it slakes itself on damp air and is then wasted. Label it clearly: to anyone who does not know, quicklime looks like an ordinary white rock.

✓ Check: Putty stored under standing water; quicklime sealed, dry, and unmistakably labelled.

Tolerances that matter

Dimension	Target	Why it matters
Shaft batter	0.9 m → 1.4 m over 3.5 m	Too little and the charge hangs then collapses into the fire; too much and it descends uncontrolled.
Stone grading	60–120 mm, uniform	Mixed sizes pack the voids and choke the draught, which stalls the whole kiln.
Calcining temperature	900–1,100 °C	Below, raw cores. Above ~1,200 °C the lime deadburns and will not slake at all.
Charge/draw interval	constant	The zones are held in place by a steady descent rate; irregular working moves them.
Lining to shell	unbonded, with a gap	Differential expansion will otherwise split the outer shell.

How this fails

Water added to quicklime. Boiling caustic slurry thrown into faces. This is the injury this build is most likely to cause and it is entirely preventable by the order of operations.

Carbon monoxide at the charging mouth. A lime kiln consumes oxygen and can run rich. People have been killed leaning over kiln tops and sheltering against warm kilns overnight. Never work the top alone, never sleep near a running kiln.

Deadburning. Overheating fuses the surface and the lime will not slake. Unlike underburning, it cannot be corrected by re-firing.

Charge hanging, then dropping. A hung charge leaves a void; when it collapses it can blow flame and dust out of the draw eyes at the person standing there.

Storing quicklime damp. It slakes itself, heats, and has started fires in stores. Sealed and dry, or slake it immediately.

Unlabelled quicklime. It looks like chalk. Children and animals have been badly hurt by it.

Relining requires demolition. If the lining was bonded to the shell, the first reline becomes a rebuild.

How you know it worked

VerificationThe kiln has run continuously for two weeks at a steady draw, product slakes vigorously from every draw, and mortar made from the resulting putty has set hard in a test wall over three months.

Scaling and variants

Smaller and far simpler: a flare kiln — a pit or a low shaft where limestone is stacked over an arch and a wood fire is burnt beneath it for a day or two — is batch-operated, much less efficient, and needs a fraction of the construction. For occasional small quantities of mortar it is the right choice, and it is how you should learn to judge burnt lime before committing to a draw kiln.

Larger: the next real step is a mechanically drawn kiln with forced draught, which is a nineteenth-century industrial plant rather than a settlement structure.

Same shaft, hydraulic lime: burn a limestone that naturally contains 10–20% clay and you get lime that sets underwater — a substantially better material for foundations, tanks and anything wet, and the direct precursor to cement. If any local limestone is visibly impure or marly, test it for this before assuming it is poor stone.

The same thing, built

**A twin-eyed limekiln bank — charged from the top of the slope, drawn from the arches below.** Limekilns near Little Argyll · 2008-01-02 · Trevor Littlewood · CC BY-SA 2.0 · Commons ↗