Lime Putty

Part of Lime & Cement

Preparing, maturing, and storing lime putty for mortar, plaster, and limewash.

Why This Matters

Lime putty is the refined, workable form of slaked lime — the material that actually goes into mortar, plaster, and limewash. While quicklime is a raw, dangerous chemical, lime putty is a smooth, creamy paste that can be stored for years and improves with age. The transformation from lumpy, freshly slaked lime to silky putty is where chemistry meets craft.

Getting this step right determines the quality of everything built with lime. Poorly prepared putty produces grainy mortar that cracks and crumbles. Well-matured putty creates mortar and plaster so smooth it can be polished to a marble-like finish. Medieval builders understood this — many specifications called for lime putty aged a minimum of one year, and some cathedral projects maintained lime pits that had been maturing for decades.

In a rebuilding scenario, establishing a lime putty supply early is a strategic decision. Start a lime pit as soon as you have a kiln, and by the time you need mortar for permanent buildings, you’ll have well-aged material ready to use.

The Chemistry of Lime Putty

When quicklime (calcium oxide, CaO) is combined with water, an exothermic reaction produces calcium hydroxide (Ca(OH)₂):

CaO + H₂O → Ca(OH)₂ + heat

This reaction generates temperatures up to 300°C and produces a thick, paste-like suspension of extremely fine calcium hydroxide crystals in water. This paste is lime putty.

What Happens During Maturation

Freshly slaked lime contains relatively large, irregular crystals of calcium hydroxide. Over time, these crystals undergo a process called Ostwald ripening:

1. Smaller crystals dissolve preferentially
2. The dissolved material deposits on larger crystals
3. Crystal shape becomes more regular (hexagonal plates)
4. Particle size becomes more uniform
5. The putty becomes progressively smoother and more plastic

Age	Crystal Character	Workability
Fresh (< 1 week)	Large, irregular, gritty	Poor — rough texture
1-3 months	Mixed sizes, still some grit	Acceptable for rough work
3-12 months	Smaller, more uniform	Good — smooth and plastic
1-5 years	Fine, hexagonal plates	Excellent — buttery texture
5+ years	Very fine, highly uniform	Superior — polishing quality

Preparing Lime Putty

Method 1: Pit Slaking (Traditional)

This is the oldest and most reliable method, producing large quantities of high-quality putty.

1. Dig a pit — Excavate a pit approximately 1m deep, 1-2m wide. Line with clay, stone, or wooden planks to prevent soil contamination. A stone-lined pit is permanent and reusable.

2. Fill with water — Add clean water to about half the pit depth.

3. Add quicklime — Wearing full protective equipment, add lumps of quicklime to the water gradually. Use a long-handled rake or hoe to push lumps under the surface. The ratio is roughly 1 part quicklime to 2-3 parts water by volume.

4. Stir vigorously — As the lime slakes, stir continuously to break up lumps and ensure complete hydration. The mixture will boil violently and produce clouds of steam. Keep stirring until the reaction subsides.

5. Screen the slurry — While still liquid, pour or ladle the slurry through a coarse screen (6-10mm mesh) into a second pit or container. This removes un-burned limestone, over-burned cores, and other impurities.

6. Allow to settle — Let the screened putty settle for 24-48 hours. Excess water will rise to the top.

7. Maintain water cover — Always keep at least 50mm of water over the surface. This prevents carbonation (hardening) and allows the putty to mature.

Safety During Slaking

Slaking is the most dangerous step in lime work. The reaction is violently exothermic. Boiling water and caustic lime can spatter over a wide area. Stand upwind. Wear goggles, heavy gloves, long boots, and full-coverage clothing. Keep vinegar nearby to neutralize lime burns on skin.

Method 2: Container Slaking (Small Batches)

For smaller quantities, use a large wooden barrel, metal drum, or stone trough.

1. Fill the container one-third full with water
2. Add quicklime slowly — no more than a few shovelfuls at a time
3. Stir with a long wooden paddle after each addition
4. Allow to react fully before adding more
5. Once all lime is added and reaction has subsided, cover and allow to settle
6. Screen through mesh into storage containers

Method 3: Drowning Method

For very pure, fine putty:

1. Use a large excess of water — 4-5 parts water to 1 part quicklime
2. This dilute slurry keeps temperatures lower and produces finer crystals
3. Allow to settle for several days
4. Decant excess water carefully
5. Results in exceptionally smooth putty, but slower to produce usable consistency

Quality Testing

Visual and Tactile Tests

Test	Good Putty	Poor Putty
Color	Bright white, uniform	Grey, speckled, or yellowish
Texture	Smooth, creamy, no grit	Grainy, lumpy, sandy feel
Consistency	Like thick cream cheese	Watery or stiff and dry
Stretch	Pulls like taffy when drawn with blade	Breaks or crumbles
Smell	Slightly earthy, clean	Sulphurous or chemical (contamination)

The Trowel Test

Spread a thin layer of putty on a flat stone or board with a trowel. Good putty will:

• Spread smoothly without dragging or tearing
• Leave a slightly shiny, even surface
• Not show individual sand-like particles
• Peel cleanly from the surface when partially set

The Ball Test

Roll putty into a ball the size of an egg. Set it on a board in the shade:

• After 24 hours it should hold its shape without cracking
• It should feel firm on the surface but still soft inside
• Excessive cracking indicates poor quality or contamination with over-burned material

Storage and Maturation

Pit Storage (Recommended)

The traditional lime pit is the best long-term storage:

1. Keep submerged — Maintain at least 50-100mm of water above the putty surface at all times. Check weekly in dry weather.
2. Cover the pit — Use wooden boards, stone slabs, or heavy canvas to keep out rain debris, leaves, and animals. Leave enough gap for air exchange.
3. Mark the date — Keep records of when each batch was slaked. Use the oldest putty first for general work; save the most aged for fine plaster.
4. Prevent freezing — In cold climates, insulate the pit with straw or earth mounded around the edges. Frozen lime putty is damaged — ice crystals disrupt the fine crystal structure.

Container Storage

For smaller quantities:

• Use wooden buckets, ceramic crocks, or plastic containers
• Fill to the top, cover with water, and seal with a lid
• Store in a cool, frost-free location
• Putty stored this way will last years

Age Improves Quality

There is no upper limit on how long lime putty can be stored wet. Some of the finest lime plaster work in history used putty aged 10-20 years. If you have the foresight, start a lime pit years before you need the material.

Common Problems and Solutions

Problem	Cause	Solution
Putty has hard lumps	Incomplete slaking; over-burned cores	Screen through fine mesh (3mm); discard lumite
Putty is gritty	Under-burned limestone; contamination	Re-screen; allow more maturation time
Putty has dried and hardened	Water cover evaporated	If only surface: remove crust, re-wet; if fully carbonated: discard and re-burn
Putty smells bad	Organic contamination	Screen and transfer to clean container
Putty is grey or brown	Impure limestone or contaminated water	Usable for rough work; use pure materials for fine work
Putty too thin/watery	Excess water during slaking	Allow to settle; decant excess water carefully

Using Lime Putty

For Mortar

Mix 1 part matured putty with 2.5-3 parts sharp sand. The putty should be stiff enough to hold its shape — if too wet, allow to dry slightly by spreading on an absorbent surface (wooden board or porous stone) for a few hours before mixing.

For Plaster

Mix 1 part putty with 2-2.5 parts fine sand for base coats. For finish coats, use 1:1 or even pure putty with only a fine aggregate like marble dust.

For Limewash

Dilute putty with water to a thin, milky consistency — roughly the viscosity of whole milk. Apply with a large brush in thin coats. Three thin coats are far better than one thick coat.

For Lime Concrete

Mix putty with a blend of sand and gravel (aggregate up to 20mm) at roughly 1 part putty to 4-5 parts combined aggregate. Use for floors, foundations, and mass fills.

Estimating Quantities

A useful rule of thumb for planning:

Application	Putty Needed (liters per m²)
Mortar for brick wall (single skin)	25-30 per m² of wall
Render/plaster (15mm thick)	15-20 per m²
Limewash (per coat)	0.3-0.5 per m²
Lime concrete floor (100mm thick)	20-25 per m²

To produce 100 liters of lime putty, you need approximately 50-60 kg of quicklime and 150-200 liters of water (accounting for water lost to evaporation and the exothermic reaction).