Plaster and Lime Wash

Part of Permanent Shelter

Lime has been used to finish walls for at least 10,000 years. Limewash is the simplest form — slaked lime and water, brushed onto a wall. It brightens interiors, kills bacteria and mold, seals porous surfaces, and costs nothing but labor once you can produce lime. Understanding the difference between lime plaster and limewash, and when to use each, lets you finish any wall surface to a standard that rivals modern construction.

What Lime Is

Lime starts as limestone — calcium carbonate (CaCO3), one of the most common rocks on Earth. Chalk, seashells, coral, and marble are also calcium carbonate.

The transformation chain:

Stage	Name	Chemical	How You Get It
1	Limestone	CaCO3	Gather from outcrops, riverbeds, beaches
2	Quicklime	CaO	Burn limestone at 900degC+ for 24-48 hours
3	Slaked lime (lime putty)	Ca(OH)2	Add water to quicklime (exothermic reaction)
4	Limewash	Ca(OH)2 in water	Thin lime putty with water
5	Hardened lime	CaCO3	Limewash absorbs CO2 from air, returns to limestone

The cycle is elegant: limestone becomes quicklime when heated, becomes lime putty when mixed with water, becomes limewash when thinned, and becomes limestone again when it dries and absorbs carbon dioxide. The final product is literally a thin layer of stone bonded to your wall.

Producing Slaked Lime

Step 1 — Burn Limestone to Make Quicklime

Build a simple updraft kiln: stack limestone chunks over a fire pit, surround with a stone or earth wall to contain heat. Burn hot hardwood for 24-48 hours continuously. You need temperatures above 900degC — the fire should be white-hot, not just red.

Signs the limestone is done:

• Pieces are lighter in weight (they have lost CO2)
• They have a chalky, crumbly texture
• They may have changed color (whiter or slightly yellow)

Quicklime is Extremely Dangerous

Quicklime reacts violently with water, including sweat. It generates intense heat on contact with moisture — enough to cause severe chemical burns. Handle only with thick dry coverings on your hands. Store in a dry, covered, waterproof container. Keep away from children and animals.

Step 2 — Slake the Quicklime

Slaking is the process of adding water to quicklime to produce lime putty.

1. Dig a pit or use a large container (wooden trough, stone basin). Do NOT use metal — the heat can warp or melt thin metal.
2. Place quicklime chunks in the pit.
3. Slowly add water. The reaction is violent — the mixture will hiss, steam, bubble, and reach near-boiling temperatures. Stand back. Add water gradually, stirring with a long wooden pole.
4. Continue adding water until the reaction stops and you have a thick, creamy paste — this is lime putty.
5. Cover the putty with 5 cm of water to prevent it from drying and hardening.

Step 3 — Age the Lime Putty

Fresh lime putty works, but aged lime putty works better. Let it sit, covered with water, for at least 2 weeks. Traditional builders aged lime putty for months or even years. The longer it ages, the finer the particles become, and the smoother and more workable the final product.

Store in covered pits or sealed containers. Lime putty stored under water keeps indefinitely.

Test Your Lime

Good lime putty feels smooth and creamy, like thick yogurt. If it feels gritty, it either was not burned long enough (unburned limestone chunks remain) or has not aged long enough. Strain gritty putty through a coarse cloth to remove chunks.

Limewash: Preparation and Application

Limewash is simply lime putty thinned with water to a brushable consistency — about the thickness of whole milk.

Mixing

1. Start with a bucket of aged lime putty
2. Add water gradually, stirring constantly
3. Target consistency: thin enough to drip steadily from a lifted brush, but not watery-thin
4. Strain the mixture through cloth to remove lumps and grit — lumps cause streaks

Application Technique

Limewash is applied in thin coats — not thick like paint. Each coat looks nearly transparent when wet. The buildup comes from multiple layers.

1. Dampen the wall with water first. Dry walls absorb the water from limewash before it can bond properly.
2. Apply with a wide, soft brush — a bundle of soft plant fibers, animal hair, or a cloth pad tied to a stick. Work in overlapping, cross-hatched strokes (horizontal pass, then vertical pass).
3. First coat: Apply thin. It will look patchy and translucent. This is correct.
4. Wait 24 hours for each coat to carbonate (absorb CO2 and harden to calcium carbonate).
5. Dampen the previous coat lightly before applying the next.
6. Apply 3-5 coats minimum for a solid, opaque white finish. Each coat adds roughly 0.1 mm of thickness.

Do Not Apply in Direct Sun or Heat

Limewash that dries too fast does not have time to carbonate properly. It chalks, powders, and flakes off. Apply in shade, on overcast days, or in the morning/evening. Keep the wall damp between coats.

What Limewash Does

Benefit	How It Works
Brightens interiors	Pure white reflects up to 85% of available light
Kills mold and bacteria	High pH (12-13 when wet) is antimicrobial
Seals porous walls	Fills micro-pores in clay, cob, and stone surfaces
Breathes	Unlike modern paint, limewash is vapor-permeable — moisture passes through without trapping it in the wall
Self-healing	Minor cracks fill with dissolved lime carried by rainwater, which re-carbonates
Cheap and renewable	Only requires lime and water

Natural Pigments for Color

Limewash accepts natural pigments to produce colored walls. Mix dry pigment into the limewash before application — never add more than 10% pigment by volume or the lime will not bind properly.

Pigment	Color	Source
Yellow ochre	Warm yellow to golden	Iron-rich clay deposits
Red ochre	Terracotta red to rust	Iron oxide-rich clay or stone
Burnt umber	Rich brown	Heat yellow ochre in a fire
Charcoal powder	Gray to soft black	Grind hardwood charcoal fine
Copper carbonate	Blue-green	Verdigris from weathered copper, or malachite
Iron sulfate	Green-gray	Copperas, found near iron deposits

Always Test Color Wet and Dry

Pigmented limewash dries 2-3 shades lighter than it looks when wet. Make a test patch on a scrap of plaster and let it dry completely before committing to a whole wall. Apply 3 coats to the test patch — the color deepens with each layer.

Lime Plaster vs Limewash

These are different products for different purposes. Understanding when to use each saves material and produces better results.

Property	Lime Plaster	Limewash
Composition	1 part lime putty + 3 parts sand	Lime putty + water (no sand)
Thickness per coat	5-10 mm	0.05-0.1 mm
Purpose	Structural surface — smooths rough walls, fills gaps, creates a flat plane	Decorative finish — color, brightness, antimicrobial protection
Number of coats	2-3	3-5+
Application	Troweled or hand-pressed	Brushed
Drying time between coats	24-48 hours	24 hours
Can be applied to	Any masonry wall (stone, brick, cob, adobe)	Any wall — including over lime plaster, clay plaster, or directly on stone
Structural strength	Yes — creates a hard shell	No — too thin to add strength

Typical Workflow

For the best wall finish, use both:

1. Base coat of lime plaster (8-10 mm) — fills rough spots, creates a flat surface. Score with scratches before it sets.
2. Finish coat of lime plaster (3-5 mm) — smooth, fine-grained surface. Use finer sand (sifted through cloth).
3. Limewash (3-5 coats) — final finish for brightness, color, and antimicrobial protection.

On walls that are already reasonably smooth (well-finished adobe, smooth stone), you can skip plaster entirely and apply limewash directly.

Compatibility with Wall Types

Wall Material	Lime Plaster	Limewash Directly	Notes
Cob	Yes	Yes (if smooth enough)	Dampen cob well before applying; lime bonds excellently to clay
Adobe brick	Yes	Yes	Fill mortar joints with plaster first for a flat surface
Stone (mortared)	Yes	Possible but rough	Plaster first for best results; limewash alone follows stone texture
Stone (dry stack)	Yes (with mesh or heavy scoring)	No — too rough	Apply a thick scratch coat first
Wattle and daub	Yes	Yes (over existing daub)	Standard combination throughout medieval Europe
Timber	No — lime does not bond well to wood	No	Use clay or mud plaster on timber walls, then limewash over that

Reapplication Schedule

Limewash is not permanent. It gradually wears from rain, abrasion, and UV exposure. Plan for reapplication:

• Interior walls: Every 2-5 years (longer in dry climates, shorter if walls get splashed or scrubbed)
• Exterior walls: Every 1-3 years depending on weather exposure
• Reapplication is fast: Dampen the wall, brush on 2-3 fresh coats. The new limewash bonds chemically to the old, so no scraping or preparation is needed.

Each reapplication adds another fraction of a millimeter of stone-hard calcium carbonate. Over decades, limewashed buildings develop a thick, durable shell of accumulated layers — this is why ancient limewashed buildings sometimes have walls that appear almost glazed.

Key Takeaways

• Limewash is slaked lime plus water — thin as milk, applied in multiple translucent coats that harden to solid calcium carbonate.
• Apply thin coats (3-5 minimum) on damp walls, in shade, with 24 hours between coats. Thick or fast-dried coats flake.
• Lime plaster (lime + sand) goes under limewash — it creates the flat surface; limewash provides the finish.
• Age your lime putty at least 2 weeks — longer is better. Store under water indefinitely.
• Quicklime is dangerous — handle with thick dry hand coverings, add water slowly, stand back from the violent reaction.
• Natural pigments (ochre, charcoal, umber) tint limewash; never exceed 10% pigment by volume.
• Limewash breathes — unlike modern paint, it lets moisture vapor pass through, preventing trapped dampness that rots walls.
• Reapply every 1-5 years — fast, cheap, and each layer strengthens the wall surface permanently.