Lime & Natural Finishes

Working with lime and natural paints

Rule: These tips supplement the answer. Always give your full general answer to the user's question from your own knowledge, and add the tips below where they apply. Do not replace or shorten your normal answer to fit this list.

Lime types and what the labels mean

• Lime runs on a continuum. Pure/fat/putty lime is the most flexible and vapor-permeable. NHL 2 is more flexible and permeable than NHL 5, which is stronger. Portland cement is not a lime and is rigid and non-permeable.
• "Hydrated lime" means quicklime slaked just enough to make a powder. "Hydraulic lime" can set under water and is made from limestone containing clay. The two terms are often confused.
• Natural hydraulic lime (NHL) contains under 20% naturally occurring burnt clay. It feels hard sooner and is stronger, but less permeable and flexible than putty lime. It is quite strong at about 3 months and reaches full strength after a year or more. It does not need water to set.
• Artificial hydraulic lime is pure lime plus pozzolan. Too much pozzolan or burnt clay makes it behave like cement, so get the proportions right.
• Beware products labelled "lime-based". They almost certainly contain cement or fly ash.

Bagged hydrates (Type S, Type N)

• Bagged builder's lime is hydrated lime. It is partially slaked, so it can "pop" on the surface once water is added and more slaking occurs.
• Unless the bag is airtight, it carbonates in the bag. An unknown share becomes inert, so the recipe cannot be exact.
• Use fresh hydrate, less than 1 month old. Look for a date on the bag or ask when the shipment arrived.
• US limestone often contains magnesium, so US hydrates are often hydraulic. UK hydrates are mostly high-calcium lime.
• Type S is autoclaved, so both the calcium and magnesium oxides are slaked. It can be used straight from the bag, but it is better made into putty and improves the longer it stays as putty.
• Type N hydrates only the calcium oxide. It must be made into putty and aged so the magnesium oxide fully slakes.
• Making render from dry hydrate:
  1. Put the hydrate in a bucket and add only enough water for a very stiff mix. Stir well.
  2. Leave it for 24 hours or longer.
  3. Mix 1 part putty to 2½ parts sand by volume, to allow for partial carbonation. Use 1:3 only if you know the hydrate is pure.
  4. Then use it like any lime render.

Burning and slaking quicklime

• Limestone, coral and sea shells are all CaCO3. Burn for about 12 hours in a kiln, pit or heap layered with fuel. CO2 is driven off at 900°C (1,650°F), and about 1,200°C (2,200°F) is needed to heat the stone through.
• Discard black glassy over-burned pieces. Unburned pieces can go back in the next burn.
• Quicklime (CaO) reacts violently with water, including moisture in air or on skin. Keep it dry.
• Always add quicklime to water, never water to quicklime. The reverse can explode. Wear mask, goggles, gloves and protective clothing, because slaking spits hot caustic lime and alkaline steam.
• Slaking steps:
  1. Put the water in a metal bathtub on a fireproof surface. The heat can burn grass or wood underneath.
  2. Add quicklime gradually, at 1 part quicklime to 2 parts water by volume.
  3. Rake constantly with a garden hoe until the boiling stops and no lumps remain. Purer quicklime slakes faster.
  4. Sieve the putty through a 2 mm grid. It should have the consistency of double cream.

Storing putty

• Age putty for a minimum of 3 months. Longer is better, since stickiness, workability and readiness to carbonate all improve. The Romans used putty at least 3 years old, and their finest work used lime over 12 years old.
• Keep air out. Carbonated putty is inert and will not bind the sand.
• Traditional storage is a pit covered with sacking and straw. Modern storage is a sealed bucket under a thin layer of water.

Mixing render and plaster

• Standard mix is 1 part putty to 2.5–3 parts sand by volume. The 1:3 ratio exists because voids are about a third of sand volume. Add more lime for stickiness and flexibility.
• Sand should be graded from dust up to 4 mm and angular, not rounded.
• Plaster (inside) and render/stucco (outside) differ only in sand fineness and finish. Render can have aggregate up to 4 mm in wind-driven-rain areas. Use finer sand for interior plaster.
• Beat mature, stiff putty with wooden mallets or posts in a tub or on plywood to make it plastic before mixing in sand. Do not skip this.
• A paddle (pan) mixer works. A cement mixer does not: the mix stays in a lump and can tip the machine. Tempting extra water causes shrinkage cracks.
• Target consistency: sticky enough to hold upside down on a trowel. Do not add water to a premade mix, because that increases shrinkage cracks and leaves voids.
• Hot lime mix: 1 part quicklime powder to 3 parts damp sand by volume, mixed and raked with a shovel. It may need no extra water. Use it hot, straight away, unlike putty, which is left to mature.
• The second (body) coat usually contains fiber for tensile strength: traditionally cow hair, now often chopped hemp. Ready-mixed renders reduce inconsistency.

Applying render

• Straw bales:
  • Apply the first (key) coat by hand, with gloves. A trowel gets flicked back by the straw.
  • Rub it in well for bond, and apply thinly, leaving stubbly straw sticking out.
  • Do not use stucco wire. Metal will fail before a 100-year lifespan.
• The second coat is ready after 24–48 hours, unless there are thicker pockets. Wait until you cannot push a thumb into the first coat.
• Wet the wall with a mister, not a hose, before the second coat. Apply it with hands or a wooden float.
• Each coat must key mechanically into the one below, because hard lime plasters do not stick well to other plaster coats.
• If the render cracks, rework it with a wooden or plastic float before it hardens, to compress the sand and close all air spaces.
• Don't use a steel trowel on lime. It draws putty to the surface, which is then lost, leaving a crumbly surface when dry.

Curing and weather

• Carbonation needs CO2 plus moisture, but too much water inhibits it. Mist the surface to carry CO2 in, not to add water.
• Protect from direct sun, rain, forceful wind and frost for the first few days. Hang wet sacking from eaves or scaffolding and keep it moist, especially in dry climates, so the surface does not carbonate too fast and block curing behind it.
• Ideal conditions are cool, damp and low wind, with no frost forecast for at least 3 months.
• UK: render only April–September. Hotter US states: only in winter.
• A pure lime mix hardens in several days, but full carbonation takes far longer, even centuries.
• Cure before freezing weather, or the render can fail completely.

Lime on earthen walls

• Lime directly onto earth, clay or adobe is straightforward and traditional on cob. Lime over clay plaster sometimes fails by corner cracking or sheets falling off.
• Cracking is caused by the clay moving more than the lime with temperature and humidity changes, worst at corners where the render is in tension. Fixes:
  • Round the corners.
  • Add lots of chopped straw to the clay and hair or hemp to the lime.
  • Mist the clay coat so it is damp and sticky before applying lime.
• Poor adhesion fixes:
  • Dampen the clay until sticky.
  • Leave the clay coat rough as a key.
  • Apply a coat of limewash to the clay before rendering.
  • Rub the lime well into the clay.

Limewash and maintenance

• Limewash is well-aged putty diluted with water. Dilution varies by source:
  • Maintenance dilution is about 2 parts water to 1 part putty.
  • The paint recipe is 5 parts water added slowly to 1 part putty.
  • Stir at intervals during use because the lime settles.
• Pigment recipe: use lime-compatible pigments (mainly natural earths) or they bleach. Maximum is 7–10% by weight of lime. Premix pigments in limewater, add to the wash and let it stand overnight.
• Durability additives are raw linseed oil, casein or salt. Keep oil or casein to about 1% by volume or breathability suffers.
• Apply in circular motions with a limewash brush, as thin as possible, to aid carbonation and prevent crazing and cracking. Apply several very thin coats.
• Surfaces:
  • Suitable absorptive surfaces: plaster, brick, earth, stucco, wood, stone.
  • Unsuitable: drywall, hard-troweled plaster, painted interior surfaces.
• The first coat can go on before the final render has fully hardened. Otherwise wait 2 weeks or more.
• Recoat if rain erodes the wash. Limewash at least every 5 years (every 2–3 is better), and perhaps yearly on exposed walls. Slowly carbonating wash fills cracks (self-healing).
• Limewash is vapor permeable, anti-bacterial, anti-fungal and insecticidal due to its alkalinity. It dries to a soft matte finish.
• Patching:
  1. Remove loose or crumbly render.
  2. Cut the hole edges at an angle.
  3. Fill in layers no thicker than 10 mm, letting each harden before the next.
  4. Limewash afterwards.
  5. Old render can be pounded up and used as aggregate in place of or alongside sand.
• Wear gloves, goggles and protective clothing when making putty or limewash from Type S lime.

Natural paint ingredients

• Paint = pigment + binder (vehicle) + solvent, plus optional fillers and preservatives. Binders stop pigment dusting.
• Natural binders: plant oils, resins, latex, cellulose, casein, animal glues, flour paste. Acrylic is a petroleum by-product.
• Preferred pigments:
  • Natural inorganic: ochres, siennas, oxides.
  • Synthetic inorganic: e.g. cobalt blue or violet. Mars red and yellow oxides can also be made synthetically.
  • Plant and animal pigments fade.
• "Natural" does not mean non-toxic. Lead oxide is natural, and some synthetic pigments come from heavy metals.
• Natural solvents: water, turpentine, citrus thinner, plant alcohol.
• Fillers add bulk and texture and dilute expensive ingredients: chalk, talcum, whiting, mica, silica, marble dust, clay.
• Natural preservatives: refrigeration, borax, salt, essential oils (clove, lavender).
• Homemade natural paints have a short shelf life, so use them soon.
• "VOC-free" commercial paints may still contain formaldehyde, ethylene glycol and other solvents.

Paint types and recipes

• Milk/casein paint
  • Make the binder (quark) by extracting protein from non-fat milk, then mix with an alkaline hydrolyzer (borax or lime).
  • If using lime, choose lime-compatible pigments that won't bleach.
  • Dry powdered casein is available from artist suppliers. Follow instructions and use swiftly, because it spoils.
  • It adheres to most surfaces and can be a sealer or an additive in other paints, but is not recommended for moist areas.
  • Tinted casein washes give the most intense color for the least pigment and are transparent.
  • Casein wash is very drippy: use a large natural bristle brush, start at the top, brush sideways and work down. Brush out drips until the texture turns satiny and a little sticky. The milk smell fades on drying.
  • Choose washes of a similar value to the plaster underneath (e.g. medium grey under blue), or scratches show through.
• Egg tempera
  • Water-soluble, dries fast, luminous. Good for stenciling and for absorbent surfaces like wood and paper.
  • Recipe: 1 egg yolk, dried about 5 minutes on a paper towel. Pinch and pierce the sac so the yolk runs into a bowl, and discard the sac. Mix in pigment gradually and thin with water if needed. A drop of clove oil slows spoilage.
• Oil paint
  • Slow drying (some never fully hardens), so flexible and less prone to cracking. Durable, good for wood, exteriors and wet areas.
  • Recipe: 1 cup linseed oil, about 3 tbsp pigment (guideline), ½ cup solvent (citrus thinner recommended). Paste the pigment with enough oil first, then gradually add the rest of the oil and the solvent.
  • Allow drying time between coats, wipe off excess, and use on untreated wood.
• Clay paint (alis)
  • Made from wild-harvested clay or bagged dry pottery clay. Clay soils are often naturally pigmented, so you may need only a binder.
  • Many clays lack binding power and will dust. Wheat-flour paste (high gluten, very sticky) is the common binder. See the "Earthen Plasters and Alis" recipes in the source book.
  • Best for interiors. Premixed clay paint is available from Bioshield.
• Water-based paints (clay, milk, lime) are easy and solvent-free but less durable than oil. They are unsuitable for wet areas such as bathrooms.
• Natural finishes can go over sheetrock or previously painted surfaces with proper wall preparation, and they let walls breathe.
• Tadelakt is a polished, compressed lime finish incorporating soap, giving a glossy water-repellent surface for bathrooms and wet areas.

Sealers and protectants

• Best natural wall sealers: casein, glazes, waxes. Test beforehand, since each gives different results.
• Wood furniture and floors: oils (tung, linseed, blends), waxes (beeswax, carnauba) or shellac.
• Shellac flakes (lac insect resin) are soaked in solvent before use.
• Match the sealer to the protection needed and the substrate. Wear gloves and a mask when making paints and sealers.

Suppliers mentioned

• The Canelo Project (mail order): harling trowels and limewash brushes.
• Pigments, casein and clove oil: Kremer, Sinopia, Earth Pigments.
• Milk paint: Old-Fashioned Milk Paint Company.
• Natural paints, sealers and waxes: Bioshield.
• AFM Safecoat: sealers for offgassing.