Earthen Plasters & Floors

Earthen plasters, alis and earthen floors

Rule for using this skill: These notes add specialist tips on top of the answer. They do not replace it. Always give your full general answer to the question first, from everything you already know, and work in the tips below where they apply. Do not shrink the answer to just these tips.

Is an earthen floor suitable?

• Floors are heavy and need a stable, strong subfloor. Usual setup is on grade over a concrete or compacted gravel (road base) pad. Raised wood subfloors work only if the structure carries the load; consult an engineer if in doubt.
• Hardness is comparable to a wood floor, not concrete. Point loads dent it and heavy objects scratch it. Put stone where people might split firewood, such as under a stove.
• Sealed floors tolerate spills and mopping, but beneath the seal the earth is unstabilized. Bathrooms are fine. Showers and basements that flood are not.
• Typical thickness is ¾–2 in. (2–5 cm). Plan for this at transitions, entryways and low headroom.
• Timing: 2 weeks is possible, 3–4 weeks is common. Drying and curing take the longest; fans, heaters and dehumidifiers speed them up.
• Sealing oils contain VOCs, which take 1–3 days to evaporate. Seal off adjacent occupied rooms meanwhile.
• Earthen floors are non-flammable and suit areas around stoves and fireplaces.

Testing a floor mix

• Ingredients: sand (the main strength), clay soil (the binder), short fiber such as chopped straw (tensile strength, less cracking).
• Start with 3 test batches at sand:clay 2:1, 3:1 and 4:1. Very clay-rich soil needs more sand.
• Measure into a bucket, mix dry, then add water slowly to a thick, spreadable "cake batter."
• Make each test patch with a wooden float, about ¾ in. (2 cm) thick and at least 18 in. (45 cm) across. Smooth with a steel trowel and label the ratio.
• Reading dry patches:
  • Lots of cracks means too much clay (shrinkage).
  • Sand grains rubbing loose means too little clay.
  • A fingernail should mark it without it crumbling away. A fragile surface means too little clay.
• Once the sand:clay ratio is chosen, retest with fiber at 10–20% of the clay/sand volume.
• More fiber makes the mix harder to spread and smooth. Too little weakens the floor. Light straw shows against the dark earth, more so at higher fiber content.
• Do a last test round that includes burnishing, oiling and waxing, to preview the finished look.

Subfloor and prep for a poured floor

• A settling or flexing subfloor will eventually crack the floor.
• Cover a wood-framed subfloor with a vapor retarder to protect it from installation moisture.
• Insulate under the subfloor, using material strong enough not to shift or compact. High-compressive-strength rigid foam is typical, and pumice has worked. Straw or straw-clay is not recommended.
• Install the floor as one of the last steps, after finish plaster, carpentry and rough plumbing.
• Protect room surfaces and set up fans and dehumidifiers before pouring.
• Mark a line on the walls at the finished height.
• To control level, use a laser level, screed rails and boards, or a marked depth-gauge stick.

Mixing and placing the floor

• Mix outside. Convert the recipe to a standard measuring container, usually a 5-gallon bucket.
• Hand mix in a wheelbarrow with two people and hoes at opposite ends.
  • Mix sand and clay dry.
  • Add water slowly to cake batter.
  • Add chopped straw, then more water if needed.
• With a mortar mixer, add water and clay first and start the mixer. Then add sand, straw and more water if needed, and dump into a wheelbarrow.
• Add water slowly either way. A dry mix is easy to wet, but fixing a too-wet mix means adding dry ingredients in correct proportions.
• Place piles covering about 2 × 2 ft. (60 × 60 cm) and flatten with a wooden float. Check thickness and level often.
• Shave high spots with a back-and-forth sawing motion. Add mix to low spots and blend with the float.
• Finish each section with a steel trowel, just a couple of light passes. Do not overwork it. Irregularities up to about 1.5 mm are removed later by burnishing.

Burnishing (optional, improves appearance)

• Burnish 24–48 hours after pouring, when the floor is damp but firm.
• If a trowel mark goes deeper than about ⅛ in. (3 mm), the floor is still too wet.
• Walk out to the far end on foam pads or steel floor pans to spread your weight.
• Work one section at a time. Mist with a spray bottle and press with a steel trowel until shiny and smooth. Fill low areas with a little wet mix.

Tamped road-base floor (no stabilizers, little water)

• Road bases differ. Pick a quarry-blended silt/clay/sand/gravel product approved for road building (for example Austin's "city base"), and test it.
• Compact the ground, level and smooth, to at least 6 in. (15 cm) below the finished floor height.
• If using a vapor barrier, put 1–2 in. (2.5–5 cm) of sand both below and above it so gravel can't puncture it.
• Lay road base in about 2 in. layers. Wet each with a hose or watering can, only enough for silt and clay to stick to the aggregate. Compact each layer.
• Compact with a plate compactor, or hand tamp (slower but quiet and fume-free). A gas compactor needs fans to vent the building. Hand tamp the edges even when using a powered compactor.
• Repeat layers to about 1 in. (2.5 cm) below finish height.
• Then sift road base through ⅜ in. (1 cm) hardware cloth, lay it on, level with screed boards, moisten and tamp so it bonds to the layer below.
• Top coat: screen through ⅛ in. (3 mm) hardware cloth. Do not compact it. Hand trowel and burnish with just enough water to bond and work. Bonding to the layer below is the most important point.
• Chopped straw reduces cracking but makes a glassy finish harder. Chop it very finely if you want a glassy finish.
• The floor can be walked on a day or two after installation.

Sealing floors: oil and wax

• The floor must be thoroughly dry before oiling. Otherwise the finish can turn cloudy, or moldy in a damp location.
• Use a drying oil, such as linseed (boiled linseed or tung also used). The oil soaks in and polymerizes, which gives hardness and water resistance.
• Prefer oils without heavy-metal drying agents that come premixed with solvent. DIY blend: 4 parts linseed oil to 1 part solvent (mineral spirits, citrus oil or turpentine).
• The unoiled floor is fragile, so walk and work on it carefully.
• Apply with a brush, paint roller or rags.
• Coat counts differ by source:
  • Four to six coats in one day, one right after another.
  • Alternatively, at least six coats, each fully dry before the next.
• Wipe up any puddled oil with a clean rag.
• Ventilate during application and curing, which takes a week or more.
• Commercial sealers may dry harder than linseed but cost more; consider them for the final coat only.
• Optional finishing after the oil has cured:
  • Lightly sand for a smoother finish.
  • Apply a fairly liquid floor wax with a brush or rag.
  • After the wax dries, buff with an electric buffer to remove excess.

Getting clay for plasters and alis

• Test native clays with a jar shake test first to check clay content.
• Levigation:
  • Put equal parts dirt and water in a barrel and stir or whip with a drill mixer to a thin-milkshake consistency.
  • Leave overnight to settle.
  • When the water is clear, skim the smooth last-settled layer; this is the clay slip.
• Hard, leather-hard clays are tough to slake. Wet-sieve through a screen, or find more saturated clay. In the desert Southwest, many clays can be sifted dry.
• Sieving is less fine than levigation. That is fine for most plasters, but a final alis coat must have no coarse particles.
• Test native-clay mixes with test blocks, because shrinkage varies widely and no recipe is guaranteed.
• If a test cracks, add any of whiting, mica, silt or fine sand.
• If a test dusts, add more clay or binder.
• Finely sifted native dirt plus a binder such as buttermilk may make a good alis as is.

Binders

• Binders make wet plaster stickier and dry plaster harder, less dusty and more waterproof. Common ones are starch pastes and milk casein.
• Flour paste (wheat flour, rice flour or cornstarch):
  • Fill a pot no more than ⅔ with water.
  • For each quart of water in the pot, whisk about 1 cup flour with 1 cup cold water in a separate bowl.
  • Pour it into the rolling-boil water while whisking, like gravy.
  • Remove from heat as soon as it thickens and turns opaque, to avoid scorching.
  • It sours and smells quickly, so make only a day's worth or refrigerate leftovers.
• Casein is stronger than starch, so use less.
  • Curdle skim or powdered milk with lemon juice or vinegar, ideally letting it sit. For speed, add vinegar while warming on the stove until it separates into curds and whey.
  • Strain through cheesecloth in a colander and rinse the curds.
  • Blend the curds with 1 tbsp dissolved borax per quart of starting milk.
  • Store the concentrate cold. Dilute with 4–6 times as much water before adding to alis or using in a wash.
• Buttermilk can substitute for casein in most recipes, but test.

Straw-clay mud (base and sculpting layer)

• Use it as a base coat on very rough, uneven or soft walls, and for sculpting. It has more straw than ordinary plaster and more clay than light straw-clay (also called "strob").
• Straw bale walls need more plaster than any other wall type.
• To prime bales, work pure clay slip into the bale or dip bales in slip before placing. The mud then sticks well.
• Mixing:
  • Fill a wheelbarrow ⅓ with water.
  • Add sifted clay, either directly or by sifting dirt through a ½ in. screen over the barrow until a small dry pile shows above the water.
  • Adding clay to water avoids dry pockets. Soak about 5 minutes.
  • Add long straw, turning with both arms, until a big handful holds its shape with only a slight slump. It takes a lot of straw. Straw should be well coated and the mix sticky.
  • Too much water won't hold together.
  • For very pure, high-shrink clay, add some sand before the straw, but don't overdo it.
• For sculpture, add manure and/or wheat flour paste for strength, smoothness and stickiness.
  • Wheat paste darkens the plaster and can ferment and stink if it stays wet too long in thick mud.
  • In exterior plaster, wheat paste adds toughness and weatherproofing. The mix suits a one-coat exterior finish.
  • For a tougher, smoother exterior, let it partly dry and compress with a trowel at leather-hard.
  • Design roof overhangs to protect exterior plaster.
• Application:
  • Dampen the wall and toss handfuls into low spots.
  • Spread and smear with the base of the palm, feathering edges so the next handful covers the thin edge.
  • It works from 1 to 4 in. thick.
  • Soon after applying, flatten with a wet 2 × 4, long darby or wooden float. Interior walls that are smooth and flat collect less dust.
  • Aim to shape the wall so only one very thin finish coat is needed.

Troweled finish plaster

• Make it by adding extra sand to an alis mix. Consistency is cake batter that slides on creamily.
• Typical proportion is about 2 parts aggregate to 1 part clay by volume. Always test, and add sand if the sample cracks.
• Finer aggregate allows a thinner plaster. Fibers can catch straw bits on the trowel.
• For very thin plasters, use aggregates and plenty of whiting with the clay.
• For a very smooth wall, apply two coats. Burnish the second with a trowel at leather-hard, spraying with water as you go.
• Japanese trowels suit burnishing. A plastic yoghurt-lid disk makes a good trowel for curves.
• For fine plaster, chop or break up straw, for example with a weed whacker.

One-coat plaster

• Use it on walls already evenly shaped and needing only ½–¾ in. (13–19 mm) of plaster: well-formed adobe, trimmed cob, straw-clay, pumicecrete.
• To trim residual bumps on cob or adobe, make a rasp from metal lath attached around a short piece of 2 × 4.
• Recipe order:
  • Clay in a color you like (add kaolin if too dark).
  • Sharp plaster sand, which has mixed grain sizes.
  • Soaked manure.
  • Chopped straw.
• Paper pulp (soaked and beaten) can replace manure, but it holds a lot of water and slows drying.
• Mixed aggregate and fiber sizes give a stronger, less cracky mix.
• Start with equal parts clay, sand and fiber and test. Add more sand if it cracks. Use less sand if the soil is already sandy or silty.
• Application:
  • The wall must be completely dry. Dampen the surface before plastering.
  • Plaster a vertical strip about 4 ft wide, from bottom to top.
  • Sponge it to remove trowel marks, waiting until it has set up. A sponge tears a surface that is too soft.
  • Move to the next strip. The first one is then usually ready for hard troweling. If still soft, wait. If too hard, dampen it.
  • Sponge the joint between sections before hard troweling.
  • Use a 7 in. (18 cm) oval steel pool trowel, which has no corners to leave marks.
• Fixing uneven color:
  • Wait until the plaster is thoroughly dry, then apply an alis or casein wash.
  • Sponging dry plaster with a damp sponge often improves it.
  • Casein mixed with whiting gives an antique look, lightens the color and hardens the surface.

Alis

• Alis (from Spanish alisar, "to polish") is a thin paste of clay slip and very fine aggregate. It is brushed on and burnished with a damp sponge as it dries.
• Basic recipe: half kaolin (porcelain) clay and half mica powder. Add cooked wheat flour paste at 25% to the water before mixing in the clay and mica.
• About 30 minutes after brushing, at leather-hard, rub with a damp tile sponge. Rinse and squeeze the sponge in warm water often to keep it clean.
• Sandy first coat:
  • Add fine sand so the alis can fill divots and repair rough or damaged surfaces without troweling each one.
  • It also covers floated gypsum base plaster, and helps alis stick to slick surfaces such as sheetrock.
  • With acrylic glue added to a sandy base alis, it will even stick to painted walls.
• Two coats are often needed for the best look. The first coat needs sponge buffing only if it is very rough. The second needs little or no fine sand.
• Whiting or marble dust (fine calcium carbonate) makes alis smoother and more spreadable.
• Mixing:
  • Start with a bucket or barrel no more than ⅓ full of water.
  • Whisk in 3 scoops clay gradually, then 1 scoop whiting, 1 scoop ground mica and 1 scoop fine silica sand.
  • Repeat until it reaches thin-cream consistency.
  • Then whisk in the binder, 1 quart (0.95 L) starch paste per gallon (3.8 L) of alis. This thickens it, so add water to reach medium-to-thick cream.
• Pigment: shake the powder with a little water in a jar, then measure exact amounts of the dissolved pigment per batch so batches match.
• Application:
  • Brush corners and edges first with a small flat natural-bristle brush.
  • Switch to a larger brush and work from the top of the wall down to avoid drips on finished areas.
  • Lay old sheets on the floor and tape over rough wood. Alis wipes off smooth surfaces before it dries.
  • Buff with a damp tile sponge at leather-hard. When almost dry, buff again with a dry flannel cloth.

Drying and cracking (plasters and alis)

• Cracking is mostly about timing. Let moisture evaporate before compressing the surface. Compressing too early traps water, so it dries unevenly and cracks.
• Troweling pulls clay to the surface. Making it slick right away will crack it. Letting it dry and shrink first greatly reduces cracks.
• Don't rush. In cold weather, run heaters or a fireplace so the wall can dry.
• Each coat of plaster or alis must be completely dry before the next, or later layers will crack.
• Lightly spray a dry wall before applying the next layer. A dry inner layer helps the next one dry faster.
• A still-wet inner layer stains the next coat and can grow mold.
• Alis especially needs a dry surface. It discolors from the wheat paste if it dries unevenly.