Material Research

Lime & Natural Plaster

Lime cycle from limestone to finish coat — the living chemistry of traditional plaster

Natural lime plaster has been applied to adobe, rammed earth, stone, and brick buildings for thousands of years — protecting earthen structures from water damage and producing interior surfaces of remarkable breath-ability, beauty, and durability. Unlike Portland cement plaster, lime plaster is a living material: it absorbs and releases moisture, continues to carbonate for years after application, and can be recycled.

Topics

earth-claylime-plasterearthen-buildingchemistrynatural-plaster

The Lime Cycle

From The Material Way 2025 workshop notes:

Limestone (석회석): Calcium carbonate (CaCO₃) — the raw material, mined from geological deposits
Quick lime / burnt lime (생석회): Heat limestone to 900°C+ → calcium oxide (CaO) + CO₂ released. Highly reactive — exothermic, dangerous.
Hydrated lime / slaked lime (소석회): Add water to quick lime → calcium hydroxide Ca(OH)₂. The hydration reaction produces significant heat (up to 300°C). This is the working form of lime for plaster.
Lime putty: Excess water added to slaked lime → smooth paste that can be aged (pitted) for months or years. Older lime putty (aged 5–10+ years) produces finer, more workable plaster.
Carbonation (hardening): Applied lime plaster absorbs CO₂ from the air and slowly reverts to calcium carbonate — essentially returning to limestone. This is why lime plaster "breathes" and continues to harden for years after application. The cycle is complete.

Natural Plaster Components

Binder: Lime putty or hydraulic lime (NHL — natural hydraulic lime, sets faster in wet conditions)
Aggregate: Sand — controls shrinkage and adds body. The particle size and type of sand determines the plaster's texture. Traditional adobe finish plasters use very fine-grained local sand.
Fiber reinforcement: Animal hair (horsehair, goat hair) in the base coat — prevents cracking during drying. Same principle as organic fiber in adobe brick.
Clay: Small amounts of clay can be added for working consistency and to improve adhesion to earthen substrates (clay-on-clay principle).

Plaster Layers

Scratch coat: Coarsest mix, applied directly to substrate. Scratched with a comb tool to provide mechanical key for the next layer.
Brown coat: Medium texture. Brings wall to flat. Applied when scratch coat has set but not fully cured.
Finish coat: Finest mix. May be left as natural white, pigmented with earth pigments, or polished (marmorino technique).

Quartz / Silica

Quartz (SiO₂, silicon dioxide) is the primary mineral in most sands used for plaster aggregates. Pure quartz sand gives a very white, hard finish. Mixed with lime (calcium hydroxide), quartz can participate in a pozzolanic reaction (at elevated temperatures) — producing calcium silicate hydrates that add strength. Historical Roman concrete used volcanic pozzolans (silica-rich ash) for this reason.

Adobe Plaster Connection

In New Mexico and traditional adobe building, the earthen wall is plastered with successive layers of natural plaster — often combining clay-based earthen plasters (for interior) and lime plaster (for exterior weathering protection). The plastered surface is what gives traditional New Mexico adobe buildings their smooth, rounded appearance — the structural adobe brick beneath is hidden by plaster layers built up over decades.

Jay's Studio Note

The lime cycle is the slowest alchemy in this material library — limestone becoming plaster becoming stone again over years and decades. Working with lime plaster means working with a material that is still becoming. The surface applied today will be harder in five years, harder still in ten. In this way, lime plaster is the exact opposite of bioplastic: one material approaches permanence; the other approaches compost.