Enduring strength
Traditional three-coat plaster is unmatched in strength and durability — the reason original walls still stand in homes more than a century old.
Plaster knowledge
Why plaster?
The science, history and soul of one of architecture’s oldest finishes — and why it remains the finest wall a home can have.
From stone to wall
How plaster is made.
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01
Quarried from the earth
Plaster begins as gypsum — calcium sulfate dihydrate (CaSO₄·2H₂O), a soft mineral holding water locked inside its crystals. Most gypsum is quarried from near-surface deposits, then drilled, blasted and hauled for processing.
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02
Crushed and ground
The rock is crushed and pulverized into a fine powder, so heat can reach every grain evenly in the next step.
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03Calcined — the dehydration
In a kettle, the powder is heated to roughly 300–350 °F. It seems to “boil” as about three-quarters of its chemically bound water is driven off as steam, leaving hemihydrate (CaSO₄·½H₂O) — the material known as plaster of Paris.
CaSO₄·2H₂O + heat → CaSO₄·½H₂O + 1½ H₂O ↑
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04
Rehydrated and set
On the job, the plasterer mixes the powder with water. The water rejoins the crystals, the mix gives off gentle heat, and it hardens back into solid gypsum — stone again, now shaped by hand into a wall.
CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O
The lime cycle
Stone, to putty, to stone again.
Before gypsum, walls were finished with lime. Limestone or oyster shell is burned into quicklime, then slaked with water into a soft putty — a reaction that releases heat. Mixed with sand and animal hair and troweled onto the wall, the lime slowly draws carbon dioxide from the air and hardens back into limestone. It can take months to cure fully, which is why gypsum, setting in minutes, took over around 1900.
- LimestoneCaCO₃
- Quicklimeburned · CaO
- Lime puttyslaked · Ca(OH)₂
- Limestonecured · CaCO₃
Built in layers
The three-coat system.
- Scratch coatPressed through wood or metal lath so it curls behind and locks on as “keys,” then scored to grip the next layer.
- Brown coatBuilds the wall straight and true. Together the first two coats are about ⅜ inch thick.
- Finish coatA thin, hard layer — about ⅛ inch — troweled to a smooth or textured surface.
Stick with plaster
Why it’s worth keeping.
Fire resistance
Plaster is a non-combustible mineral surface, and historic plaster walls are valued for the fire resistance they add to a building.
Quiet rooms
Dense, solid plaster reduces sound transmission between rooms far better than thin panels.
Naturally antimicrobial
Fresh lime is strongly alkaline (around pH 12.4), an environment that inactivates bacteria and viruses — and plaster has no paper facing for mold to feed on.
A breathing wall
Lime plaster is vapor-permeable, helping older masonry walls release moisture rather than trap it.
Repairable forever
Plaster can be patched, re-keyed and refinished again and again — it is restored, not thrown away.
“Plaster in a historic building is like a family album.”— National Park Service, Preservation Brief 21
Ten thousand years of walls
A brief history of plaster.
- c. 8000 BCE
The earliest known gypsum plasters appear in Anatolia.
- Antiquity
Egyptian builders use gypsum plaster; later, lime plasters finish Greek and Roman walls.
- Paris
Gypsum quarried in the Montmartre district gives “plaster of Paris” its name.
- c. 1900
Gypsum plaster is widely adopted in America because it sets in minutes rather than months.
- 1930s–40s
Plaster remains the standard wall surface for nearly every building — until drywall begins to replace it.
- Today
Fewer craftsmen practice the trade. We are keeping it alive.
Sources
Free estimates · Residential & commercial
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