Glossary of Plaster Repair Terms
There are lots of tools, processes and materials specific to plaster repair. Some of their names you may recognize, some may sound vaguely familiar, and some may be completely foreign.
Thus, we present to you a plaster repair glossary. Read them, study them, memorize them and wow all your friends at cocktail parties when you explain that a “hawk” is not only a bird of prey of the Falconiformes order, but also a flat sheet of metal with a handle, made to hold globs of wet plaster during the application process.
Plaster Repair Glossary
Aggregate: Mineral based, originally sand, serves as a shrinkage compensator, stabilizer, and bulking agent for plaster.
Building Efficiencies: Historically structures were built with materials that could be obtained locally usually for economies sake. People used what they had close by.
Carrying Capacity: The amount of lime (or gypsum) that any sand has the ability to carry for the perfect plaster (mortar) formulation for particular sand. This is determined by a volumetric test.
Crazing: Tiny cracks or fissures usually caused by rapid dry out while curing.
Delamination: The separation of plaster layers, where one layer separates from another or plaster from its lath.
Efflorescence: is a surface deposit of salts (it looks like mold on the surface of the plaster) carried by moisture. The salts either come from the sand or gypsum in interior plasters.
Emley Rating: is a measurement of plasticity; a rating of 400 is considered good.
First Coat, Scratch Coat: The first layer of plaster applied, its function is to hang onto the lath and when scratched provide a mechanical bond for the brown coat to hang on.
Fiber: a binder used in lime plaster, must not be alkaline resistant.
Hair: in this country cattle hair was used mostly, followed by goat hair in frequency. Lime plaster was “haired up” just prior to application as the lime would dissolve the hair when wet.
Hemp: was the strongest fiber used in lime plaster; it gets stronger in an aqueous lime environment.
Gypsum: CaSO4•2H2O sedimentary rock processed by crushing and heated (calcined) to boil off the water within the crystal structure.
In-Kind: is the ideal composition for replacement plaster, this ideal trumps ASTM and Master Specs for matching historic plaster. It is more important to be similar than textbook “correct” or “ideal” as historic buildings were not built according to ASTM.
Lath: is the material that the plaster is applied to. Wood laths are sawn roughly 1 1/2" wide by 1/4” thick and 3-4 feet long made form whatever wood was available locally. In early New England accordion lath was used -- 3/8” boards were applied to the framing, split with a hand axe and stretched, opening up keyways in the board to hang the plaster on. This is also known as "building efficiencies" when there was limited man power and abundant mechanical water power. Down south riven lath was used into the mid-19th century, where there was abundant manpower and limited water (mechanical) power.
Metal lath, pierced metal lath was used from the turn of the 19th century to the 1940s, depending on where in the country the building was located. Welded wire lath was patented in England in the 1790’s; today it is known as hardware cloth. Diamond lath is another name for expanded metal lath and is the most widely used modern lath.
Lime: either CaOH, CaCO3, or CaO. High calcium is when the lime is up to72%- 98% calcium carbonate. Dolomitic is a lime that can have about 35-40% magnesium carbonates. Lime stone CaCO3 native sedimentary form of lime found in quarries; metamorphic form marble. Quicklime CaO, the initial product from the burning of limestone; a process which drives off the water, carbon dioxide, and carbolic acid.
Lime Cycle: limestone is burned into calcium oxide, slaked to lime putty or calcium hydroxide, mixed into plaster and carbonates into a limestone-like material.
Lime Putty: CaOH is calcium oxide that has had its need for water satisfied and is turned into a paste which is the basic building block for traditional plastering.
Map Cracking: Wide cracking that looks like a road map; often seen in turn of the century lime plaster over terra cotta as well as developing over time in historic plaster ceilings.
Plaster: is a paste-like mixture of lime and/or gypsum, sand, and water, sometimes with fiber (hair or plant) added, that hardens to a smooth solid wall/ceiling coating.
Mud: plaster, mixture of earth and water, often containing some but not necessarily all of the following: spanish moss, animal hair, sand, clay, and plant fibers, peat, straw.
Lime Plaster: a plaster mixture containing chemically active lime, hair or plant fiber, and sand…a thin bodied plaster must be applied in layers no more than 5/16” thick to allow for crystallization by interaction with atmospheric carbon dioxide, the lime must have a large surface area that allows capillary action to develop the slow evaporation of water at the same time as the air is allowed to come in for crystallization.
Gypsum Plaster: a plaster mixture containing chemically active gypsum, and aggregate, sand or perlite, a thick bodied plaster sets by the addition of water into its crystalline structure; it must have enough water and body to supply to moisture for crystallization, suction from the substrata, and evaporation.
Plasticity: is plaster flow characteristics or how easily the plaster flows from the trowel and stays where it is placed. On a micro level the water acts a lubricant between the particles of mineral allowing them to flow under high shear and to stay in place under low shear, taking advantage of water's surface tension and cement interaction.
Reattachment: Adhesive is specifically designed to secure and stabilize historic plaster to its lath, either wood, brick, terra cotta, or masonry; one manufacturer is Big Wally’s Adhesives, Inc.
Second Coat: also called Float Coat, or Brown Coat applied to the scratch coat this layer is flat and level, floated to a light texture for the finish layer to be applied to… its purpose is to fill in the scratch coat level, plumb and square.
Secure and Stabilize: is the basis for plaster repair. Once the plaster is stable everything else is cosmetic. This is accomplished by reattaching the plaster to the lath, using the lath to bridge cracks, reinforcing the whole plaster/lath structure.
Suction: is the amount of moisture the substrata pulls from the plaster; this can be the lath, the air or another layer of plaster.
Stress Cracks: are caused from building movement overcoming the ability of the plaster to stay intact.
Volumetric Test: Determines the carrying capacity of the aggregate (sand) allowing for an ideal (proper) plaster composition by measuring the volume of air space in any given aggregate. This air space represents the correct amount of cementious material in ratio to a specific aggregate.
White Coat: also called the Finish Layer, Setting Coat, or Putty Coat -- this is a thin layer used to fill in the cosmetic defects of the brown coat resulting in a smooth white finish that is occasionally left unpainted. Its purpose is mostly cosmetic while providing an abrasion-resistant surface.