Joint Deformation
Deviation of the formed joint from a plane surface; may be referred to as “ridging,” “beading,” “lipping”
or “stepping” of the joint. The condition may be caused by improper lath attachment; improper joint treatment to achieve
reinforcement; irregular water absorption over the joint; abnormal substrate movement in response to thermal, hygrometric
(humidity), structural and other construction influences.
Journeyman
A plasterer or lather who, through training and experience, has become thoroughly skilled in the trade.
Keenes
Cement
An anhydrous gypsum plaster of low consistency that, when mixed with lime putty, provides a dense, hard
finished surface. This material is unique in that it may be retempered using water and reworked with a trowel to resume the
plastic, workable state.
Knock-Down
An intermediate plaster finishing step where sizable surface imperfections of the partially stiffened plaster
application are removed prior to final finish troweling.
Land Plaster
Coarse-ground natural gypsum that is calcined for the manufacture of gypsum plasters.
Latex Modifiers
Dry powder or emulsified admixtures that improve plaster plasticity, surface hardness and compressive
strength of the set material, and that improve bond between the plaster and base.
Lath
A metal or gypsum material applied separately to a structure that serves as a base for plaster.
Lean Mixture
Any plaster which has a high ratio of aggregate to cementitious material. Usually exhibits poor or hard
working characteristics.
Lightweight Aggregate
Low-density, inert aggregates such as vermiculite and perlite (as opposed to silica sand and pumice).
Lime
The principal product derived from calcining various types of limestone consisting of oxides or hydroxides of calcium
or magnesium.
Lime Plaster
A basecoat plaster consisting of lime and aggregate.
Lime Putty
A finishing material resulting from slaking of quicklime or soaking and mixing hydrated lime with water to form
a plastic mass; usually mixed with a gauging plaster or gauging and aggregate as a finish over basecoat.
Machine Application
Plaster application by mechanical means; generally by pumping and spraying.
Mason’s Lime
See: Type “N” Hydrated Lime.
Mechanical Bond
The physical kerfing of one plaster coat to another, or to a plaster base; e.g., clinching of plaster keys to
expanded metal lath, and the bond obtained between adjacent plaster coats by brooming or crossraking the first coat prior
to set.
Metal Lath
A metallic plaster base manufactured from sheet metal by slitting and expanding, or by punching and forming
Mortar
A plastic mixture composed of water and a cementitious material, which may be machine or hand applied, and
which hardens in place.
Neat
A basecoat gypsum plaster product produced and supplied without aggregate. It is intended to be job-mixed with
aggregate, such as sand, perlite or vermiculite.
Moulding Plaster
A fine-grind, high-consistency hemihydrate gypsum that, when mixed with water, yields a pourable slurry
for casting ornamental trim units or decorative plaster runwork.
One-Coat System
Veneer plastering; the veneer finish is applied to veneer gypsum base or other approved substrate in a
scratch and double-up method (both “coats” from the same batch), to a full thickness of 1/16" (1.6 mm) to 3/32" (2.4 mm).
Open Time
The amount of time, from the point of mixing, during which a plaster retains a plastic workable consistency, until
it becomes too stiff to be applied.
Parge
To coat with plaster; usually refers to application to foundation walls and rough masonry.
Pencil Rods
Mild steel rods of 3/16" (4.8 mm), 1/4" (6.4 mm), or 3/8" (9.5 mm) diameter, used to provide rigid internal
reinforcement of the base for plaster application.
Perlite
A siliceous volcanic glass that, when expanded by heat, is used as a lightweight plaster aggregate.
Plaster
A cementitious material or combination of cementitious materials and aggregate that, when mixed with water, forms
a plastic mass. When applied to a surface, plaster adheres to it and subsequently sets or hardens, preserving in a rigid state
the form or texture imposed during the period of plasticity.
Plaster of Paris
CaSO
4
• 1/2H
2
O or hemihydrate gypsum without set control additives, used in casting and industrial
applications.
Plasticity
The property of plaster that permits continuous and permanent deformation in any direction. As opposed to fluidity,
a plastic material requires a measurable force (yield value) to initiate flow. A material of low plasticity is usually described as
being “poor” or hard working; high plasticity is described as “rich” or easy working.
Plasticizer
An admixture to improve plasticity, workability, and spread under the trowel.