Plaster is a building material used for coating walls and
ceilings. Plaster starts as a dry powder similar to mortar or cement and like
those materials it is mixed with water to form a paste which liberates heat and
then hardens. Unlike mortar and cement, plaster remains quite soft after
setting, and can be easily manipulated with metal tools or even sandpaper.
These characteristics make plaster suitable for a finishing, rather than a
load-bearing material.
The term plaster can refer to gypsum plaster (also known as
plaster of Paris), lime plaster, or cement plaster.
Types
Gypsum plaster (plaster of Paris) Gypsum plaster, or plaster of Paris, is produced by heating
gypsum to about 300°F (150 °C):
2CaSO4·4H2O + Heat
→ 2CaSO4·H2O + 3H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms
into gypsum. The setting of unmodified plaster starts about 10 minutes after
mixing and is complete in about 45 minutes; but not fully set for 72 hours.
If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which
will also re-form as gypsum if mixed with water.
A large gypsum deposit at Montmartre
in Paris led gypsum plaster to be
commonly known as "plaster of Paris".
Plasterers often use gypsum to simulate the appearance of
surfaces of wood, stone, or metal, on movie and theatrical sets for example.
Nowadays, theatrical plasterers often use expanded polystyrene, although the
job title remains unchanged.
Plaster of Paris can be used to impregnate gauze bandages to
make a sculpting material called modroc. It is used similarly to clay, as it is
easily shaped when wet, yet sets into a resilient and lightweight structure.
This is the material which was (and sometimes still is) used to make classic
plaster orthopedic casts to protect limbs with broken bones. Set modroc is an
early example of a composite material.
Lime plaster
Lime plaster is a mixture of calcium hydroxide and sand (or
other inert fillers). Carbon dioxide in the atmosphere causes the plaster to
set by transforming the calcium hydroxide into calcium carbonate (limestone).
Whitewash is based on the same chemistry.
To make lime plaster, limestone (calcium carbonate) is
heated to produce quicklime (calcium oxide). Water is then added to produce
slaked lime (calcium hydroxide), which is sold as a wet putty or a white
powder. Additional water is added to form a paste prior to use. The paste may
be stored in air-tight containers. Once exposed to the atmosphere, the calcium
hydroxide turns back into calcium carbonate, causing the plaster to set.
Lime plaster was a common building material for wall
surfaces in a process known as lath and plaster, whereby a series of wooden
strips on a studwork frame was covered with a semi-dry plaster that hardened
into a surface. The plaster used in most lath and plaster construction was
mainly lime plaster, with a cure time of about a month. To stabilize the lime
plaster during curing, small amounts of plaster of Paris were incorporated into
the mix. Because plaster of Paris sets quickly, "retardants" were
used to slow setting time enough to allow workers to mix large working
quantities of lime putty plaster. A modern form of this method uses expanded
metal mesh over wood or metal structures, which allows a great freedom of design
as it is adaptable to both simple and compound curves. Today this building
method has been partly replaced with drywall, also composed mostly of gypsum
plaster. In both these methods a primary advantage of the material is that it
is resistant to a fire within a room and so can assist in reducing or
eliminating structural damage or destruction provided the fire is promptly
extinguished.
Lime plaster is used for frescoes, where pigments, diluted
in water, are applied to the still wet plaster.
Cement plaster
Cement plaster is a mixture of suitable plaster, sand,
portland cement and water which is normally applied to masonry interiors and
exteriors to achieve a smooth surface. Interior surfaces sometimes receive a
final layer of gypsum plaster. Walls constructed with stock bricks are normally
plastered while face brick walls are not plastered. Various cement-based
plasters are also used as proprietary spray fireproofing products. These
usually use vermiculite as lightweight aggregate. Heavy versions of such plasters
are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges
and vessel skirts.
Uses
In architecture
Plasterwork
Plaster may also be used to create complex detailing for use
in room interiors. These may be geometric (simulating wood or stone) or
naturalistic (simulating leaves, vines, and flowers) These are also often used
to simulate wood or stone detailing found in more substantial buildings.
In art
Many of the greatest mural paintings in Europe, like
Michelangelo's Sistine Chapel ceiling are executed in fresco, meaning they are
painted on a thin layer of wet plaster, called intonaco (in fact the general
term for plaster in Italian); the pigments sink into this layer so that the
plaster itself becomes the medium holding them, which accounts for the
excellent durability of fresco. Additional work may be added a secco on top of
the dry plaster, though this is generally less durable.
Plaster may be cast directly into a damp clay mold. In
creating this piece molds (molds designed for making multiple copies) or waste
molds (for single use) would be made of plaster. This "negative"
image, if properly designed, may be used to produce clay productions, which
when fired in a kiln become terra cotta building decorations, or these may be
used to create cast concrete sculptures. If a plaster positive was desired this
would be constructed or cast to form a durable image artwork. As a model for
stonecutters this would be sufficient. If intended for producing a bronze
casting the plaster positive could be further worked to produce smooth
surfaces. An advantage of this plaster image is that it is relatively cheap;
should a patron approve of the durable image and be willing to bear further
expense, subsequent molds could be made for the creation of a wax image to be
used in lost wax casting, a far more expensive process. In lieu of producing a
bronze image suitable for outdoor use the plaster image may be painted to
resemble a metal image; such sculptures are suitable only for presentation in a
weather-protected environment.
Plaster expands while hardening, then contracts slightly
just before hardening completely. This makes plaster excellent for use in
molds, and it is often used as an artistic material for casting. Plaster is
also commonly spread over an armature (form), usually made of wire, mesh or
other materials, a process raised details. For these processes, limestone or
acrylic based plaster may be employed.
In medicine
Plaster is widely used as a support for broken bones; a
bandage impregnated with plaster is moistened and then wrapped around the
damaged limb, setting into a close-fitting yet easily removed tube, known as an
orthopedic cast; however, this is slowly being replaced by a fibreglass
variety.
Plaster is also used within radiotherapy when making immobilization
casts for patients. Plaster bandages are used when constructing an impression
of the patients head and neck, and liquid plaster is used to fill the
impression and produce a plaster bust. Perspex is then vacuum formed over this
bust creating an immobilization shell.
In dentistry, plaster is used for mounting casts or models
of oral tissues. These diagnostic and working models are usually made from
dental stone, a stronger, harder and denser derivative of plaster which is
manufactured from gypsum under pressure. Plaster is also used to invest or
flask wax dentures, the wax being subsequently removed and replaced with the
final denture base material which is cured in the plaster mold
In fire protection
Plasters have been in use in passive fire protection, as
fireproofing products, for many decades.
The finished plaster releases water vapor when exposed to
flame, acting to slow the spread of the fire, for as much as an hour or two
depending on thickness. It also provides some insulation to retard heat flow
into structural steel elements, that would otherwise lose their strength and
collapse in a fire. Early versions of these plasters have used asbestos fibres,
which have by now been outlawed in industrialized nations and have caused
significant removal and re-coating work. More modern plasters fall into the
following categories:
fibrous (including
mineral wool and glass fiber)
cement mixtures
either with mineral wool or with vermiculite
gypsum plasters,
leavened with polystyrene beads, as well as chemical expansion agents to
decrease the density of the finished product.
One differentiates between interior and exterior
fireproofing. Interior products are typically less substantial, with lower
densities and lower cost. Exterior products have to withstand more extreme fire
and other environmental conditions. Exterior products are also more likely to
be attractively tooled, whereas their interior cousins are usually merely
sprayed in place. A rough surface is typically forgiven inside of buildings as
dropped ceilings often hide them. Exterior fireproofing plasters are losing
ground to more costly intumescent and endothermic products, simply on technical
merit. Trade jurisdiction on unionized construction sites in North
America remains with the plasterers, regardless of whether the
plaster is decorative in nature or is used in passive fire protection.
Cementitious and gypsum based plasters tend to be endothermic. Fireproofing
plasters are closely related to firestop mortars. Most firestop mortars can be
sprayed and tooled very well, due to the fine detail work that is required of
firestopping, which leads their mix designers to utilise concrete admixtures,
that enable easier tooling than common mortars.
In dentistry
Dentures (false teeth) are made by first taking a dental
impression using a soft, pliable material that can be removed from around the
teeth and gums without loss of fidelity and using the impression to creating a
wax model of the teeth and gums. The model is used to create a plaster mold
(which is heated so the wax melts and flows out) and the denture materials are
injected into the mold. After a curing period, the mold is opened and the
dentures are cleaned up and polished.
Safety issues
The chemical reaction that occurs when plaster is mixed with
water is exothermic in nature and, in large volumes, can burn the skin. In
January 2007, a student sustained third-degree burns after encasing her hands
in a bucket of plaster as part of a school art project. The burns were so
severe she required amputation of both her thumbs and six of her fingers.
Some variations of plaster that contain powdered silica or
asbestos may present health hazards if inhaled. Asbestos is a known irritant
when inhaled in powder form can cause cancer, especially in people who smoke,
and inhalation can also cause asbestosis. Inhaled silica can cause silicosis
and (in very rare cases) can encourage the development of cancer. Persons
working regularly with plaster containing these additives should take
precautions to avoid inhaling powdered plaster, cured or uncured. (Note that
asbestos is rarely used in modern plaster formulations because of its
carcinogenic effects.
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