Plaster
Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "render" commonly refers to external applications. The term stucco refers to plasterwork that is worked in some way to produce relief decoration, rather than flat surfaces.
The most common types of plaster mainly contain either gypsum, lime, or cement, but all work in a similar way. The plaster is manufactured as a dry powder and is mixed with water to form a stiff but workable paste immediately before it is applied to the surface. The reaction with water liberates heat through crystallization and the hydrated plaster then hardens.
Plaster can be relatively easily worked with metal tools and sandpaper and can be moulded, either on site or in advance, and worked pieces can be put in place with adhesive. Plaster is suitable for finishing rather than load-bearing, and when thickly applied for decoration may require a hidden supporting framework.
Forms of plaster have several other uses. In medicine, plaster orthopedic casts are still often used for supporting set broken bones. In dentistry, plaster is used to make dental models by pouring the material into dental impressions. Various types of models and moulds are made with plaster. In art, lime plaster is the traditional matrix for fresco painting; the pigments are applied to a thin wet top layer of plaster and fuse with it so that the painting is actually in coloured plaster. In the ancient world, as well as the sort of ornamental designs in plaster relief that are still used, plaster was also widely used to create large figurative reliefs for walls, though few of these have survived.
History
Plaster was first used as a building material and for decoration in the Middle East at least 5,000 years ago. In Egypt, gypsum was burned in open fires, crushed into powder, and mixed with water to create plaster, used as a mortar between the blocks of pyramids and to provide a smooth wall facing. In Jericho, a cult arose where human skulls were decorated with plaster and painted to appear lifelike. The Romans brought plaster-work techniques to Europe.Types
Clay plaster
Clay plaster is a mixture of clay, sand and water often with the addition of plant fibers for tensile strength over wood lath.Clay plaster has been used around the world at least since antiquity. Settlers in the American colonies used clay plaster on the interiors of their houses: "Interior plastering in the form of clay antedated even the building of houses of frame, and must have been visible in the inside of wattle filling in those earliest frame houses in which … wainscot had not been indulged. Clay continued in use long after the adoption of laths and brick filling for the frame." Where lime was not easily accessible it was rationed and usually substituted with clay as a binder. In Martin E. Weaver's seminal work he says, "Mud plaster consists of clay or earth which is mixed with water to give a 'plastic' or workable consistency. If the clay mixture is too plastic it will shrink, crack and distort on drying. Sand, fine gravels and fibres were added to reduce the concentrations of fine clay particles which were the cause of the excessive shrinkage." Manure was often added for its fibre content. In some building techniques straw or grass was used as reinforcement.
In the Earliest European settlers' plasterwork, a mud plaster was used McKee wrote, of a circa 1675 Massachusetts contract that specified the plasterer, "Is to lath and siele the four rooms of the house betwixt the joists overhead with a coat of lime and haire upon the clay; also to fill the gable ends of the house with ricks and plaister them with clay. 5. To lath and plaster partitions of the house with clay and lime, and to fill, lath, and plaister them with lime and haire besides; and to siele and lath them overhead with lime; also to fill, lath, and plaster the kitchen up to the wall plate on every side. 6. The said Daniel Andrews is to find lime, bricks, clay, stone, haire, together with laborers and workmen." Records of the New Haven colony in 1641 mention clay and hay as well as lime and hair also. In German houses of Pennsylvania the use of clay persisted.
Old Economy Village is one such German settlement. The early Nineteenth-Century utopian village in present-day Ambridge, Pennsylvania, used clay plaster substrate exclusively in the brick and wood frame high architecture of the Feast Hall, Great House and other large and commercial structures as well as in the brick, frame and log dwellings of the society members. The use of clay in plaster and in laying brickwork appears to have been a common practice at that time not just in the construction of Economy village when the settlement was founded in 1824. Specifications for the construction of, "Lock keepers houses on the Chesapeake and Ohio Canal, written about 1828, require stone walls to be laid with clay mortar, excepting 3 inches on the outside of the walls … which to be good lime mortar and well pointed." The choice of clay was because of its low cost, but also the availability. At Economy, root cellars dug under the houses yielded clay and sand, or the nearby Ohio river yielded washed sand from the sand bars; and lime outcroppings and oyster shell for the lime kiln.
The surrounding forests of the new village of Economy provided straight grain, old-growth oak trees for lath. Hand split lath starts with a log of straight grained wood of the required length. The log is split into quarters and then smaller and smaller bolts with wedges and a sledge. When small enough, a froe and mallet were used to split away narrow strips of lath. Farm animals provided hair and manure for the float coat of plaster. Fields of wheat and grains provided straw and hay to reinforce the clay plaster. But there was no uniformity in clay plaster recipes.
Manure provides fiber for tensile strength as well as protein adhesive. Unlike casein used with lime plaster, hydrogen bonds of manure proteins are weakened by moisture. With braced timber-framed structures clay plaster was used on interior walls and ceilings as well as exterior walls as the wall cavity and exterior cladding isolated the clay plaster from moisture penetration. Application of clay plaster in brick structures risked water penetration from failed mortar joints on the exterior brick walls. In Economy Village, the rear and middle wythes of brick dwelling walls are laid in a clay and sand mortar with the front wythe bedded in a lime and sand mortar to provide a weather proof seal to protect from water penetration. This allowed a rendering of clay plaster and setting coat of thin lime and fine sand on exterior-walled rooms.
Split lath was nailed with square cut lath nails, one into each framing member. With hand split lath the plasterer had the luxury of making lath to fit the cavity being plastered. Lengths of lath two to six foot are not uncommon at Economy Village. Hand split lath is not uniform like sawn lath. The straightness or waviness of the grain affected the thickness or width of each lath, and thus the spacing of the lath. The clay plaster rough coat varied to cover the irregular lath. Window and door trim as well as the mudboard acted as screeds. With the variation of the lath thickness and use of coarse straw and manure, the clay coat of plaster was thick in comparison to later lime-only and gypsum plasters. In Economy Village, the lime top coats are thin veneers often an eighth inch or less attesting to the scarcity of limestone supplies there.
Clay plasters with their lack of tensile and compressive strength fell out of favor as industrial mining and technology advances in kiln production led to the exclusive use of lime and then gypsum in plaster applications. However, clay plasters still exist after hundreds of years clinging to split lath on rusty square nails. The wall variations and roughness reveal a hand-made and pleasing textured alternative to machine-made modern substrate finishes. But clay plaster finishes are rare and fleeting. According to Martin Weaver, "Many of North America's historic building interiors … are all too often … one of the first things to disappear in the frenzy of demolition of interiors which has unfortunately come to be a common companion to 'heritage preservation' in the guise of building rehabilitation."
Gypsum plaster
Gypsum plaster, also known as plaster of Paris, is a white powder consisting of calcium sulfate hemihydrate. The natural form of the compound is the mineral bassanite. Although it is created out of gypsum, the powder is not itself gypsum, which is calcium sulfate dihydrate rather than hemihydrate.The name "plaster of Paris" was given because it was originally made by heating gypsum from a large deposit at Montmartre, a hill in the north end of Paris.
Gypsum plaster is produced by heating gypsum to about in a kiln:
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Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water.
Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time.
When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume. It is used in making casts for statues, toys, and more. The initial matrix consists mostly of orthorhombic crystals: the kinetic product. Over the next 72 hours, the rhombic crystals give way to an interlocking mass of monoclinic crystal needles, and the plaster increases in hardness and strength. If plaster or gypsum is heated to between, hemihydrate is formed, which will also re-form as gypsum if mixed with water.
On heating to, the nearly water-free form, called γ-anhydrite is produced. γ-anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above, the completely anhydrous form called β-anhydrite or dead burned plaster is formed.
Uses include:
- For making surfaces like the walls of a house smooth before painting them and for making ornamental designs on the ceilings of houses and other buildings.
- For making toys, decorative materials, cheap ornaments, cosmetics, and black-board chalk.
- A fire-proofing material.
- An orthopedic cast is used in hospitals for setting fractured bones in the right position to ensure correct healing and avoid nonunion. It keeps the fractured bone straight. It is used in this way, because when plaster of Paris is mixed with a proper quantity of water and applied around the fractured limb, it sets into a hard mass, thereby keeping the bones in a fixed position. It is also used for making casts in dentistry.
- chemistry laboratory for sealing air-gaps in apparatus when air-tight arrangement is required.