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" means a material used for the interiors of buildings, while "render" refers to external applications. Another imprecise term used for the material is stucco, often used for plasterwork, worked in some way to produce relief decoration, rather than flat surfaces; the most common types of plaster 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 before it is applied to the surface; the reaction with water liberates heat through crystallization and the hydrated plaster hardens. Plaster can be easily worked with metal tools or sandpaper, can be moulded, either on site or to make pre-formed sections in advance, which are put in place with adhesive. Plaster is not a strong material. Forms of plaster have several other uses.
In medicine plaster orthopedic casts are still used for supporting set broken bones. In dentistry plaster is used to make dental impressions. Various types of models and moulds are made with plaster. In art, lime plaster is the traditional matrix for fresco painting. In the ancient world, as well as the sort of ornamental designs in plaster relief that are still used, plaster was widely used to create large figurative reliefs for walls, though few of these have survived. Clay plaster is a mixture of clay and water with the addition of plant fibers for tensile strength over wood lath. Clay plaster has been used since antiquity. Settlers in the American colonies used clay plaster on the interiors of their houses: “Interior plastering in the form of clay antedated the building of houses of frame, 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 the use long after the adoption of laths and brick filling for the frame."
Where lime was not available or accessible it was rationed or substituted with other binders. In Martin E. Weaver’s seminal work he says, “Mud plaster consists of clay or earth, mixed with water to give a “plastic” or workable consistency. If the clay mixture is too plastic it will shrink and distort on drying, it will probably drop off the wall. Sand and fine gravels were added to reduce the concentrations of fine clay particles which were the cause of the excessive shrinkage.” Straw or grass was added sometimes with the addition of manure. In the Earliest European settlers’ plasterwork, a mud plaster was used or more a mud-lime mixture. McKee writes, 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. 5. To lath and plaster partitions of the house with clay and lime, to fill and plaister them with lime and haire besides. 6. The said Daniel Andrews is to find lime, clay, haire, together with laborers and workmen….”
Records of the New Haven colony in 1641 mention 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, used clay plaster substrate 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 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 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.
Other required building materials were sourced locally. 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 spit into quarters and 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 - unattainable with field trees and their many limbs. Farm animals pastured in the fields cleared of trees provided the hair and manure for the float coat of plaster. Fields of wheat and grains provided straw and other grasses for binders for the clay plaster, but there was no uniformity in clay plaster recipes. Straw or grass was added sometimes with the addition of manure providing fiber for tensile strength as well as protein adhesive. Proteins in the manure act as binders; the hydrogen bonds of p
Fresco is a technique of mural painting executed upon freshly laid, or wet lime plaster. Water is used as the vehicle for the dry-powder pigment to merge with the plaster, with the setting of the plaster, the painting becomes an integral part of the wall; the word fresco is derived from the Italian adjective fresco meaning "fresh", may thus be contrasted with fresco-secco or secco mural painting techniques, which are applied to dried plaster, to supplement painting in fresco. The fresco technique has been employed since antiquity and is associated with Italian Renaissance painting. Buon fresco pigment is mixed with room temperature water and is used on a thin layer of wet, fresh plaster, called the intonaco; because of the chemical makeup of the plaster, a binder is not required, as the pigment mixed with the water will sink into the intonaco, which itself becomes the medium holding the pigment. The pigment is absorbed by the wet plaster; the chemical processes are as follows: calcination of limestone in a lime kiln: CaCO3 → CaO + CO2 slaking of quicklime: CaO + H2O → Ca2 setting of the lime plaster: Ca2 + CO2 → CaCO3 + H2O In painting buon fresco, a rough underlayer called the arriccio is added to the whole area to be painted and allowed to dry for some days.
Many artists sketched their compositions on this underlayer, which would never be seen, in a red pigment called sinopia, a name used to refer to these under-paintings. Later,new techniques for transferring paper drawings to the wall were developed; the main lines of a drawing made on paper were pricked over with a point, the paper held against the wall, a bag of soot banged on them on produce black dots along the lines. If the painting was to be done over an existing fresco, the surface would be roughened to provide better adhesion. On the day of painting, the intonaco, a thinner, smooth layer of fine plaster was added to the amount of wall, expected to be completed that day, sometimes matching the contours of the figures or the landscape, but more just starting from the top of the composition; this area is called the giornata, the different day stages can be seen in a large fresco, by a sort of seam that separates one from the next. Buon frescoes are difficult to create because of the deadline associated with the drying plaster.
A layer of plaster will require ten to twelve hours to dry. Once a giornata is dried, no more buon fresco can be done, the unpainted intonaco must be removed with a tool before starting again the next day. If mistakes have been made, it may be necessary to remove the whole intonaco for that area—or to change them a secco. An indispensable component of this process is the carbonatation of the lime, which fixes the colour in the plaster ensuring durability of the fresco for future generations. A technique used in the popular frescoes of Michelangelo and Raphael was to scrape indentations into certain areas of the plaster while still wet to increase the illusion of depth and to accent certain areas over others; the eyes of the people of the School of Athens are sunken-in using this technique which causes the eyes to seem deeper and more pensive. Michelangelo used this technique as part of his trademark'outlining' of his central figures within his frescoes. In a wall-sized fresco, there may be ten to twenty or more giornate, or separate areas of plaster.
After five centuries, the giornate, which were nearly invisible, have sometimes become visible, in many large-scale frescoes, these divisions may be seen from the ground. Additionally, the border between giornate was covered by an a secco painting, which has since fallen off. One of the first painters in the post-classical period to use this technique was the Isaac Master in the Upper Basilica of Saint Francis in Assisi. A person who creates fresco is called a frescoist. A secco or fresco-secco painting is done on dry plaster; the pigments thus require a binding medium, such as egg, glue or oil to attach the pigment to the wall. It is important to distinguish between a secco work done on top of buon fresco, which according to most authorities was in fact standard from the Middle Ages onwards, work done a secco on a blank wall. Buon fresco works are more durable than any a secco work added on top of them, because a secco work lasts better with a roughened plaster surface, whilst true fresco should have a smooth one.
The additional a secco work would be done to make changes, sometimes to add small details, but because not all colours can be achieved in true fresco, because only some pigments work chemically in the alkaline environment of fresh lime-based plaster. Blue was a particular problem, skies and blue robes were added a secco, because neither azurite blue nor lapis lazuli, the only two blue pigments available, works well in wet fresco, it has become clear, thanks to modern analytical techniques, that in the early Italian Renaissance painters quite employed a secco techniques so as to allow the use of a broader range of pigments. In most early examples this work has now vanished, but a whole painting done a secco on a surface roughened to give a key for the paint may survive well
Hydraulic lime is a general term for varieties of lime, or slaked lime, used to make lime mortar which set through hydration. These contrast with varieties of air lime, the other common types of lime mortar, which set through carbonation. Hydraulic lime provides a faster initial set and higher compressive strength than air lime and eminently hydraulic lime will set in more extreme conditions including under water; the terms hydraulic lime and hydrated lime are quite similar and may be confused but are not the same material: hydrated lime is any lime, slaked whether it sets through hydration, carbonation, or both. Calcium reacts in the lime kiln with the clay minerals to produce silicates that enable some of the lime to set through hydration. There are two basic types of hydraulic limes: Natural hydraulic lime is produced by heating limestone that contains clay and other impurities: no materials may be added to create the hydraulicity. In the United States NHL may be called hydrated hydraulic lime per ASTM C-141 Standard Specification for Hydrated Hydraulic Lime for Structural Purposes.
Artificial hydraulic lime or artificial lime becomes hydraulic when hydraulic and/or pozzolan materials are added either before or after burning in a lime kiln. Artificial limes are more identified as hydraulic lime, as defined European Norm 459, "Consists of lime and other materials such as Portland cement, blast furnace slag, fly ash, limestone filler and other suitable materials.". It is identical to HL but its composition must be declared on the CE marking.". Consists of hydrated lime with one or more pozzolans with possible inclusion of inert filler; when Portland cement traces, is present, it has to be labeled as'PHLc'." Hydraulic lime is a useful building material for the following reasons: It has a low elastic modulus. There is no need for expansion joints, it allows buildings to "breathe", does not trap moisture in the walls. It has a lower firing temperature than Portland cement, thus consumes less energy. Stone and brickwork bonded with lime is easier to re-use. Lime acts sacrificially in that it is weaker and breaks down more than the masonry, thus saving weaker stone such as sandstone and limestone from the harmful effects of temperature expansion and mortar freeze.
It is thus less cold bridging. Lime re-absorbs the carbon dioxide emitted by its calcination, thus offsetting the large amount emitted during its manufacture; the more hydraulic a lime, the less CO2 is reabsorbed during set, for example, 50% of CO2 is reabsorbed by NHL 3.5 during the set, compared to 100% of CO2 being reabsorbed by pure calcium hydroxide. Hydraulic lime concretes have been in use since Roman times, either as mass foundation concretes or as lightweight concretes using tufa or pumice as aggregates and a wide range of pozzolans to achieve different strengths and speeds of set; this meant that lime could be used in a wide variety of applications including floors and vaults or domes. An example is the Pantheon in Rome; the dome's diameter is equal to its height from the floor. It is constructed from six different lime mixes, which change the properties and lightness of the material. Natural hydraulic lime is classified for different uses the first two of which are sometimes called semi-hydraulic lime because they set with water but continue to set in contact with carbon dioxide in the air.
Feebly hydraulic lime is used for external work in sheltered areas. Feebly hydraulic lime contains up to 10% clay/ clay mixed with other impurities, it might take one week or more to set after the addition of water. Setting is the process of permanently taking the shape into. Moderately hydraulic lime can be used for external work in most areas. Moderately hydraulic lime contains clay in the range of 11% to 20%; this type of lime sets within a few days after the addition of water. Eminently hydraulic lime is used for external work in exposed areas, such as chimneys and for floor slabs/underpinning. Eminently hydraulic lime contains clay in the range of 21% to 30%. Properties of eminently hydraulic lime are close to those of cement. Eminently hydraulic lime sets within one day after the addition of water. Hydraulic limes gain strength over time hence providing flexibility and avoiding the need for expansion joints. Considered to be more environmentally friendly than cement as they are burnt at a lower temperature and uniquely re-absorb some of the carbon dioxide given off during burning as they cure/carbonate in/on the wall.
Enable building components to be reclaimed and reused as they are'softer' than cement. Set under water hence making them ideal for applications in contact with the sea, rivers etc. Lime plaster Lime mortar Quick lime Hempcrete The Technology and Use of Hydraulic Lime, by John Ashurst The National Lime Association How to use hydraulic lime and PDF application fact sheets The Scottish Lime Centre
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
The Tarxien Temples are an archaeological complex in Tarxien, Malta. They date to 3150 BC; the site was accepted as a UNESCO World Heritage Site in 1992 along with the other Megalithic temples on the island of Malta. The Tarxien consist of attached, temple structures; the main entrance is a reconstruction dating from 1956. At the same time, many of the decorated slabs discovered on site were relocated indoors for protection at the Museum of Archaeology in Valletta; the first temple has been dated to 3100 BC and is the most elaborately decorated of the temples of Malta. The middle temple dates to about 3000 BC, is unique in that, unlike the rest of the Maltese temples, it has three pairs of apses instead of the usual two; the east temple is dated at around 3100 BC. The remains of another temple and older, having been dated to 3250 BC, are visible further towards the east. Of particular interest at the temple site is the rich and intricate stonework, which includes depictions of domestic animals carved in relief and screens decorated with spiral designs and other patterns.
Demonstrative of the skill of the builders is a chamber set into the thickness of the wall between the South and Central temples and containing a relief showing a bull and a sow. Excavation of the site reveals that it was used extensively for rituals, which involved animal sacrifice. Interesting is that Tarxien provides rare insight into how the megaliths were constructed: stone rollers were left outside the South temple. Additionally, evidence of cremation has been found at the center of the South temple, an indicator that the site was reused as a Bronze Age cremation cemetery; the large stone blocks were discovered in 1914 by local farmers ploughing a field. After the accidental discovery of the nearby Tarxien hypogeum in 1913, the proprietor of the land underneath which the temples were buried figured that the large stones that were continually struck by workers' ploughs may have had some archaeological value. On that notion, he contacted the director of the National Museum, Themistocles Zammit, who began to dig on his first inspection of the site, where he discovered the center of the temple compound.
It was not long before Zammit found himself standing in what appeared to be an apse formed by a semicircle of enormous hewn stones. Over the course of three years, Zammit enlisted the help of local farmers and townspeople for an excavation project of unprecedented scale in Malta. By 1920, Zammit had identified and carried out restoration work on five separate but interconnected temples, all yielding a remarkable collection of artifacts, including the famous "fat lady" statue, a representation of a Mother Goddess or a fertility charm and several unique examples of prehistoric relief, including ships; the temples were included on the Antiquities List of 1925. Further excavations at the temples were conducted in the post-World War II period under the directorship of J. G. Baldacchino. Protective tent-like shelters, similar to those at Ħaġar Qim and Mnajdra, were built around the Tarxien Temples in 2015, were completed in December of that year; the discovery of the complex did much to further Malta's national identity, solidly confirming the existence of a thriving ancient culture on the island.
The general interest aroused by the finds engendered for the first time a public concern for the protection of Malta's historical treasures, including a need for management of the sites, the promulgation of laws, other measures to protect and preserve monuments. At the same time, Themistocles' thorough method in excavating the site paved the way for a new scientific approach to archaeology. List of megalithic sites Listing at UNESCO World Heritage Center Official Tourist information National Inventory of the Cultural Property of the Maltese Islands
Amarna is an extensive Egyptian archaeological site that represents the remains of the capital city newly established and built by the Pharaoh Akhenaten of the late Eighteenth Dynasty, abandoned shortly after his death. The name for the city employed by the ancient Egyptians is written as Akhetaten in English transliteration. Akhetaten means "Horizon of the Aten"; the area is located on the east bank of the Nile River in the modern Egyptian province of Minya, some 58 km south of the city of al-Minya, 312 km south of the Egyptian capital Cairo and 402 km north of Luxor. The city of Deir Mawas lies directly west across from the site of Amarna. Amarna, on the east side, includes several modern villages, chief of which are el-Till in the north and el-Hagg Qandil in the south; the area was occupied during Roman and early Christian times. The name Amarna comes from the Beni Amran tribe that lived in the region and founded a few settlements; the ancient Egyptian name was Akhetaten. English Egyptologist, Sir John Gardner Wilkinson visited Amarna twice in the 1820s and identified it as'Alabastron', following the sometimes contradictory descriptions of Roman-era authors Pliny and Ptolemy, although he was not sure about the identification and suggested Kom el-Ahmar as an alternative location.
The area of the city was a virgin site, it was in this city that the Akhetaten described as the Aten's "seat of the First Occasion, which he had made for himself that he might rest in it". It may be that the Royal Wadi's resemblance to the hieroglyph for horizon showed that this was the place to found the city; the city was built as the new capital of the Pharaoh Akhenaten, dedicated to his new religion of worship to the Aten. Construction started in or around Year 5 of his reign and was completed by Year 9, although it became the capital city two years earlier. To speed up construction of the city most of the buildings were constructed out of mud-brick, white washed; the most important buildings were faced with local stone. It is the only ancient Egyptian city which preserves great details of its internal plan, in large part because the city was abandoned after the death of Akhenaten, when Akhenaten's son, King Tutankhamun, decided to leave the city and return to his birthplace in Thebes; the city seems to have remained active for a decade or so after his death, a shrine to Horemheb indicates that it was at least occupied at the beginning of his reign, if only as a source for building material elsewhere.
Once it was abandoned it remained uninhabited until Roman settlement began along the edge of the Nile. However, due to the unique circumstances of its creation and abandonment, it is questionable how representative of ancient Egyptian cities it is. Amarna was hastily constructed and covered an area of 8 miles of territory on the east bank of the Nile River; the entire city was encircled with a total of 14 boundary stelae detailing Akhenaten's conditions for the establishment of this new capital city of Egypt. The earliest dated stele from Akhenaten's new city is known to be Boundary stele K, dated to Year 5, IV Peret, day 13 of Akhenaten's reign, it preserves an account of Akhenaten's foundation of this city. The document records the pharaoh's wish to have several temples of the Aten to be erected here, for several royal tombs to be created in the eastern hills of Amarna for himself, his chief wife Nefertiti and his eldest daughter Meritaten as well as his explicit command that when he was dead, he would be brought back to Amarna for burial.
Boundary stela K introduces a description of the events that were being celebrated at Amarna: His Majesty mounted a great chariot of electrum, like the Aten when He rises on the horizon and fills the land with His love, took a goodly road to Akhetaten, the place of origin, which had created for Himself that he might be happy therein. It was His son Wa'enrē who founded it for Him as His monument when His Father commanded him to make it. Heaven was joyful, the earth was glad every heart was filled with delight; this text goes on to state that Akhenaten made a great oblation to the god Aten "and this is the theme, illustrated in the lunettes of the stelae where he stands with his queen and eldest daughter before an altar heaped with offerings under the Aten, while it shines upon him rejuvenating his body with its rays." Located on the east bank of the Nile, the ruins of the city are laid out north to south along a "Royal Road", now referred to as "Sikhet es-Sultan". The Royal residences are to the north, in what is known as the North City, with a central administration and religious area and the south of the city is made up of residential suburbs.
If one approached the city of Amarna from the north by river the first buildings past the northern boundary stele would be the North Riverside Palace. This building ran all the way up to the waterfront and was the main residence of the Royal Family. Located within the North City area is the Northern Palace, the main residence of the Royal Family. Between this and the central city, the Northern Suburb was a prosperous area with large houses, but the house size decreased and became poorer the furth
Drywall is a panel made of calcium sulfate dihydrate, with or without additives extruded between thick sheets of facer and backer paper, used in the construction of interior walls and ceilings. The plaster is mixed with fiber, foaming agent, various additives that can reduce mildew and water absorption. Drywall construction became prevalent in North America as a time and labor saving alternative to traditional lath and plaster; the first plasterboard plant in the UK was opened in 1888 in Kent. Sackett Board was invented in 1894 by Augustine Sackett and Fred Kane, graduates of Rensselaer Polytechnic Institute, it was made by layering. Sheets were 36 by 36 by 1⁄4 inch thick with open edges. Gypsum board evolved between 1910 and 1930 beginning with wrapped board edges and elimination of the two inner layers of felt paper in favor of paper-based facings. In 1910 United States Gypsum Corporation bought Sackett Plaster Board Company and by 1917 introduced Sheetrock. Providing efficiency of installation, it was developed additionally as a measure of fire resistance.
Air entrainment technology made boards lighter and less brittle, joint treatment materials and systems evolved. Gypsum lath was an early substrate for plaster. An alternative to traditional wood or metal lath, it was a panel made up of compressed gypsum plaster board, sometimes grooved or punched with holes to allow wet plaster to key into its surface; as it evolved, it was faced with paper impregnated with gypsum crystals that bonded with the applied facing layer of plaster. In 1936 US Gypsum trademarked ROCKLATH for their gypsum lath product. In 2002 the European Commission imposed fines totaling €478 million on the companies Lafarge, BPB, Knauf and Gyproc Benelux, which had operated a cartel on the market which affected 80% of consumers in France, the UK, Germany and the Benelux countries. A wallboard panel consists of a layer of gypsum plaster sandwiched between two layers of paper; the raw gypsum, CaSO4·2 H2O, is heated to drive off the water slightly rehydrated to produce the hemihydrate of calcium sulfate.
The plaster is mixed with fibre, foaming agent, finely ground gypsum crystal as an accelerator, EDTA, starch or other chelate as a retarder, various additives that may decrease mildew and increase fire resistance, wax emulsion or silanes for lower water absorption. The board is formed by sandwiching a core of the wet mixture between two sheets of heavy paper or fibreglass mats; when the core sets it is dried in a large drying chamber, the sandwich becomes rigid and strong enough for use as a building material. Drying chambers use natural gas today. To dry 1 MSF of wallboard, between 1,750,000 and 2,490,000 BTU is required. Organic dispersants/plasticisers are used so the slurry will flow during manufacture, to reduce the water and hence the drying time. Coal-fired power stations include devices called scrubbers to remove sulphur from their exhaust emissions; the sulphur is absorbed by powdered limestone in a process called flue-gas desulphurization, which produces a number of new substances. One is called "FGD gypsum".
This is used in drywall construction in the United States and elsewhere. Drywall panels in the United States are manufactured in 48,-54,-and-96-inch wide panels in varying lengths to suit the application, though 48-inch is by far the most common width. Lengths up to 16 feet are available, though the most common length is 8 feet. Common panel thicknesses are 1⁄2 and 5⁄8 inch, 1⁄4, 3⁄8, 3⁄4, 1 inch thicknesses are used in specific applications. In Europe, most plasterboard is made in 120-centimetre-wide sheets, though 60-and-90-centimetre-wide sheets are made. Plasterboard 120 cm wide is most made in 240 centimetres lengths, though 250, 260, 270, 280, 300 centimetres or longer are available. Thicknesses of plasterboard available are 9.5 to 25 millimetres. Plasterboard is made with one of three different edge treatments: tapered edge, where the long edges of the board are tapered with a wide bevel at the front to allow for jointing materials to be finished flush with the main board face. However, four-side chamfered drywall is not offered by major UK manufacturers for general use.
The term plasterboard is used in Australia and New Zealand, in the latter country it is known as Gibraltar board. Panels are sold in 1200 × 2400 mm, 1200 × 4800 mm, 1200 × 6000 mm sheets. Sheets are secured to either a timber or cold-formed steel frames anywhere from 150 to 300 mm centres along the beam and 400 to 600 mm across members. Various companies, such as Boral and CSR, manufacture plasterboard under various brand names including Gyprock; as an alternative to a week-long plaster application, an entire house can be drywalled in one or two days by two experienced drywallers, drywall is easy enough to use that it can