ABS PLASTICS

Post 284 –   by Gautam Shah

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Elektroauto_Hotzenblitz_Buggy_mit_ABS-Karosserie

ABS (Acrylonitrile-butadiene-styrene) has been described as wonder or Industrial plastic. Styrene Acrylonitrile copolymers were available since 1940’s, but introduction of a Butadiene as a third component in the 1950s created a range of ABS plastics.

ABS high impact resistance cover

ABS polymers are not affected by water, salts, inorganic acids, food acids and alkalis, alcohols and animal, vegetable and mineral oils. ABS plastics are soluble in ketone, swell or soften in some chlorinated hydrocarbons, esters, aromatics and aldehydes.

ABS in home goods

ABS can be moulded, extruded, vacuum-formed, blow moulded, rotational moulded and chrome plated. Moulding at a high temperature improves the gloss and heat resistance; however, high impact resistance and strength, are available by moulding at low temperature. ABS plastics are used largely for mechanical purposes, they also have remarkable electrical properties that are fairly constant over a wide range of frequencies and unaffected by temperature and atmospheric humidity. ABS is damaged by sunlight, causing a widespread and expensive recall of automobiles in US history.

Automobile Interior trims of Plain and Chrome plated ABS

ABS is used for brief cases, suit cases, printer bodies, remote control bodies, television bodies, hair dryers, textile bobbins, toys including Lego and Kre-O bricks, plated car linings and trims, bumpers, bars, furniture, protective headgear, hardware, water-taps, sanitary ware, golf-club heads. ABS raw material colour is ivory to white, but can be pigmented. ABS plastic ground down to a dia. of less than one micrometer is used as the vivid colourant in some tattoo inks.

ABS LEGO bricks

Box Handle Isolated Luggage

Its glass transition temperature is 105 °C. ABS. It is amorphous and therefore has no true melting point. ABS can be used between -20 and 80 °C, though its mechanical properties vary with temperature. ABS plastics are self-extinguishing, but flammable at high temperatures. It melts, then boils, when its vapours may burst into flames. On burning ABS does not produce any organic pollutants except carbon monoxide and hydrogen cyanide.

Automobile parts

ABS is a Terpolymer (three-way polymer) made by polymerizing styrene and acrylonitrile in the presence of poly-butadiene. It is formed of a long chain of poly-butadiene crossed with shorter chains of poly-styrene-co-acrylonitrile, but in varying proportions (from 15-35% acrylonitrile, 5-30% butadiene and 40-60% styrene), to achieve quality variants. The Acrylonitrile imparts chemical resistance and surface hardness, Butadiene contributes the impact strength and over all toughness, and Styrene helps in processing. ABS combines the strength and rigidity of acrylonitrile and styrene polymers with the toughness of poly-butadiene rubber.

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FLOOR SPREADS –TYPES, SIZES and SHAPES

FLOOR SPREADS –TYPES, SIZES and SHAPES

Post 283 ⇒   by Gautam Shah  →

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The words ‘Carpets, Rugs, Durries, Floor Spreads and Heavy Tapestries’ are often used synonymously. The quality, the sizes and the uses differentiate one from the other.

Early (pre 1900s) Carpets, Rugs, Durries, Floor Spreads and Heavy Tapestries were traditionally made for specific purposes and clientèle. Some of these were used as table covers, and floor spreads were hung. With industrial revolution, looms were automated and users increased manifold. The sizes, yet varied from country to country, and for the niche markets being served. The carpets and other items were earlier sold at places of production to tourists, but now vast quantities were sold in major commercial centres of the world.

Carpets are larger, nearly cover the room space or are extended ‘wall to wall’. Modern day carpets are made heavier, stiffer and softer, by the backing materials system. Some backing systems are fixed to the back face of the carpet, whereas others such as soft padding are laid on the floor. Backing system often include a static discharge network with earthed wires. Some of the pile and tufted carpets can have very heavy body construction, and may not require backing. Carpets either remain on the floor due to gravity, non skidding backing surface, or the large extensive surface. Most carpets are, however, affixed with carpet tacks or hooks at the periphery or by adhesives.

Rug

Rugs are smaller and nearly similar to carpets in terms of constitution. Rugs are simply spread on the floor, without backing or padding. Rugs being thinner and smaller can be rolled up. Rugs are also called throw-rugs because by throwing up in air one can unroll and spread it on the floor. Rugs are floor-spreads placed at specific locations within a room. Rugs are chiefly used for under the feet comfort. Rugs are also used to create places of interest or focus. Rugs are often laid over the carpet for the same reason. Rugs are spread over railings or parapets or hung on the walls. Rugs are thinner and simply reversing it dust can be removed. Rugs are formed as circular, elliptical and diamond shaped. The material is cut from larger woven material, and the edges are seamed with stitches, often with additional borders.

Durries

Durries are thinner than rugs. Durries can be folded up to 4/6 folds. Durries have tied weft ends or stitched edges. Durries have a simple weave and patterns are generally identical on both the faces. Traditional durries were made from waste, carded or combed wool, cotton or jute of small length or staple fibres. The coarseness of the yarn provides a dull finished surface. Durry could be very small 500x 500 mm (in India called Aasan), for one person to seat during religious ceremonies. Durries with finer cotton fibres are called Shetrunjees (India). These are small enough for a person to sleep (800x1800mm) and larger enough (2700x4500mm) to seat 20 people.

Cotton Rugs – Shetrunjees – floor spreads

Floor Spreads are made from cotton yarns, thin grasses and reeds, and synthetic flat yarns (such as polyester or Polypropylene). Floor-spreads are used for floor level dining, seating and sleeping. Reed floor-spreads, are called Chatais. Chatais are used as under spreads for mattresses, as prayer mats, dead body covers for burial and as hung screens over passages and openings. Modern day floor-spreads are non-woven mats of polypropylene fibres. Foamed base Rexine and other extruded composites.

Chatai -woven mat

Carpets of Asian countries came to Europe after 11th C. These were very rare and precious items to be placed under the feet. Carpets were display items as table covers and wall hangings. Persian carpets are categorised as for Farsh or floors, (5500x2500mm) are the larger units placed at the centre. This is flanked by two runners or strip carpets called Kanarehs, (5000x900mm) used for walking. The end piece of three carpet arrangement is called Kellegi, (3700x1800mm). Other then these are nomadic-rug (Qa-licheh=small rug) or carpets (called Klim or Saumak), and prayer spreads called Namazik.

Modern loom carpets are 12 Feet (3.7 m) and 15 feet (4.6 m) in the USA, and 4.0 and 5.0 Mts in Europe. Carpet tiles are cut from woven material or non-woven pressed fiber sheets. The edges are seamed by stitching, adhesives or heat-fused. Commercial sizes are 400x400mm. For very large spaces like halls (greater than 3.5mts) pieces of carpets are seams joined by stitching or adhesives.

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PLASTICS -the beginnings

Post 282 – by Gautam Shah

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Plasticity of Clay

Man had Clay as the first material that could be formed to desired shape. Clay gains ‘plasticity’ -moulding or shaping capacity, due to its grain shape, size and distribution and addition of water. A natural metal nodule or a purified one from the ore, on heating also became, ‘plastic’. This property was unavailable with materials like wood and stone. Materials like Bamboo or Cain, have the capacity to bend but cannot be shaped or moulded.

640px-Fountain_pens_and_converters

Historically there were few natural materials that exhibited the plastic behaviour. Bitumen was used as a water proofing material for boats and also as a joint material for masonry. Plant-based starch materials on being cooked showed flow behaviour.

Bitumen

A plastic material can ‘flow’, is ductile and so can be moulded or shaped, with application of pressure or heat. Plasticity is a property of material to be deformed repeatedly without rupture by the action of a force, and remain deformed after the force is removed. Materials commonly known as ‘plastics’ are polymeric compounds that show elastic and viscous components.

Natural Rubber

Several natural plastic materials were known from ancient times, but without clear perception of categorical behaviour. Lac, an insect exudate was used as gum or joining material in India. The lac was used for cast mouldings since 1868. Rubber, a plant exudate was used since 1535, as water proofing material and for shoe making. Cattle’s horns were used for Lanterns during middle ages, however, materials with similar properties of horns were developed by treating casein -milk-proteins with lye. Casein was also used as gum material. During 1851 Rubber was combined with sulphur to form Ebonite. Several natural oils, such as Linseed, Castor, etc., was polymerised to form longer chain products.

The development of plastics actually began with formation a cellulose nitrate plasticized with camphor, as a substitute material for than widely used ivory balls for billiards in 1860. The product was patented under the trademark Celluloid. It was also used later in the manufacture of objects ranging from dental plates to men’s collars. Celluloid was commercially successful, despite its flameability and capacity deteriorate when exposed to light. Cellulose used for manufacturing the Celluloid was of plant origin. The first totally synthetic plastic was the phenol-formaldehyde resin, Bakelite. Other plastics introduced during this period include modified natural polymers such as a rayon, made from cellulose products.

Spectacles 1800-1840 probably US cattle horn Winterthur Museum

Bakelite electrical switches were part of early electrification. These were black to dark chocolate brown electrical switches mounted on ceramic base and had brass internal fittings. Bakelite was used for garment buttons, telephones and electric current proof handles. Celluloid films were part of early reels of movies. These were highly combustible and required extra ordinary protection in cinema projection rooms. Celluloid films were later replaced with polyester films. Celluloid white balls were used in Table tennis or ping-pong, as it was known in 1940-50s.

The development of plastics has evolved from the use of natural plastic materials such as shellac to chemically modified natural materials such as the Rubber, Nitrocellulose, etc. to completely synthetic materials such as Bakelite, etc. During 1800s, Goodyear developed a process of vulcanization of natural rubber, accelerating the development of plastics.

Natural Rubber crafts

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SPECIFICATIONS FORMATS

SPECIFICATIONS FORMATS

Post 281 ⇒   by Gautam Shah  →

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In Commerce, Industry and Government many different types of specifications are used for acquisition of products or assignments. The process depends on nature of relationship between the acquirer, or assigner on one side and the vendor, supplier, assembler or manufacturer on the other side. It depends on the traditions in the field. Format of specification is formed by the technology and quality of available public specifications used for the product or process.

BRAND-NAME SPECIFICATIONS

These are restrictive kind of specification limiting the bidding to equivalent products of a given one or few brands. The brands chosen are presumed to be the best, adequate or representative one in the market. It is killing the competition. The supplier of unspecified brand is required to show that offered product is indeed equivalent. To supply goods of the named brand, various dealers of the same product will have to contest, a condition unlikely to be allowed by the main distributor. Such a restricted practice is employed by Government Departments and organizations who do not wish to spend time or effort to define-search for product specifications. The procuring agency automatically gets the right to determine equivalence. This process has doubtful legitimacy, but is none the less practised.

LIST OF QUALIFIED PRODUCT (QPL)

Lists of qualified products are mainly formed by Government agencies as recommendation to their departments for purchase of commonly used items. In some countries Non governmental organizations (NGOs) set up filters to identify undesirable products and processes. Such lists often suggest uses and typical price range for such items. The criteria for formulating and updating a QPL are in a public domain, and periodically updated. A purchase mentions the code and calls for + or – over the standard price tag is usually in public domains.

In India, The Central Purchase Organization DGS&D (Directorate General of Supplies & Disposals) typically creates lists goods with approved rates. The term goods used in this manual apply generally to all articles, material, commodities, livestock, furniture, fixtures, raw material, spares, instruments, machinery, equipment, industrial plant etc. purchased or otherwise acquired for the use of Government but excluding books, publications, periodicals, etc. for a library.

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DESIGN SPECIFICATIONS

Design specifications mention dimensional and other physical requirements of the item. Design here means a method or scheme of creating or putting together an item. It is the most traditional kind of specification. Design specifications are prescription of what an entity should be in its completed form. These are also called Item Specifications, as the design details are itemised in terms of the execution, material’s technology or mode of execution. Design specifications show how the item must be created, and often with the additional information (but, usually less effectively) what the final product is intended to be (goal, dreams, perception). Here the problems arise, because a manufacturer or supplier is emphatically told what and how to produce or deliver. In most cases this means a demand for a very customised Item. It leaves no chance for the manufacturer or supplier to offer, technologically or economically superior item, or one from their own standard range.

Indian Space Programme Design specification assembly

PERFORMANCE SPECIFICATIONS

Performance specifications list the expectations how an entity should function or what it must deliver. Here the user communicates the requirements as to What will be an acceptable product, and How the adequacy of the product will be judged. The performance specification is more related to how a product performs or functions and at what cost, and less related to its dimensions, materials or configuration. The vendor gets substantial freedom in offering the most appropriate technology. For such specifications it is mandatory to explain in detail the results required and how these be will be checked. All performance requirements must be matched with tests for adequacy. There is a tendency to demand performance requirements that are very high in comparison to actual projections, which leads to cost escalation.

Problems arise when test methods for judging adequacy of a product could require a ‘Destructive Testing’ or a ‘Laboratory or Plant-based facility’. Full activation or critical testing of an atomic reactor may not be feasible, or a long term performance of material cannot be checked in any setup. The provision of assurance by the supplier becomes very important.

Performance requirements

OPERATIONAL SPECIFICATIONS

Operational specifications have lesser bearing on how an item is created or procured, but relate to the working of a system. These relate to the functioning of the item, and for that reason product formation, delivery, installation processes must have built in strategy for operation or conduction with optimum efficiency. Operational specifications are not performance specifications, but details about mitigating risks arising out of operation of a system.

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WINDOWS and VENTILATION

Post 280 – by Gautam Shah

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640px-Smoke_-_Padaung_Ethnic_Group_-_Myanmar_Travel_Essentials

Windows served two main functions for interior spaces: Ventilation and Illumination. To this was added the view out with the advent of glass. The window became part of shop front and it served the purpose of view in.

628px-Varanasi_through_a_window_(8015855328)

For several centuries a window was a minor entity for Illumination of domestic interiors. The door provided enough day time illumination. Domestic finer activities, such as the needle craft were conducted just inside or outside the threshold of the door. In deeper spaces, such as inner rooms, roof holes provided basic illuminance. In early public buildings, illumination was provided through smaller openings covered with parchment or alabaster. The areas of window opening though small, was distributed over a larger surface made available through increased interior heights.

Door as the only opening in the dwelling

In a tropical house admission of light is usually accompanied by heat gain, but the breeze coming through a door balances the interior environment. In tropical climates interiors tend to be darker to reduce the heat gain compared to colder climates where greater illumination is perceived as warmth. Naturally illuminated lit spaces are perceived to be healthier.

Ventilation in extreme climates such as very warm and cold, occurs through the temperature gradient between outside and inside. In hot and humid climates, the temperature gradient is not acute enough to cause natural air movement of a sufficient quantum. The need for large volume air movement is significant for moisture control in hot humid areas.

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Ventilation requirements of an interior space vary depending on the number of occupants, nature of indoor cooking activities, fuels used for indoor heating and cooking, duality of distinct entry and exit points, the structure of the dwelling and scope for micro passive ventilation. To a smaller extent the ventilation needs are governed by the siting of the dwelling, such as the densely populated urban colony. Ventilation also depends on the nature of opening (cracks, crevices, holes), size of opening, number, distribution, location, orientation, and external climatic conditions (snow, rain, windy).

Micro ventilation

Cracks and gaps being unintentional are usually insufficient for heavier needs of ventilation and cooling or heating of spaces such as for toilets, kitchens, production areas and public spaces. Planned openings like windows on external face provide for such needs at the location, elevation, depth and in required quantity (such as a rate of air change -dilution, and the rate of air flow). The effectiveness of windows in achieving desired ventilation depends also on which windows are opened, how far they open, and the nature of shutter fixing.

steep-roof-3443865_1280

Ventilation requirements for a dwelling are regulated by the cooking activity. In hot arid climates cooking is done outside the house, in an attached facility or semi open lean-to shades. Kitchen areas are sited in isolated spaces or corners. Cooking with a centric hearth occurred where it also contributed heat for warming. Moisture dilution is an important factor of ventilation requirements. In hot humid climates water utilities like storage and usage (bathing and washing) are placed in the Chowk like interior courtyards, outside or away from the dwelling. According to cannons of Building design, the Vastu Shastra, place of water is in the North-East side. This orientation provides for exposure to south-west face, the warmest or Sun side in the Northern hemisphere.

Punjab India -Open air – outdoor cooking minimises internal ventilation needs

Ventilation is required to dilute the odours, moisture, carbon dioxide, airborne pollutants such as dust, smoke, volatile organic compounds (VOCs), latent heat from air, objects, etc. and encourage evaporation of body moisture and thereby cause cooling.

Windows provide ventilation, more effectively in rooms with internal doors (that is a door not opening to an exterior face), and especially when the exterior face door is closed for security reasons, such as at night. Movement of air between indoor spaces, and not the outside, is called transfer-air. Transfer-air has very little role in diluting the polluted air.

Windows placed on opposite sides and on same axis are better ventilating devices. The position of window vis a vis the work plane or task is determined whether one wants a draught-breeze over the body and the task, or avoid it. The nature of shutter opening also determines the direction of the internal air movement.

481px-Ilsted_The_open_door

Windows with shutters opening outward often obstruct the wind path, but double hung sash windows and sliding shutters which open within the frame are better as receptors. Casement window shutters with offset hinges or friction stay which create a small gap on the jamb side help in catching the breeze. Hoppers, awning and jalousie windows direct the breeze due to the angle of opening. The depth of a window and its surround also affects the nature of ventilation. Splayed sides create funnel effect to catch the breeze.

Mumbai Houses -One face for ventilation

Most building codes suggest minimum opening area (including doors, windows, etc.) @4 to 5% of the floor area. But actual ventilation requirements are higher such as during rainy days, moisture content is very high, or when during celebrations and social events lot of people gather in a room. Nominally openings (including doors) @20% of the floor area, are sufficient for the purpose of ventilation, provided some sections of the openings are located within the human height (1.75 mts). Even in unoccupied rooms some ventilation is required to remove fumes and moisture generated by materials, plants and condensation. Minimum volumetric requirements for ventilation are 23 to 25 CMt per person per hour, and 12 to 16 CMt per Kg of fuel burnt. Large sized openings create turbulent air movements, whereas cracks and crevices create a viscous or laminar flow.

Commonly ventilation is measured in terms of entire interior volume of air gets replaced per hour, it is called air changes per hour, ACH, but requirements for air for well being per person are also specified. Minimum 0.35 ACH, but the supplied air must be no less than 15 cfm/person or 7.5 l/s/person. Since 2003, the standards for ventilation have been changed on floor area basis which is from 3 CFM/100 sq. ft. or 15 l/s/100 sq. m. to the 7.5 CFM/person or 3.5 L/s/person. To find the total amount of outside air required, one needs to add 3 cfm/100 sq. ft. or 15 l/s/100 sq. m. to the 7.5 cfm/person or 3.5 l/s/person. Thus, the air change rate requirement will vary by the size of the house and the occupancy.

Ventilation is required for a fire emergency from areas like corridors, stairs, etc. Openings for ventilation are necessary for all climate conditions, but control requirements are very acute in warm and extremely cold climates, due to outward leakage of internal air.

1965_AMC_Ambassador_detail_of_vent_window

For adequate ventilation the building must take full advantage of prevailing breezes on the site. This includes consideration of: seasonal and diurnal wind patterns, land contours and other topographical features, shape and form of the building, height of the openings, axial position of the openings, work or task plane, physical state and age of the occupants, etc. Other important conditions are position of the window, the form of the surrounds and projections and design of the window shutter.

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JOB HANDLING in DESIGN ORGANIZATIONS

Post 279 – by Gautam Shah

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Organizations exploit both, the individual talents and traits of their employees. FIRST, persons with only required qualities are sought. SECOND, better compensations are offered for hiring specific qualities. THIRD, incentives are offered to individuals who show readiness to reformat their talents and traits. FOURTH, employees unable to convert are punished, or shifted out of the organization. Employees of the organization are motivated in different ways to modify or upgrade their expertise.

Rem_Koolhaas_SCL

Beyond paying out incentives, organizations use Job assignment as the key method, to exploit the human resources. Organizations divide their projects, assignments etc. into manageable lots or jobs of various skill and resources-based specialities. These are then assigned to individuals or teams as distinct roles. Jobs are presented as an opportunity, challenge, and incentive to a person or a team.

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Design creation processes have many stages such as project formulation, concept evolution, planning, detailing, job award processes, execution, client and consultant relationships. In very small offices few individuals take on many of these roles. In very large offices there could be several individuals or departments to handle these functions.

design process

A leader of the organization or project manager diverts a job from one to another person, to achieve diverse results. Jobs are assigned to remove the tedium of repetitions, or to provide new training or exposures. Jobs are also given out, to infuse new thoughts, work methods, and utilize different resources (plant, equipments, tools, talents). An organization becomes innovative and creative through such shifting of the personnel.

In design organizations personnel are identified in terms of their talent, and experience. In medium to large organizations common pools of human and other resources are formed. Project managers draw from such pools their requirements of human and other resources.

An organization is formed of employees of different talents and personality traits. These manifest in their attitude and conduct. A person may reflect multiple characteristics within a situation, or show a different personality if adequately motivated and conditioned.

sly-fox

Dream-weavers are prolific generators of ideas and new concepts, but lack the skill to detail them. The dream-weavers are mercurial and often have a fear of failure. A dream weaver must be an extrovert otherwise never gets acknowledged.

Technocrats have a talent of visualizing structured entities. For them an entity is conceivable, if it is structured and so practicable. Technocrats are fastidious, uncompromising, and hard-headed. A technocrat may get entwined in detailing the parts, and may lose the grasp of the holistic scheme.

Exponents enjoy advocating ideas or schemes, without bothering either its authorship or practicability. They feel that the public attention received through the advocacy is the measure of their skill and success.

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Patrons are not necessarily resourceful people, but are ready to support any new activity that takes them away from their routine chores, provides a novel experience, and keeps them busy. A person may become a sponsor by virtue of the position and powers to allocate resources. Such people are motivated by strategic gains through various sponsorships.

Arrangers are expert manipulators, and keenly look for a chance to jump into any difficult situation to manage it. As a risk taker they collect a lot of benefits, and very fast.

Orthodox are very over careful by personality. Their conservatism is due to a struggle less life or due to old age lethargy. They detest change, but if instrumental of causing even minor innovation, take a great pride.

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GYPSUM – PLASTER of PARIS

GYPSUM – PLASTER of PARIS

Post 278 ⇒   by Gautam Shah  →

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Gypsum mineral is found as layered sedimentary deposits in association with halite, anhydrite, sulphur, calcite and dolomite. It is a very common sulphate mineral found in all regions of the world, and is of great commercial importance.

Gypsum formation Capo Bianco

In well-developed crystals the mineral is selenite. The fibrous massive variety, called a satin spar is translucent and opalescent, and is used in jewellery. The fine-grained rock like variety is called alabaster, which is carved and polished for statuary and decorative items. Gypsite is the earthy and powdery variety.

Selenite crystal

Gypsum is in use by man, in the form of plastering and cementing (binding) material and in the form of alabaster for beads and other decorative items, for more than 12000 years.

The word Gypsum derives from the Greek, gyps =burned or baked (calcined) mineral, or the Greek word gypsos =chalk or plaster. Gypsum as a rock-material was known in Old English as spear stone due to its crystalline projections. Selenite was used in Greek, alluding to the pearly lustre (moon light) on cleavage fragments.

Plaster was discovered in Catal-Huyuk in Asia in an underground fresco, and in Israel Gypsum floor screeds were found from 7000 B.C. During the time of the Pharaohs, Gypsum was used as mortar in the construction of the Cheops Pyramid (3000 B.C.). In the Middle Ages and the Renaissance, decorations and artistic creations were made of plaster.

ALABASTER

Modern uses of Gypsum include, as sulphur donating additive in fertilizers, filler in paper and textiles, and setting time retarder in Portland cement. Substantial part of Gypsum in calcined form is used as plaster of Paris, Keene’s cement, board products, and ceiling tiles and blocks.

Common Gypsum is composed of Hydrated Calcium Sulphate (CaSO4 ·2H2O). Gypsum plaster is a white cementing material made by partial or complete dehydration of the mineral gypsum, sometimes with additional setting time retarding or hardening agents. Applied in a plastic state (with water), it sets and hardens by chemical recombination of the gypsum with water.

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PLASTER of PARIS

Plaster of Paris is a hemi-hydrate of calcium sulphate, composition CaSo4, ½ H2O made by calcining the mineral gypsum, at temperatures 120°–180° C (248°–356° F), creating a composition of CaSo4, 2H2O. The hemi hydrate calcium sulphate, in commercial forms contain many impurities, which govern the chemical behaviour. The powdered hemi-hydrate is mixed with water to form a paste or slurry, the calcining reaction is reversed and a solid mass of interlocking gypsum crystals with moderate strength is formed. The mixing of water with plaster causes an exothermic chemical reaction that releases heat. This heat helps to harden the Plaster of Paris, allowing it to set. Upon setting there is a slight contraction. This dimensional consistency makes it suitable for casts and its release. Different types of Plaster of Paris are available for different applications that have varying setting time, required quantity of mixing water, and hardness on drying. These characteristics are controlled by the calcination, additives, impurities, granular size-distribution, amount and ambient temperature conditions.

Mask making from Plaster of Paris

Rapid setting qualities may set even before complete hydration has taken place and so may not leave any time for tempering and rendering. Insufficiently hydrated mass attracts moisture later on, resulting in expansion, warping and peel off. Such failures can occur in low setting qualities, when for any reason the amount of hydration water and mixing efforts are insufficient. Particle size distribution is an important factor governing the behaviour of the Plaster of Paris. Too many fine particles quicken the setting without allowing full hydration of the mass. Sand and mineral lime particles improve the wettability of the initial mass and workability during casting or rendering. Hydraulic Lime on getting carbonated can increase the surface hardness at a later date.

Gypsum Ganesh Statue making

It is called Plaster of Paris because Paris became a centre for production of plaster material during the 1700s. It was than mainly used for coating masonry surfaces, as it set at a much faster rate than Lime plaster. It was also used to cover wooden surfaces to make them fire resistant. Plaster of Paris releases water vapour when exposed to a flame, making it ideal as a fire resistant material.

Dental cast

Plaster of Paris is used to make ceiling boards and ornamental cornices. It is also used for making casts and moulds for sculptures, forensic investigations, dentistry, jewellery and for immobilizing broken bones. Gypsum plasters are now used as leveller coating before painting, micro cracks and crevices filler.

Large surfaces of Plaster of Paris require some inter-mass reinforcing and substrate keying. Jute, cotton, threads, coir, hair, viscose, fibreglass, are some of the commonly used fibres for reinforcement. Masonry surfaces of bricks, rough stones and cement plasters offer sufficient substrate bonding. Wood, Metal and Polished stone surfaces require substrate keying through wood, metal lattices, netting or laths. Very thick sections or massive items require some technique for discharge of heat during hydration. This is done layer by layer application, use of cold water mixing, or colder atmosphere application. An entrapped heat quickens the rate hydration of wetted outer mass, but blocking the hydration of rest of the inner core mass. It may also crack the mass due to localized expansion.

For specific hard finish surfaces such as in commercial Gyp or Gypsum boards, the gypsum is completely dehydrated at high temperature, and with use of chemicals such as alkali sulphate, alum, or borax. For surface integrity fibres (short staples), semi digested paper pulp, lime or clay may be added to the plasters during manufacture. Gypsum boards are reinforced with paper or synthetic films on one or both sides.

Rodin Plaster form of The Three Shades

Many famous historical paintings in Europe are painted on a thin layer of wet plaster or Gesso. It is a white paint like coating of a binder gum mixed with chalk, gypsum, and sometimes a colourant, or any combination of these. It is preparatory surface for drawing or impressing outlines of artwork. The coating was applied on wood panels, canvas, sculptures and masonry walls and ceilings. Gypsum is used as a coating material over papier-mache forms.

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