Module 2 Unit 11 Glass and Ceramics PDF

UNIT 11: Glass and Ceramics TLO 7: Identify correctly what kind of glass they are encountering in their daily activities. Classify the actual ceramics from the concretes and glass based on the characteristics and properties of a ceramics. Analyze the safety...

UNIT 11: Glass and Ceramics TLO 7: Identify correctly what kind of glass they are encountering in their daily activities. Classify the actual ceramics from the concretes and glass based on the characteristics and properties of a ceramics. Analyze the safety and importance of having and handling sulfuric products. GLASS INDUSTRY Glass is a rigid, under cooled liquid having no definite melting point and sufficiently high viscosity to prevent crystallization. Glass is also found in nature, as the volcanic material obsidian and as the enigmatic objects known as tektites.  Glass in the common sense refers to a hard, brittle, transparent amorphous solid, such as that used for windows, many bottles, or eyewear.  In the technical sense, glass is an inorganic product of fusion which has been cooled to a rigid condition without crystallizing 126 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.   In the scientific sense the term glass is often extended to all amorphous solids (and melts that easily form amorphous solids), including plastics, resins, or other silica-free amorphous solids.  The term glass developed in the late Roman Empire. It was in the Roman glassmaking center at Trier, Germany, that the late-Latin term glesum originated, probably from a Germanic word for a transparent, lustrous substance.  Glass, chemically, is actually more like a liquid, but at room temperature it is so viscous or 'sticky' it looks and feels like a solid. At higher temperatures glass gradually becomes softer and more like a liquid. It is this latter property which allows glass to be poured, blown, pressed and moulded into such a variety of shapes.  It is neither a solid nor a liquid but exists in a vitreous, or glassy, state in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity.  Glass is cooled to a rigid state without the occurrence of crystallization; heat can reconvert glass to a liquid form.  Usually transparent, glass can also be translucent or opaque. Color varies with the ingredients of the batch. Reasons for the Usefulness of Glass Modern life just would not be possible without glass. Glass also plays an essential role in various scientific fields and in industry. The optical and physical properties of glass make it suitable for different applications  because of its transparency  high resistance to chemical attack  effectiveness as an electrical insulator  ability to contain a vacuum Types of Glass 1. Fused silica (or Vitreous silica)  Sometimes erroneously referred to as quartz glass  Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700°C. 127 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.   Because this glass has a high melting point and does not shrink or expand greatly with changing temperatures, it is suitable for laboratory apparatus and for such objects subject to heat shock as telescope mirrors 2. Alkali Silicates  Sand and soda ash are simply melted together  The only two-component glass of commercial importance  The addition of sodium carbonate (Na2CO3), known as soda ash, to produce a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the temperature of fusion to about 800°C.  However, a glass of this composition is water-soluble and is known as water glass.  uses include fireproofing, sealant, adhesive for paper, for detergents and as soap builders 3. Soda-Lime Glass (Commercial glass)  Most manufactured glass is a soda- lime composition used to make bottles, tableware, lamp bulbs, and window and plate glass.  Commercial glass is normally colorless, allowing it to freely transmit light, which is what makes glass ideal for windows and many other uses. Additional chemicals have to be added to produce different colours of glass such as green, blue or brown glass.  Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron(II) oxide. Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron(II) oxide (FeO) impurities of up to 0.1 wt% produce a green tint which can be viewed in thick pieces or with the aid of scientific instruments. 128 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  4. Lead Glass  The fine-quality table glass known as crystal is made from potassium-silicate formulas that include lead oxide.  Lead glass is heavy and has an enhanced capacity to refract light, which makes it suitable for lenses and prisms, as well as for imitation jewels.  Because lead absorbs high- energy radiation, lead glasses are used in shields to protect personnel in nuclear installations.  Commonly known as lead crystal, lead glass is used to make a wide variety of decorative glass objects.  It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide. The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be described as lead crystal.  Glass containing less than 24% PbO, is known simply as crystal glass. The lead is locked into the chemical structure of the glass so there is no risk to human health  In 1676, an Englishman named George Ravenscroft discovered that by adding lead oxide to the glass composition, a far more brilliant, sparkling glass could be produced than had ever been made before  Lead glass has a high refractive index making it sparkle brightly and a relatively soft surface so that it is easy to decorate by grinding, cutting and engraving which highlights the crystal's brilliance making it popular for glasses, decanters and other decorative objects.  These glasses are of very great importance in optical work because of their high index of refraction and dispersion  Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation.  Used for the production of electric light bulbs and neon-sign tubing 5. Borosilicate Glass 129 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.   Borosilicate glass contains borax as a major ingredient, along with silica and alkali. Noted for its durability and resistance to chemical attack and high temperatures, borosilicate glass is widely employed for cooking utensils, laboratory glassware, and chemical process equipment.  glass in the form of ovenware and other heat-resisting ware, better known under the trade name Pyrex  has a low expansion coefficient, superior resistance to shock, excellent chemical stability and high electrical resistance. 6. Glass Fibers  Glass fibers are produced from special glass compositions that are resistant to weather conditions  The very large surface area of the fibers makes them vulnerable to attack by moisture in the air.  Glass fiber has many uses from roof insulation to medical equipment and its composition varies depending on its application.  It is possible to produce fibers that can be woven or felted like textile fiber by drawing out molten glass to diameters of a few ten- thousandths of an inch. Both long, continuous multifilament yarns and short-staple fibers 25 to 30 cm (10 to 12 in) long may be produced.  Woven into textile fabrics, glass fibers make excellent drapery and upholstery materials because of their chemical stability, strength, and resistance to fire and water. Glass fabrics alone, or in combination with resins, make excellent electrical insulation. By impregnating glass fibers with plastics, a composite fiberglass is formed that combines the strength and inertness of glass with the impact resistance of the plastic. 130 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  7. Special glasses Optical Glass  Optical glasses will be found in scientific instruments, microscopes, fighter aircraft and most commonly in spectacles.  Optical glass differs from other glass in the way in which it bends, or refracts, light. The manufacture of optical glass is a delicate and exacting operation. The raw materials must be of the highest purity, and great care must be taken so that no imperfections are introduced in the manufacturing process  The most important properties are the refractive index and the dispersion. The index is a measure of how much the glass bends light. The dispersion is a measure of the way the glass splits white light into the colors of the rainbow. Glass makers use the variations in these characteristics to develop optical glasses. Photosensitive/ Photochromic Glass  Photosensitive glass is similar to photographic film in that gold or silver ions in the material will respond to the action of light. This glass is used in printing and reproducing processes. Heat treatment following an exposure to light produces permanent changes in photosensitive glass.  Photochromic glass darkens when exposed to light but fades to its original clear state when the light is removed. This behavior is achieved by the action of light on extremely small silver chloride or silver bromide crystals distributed throughout the glass.  Photochromic glass finds a natural use in spectacle lenses that darken into sunglasses when in the sun and lighten again when removed from sunlight. The field of electronics also finds uses for this kind of glass. Glass Ceramics  Glass containing certain metals will form a localized crystallization when exposed to ultraviolet radiation. If heated to high temperatures, the glass will convert to 131 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  crystalline ceramics with mechanical strength and electrical insulating properties greater than that of ordinary glass.  Such ceramics are now made for such uses as cookware, rocket nosecones, and space-shuttle tiles. Other metallic glasses— including alloys of pure metals— can be magnetized, are strong and flexible, and prove very useful in high- efficiency electrical transformers.  Some of these "Glass ceramics", formed typically from lithium aluminosilicate glass, are extremely resistant to thermal shock and have found several applications where this property is important, including cooker hobs, cooking ware, windows for gas or coal fires, mirror substrates for astronomical telescopes and missile nose cones.  An essential feature of glass is that it does not contain crystals. However, by deliberately stimulating crystal growth in glass it is possible to produce a type of glass with a controlled amount of crystallization that can combine many of the best features of ceramics and glass. Raw Materials Classification of Raw Materials: Flux – compounds that lower the melting point of sand thereby facilitating the melting of the raw materials – compounds that promote fusion Stabilizers – compounds added to increase the durability of the glass Manufacturing Procedure A. MIXING AND MELTING After careful preparation and measurement, the raw materials are mixed and undergo initial fusion before being subjected to the full heat needed for vitrification. Most glass is melted in large tank furnaces, first introduced in 1872, that can hold more than 1,000 metric tons of glass and are heated by gas, oil, or electricity. The glass batch is fed continuously into an 132 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  opening (doghouse) at one end of the tank, and the melted, refined, and conditioned glass is drawn out the other end. B. SHAPING / FORMING When working glass in its plastic state, five basic methods are employed to produce an almost limitless variety of shapes: casting, blowing, pressing, drawing, and rolling. Casting In this process, known to the ancients, molten glass is simply poured into a mold and allowed to cool and solidify. In modern times centrifugal casting processes have been developed in which the glass is forced against the sides of a rapidly rotating mold. Capable of forming precise, lightweight shapes, centrifugal casting is used for the production of television-tube funnels. Glassblowing The revolutionary discovery that glass could be insufflated and expanded to any shape was made in the third quarter of the 1st century BC, in the Middle East along the Phoenician coast. Glassblowing soon spread and became the standard way of shaping glass vessels until the 19th century. The necessary tool is a hollow iron pipe about 1.2 m (about 4 ft) long with a mouthpiece at one end. The glassblower, or gaffer, collects a small amount of molten glass, called a gather, on the end of the blowpipe and rolls it against a paddle or metal plate to shape its exterior (marvering) and to cool it slightly. The gaffer then blows into the pipe, expanding the gather into a bubble, or parison. By constantly reheating at the furnace opening, by blowing and marvering, the gaffer controls the form and thickness. Pressing In this process, a gather of glass is dropped into a mold, and a plunger then squeezes the glass between itself and the outer mold and forms the final shape. Both the mold 133 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  and the plunger may be patterned to impart decorative design to the object being made. Drawing Molten glass can be drawn directly from the furnace to make tubing, sheets, fibers, and rods of glass that must have a uniform cross section. Tubing is made by drawing out a cylindrical mass of semifluid glass while a jet of air is blown down the center of the cylinder. Rolling Sheet glass, and plate glass in particular, was originally produced by pouring molten glass on a flat surface and, with a roller, smoothing it out prior to polishing both its surfaces. Later it came to be made by continuous rolling between double rollers. C. ANNEALING After being formed, glass objects are annealed to relieve stresses built up within the glass as it cools. In an oven called a lehr, the glass is reheated to a temperature high enough to relieve internal stresses and then slowly cooled to avoid introducing new stresses. D. FINISHING After annealing, a glass object may be embellished in a number of ways. Some of them are as follows: - In cutting, to produce cut glass, facets, grooves, and depressions are ground into the surface with rotating disks of various materials, sizes, and shapes and a stream of water with an abrasive. The steps are marking the pattern, rough cutting, smoothing, and polishing. - Designs are engraved by means of a diamond point or a metal needle, or with rotating wheels, generally of copper.  In the etching process intaglio decoration is achieved with acid, the results varying from a rough to mat finish. 134 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.   In sandblasting, fine grains of sand, crushed flint, or powdered iron are projected at high speed onto the glass surface, leaving a design in mat finish.  In cold painting, lacquer colors or oil paints are applied to glass but are not affixed by firing.  In enamel painting, enamel colors are painted and then fused onto the surface in a low-temperature firing.  In gilding, gold leaf, gold paint, or gold dust is applied to glassware and sometimes left unfired; low-temperature firing, however, is necessary for permanency. CERAMIC INDUSTRY 135 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  Sometimes referred to as the clay products or silicate industries. The word ceramic may be used as a singular noun but is more often used as an adjective meaning inorganic, non- metallic. Origin: Egypt and Mesopotamia - cradle of pottery development China: Chou and Han Dynasty – started developing the ceramic industry POTTERY MAKING – is one of the most ancient of human industries Characteristics of Ceramic Products 1) Withstand high temperatures 2) Resist greater pressures 3) Have superior mechanical properties 4) Possess special electrical characteristics 5) Can protect against corrosive chemicals Types of Ceramic Products 1) Whitewares - is a generic term for ceramic products which are usually white and of fine texture. - selected grades of clay bonded together with varying amounts of fluxes and heated to a moderately high temperature in a kiln (1200 to 1500 C) (a) Earthenware - sometimes called semi- vitreous dinnerware, is porous and non- translucent with a soft glaze (b) Chinaware - a vitrified translucent ware with a medium glaze which resists abrasion to a degree; it is used for non- technical purposes (c) Porcelain - is a vitrified translucent ware with a hard glaze which resists abrasion to the maximum degree - it includes chemical, insulating and dental porcelain - it is the highest quality of clayware (the glaze and body are fused into one) 136 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  (d) Sanitary ware - formerly made from clay, usually porous; hence a vitreous composition is presently used. - prefired and sized vitreous grog is sometimes included with the triaxial composition (e) Stoneware – one of the oldest ceramic wares, regarded as crude porcelain not so carefully fabricated from raw material of poorer grade (f) Whiteware tiles – generally classified as floor tiles, which are resistant to abrasion and impervious to stain penetration and may be glazed or unglazed 2) Structural Clay Products - building bricks, face brick, terra-cotta, sewer pipe and drain tile - made from the cheapest of common clays with or without glazing. 3) Refractories - acidic, basic or neutral; materials of high resistance to thermal, physical and chemical effects suitable for furnace construction - sold in the form of firebricks (neutral), silica (acid), chromite, magnesite (strongly basic), magnesite-chromite bricks; silicon carbide and zirconia refractories, aluminum silicate and alumina products 4) Specialized ceramic products 5) Enamels and enameled metal In 1676, before. lead more produced oxide an Englishman brilliant, to than discovered thesparkling had named glass ever composition, that glass been bycould adding made a far be Enamel - a smooth, durable coating made of melted and fused glass powder  Ravenscroft George Basic Raw Materials The three main raw materials in making ceramic products are clay, feldspar and sand. 1) CLAY MINERALS 137 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  Clay - an earthly or stony aggregate consisting essentially of hydrous silicates of alumina and it is plastic when sufficiently pulverized and wetted, rigid when dry, and vitreous when fired at sufficiently high temperature (a) Kaolinite - Al2O3.2SiO2.2H2O (b) Montmorillonite - (Mg, Ca)O.Al2O3.5SiO2.nH2O (c) Illite - K2O, MgO,Al2O3,SiO2,H2O in variable amounts 2) FELDSPAR - fluxing constituent in ceramic formulas 3 common types: (a) Potash feldspar (Microline) – K2O.Al2O3.6SiO2 (b) Soda feldspar (Albite)- Na2O.Al2O3.6SiO2 (c) Lime feldspar (Anorthite) – CaO. Al2O3.6SiO2 3) SAND or FLINT or QUARTZ - a refractory constituent which must contain low iron if chosen for light-colored ceramic products Nepheline Syenite – (Na,K)2Al2Si2O8 or K2O.3Na2O.4Al2O3 - a quartz-free igneous rock used extensively in whitewares - it is a more active flux than feldspar 4) Fluxing agents and special refractory ingredients (variety of other minerals, salts and oxides) - Additives to facilitate melting (mixtures have lower melting points than the respective pure components) - Principal refractory oxides – SiO2, Al2O3, CaO,and MgO - Principal fluxing oxides – Na2O, K2O, B2O3 and SnO2 with fluorides. 138 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  Chemical Conversions 1)Drying – 20 – 150°C 2)Dehydration or chemical smoking – 150 - 650°C (Removal of chemically combined water – 600 -650°C) 3)Calcination 600 - 900°C 4)Oxidation of ferrous iron and organic matter –350 - 900°C 5)Amorphous alumina transferred to crystalline alumina at 940°C 6)Silicate formation – 900 or 1000°C and above Dehydration of clay: Al2O3.2SiO2.2H2O → Al2O3 + SiO2 + 2H2O ( 600- 650°C) Formation of mullite at 1000°C: 3(Al2O3.2SiO2.2H2O) → 3Al2O3.2SiO2 + 4SiO2 + 6H2O Kaolinite mullite cristobalite NOTE: - Any ceramic body is composed of a vitreous matrix plus crystals of which mullite and cristobalite are two of the most important. - The degree of vitrification of ceramic products depends upon the relative amounts of refractory and fluxing oxides in the composition, the temperature and the time of setting Manufacturing Procedure I. STORING - no foreign materials / impurities especially iron should enter the raw materials II. CRUSHING and GRINDING – only coarse raw materials like quartz and feldspar are crushed and ground before keeping them into the fines storage III. BATCHING - proper amount of each raw material are weighed and fed in ball mills with the addition of water amounting to 15% to as high as 50%. Deflocculants in the form of sodium silicates or sodium carbonate amounting to 0.3% are added 139 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  (Deflocculants are added to allow uniform settling irrespective of particle size and therefore assures uniformity of slip composition and to control fluidity) SLIP - creamy mixture of raw materials IV. FINE GRINDING (BALL MILLING) - the raw materials are milled in ball mills V. SCREENING - the slip is run through a vibrating screen which removes bits of woods, metal, lignate and other solids and organic impurities VI. MAGNETIC SEPARATION - the slip is passed over a magnetic separator consisting of strong electromagnets to remove iron particles which may stain the fired ware VII. SLIP STORAGE - the slip is kept in storage cisterns provided with paddles which continuously stir the slip in order to provide uniform composition of the slip. VIII. FORMING THE WARES – from storage, the slip is pumped to the various forming departments Methods of forming: 1. slip casting method - the slip usually containing 35% water is pumped from storage to the slip tank from where the pouring may be done with a rubber hose or with a suitable container to the dry plaster molds - the products made by slip casting are table wares, art wares, chemical and electrical porcelain, sanitary wares and glass pots 2. press method (stiff-mud process) 1) From storage, the slip is pumped to the filter presses where water is removed forming a cake with a final water content of 18% to 25% (Pressure = 80 – 120 psi) 2) The cake is dried by air to remove as much water as possible. 3) The dried cake are ground to control particle size 4) Moisture is then sprayed while mixing until the moisture content is 4 – 12% and can be as high as 20% 5) Cake is then formed to wares press (screw press, toggle press, semi- automatic and fully-automatic) 3. Jiggering method 140 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  1) After filtering, the cake is kneaded in a pug mill or wet pan to allow the uniform distribution of water throughout the mass because the cake is usually wetter in the inside than in the outside, also to remove air 2) Aging – the kneaded cake are aged to ensure uniform water distribution and to develop maximum plasticity of the mass by hydration and bacterial action 3) De-airing – done by de-airing machines to ensure complete removal of entrapped and absorbed air from the cake which has been removed by kneading 4) Jiggering – potter’s wheel or a jigger machine which is a modified of potter’s wheel is used to form the wares. A mold may or may not be used. The products made by jiggering are dishes, hallow table wares, insulators, crucibles 4. Extrusion method (stiff-mud process) -after filtration, kneading, aging and de-airing, the cake is extruded to form the wares 5. Hand-molding (soft-mud process) - after filtration, the cake maybe formed into wares by hand with the aid of a potter’s wheel THROWING - the method of shaping the mass by hand molding with the aid of potter’s wheel - the products of hand-molding are art wares, figurines, ceramic jewelry, other small ware articles IX. DRYING - removal of water (shrinkage water) or pore water X. FINISHING STEPS (a) trimming (fettling) – the fins at the joints or edges are trimmed while the body is “leather hard” (b) burnishing – surface is rubbed with a hard or smooth object to polish the surface (c) wet and dry finishing wet – wiping off hollows, pimples due to pin holes and rounding drain holes and top edges with a wet sponge dry – humps are removed with sand paper and dusts by air-blasting or brushing 141 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  (d) repairing a piece – instead of discarding a defective piece, justifiable repair may be performed on wares when much work has already gone into the ware XI. FINAL DRYING - to increase efficiency of the kiln and minimize defects due to firing with moisture, the ware is placed in a warm place for several days. XII. FIRING – the wares are fired in kilns XIII. UNDERGLAZED DECORATIONS - colors and decorations are applied to the wares before glazing them with transparent glazes and firing the color and the glaze together. Color should not react with the glaze. XIV. GLAZING – glazes are prepared like clay slips. The glaze composition must bond with the ware and must have a coefficient of expansion sufficiently close to that of the ware to avoid defects. Purposes: 1. To increase the durability of the product 2. increase the usefulness 3. increase the beauty of product by producing a variety of colors and texture not possible on the body itself 4. Enhance decorative usefulness of product 5. Improve sanitation and cleanliness 6. Improve electrical and other physical properties 7. To conceal texture and color of the body Methods of glazing: 1) Brushing – difficult to attain an even layer (small amount of slip is used) 2) Dipping – even coating is attainable (large amount of slip is required) 3) Pouring the glaze slip – slower than dipping but requires smaller amount of slip than dipping 4) Spraying (best method) – just enough amount of glaze to cover the piece is required and even coating is attained 5) Vapor glazing – the vapors from glaze deposit a very thin coat into the piece 6) Salt glazing – adding to the fire common salt which at once volatilizes and combines with the clay surface, done for stone wares and special kiln must be used for this purpose 142 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.  Glazing defects: 1) Crawling / creeping – glaze form into beads 2) Crazing – fine cracks due to high coefficient of expansion of the glaze 3) Shivering – reverse of crazing, glazes flakes off particularly on curved surfaces due to low coefficient of expansion 4) Pinholing – small holes on glazed surface due to rapid expansion 5) Blistering (bleb) – defects which causes pinholing and small bubbles 6) Peeling – due to soluble salts, glaze peels off the body 7) Spitting out – glazes becomes rough due to ware being fired damped Colors of glazes: 1) cobalt oxide (CoO or Co2O3) – various shades of blue, violet or black 2) copper oxide (CuO) – shade of green 3) chromium oxide (Cr2O3) – greens, red and pink 4) Iron oxide (FeO) – yellow to brownish red 5) Gold chloride (AuCl3) – pink rose, purple, gold luster 6) Manganese oxide (MnO2) – shades of violet, red, black brown 7) Antimony oxide (Sb2O3) - yellow and orange 8) Barium chromate (BaCrO4) – lemon yellow to pale green 143 Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.