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Salt, soda firing
Salt firing is a process where unglazed ware is fired to high temperatures and salt fumes are introduced into the kiln chamber (normally by a spray in the burner ports). The sodium in the salt combines with the silica and alumina in the clay to form a glaze. Salt glazed ware often has marbled and variegated surface effects and has a very distinctive look. Salt glazed ware is suitable for functional use.
Sodium vapor glazing using compounds other than sodium chloride (table salt) is practiced by many people. Many books are available on this topic and an increasing number of web pages on the Internet extol the process.
There has been quite a bit of discussion about the safety and environmental concerns of salt vs. soda firing. It does not appear to be a foregone conclusion that soda is better than salt or even that chlorine is released in salt firings (rather than HCL vapor). Recent reports on the Internet claim that measurements done on kilns have demonstrated that salt firing is as clean or cleaner than fuel reduction firing.
Pictures
Salt glazed vase
Plainsman P580, P600, H570 soda fired samples
Soda and salt kilns at the Medalta International Artists in Residence in Medicine Hat, Alberta, Canada. Designed by Aaron Nelson.
Rear of soda and salt kilns at the Medalta International Artists in Residence in Medicine Hat, Alberta, Canada. Designed by Aaron Nelson.
Semi-Matte Glaze
It is difficult to draw a line between what is matte and what is semi-matte from a visual inspection point of view. However from a production point of view it is much easier. Glazes generally want to be glossy, the vast majority of random glaze formulations would be glossy. Matte glazes, on the other hand, are difficult to create, there is a narrow range of chemistries wherein matte effects will develop such that the glaze is still well melted and does not cutlery mark or craze. Matte glazes can be such because of a micro-wavy light-scattering surface or because of crystallization, each mechanism has its own firing and process challenges to maintain. Companies generally configure their process to make the glaze as matte as possible while still having good technical properties (actually some do relax the technical properties and tolerate some cutlery marking, for example). Thus, to them, a semi-matte is a relaxing of the stringent requirements of the matte effect, a movement toward an easier-to-manufacture product. The semi-matte range is quite narrow, small chemistry changes toward more gloss produce large shifts toward actual gloss in the fired glaze. Companies can measure the amount of semi-matteness by measuring the amount of reflected light from a glaze surface or comparison of surface micrographs.
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- (Glossary)
Matte Glaze
A glaze that is not glossy. Of course, unmelted gl... - (Articles)
G1214Z Cone 6 Matte Base Glaze This glaze was developed using the 1214W glossy as a starting point. This article overviews the type...
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Shino
The common reduction fired Shino glazes in North America are mostly gloss or semigloss, fat, white crackle glazes with some orange to red and dark red from iron in the clay body or iron slips under the glaze. Shinos were born in Japan as almost pure high fired feldspar glazes. Shino on porcelain requires underglaze iron bearing slips, or in-glaze iron (i.e. from an iron-containing kaolin). Shinos are fluxed with soda spars and nepheline syenite and do not contain calcium because it dissolves the iron and inhibits the red color. Shinos usually do not have added silica, the silica being contributed by the feldspars and the clays in the glaze. Some Shinos contain Spodumene, which may be added to balance the high expansion soda spars. A simple Shino recipe is 70% Nepheline Syenite and 30% Kaolin.
Shinos crawl, craze, and pinhole in ways that are often decorative and pleasing. Carbon trapping is common with Shinos. The trapped carbon leaves random dark, shadowy areas in the glaze.
People who use Shino glazes then to be philosophers!
Shivering, peeling
A defect in glazed ware where the glaze is compressed by a body having a higher thermal expansion. While it is normal, even necessary for glazes to be under some compression to avoid crazing and improve ware strength, over compression will actually cause the glaze to peel off the ware on edges to relieve the stress. Shivering is the opposite of crazing.
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- (Glossary)
Crazing
Small hairline cracks in glazed surfaces that usua... - (Tests)
CIGF - Boiling Water:Ice Water Glaze Fit Test
- (Glossary)
Glaze Compression
Every solid has a thermal expansion, that is, an a...
Pictures
Example of serious glaze shivering using G1215U low expansion glaze on a high silica body at cone 6.
Si:Al Ratio
This number is reported by INSIGHT software as part of its calculation of the chemistry of a batch recipe. It singles out the silica and alumina oxide molecules and gives their relationship. For example, if there is 5.0 SiO2 and 0.5 Al2O3, then the ratio is 5.0:0.5 or 10:1, or just 10. This ratio is significant in stoneware glazes, for example, because high silica tends to produce glossy glazes when alumina is low and high alumina creates matte glazes when silica is low. It thus follows that the higher the Si:Al ratio the glossier a glaze will be.
However we must recognize that this ratio is not a general or fool-proof measure of gloss. Validity of the Si:Al ratio assumes a transparent glaze that is melting well and relatively free of other mechanisms that create matteness (e.g. high magnesia or calcia in low fire). Also remember that very low alumina glazes are fluid and encourage crystal formation during cooling; if these crystals are small they can completely cover the surface turning it matte.
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SiB:Al Ratio
This number is reported by INSIGHT software as part of the chemistry calculation of a batch recipe. It refers to the Silica:Boron combination campared to the amount of alumina. Since boron is also a glass former it needs to be considered in low and medium fire glazes. This ratio is not as clear an indicator of fired gloss as Si:Al because low fire glazes generally contain significant boron and thus operate by a 'different set of rules' than stoneware ones. For example, boron gloss glazes can take a lot more alumina into solution inthe melt than stoneware glazes even though alumina is very refractory. It is not typical to make a high alumina matte at low fire, generally mattes are made using high calcia or magnesia.
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Sieve, screen, lawn
Sieves are usually made from bronze or stainless steel wires. They are available in varying degrees of fineness and sizes are quoted according to the size of the opening or in wires per inch. An 40-60 mesh sieve is normally required to screen glazes to make sure they have no coarse particles that could disrupt the fired surface. In order for a porcelain to fire speck-free it would normally need to pass a 200 mesh ( about 75 micron opening) or finer screen.
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Silica:Alumina Ratio (SiO2:Al2O3)
The ratio of silicon dioxide to alumina oxide is often used as an indicator of glaze matteness. A glaze with high alumina thus has a low silica:alumina ratio. This ratio has some value because alumina stiffens the glaze melt (stiffer melts do not smooth out as well on cooling thus creating a fired surface that scatters light). There are obvious limitations to using this indicator since there are other mechanisms and influencers of glaze matteness (e.g. crystallization, firing temperature). Generally, the mechanism of matteness is never purely the alumina:silica ratio. The presence of boron can gloss even a high alumina mix. Of course firing temperature affects gloss (a glossy glaze will be matte if underfired). Firing range is another variable: at low temperatures high temperature fluxing oxides (e.g. CaO, MgO) turn into matting agents (via melt stiffening). In addition, the presence of zinc can cause crystallization and matteness in an otherwise glossy glaze that even has a very high ratio. CaO in large amounts is a matting agent in high and medium temperature glazes (via crystallization). Opacifers will affect mattness. Thus, if you are aware of all the mechanisms and interactions present, there are many cases where you can make use of the SiO2:Al2O3 ratio to some degree to control matteness by adjusting the ratio. The G1214Z recipe is a good example of this.
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- (Articles)
G1214Z Cone 6 Matte Base Glaze This glaze was developed using the 1214W glossy as a starting point. This article overviews the type... - (Glossary)
Matte Glaze
A glaze that is not glossy. Of course, unmelted gl... - (Glossary)
Dolomite Matte
Reduction fired cone 10 glazes that have a pleasan...
Silk screen printing
Screen printing is a technique to reproduce multicolor designs on tiles and flat surfaces. It is used in the sign painting industry and it is easy to find books on the process. Ceramic inks are available from suppliers or can be mixed from stain pigments and oil or glycerin bases.
Screens may be printed "on contact" or "off contact" The latter gives a sharper image, the screen is held off from the printed surface by a small distance (1/8" to 1/4") and as the squeegee is pulled, the screen is stretched down to make contact with the printed surface. After the squeegee passes the screen snaps back up. Improvisation is often to key to this process and incredible results are possible.
Sinter, sintering
Sintered clay has been fired high enough so that it no longer will slake or break down when exposed to water. Bisque fired ware is sintered. However the term sintering refers more to the particle bonding mechanism where particles are not glued together by the melting of a flux. Rather adjacent particles bond by the migration of species across the connection and by the deposition and buildup of material that has become gaseous in the kiln atmosphere. Refractories are often sintered to considerable strength. Sintered alumina bodies are very porous yet they can have a 'ring' like that of a fine porcelain.
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Slake, Slaking
'Slaking' refers to the breakdown that normally occurs when you immerse dried clay chunks or lumps in water (damp or wet lumps will not normally break down in the same manner because the wet clay resists the penetration of water). Typically the water attacks the surface and particles simply fall away. When slaking is complete a pile of fine material will be found settled on the bottom of the container of water, power mixing will then produce a slurry. Clays that slake well will break down in minutes if chunks are less than about 1 cm in size and all have exposure to the water. Very plastic clays may not slake since the wetting of the surface will cause swelling and act as a barrier to further water penetration. The slaking phenomenon makes it possible to slurry a raw clay mix and screen impurities from the slurry and then dewater, all without the need of grinding equipment.
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Slip Trailing, Incising
A technique of adding a relief pattern to a clay surface by applying a slip using a tube or nozzle (like cake decorating). Incising does the opposite, the pattern is cut into the surface using a ribbon tool. These techniques are often coupled with the use of contrasting colors or an overglaze that changes color at the edges of the relief to highlight the design.
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Slip, slurry, suspension
A slip or slurry is a suspension of clay and mineral particles in a water medium. It is typically either:
A glaze consisting primarily of clay ingredients. It is applied to once-fire ware. Slip glazes can be glossy or matte and any color or texture.
A clay slurry poured into molds to be cast into shapes. The slip is usually deflocculated to minimize water content and fine tune viscosity. The deflocculation process involves using special chemicals that enable you to create a fluid clay-water slurry with a very low water content.
Slurry Mixer, Propeller Mixer
An important tool in ceramics for mixing slurries. These come in lab sizes to huge industrial devices. Good mixers are invaluable in a lab (for mixing glaze and clay specimens) and they are fairly expensive (although they can be found on ebay.com).
Pictures
Simple propeller mixer with mount and switch (this 1/3 hp mixer can handle up to 10 gallons).
Soaking
The practice of holding the kiln at final firing temperature for a period of time. This is usually done to mature the clay and give the glaze opportunity to flow and heal imperfections. The advent of electronic kiln controllers has made it possible for anyone to soak. Soaking is especially advantageous for glazes with a stiff melt (i.e. low temperature zirconia whites) and for porcelains that require translucency, density, and glassy surfaces.
Soluble Colors, Sulfate Colors
Water soluble salt colors are used in porcelain tile for the surface decoration and in automated application systems (inkjet printing). Polyethylene glycol additive may be used to maintain viscosity and CMC gum for binding. In porcelain tiles these soluble salts penetrate into the surface and after subsequent polishing the design appears.
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- (Materials)
CMC Gum - GUM, Aqualon, C.M.C., C. M. C.
C.M.C.
Specific gravity
A comparison of the weights of equal volumes of a given liquid and water. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the container full of water and then full of the liquid of unknown specific gravity. Subtract the weight of the container from each weight and divide the weight of the liquid being measured by the weight of the water.
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- (Tests)
RHEO - Rheology of a Ceramic Slurry
Splitting
Refers to a phenomenon where a plastic clay develops cracks and splits open to relieve the self-support stresses it is subjected to. The process often occurs over a period of minutes. This typically happens on the bellies of thrown vases, rims of overhung bowls, on applied handles. This phenomenon is most pronounced in bodies having a particle size distribution exhibiting large percentages in the plus 100 mesh size range and where straight water is being applied to or left on the surface (the particles provide discontinuities in the surface where the water can enter). For example, if handles are being hand pulled from fresh clay and then applied immediately to the ware, the combination of water left on the surface and the weight of the handle itself will start at split at the point where the surface is being stretched the most. While meticulous avoidance of leaving water on any stressed surface can alleviate the problem on the bellies and necks of vessels, for handles it is generally better to pull handles first, and set them out to harden a little before applying them (using slip, not water).
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- (Tests)
SADR - Sieve Analysis Dry
- (Tests)
SIEV - Sieve Analysis 35-325 Wet
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Pictures
An example of how water can start a split in a plastic clay. This complete process occurred in about 1 minute.
Stain, Pigment
Stains are man-made colored powders used in glazes, bodies and engobes. They are manufactured by sintering (or even melting) components in special furnaces at high temperatures, this process renders them stable against dissolving in glaze melts or melting (unlike the metal oxides from which they are made). After firing the material is ground in such a way as to control the particle size within a specific range (often unique to each type of stain). Some stains are also acid washed after grinding. Different types of stains have differing levels of stability against temperature. For many colors there are a variety of stain chemistries that can produce it, each has advantages and disadvantages. Some colors, however, can only be made use one specific technology or chemistry. Many stains require a specific chemistry in the host glaze to develop the color.
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Stoneware
A high fired (about 1150C+) ceramic clay that is semi-vitreous (not translucent and not zero porosity). Industrial stonewares are typically refined and fire white or grey and are glazed with opaque and colored glazes. There is a fuzzy line between stoneware and whiteware (the latter has higher porosities and lower strengths because it is not fired to as high temperatures). Stonewares used by potters and traditional potteries generally employ less refined materials, they can fire brown, grey, buff or white. They also commonly have some speckle impurities and some particulate material such as sand or fine grog the impart a tactile surface. Stonewares are noted for their excellent working properties, stoneware throwing bodies, for example, are prized by potters (the clays used are ball clay based rather than kaolin based). The workability of these materials results from the fact that their recipe does not need to contain alot of feldspar or silica (these cut plasticity), the clay materials contain these naturally. These bodies generally cannot be fired to zero porosity without bloating.
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- (Glossary)
High Temperature Glaze
In functional ceramics this term generally refers ... - (Glossary)
Porcelain
A comparatively white burning clay body (unless st... - (Glossary)
Bloating
Bubbling that occurs in clay bodies if they are ov...
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Pictures
Brown and buff stoneware clays compared to a porcelain at 1300C in reduction. Courtesy of Plainsman Clays.
Stull Chart
These attempt to show graphically, on an x-y coordinate system, the fired results of a range of oxide mixtures. These graphs are generally used to help isolate, explain and predict how to achieve a specific result. For example, a graph may display results for a constant amount of flux with varying amounts of SiO2 and Al2O3 (the latter corresponding to the x and y of the graph). Within the graphic notations might explain zones of matteness, glossy, crystallization, etc.
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Pictures
Stull chart showing the SiO2-Al2O3-(0.7CaO+0.3KNaO) system
Sulfate Scum
A yellow coloration can appears on the dry glaze surface, or on the bisque clay surface on the opposite side of a vessel wall that has just been glazed. The coloration does not appear immediately, but some time during the dryout of the piece. This is a product of soluble or partially soluble sulfates in the clay which, when given extended contact with the water from the applied glaze, dissolve and migrate to the surface with it. This phenomenon can happen even with porcelains and it is the product of drying the ware too slowly. In industry, care is take to dry ware quickly so the solution and migration does not occur. This can be done by heating the ware or putting it in a drying chamber immediately after glazing.
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Sulfates, Sulphates
Sodium, potassium, magnesium sulfates can be found in many clays. These are soluble and often dealt with by the addition of barium carbonate to precipitate them. However, while the reaction that occurs produces insoluble chlorides of sodium, potassium, magnesium, these can have their own issues (eg. firing them generates of gases and fumes harmful to kiln refractories). Heavy clay industries can tolerate clays with higher sulfate contents, but other industries, such as tile, need lower contents).
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