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Words beginning with S
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.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. It is the opposite of crazing. Our Magic of Fire book has many chapters on adjusting the thermal expansion of glazes. Also check the Education area of this site.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.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.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.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.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.Stoneware
A high fired ceramic that is vitreous or semi-vitreous, not translucent, and often made of clays that are not highly refined. Stonewares can be brown, buff or white. Stonewares commonly have some speck and some particulate material such as sand or fine grog.
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