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All About Pottery Glazes

Shelling - flaking or peeling.  A glaze or glaze and slip defect in which the glaze falls from the body in flakes. It is caused by insufficient bond between glaze and body, usually the result of under firing.

Shivering - a defect in which the fired glaze pulls away from the pottery taking some of the body with it in the form of slivers. This generally occurs on sharp rims and edges of handles, and is due to improper glaze fit caused by too much compression by the body. 

Short - non-plastic clay poor in working qualities.

Shrinkage - the decrease in the size of clay pottery due to drying and firing.  Dry shrinkage is reversible with the return of water, but firing shrinkage is permanent due to chemical and physical changes the clay undergoes when exposed to heat.

Silica - the primary glass forming oxide used in pottery.  Boron is the other glass forming oxide used although more commonly as a flux than as a glass former due to its low melting point, 1063 F (577 C).  A glass forming oxide must be present in any glaze and as silica’s melting point is, 3272 F (1800 C), a flux is always present to reduce the melting point to a workable range.  Pure boron glasses are water-soluble so of little use but boro-sillicate glasses have a very low thermal expansion and are the main constituent of 'Pyrex' etc.

Silicate - a compound containing silicon, oxygen and a metal.

Sintering - is a method for making objects from powder, by heating the material below its melting point until its particles adhere to each other.

Slabs - flat, thin 1/8" to 1/4" sections of clay which are produced by hand or with a mechanical device called a slab roller.

Slab Built Pottery- pottery constructed wholly or mostly from sections of clay slabs.

Slake - to moisten dry clay with water.

Slip - this is clay that has been thinned by the adding water to the clay.

Slip Casting - plaster molds are filled with a deflocculated slip; deflocculation reverses the electric charges in the clay particles, which reduces the water content in a slip to that of most plastic clays, around 30% of total weight.  A common deflocculant is Sodium Silicate.  The plaster absorbs sediment of clay leaving the remaining moisture over the entire interior surface of the mold.  The excess slip is drained off and the cast can be removed from the mold soon after.  This approach is used widely by industry and some pottery workers.

Slip Trailing - another decorating method.  Slip is applied to the greenware through a tube or nozzle, much like icing a cake.

Slipware - a traditional English decorative technique associated with red earthenware and lead glaze.  Colored slip is piped onto the leather hard pottery much like cake decoration. The most noted exponent of slipware was the 18-century pottery worker “Thomas Toft”. His dishes set a standard that few modern pottery workers can compete with.

Slumping - firing a piece of pottery or ceramic to a very high temperature which causes the piece to drop or sink, and basically melts.

Slurry - thick slip.

Small Cones - can also be used on the kiln shelf as mini witness cones where space is limited.  They require mounting in cone holders or plaques at an 8 degree angle so 15/16" of the cone is exposed above the cone holder.  Small cones deform at slightly higher temperatures than large or self-supporting cones of the same number.

Smoking - greying or discoloration of a glaze caused by underglazing.

Soaking - by balancing the fuel intake in a kiln, fuel intake does not pertain to a electric kiln, vitrification temperature can be maintained.  Instead of shutting off the kiln, or cooling down when the desired temperature is reached, you would leave it for ten minutes or so, soaking. This gives the glaze a more mature appearance.

Soft Fire - firing greenware cooler than cone 06 making it less fragile, yet can be cleaned.

Soluble - capable of being dissolved in water.

Spongeware - is where the color is applied with a sponge or rag in a random or precise pattern.

Sprigging - this is a technique used by Wedgwood, it is a molded piece of clay applied to the greenware with slip.

Spur Marks - the marks left by the stilts used to support pottery in the kiln. Usually seen as three dots in the form of an triangle.

Stacking - loading a kiln with pots.  Large and small items are carefully positioned to use the space most efficiently.

Stains - a suspension of metallic oxides, clays and other materials with water used to add color to the surface of clay and glaze.

Stilts - ceramic bars or circles with high heat wire protruding, that pottery is set on so it won't adhere to the kiln shelf when firing glazes.

Stoneware - non-porous, this pottery is fired to temperatures of about 2372 F (1300 C).  It is made from some clays which have some impurities in them so that they burn to a dark color.  Will stand a relatively high temperature and burn to a dense hard body, as hard and non-porous as china, but lacking the light color, delicacy, and translucency.  They are usually covered with a salt or lead glaze.  At the point of vitrification the glaze and body are partially fused together.

Talc - useful as a flux in low temperature clay bodies and in glazes at both high and low temperatures. Also functions as an opacifier in glazes and reduces thermal expansion.

Tenmoku - stoneware glaze that is stained by iron oxide. Generally Tenmoku’s are shiny glazes that are black or dark brown.

Temper  - the addition of coarse non-plastic materials that can improve the working and firing of the clay.

Terra Cotta  - (from the Italian meaning "cooked earth") is a hard, semi-fired and absorbent clay used for both decorative and construction products.  The colors can range from grayish to dark reddish-orange, light to medium reddish-brown, or strong brown to brownish or deep orange

Terra Sigillata  - a slip comprised of the smallest particles of clay, which consequently resembles a burnished surface.  The technique was used to impressive effect in the Greco-Roman period.

Throwing - to make pottery by hand on a kickwheel. A delicate balance, which defies gravity and centrifugal force as clay is coaxed up by hand from a spinning turntable.

Throwing Bats - circular disks that can be fitted to the head of a kickwheel so that finished pottery pieces, particularly broad bottomed pieces such as plates or platters, can be easily lifted off the kickwheel.  Bats can be made of plaster of paris, wood, or Bakelite.

Tin Glaze - this is white and opaque glaze, made this way by the addition of tin oxide to the glaze.

Tin Oxide - dependable and widely used as a glaze opacifier.  Tin oxide has a refractory effect on glazes and can increase craze resistance.

Titanium Dioxide - used primarily as an opacifier in glazes, titanium dioxide produces matt surfaces. It is also used as a seeding agent in crystalline glazes.

Transfer Printing - a method of decoration where a pattern or picture is printed onto the gelatin coating of paper and then, when wet, is slid onto the surface of a pot. The gelatin can be sensitized with silver halide allowing the same process to be used photographically.

Translucent - diffusing light sufficiently to cause images to be blurred.

Transparent - capable of transmitting light so objects on the other side can be seen clearly.

Trimming, or Turning - certain forms made on the kickwheel will not support themselves unless excess clay is left at the base, alternatively, extra definition on the foot of a pot may be needed. The solution to both these problems is turning, which is done at the leather hard stage. The pottery is inverted onto a kickwheel and a metal cutting tool is applied to the bottom of the pottery until the desired finish is achieved.

Under Fire - firing a piece of pottery below the temperature required to harden it or if glazing below the temperature required to melt it.

Under Glaze - a decorative technique in which one or more colored glazes are applied to pottery then an overglaze of one or more clear or translucent glazes are applied over them either before or after the first glaze firing.  If there are multiple glaze firings, each subsequent glaze firing is done at a lower and lower temperature so that each underlying design is not affected.  Print and paint is where the outline of a design is printed on bisque and filled in with color by hand.  It is a popular decorating method, giving the pottery worker wide creativity in design and color.

Unglazed - a piece of pottery that is fired but no finish is fired on the out side.

Vent Holes -  a hole made so that air and gasses can escape where two pieced of pottery has been attached together forming a sealed pocket.

Viscosity - the ability of a liquid to flow, the term is used by the pottery worker in relation to molten glazes, glaze suspensions and slips.  A stiff molten liquid glaze is one of high viscosity, while a runny molten liquid glaze is one of low viscosity.

Viscosity Cup - a small cup with hole in bottom used to determine if glaze is the right consistency.  A range of 19-23 seconds is normal.  If glaze remains in the cup and reading is below or within this range, or if it takes longer than 23 seconds to drain, the glaze is too thick.  Add small amounts of water as needed.

Vitrification - the temperature that occurs during this process varies with each different clay.  During firing, vitrification is simply the fusion of a clay body.  This is necessary so that the clay body will reach the exact point of hardness at the same temperature that the glaze will melt to form the individual shiny coating over the pottery.  It is a glass-like non-porous ware, which has been fired at a higher temperature than earthenware and contains silica which makes a body non-absorbent. After vitrification the clay can no longer be recycled although it may be used as grog.

Wax Resist - water-soluble wax emulsions used to resist glazes, under glaze, stains, etc on foot rings, lid flanges and lids or for masking specific areas of a piece for multi-layered glazing techniques.  Once applied, wax cannot be removed except by firing off in the kiln.

Wedging - to knead or mix plastic clay by hand.  A hand process used to homogenize the clay and remove air bubbles, thus making it workable.  The techniques for wedging are called Spiral,  Chrysanthemum wedging, Rams head, Monkey face wedging and wire/slab wedging.  Both Rams head and Spiral wedging involves the folding of the clay on itself too build up an ever-tightening spiral of clay platelets.  Wire wedging builds up increasing layers of clay platelets and is the best for introducing other clays and fillers into an already plastic clay body.

White Granite - has ground stone used in the body.  The color varies from creamy white to bluish or grayish-white.  The process makes the pottery harder and stronger than earthenware.  White Granite is also referred to as White Ironstone, Pearl China, Pearl Granite, Porcelain Granite, Flintware and Opaque.  In China all commercial names intended to inspire confidence in the strength of the pottery.

Whiting - a washed and ground pure white chalk used for paints.

Whiteware - is pottery or porcelain with a white body, so called to distinguish it from redware or yellowware.

Yellowware - the body color varies in hue from ecru to mustard and is made from a naturally occurring clay. 

Zinc Oxide - generally used as a flux in high temperature glazes, zinc oxide lends opacity and encourages crystal growth when used in large amounts.  A common constituent of matt glazes, it also has a marked effect on colorants, promoting brilliance with copper and dulling iron and chrome.  Zinc oxides can assist in reducing crazing of glazes.  Not recommended for use with some ceramic stains.

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