Pottery and Ceramics
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All About Pottery Glazes
Alumina Hydrate
- used as a source of alumina in glazes, alumina hydrate promotes opacity and a matte finish and increases hardness and durability.Very refractory.
It can also be used in kiln wash or to protect kiln furniture in salt firings.
Barium Carbonate
- functions as a high temperature flux.It can produce satin matt glazes, but an excess will cause a rough, dry glaze surface.
Bentonite
- a highly plastic clay which is used in small amounts as a plasticizer in clay bodies and a suspension agent in glazes.More than 3% can cause cracking of the unfired glaze layer due to the high drying shrinkage rate.
Bone Ash
- typically made from ground, calcined animal bones, bone ash is a source of calcium phosphate.It is used as a flux in clay bodies - notably bone china, which is known for its thinness and translucency.
Also used as an opacifier in glazes and to assist in formation of opalescence.
Borax
- a source of boric oxide and soda in glazes.Small amounts can increase fluidity of a glaze; larger amounts can lower the maturing temperature.
CER Veegum (Glaze Hardener)
- used in glazes as a suspending agent, viscosity stabilizer and surface hardener.Recommended use: add 1.0% to 1.5% based on the dry weight formula. Disperse in water or add directly to dry mix.
Chrome Oxide (Green)
- a powerful and versatile glaze colorant that can produce a variety of colors including green, yellow, pink, brown, and red depending upon glaze formula, atmosphere, and temperature of firing.At temperature above Cone 6, chrome oxide becomes volatile and may affect adjacent ware in the kiln.
Powdered Gum
- an organic cellulose gum used in slips and glazes as a binder, thickener, and suspension agent.Recommended use: mix 1 to 2 tablespoons per gallon of warm water and let stand for 24 hours.
Use approximately one part of this solution to 2 parts water when mixing a glaze to improve brush ability.
Use lesser amounts for dipping, pouring or spraying glazes.
It will increase drying time which is not always beneficial to these application techniques.
Cobalt Carbonate
- used as a slip and glaze colorant, cobalt carbonate produces various shades of blue or purples where manganese is present. Because of its extremely fine particle size, it is more easily dispersed in glazes than cobalt oxide.Cobalt Oxide
- is the most powerful ceramic pigment available.Used in slips and glazes in very small amounts, it produces h6 blue colors. Because of its intense coloring power and larger particle size, it can cause a speckled effect.
Copper Carbonate
- a versatile, fine particle glaze and slip colorant which can produce greens, blues, or reds depending upon glaze composition and firing conditions.Copper Oxide (Black)
- is more concentrated than copper carbonate, copper oxide is a h6 colorant and a flux.In an oxidizing or neutral atmosphere it produces greens. Under reduction conditions red colors can be obtained.
When used in an alkaline glaze copper oxide gives a turquoise blue color. Above 1900 degrees F copper oxide becomes increasingly volatile and can affect the color of other glazed pieces in the kiln.
Copper Oxide
- made by heat treating Black Copper Oxide.Red Copper is insoluble in water, making it more difficult to work with.
Cornwall Stone
- an English feldspathoid used as a flux in clay bodies and as a major constituent in glazes.Like feldspar, it introduces the fluxes potash, soda and lime into a glaze, but with more alumina and silica than is introduced by feldspar.
Darvan No. 7
- long-chain sodium poly electrolyte best suited to deflocculate low-fire, white hobby clays.It produces slips with a long casting range, and excellent mold-release properties.
Pottery can be removed as soon as one to two hours depending on size or left as long as 6 to 24 hours provided that the drain hole is covered.
This allows larger or more intricate forms extra setup time for safer handling and reduced warping.
Slips made with it have an increased shelf life because they don’t thicken or become hard upon standing, which is a common problem with many hobby slips.
During casting, there is minimal absorption of Darvan No. 7 by the plaster resulting in greatly increased mold-life as well as scrap that can be easily reclaimed sometimes by adding only water.
It is very powerful requiring only a .5 to 1% addition based on dry body weight to the formula.
Even so, adding excess Darvan will not cause the slip to re-flocculate.
In addition, it is an excellent deflocculant dispersing agent in glazes to be applied by spraying, providing increased fluidity without the addition of excessive amounts of water, and keeping the materials evenly dispersed in the glaze while you work.
It has a shelf life of 1 year or more if stored in a tightly closed container. Temporary freezing will not effect its properties but the thawed material must be thoroughly re-mixed before using.
Darvan 811
- a short-chain sodium poly electrolyte used to deflocculate mid to high fire porcelain and stoneware slips as well as low-fire red casting slips.It has many of the same benefits as Darvan No.7, such as longer casting range, increased mold life, easier to reclaim and improved viscosity stability.
It also reduces hard spots and eliminates calcium/silica scum for better glaze adhesion.
It can also be used in glazes as a deflocculant dispersing agent to improve working properties during spray application.
It has a shelf life of one year or more and is not adversely effected by short-term freezing.
Thaw completely and remix thoroughly.
Deflocculant
- material that acts on the molecular structure of clay causing clay to flow as a liquid without the addition of excessive amounts of water.Primarily used in the production of casting slips to create and maintain fluidity without the increased shrinkage and weakness that excess water will cause in the resulting green ware.
Dolomite
- dolomite is a naturally occurring mineral that contains calcium carbonate and magnesium carbonate.It is used as a flux in high temperature glazes wherever both magnesia and lime are required.
Feldspar, Custer
- a potash feldspar used in both clay bodies and glazes.It can function as a flux or as a refractory depending upon formula and firing temperature.
Feldspar, Kona
- a soda feldspar used in clay bodies and glazes where a sodium flux is required.Feldspar
- a potash feldspar used in glazes and clay bodies.Flint, Silica, Silicon Dioxide
- the fundamental constituent of glazes and component of clay and other ceramic raw materials.Used in glazes, flint will raise the maturing temperature, increase the viscosity of the melt, and increase hardness and craze resistance.
The addition of silica to clay bodies makes the fired clay harder and more durable and improves glaze fit.
Frits
- they are combined raw materials that are mixed, fired, melted, crushed and ground into a powder.The reason for fritting is to render soluble materials insoluble and decrease toxicity of raw materials such as lead, barium and zinc, to reduce fusion point of glazes and to avoid volatilization of unstable substances.
Frits are often used in glazes as fluxing agents but can also be used in slips and clay bodies.
Gerstley Borate
- a sodium/calcium/borate compound used as a flux in glazes.It is used to introduce boric oxide in an insoluble form and as a more stable substitute for colemanite.
Grog
- ground, fired clay added to clay bodies to improve workability and strength and to give texture.The addition of grog also reduces shrinkage, warping, and cracking in drying and firing.
Ilmenite
- the ore of titanium and iron.Granular ilmenite is used to produce a speckled effect in clay bodies and glazes.
In small amounts it can promote or seed the growth of crystals in glazes especially those containing rutile.
Iron Chromate
- a form of iron usually used when darker browns and red-browns are desired.Iron Oxide
- a ferric oxide.Generally used as a colorant in clays and glazes where iron is required.
It produces a broad range of color from yellows, browns, red-browns, and blacks in oxidation to grays, blues, greens, and blacks in reduction.
Red iron has a finer particle size than black iron which makes it disperse more easily in a glaze.
In mid-high temperature ranges iron oxide will act as a flux, noticeably lowering the firing range of glazes.
Iron Oxide
- a ferric oxide that produces the same range of colors as red iron when used in glazes but is preferred by many potters when making celadon glazes for reduction firing.Kiln Wash
- a refractory mixture of kaolin and flint or alumina applied to kiln floors and furniture to prevent adhesion of melted glaze.Lithium Carbonate
- a slightly soluble source of lithium. When used as a flux in glazes it can extend firing range, reduce thermal expansion, and promote brilliant colors.Magnesium Carbonate
- this is used in glazes as a source of magnesium oxide.A refractory at low temperatures, it becomes an active flux at high temperatures.
Magnesium carbonate produces matt surfaces, increases glaze viscosity, and improves adhesion of glaze to clay.
Manganese Dioxide
- a fine particle ceramic colorant which usually produces browns in glazes but can give purples in combination with cobalt or alkaline fluxes.Molochite
- a white porcelain grog.Reduces shrinkage, warpage, and cracking, while maintaining color in white clay bodies.
Nepheline Syenite
- a useful substitute for feldspar, nepheline syenite melts at a lower temperature.It is widely used as a flux in both clay bodies and glazes.
Nickel Carbonate
- when used alone as a glaze colorant, nickel carbonate usually produces greenish or brownish grays.It is especially useful as a modifier for brighter colorants to produce more muted tones.
Nickel Oxide
- more concentrated than nickel carbonate, nickel oxide is used in small percentages to produce mute colors in glazes.It is refractory and can raise the maturing temperature of glazes if used in large amounts.
Nickel Oxide
- reverts to green nickel at around 1100°F when used in a glaze producing very similar results.Increased amounts may be necessary if substituting black for green in a recipe.
Nylon Fibers
- a fiber which can be added to clay bodies to increase wet strength.Especially useful for hand building large slab forms, the fibers create an inner structure in the clay allowing for easier handling of larger slabs.
The fibers are approximately ½ inch long and burn out during firing. Recommended use: add 1.5% to 2% by weight.
Fibers are most easily added to dry mix but can be wedged into wet clay if necessary.
Ochre, Yellow
- an iron oxide ore used as a colorant in slips, clay bodies, and glazes.Produces colors similar to iron oxide.
Pottery Plaster
- the standard of the industry, equivalent to No. 1 Pottery Plaster.Recommended for most slip casting applications for h6, long-lasting molds.
Measuring Guidelines: for slip casting molds, throwing bats and wedging tables, a good water to plaster ratio is 1:1-3/8 by weight. Using 1 lb. as the weight of 1 part of water, some general ratios are:
1 part water to 1 lb 6 oz dry plaster
1 Qt. water to 2 lbs 12 oz dry plaster
1 Gal water to 11 lbs dry plaster
To estimate how much water you’ll need, use this guideline:
One quart water for every 80 cu. ins. of mixed plaster.
To compute the volume of your form in cubic inches use this formula;
length x width x height and divide by 80
.This equals the number of quarts of water required
Mixing Instructions
: measure the appropriate amount of water into a large container and the correct amount of plaster as computed above in a separate container.Slowly sift or sprinkle the plaster evenly over the surface of the water and allow to sink.
You should see a noticeable increase in the length of time the plaster floats on the surface before sinking as additions continue.
When all the plaster is added and submerged, mix slowly but thoroughly and be sure to break up all of the chunks using your hand or a power mixer on a low setting and try not to introduce extra air into the mix.
Continue mixing until the plaster thickens slightly, until a finger drawn across the surface leaves a trace trail, and then pour slowly into your form. Tap or lightly shake the form to bring air bubbles to the surface.
Let set, remove from form and dry completely before using.
Pyrophyllite
- a hydrated aluminum silicate used in clay bodies to decrease thermal expansion, shrinkage, and cracking and to extend firing range.Rutile
- a natural ore containing titanium dioxide and a small amount of iron.Used as a slip and glaze colorant, rutile produces creams, tans and browns in oxidation and various bright oranges and blues in reduction.
It tends to produce mottled or broken colors, increase opacity, and promote the growth of crystals in glazes.
Silica Sand
- a very coarse form of silica used to add texture and strength to clay bodies.Silicon Carbide
- a powdered form, used as a glaze ingredient for special effects such as crater glazes and localized reduction.Silicon Carbide
- specially fine-ground for improved dispersion. Recommended for producing more even, consistent localized reduction effects such as are required to produce oxidation copper reds.Sodium Silicate
- a water-soluble solution of soda and silica traditionally used to deflocculate casting slips.Often used in conjunction with soda ash, it is equally effective in both high and low temperature slips.
Care must be used when using Sodium Silicate because it will actually reflocculate the slip if too much is added.
It is also readily absorbed by plaster leading to deterioration and the eventual breakdown of slip casting molds.
Soda Ash
- sodium carbonate.Frequently used as a soluble deflocculant in slip-casting bodies, it increases strength and reduces shrinkage.
Spodumene
- a lithium feldspar used in glazes to reduce shrinkage and lower vitrification temperature.In clay bodies it decreases thermal expansion.
Strontium Carbonate
- a glaze flux which promotes more craze and scratch resistant glaze surfaces.Superpax
- a fine-particle zircon opacifier used in slips and glazes.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.
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.
Rutile is titanium with iron impurities.
Burnt Umber
- a hydrated ferric oxide with manganese dioxide and clay. It can be used as a slip or clay body colorant and produces a reddish-brown color.Veegum-T
- an iron-free colloidal magnesium aluminum silicate used as a plasticizer lubricant in white clay bodies.A good binder for extruder bodies.
It can also be used as a suspending agent for glazes.
Recommended use: add 0.5% to 1.5% based on the dry weight formula. Should be used in water as a dispersion addition.
Volcanic Ash
- a naturally occurring, fine particle material used in glazes to replace feldspar and flint.Single color changes may result from the iron oxide content of the ash.
Wax Resist
- water-soluble wax emulsions used to resist glazes, underglaze, stains, etc on footrings, 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.
Whiting
- calcium carbonate.Whiting is a widely used source of lime in glazes and a primary high temperature flux.
Wollastonite
- a naturally occurring calcium silicate; wollastonite is used in both clay bodies and glazes to reduce drying and firing shrinkage and improve fired strength.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.
Zircopax
- a zirconium silicate used as a slip and glaze opacifier.Zircopax Plus
- current substitute for Superpax and original Zircopax.A fine-particle zirconium silicate used as a glaze and slip opacifier. Disperses very easily.
Substitutes 1:1 for Superpax; may require less when substituted for original Zircopax.
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