Clays and Chemicals
for Pottery and Ceramics
Clay is usually packaged as two 22 lb. blocks per 44 lb. box, each block is pugged, which is working all the air out of the clay and sealed in plastic.They are clearly marked with name, cone rating, batch numbers and date. Moist Clay is a favorite with schools, potters and ceramist.
When choosing a clay, first determine what temperature range you will be firing.
The temperature range you select is determined by the type of work you do and the equipment available to you.
You should also select clays and glazes that mature in the same temperature range, especially if you are producing work that will be used everyday.
Tools for Pottery
Tips & Techniques for Pottery and Ceramics
Clay Pottery Craft Projects
Clay Pottery Articles of Interest
Old Time Pottery History
Pottery and Ceramics
Featured Potters Gallery
Pottery and Ceramics Definitions
All About The Clay
All About Pottery Glazes
These are three basic temperature ranges:
Low Fire
Earthenware
Cone 06-3 (1850°-2135°F)
Mid Range
Stoneware & Porcelain
Cone 4-7 (2160°F-2290°F)
High Fire
Stoneware & Porcelain
Cone 8-10 (2315°F-2380°F)
High fire clay bodies are often described as having a wide firing range which would be Cone 2-10.
Any clay body can be fired below its maturation point for various reasons, but it may not be suitable for functional ware when under fired.
A high fire clay may have a 2% absorption rating at Cone 10, but 6% or more at Cone 4.
For pottery pieces that are used everyday, a general guideline is that absorption should be 3% or less for stoneware and 1% or less for porcelain.
A clay that is too porous will tend to absorb excess moisture and contaminants during use.
Under fired clay bodies also tend to have more glaze body fit problems such as crazing.
A crazed glaze allows moisture and contaminants to seep through into the underlying clay and become trapped, causing premature weakening of the piece and possible bacterial growth if food items are involved.
We have found that using glazes and clays formulated to mature at the same temperature reduces the occurrence of some of these problems when durable, functional work is desired.
A Word About Clay Mines
The majority of chemicals used in clay bodies and glaze are mined from the earth for industry, not for pottery workers.
Some materials will pick up impurities from time to time, which may cause slight changes in clays and glazes.
There is nothing that can be done about this, as the mines will not insure absolute uniformity of materials.
If you have very specific requirements for your clay or glazes, we recommend you test each new batch of clay or bagged chemical to avoid possible disappointment.
Moist clay suppliers do everything possible to maintain a high level of quality and purity with their clays.
Many of the raw materials used by pottery workers range in toxicity from mild to serious.
Hazardous materials include dusts, fumes, and vapors that can enter the body through inhalation, ingestion, and absorption through the skin.
Toxic materials are most dangerous in the form of dry dust, airborne mists and kiln fumes.
Anyone using clay and glaze related materials should be aware of the potential health hazards and protect themselves from exposure.
Some of the most potentially toxic materials include long term inhalation of free silica which is contained in clay dust, flint, feldspar and talc.
The metallic oxides, stains and carbonates through ingestion, absorption, or inhalation, can cause illness, especially barium, cadmium, chrome, cobalt, lead, lithium, manganese, nickel, potassium, selenium and vanadium.
Care must be taken to minimize exposure especially if you intend to spend a lifetime around these materials.
You must take a responsible attitude and wear masks and protective clothing when necessary, and keep a clean, dust-free studio.
Tips to Follow When Working With Clay:
Keep Your Workspace Clean
Keep chemicals in covered containers, clean frequently using a wet mop or toxic dust vacuum.
Never sweep dry clays and chemicals
Don’t smoke or eat in your workspace.
Wash skin and clothing frequently.
Wear Protective Clothing and Masks When Necessary
Wear a respirator when mixing clay or glazes and when spraying glazes.
Wear heat resistant clothing, gloves, and goggles when firing kilns.
Wear rubber gloves to protect hands from chemicals.
Ventilation
Isolate kilns, spray booths and dust producing materials.
Install exhaust fans where necessary.
Know Your Materials
Slip Clays
Albany Slip 1
- a chemical makeup which produces a glaze nearly identical to 100% Albany at Cone 10.May be used alone or as a component in clays or glazes.
Alberta Slip
- a beautiful dark slip clay that can be substituted 1 to 1 for Albany Slip 1 in recipes.Alberta Slip is extremely craze resistant, applies well to green ware or bisque, and can be used with ceramic stains for endless variations.
At 325 mesh, it mixes and sprays very well.
It is fusible and, when mixed only with water, produces beautiful satin-gloss brown glazes at Cone 10 ranging from bean pot to tenmoku, depending on atmosphere and the underlying clay body.
Ravenscrag Slip
- a light-colored, fine-grained, silty clay with low plasticity and a high melting point Cone10.It has excellent working properties and can be applied with ease to green ware or bisque.
Even multiple layers won’t crack or crawl during drying.
100% Ravenscrag produces a light-colored silky matte surface at Cone 10. Small amounts of feldspar can be added to improve flow and smooth the surface 5% or add gloss 10%.
Boron frits don’t work well with this material at Cone 10. For Cone 6, the addition of ferro fritwill yield a silky matte 10% or add gloss 20%.
Colorants and opacifierscan be added for infinite variations and effects. Because Ravenscrag is more than 70% clay, it requires sufficient fluxing and heat work to allow complete burn-out of gasses during firing.
Ravenscrag contains very little sodium and plenty of magnesia, making it a relatively problem-free base for glaze development.
Blackbird Slip
- an iron saturated, self glazing slip clay.Used alone as a slip or stain or as a colorant in glaze and clay recipes, it fires brown or black depending on temperature and atmosphere.
Vitrifies at Cone 4 -10.
Earthenware
Red Art
- fine particle, low fire clay with a very high iron content.Not very plastic on its own but is used frequently in terra cotta clay bodies and, in smaller amounts, for color in hi fire stoneware.
Red Art will flux at high temperatures and can be used in slip-glaze recipes.
It has a 14% shrinkage at Cone 1.
Fires beautiful red to red/brown depending on temperature.
Ball Clays
Kentucky
- a very plastic secondary clay used in many clay bodies and glazes.It has a 15% shrinkage at Cone 10 and fires gray white.
Mississippi
- a very fine particle clay used to improve plasticity and strength in clay bodies; however, usually limited to less than 10% of a formula since larger quantities may increase warp age and shrinkage.Tennessee
- a plastic secondary clay used in both glazes and clay bodies.It has a 14% shrinkage at Cone 10 and fires white.
Tennessee No. 5
- a plastic ball clay used both in clay and glaze formulas.It has a 15% shrinkage at Cone 10 and fires gray white.
XXSagger
- a fine grain secondary clay with good plasticity and a very low iron content.Used mostly in clay bodies.
It has a 13% shrinkage at Cone 10 and fires light cream to white.
Gold Art
- a air-floated secondary clay, very plastic but with a high sulphur content.Used both as a ball clay or fire clay.
It has a 7% shrinkage at Cone 8 and fires a light cream color.
Fire Clays
Missouri Fire Clay
- a sedimentary clay with good vertical plasticity used mostly in Cone 10 bodies as a refractory clay.Sometimes used as a mortar when mixed with water.
It has a 11% shrinkage at Cone 10 and fires light cream color.
North American Fire Clay
- a nonplastic refractory clay that fires light tan.Hawthorne Fire Clay
- fine particle Missouri Fire Clay with good plasticity.It has a 10% shrinkage Cone 10 and fires light.
Cedar Heights Bonding
- a h6, fairly plastic fire clay.Low in sulphur, cleaner than AP Green or Hawthorne Bond.
Kaolins
Edgar Plastic Kaolin
- a fine-particle, unusually plastic kaolin, the most common choice when a glaze or clay formula requires kaolin but does not specify a type.It has a 13% shrinkage at Cone 9 and fires creamy white.
Kaolin
- a very clean, plastic kaolin with excellent green strength.Good translucent qualities, used frequently in porcelain bodies.
Fires very white.
Georgia Diamond
- a replacement for Georgia Pioneer Kaolin which is no longer mined.Clean firing with high green strength.
An excellent kaolin for glaze applications.
It has a 10% shrinkage at Cone 10 and fires white.
Grolleg English Kaolin C (China Clay)
- mined in England, one of the purest kaolin's with excellent translucent properties but poor plasticity.Most often used for translucent porcelain bodies.
It has a 14% shrinkage at Cone 9.
It is the whitest firing kaolin there is.
Velvacast
- a coarse particle kaolin used in casting slips to reduce warpage and improve mold release properties.
If at first you don't succeed, check the wastebasket for the directions. |
