Understanding Clay and Glaze Materials: You Don’t Have to Be a Super Genius

Today, we live in an age of super abundance of ceramic raw materials. Innumerable clays and glaze materials offer us a bewildering array of choices. Far from understanding these materials as familiar rocks, feldspars, and clays, each with unique personalities, we know them only as white, gray, or brown powders neatly packaged in uniform bags.

But we all know that getting to know them better can only improve our work. Happy learning! – Jennifer Harnetty, editor.


There are probably as many kinds of clay as there are riverbanks, creekbeds, roadcuts, abandoned coal mines and backyard gullies, but most of the clays that many of us use on a regular basis are commercially mined.

Because not all materials are available through all suppliers, this chart is meant to provide data for the most common clays used in recipes you are likely to come across. You can use these data to compare the materials available through your supplier, or those you have on hand, with materials in the published recipes.

While the satisfaction, discovery and personal control that is possible through prospecting and processing your
own clay are certainly valid reasons for the effort, most of us rely on the consistency and (relative) reliability of air-?oated materials mined in large quantities. Even though the reasons for using commercially mined clays are most often based on a desire for a trouble-free product, the properties of clay as a natural material can make this goal somewhat elusive.

The following chart contains recent information, however, because the chemical and physical makeup of naturally mined materials can change across a given deposit, this chart is meant to be used as a starting point for clay substitutions. In order to precisely recalculate a recipe using a substituted clay, you will need to obtain a current data sheet for all materials you purchase from your supplier.

Please note that the clays are presented in alphabetical order, and the formulae are presented with alumina (Al2O3) in unity (totaling 1). This makes it easier to immediately see the ratio of alumina to silica, and also more accurately compares the relative amounts of all other components in the clays.



Unity Molecular Formulas of Clay Materials


Primary Function of Common Ceramic Raw Materials
Material Glaze Function Substitute Comment
Barium Carbonate Flux Strontium carbonate
Bentonite Suspension agent Ball Clay Do not exceed 3%
Bone Ash Opacifier
Borax Flux, glassmaker Boron frits
Chrome Oxide Colorant Green
Cobalt Carbonate Colorant Cobalt oxide Blue
Copper Carbonate Colorant Copper oxide Greens, copper reds
Cornwall Stone Flux, opacifier
Custer Feldspar Glaze core Potash feldspar (G-200)
Dolomite Flux, opacifier Whiting Many brands
EPK Kaolin Alumina, opacity Kaolin
Ferro Frit 3110 Glaze core, flux Pemco P-IV05, Fusion F-75 Crystalline glazes
Ferro Frit 3124 Glaze core, flux F-19, P-311, Hommel 90 Boron frit
Ferro Frit 3134 Glaze core, flux F-12, P-54, Hommel 14 Boron frit
Ferro Frit 3195 Glaze core, flux Hommel 90, Fusion F-2 Complete glaze
Ferro Frit 3269 Flux, glaze core Pemco P-25
Ferro Frit 3278 Flux, glaze core Fusion F-60, Pemco P-830
G-200 Feldspar Glaze core Potash feldspar (Custer)
Green Nickel Oxide Colorant Black nickel oxide Blues, tan, browns, greens, grays
Kentucky OM4 Ball Clay Alumina, opacity Ball Clay
Kona F-4 Feldspar Glaze core Soda feldspar
Lithium Carbonate Flux
Magnesium Carbonate Flux, opacifier Promotes crawling
Manganese Dioxide Colorant Purple, red, yellow-brown
Nepheline Syenite Glaze core
Red Iron Oxide Colorant Celadon green to brown
Rutile Colorant Ilmenite
Silica Glass former, glaze fit Flint Use 325 mesh
Spodumene Lithium glaze core
Strontium Carbonate Flux Barium carbonate
Talc Flux, opacifier Many brands
Tin Oxide Opacifier Zircopax
Titanium Dioxide Opacifier
Whiting Flux, opacifier Wollastonite, Dolomite Many brands
Wollastonite Flux, opacifier Whiting, dolomite
Wood Ash Glaze core, flux, colorant Whiting Results vary by type
Zinc Oxide Flux, opacifier
Zircopax Opacifier Superpax, Ultrox


Notes

1. Substituting glaze ingredients may alter color,
texture, opacity, viscosity, and/or sheen, as well as create pinholing,
crazing, black spotting, and/or pitting. In most cases, additional
adjustments to other ingredients need to occur when substituting.

2. Test and record your results.

3. Materials vary from supplier to supplier and batch to batch.

Comments
  • gosh one just about needs to be a scientist but this is quite well explained
    thankyou j

  • gosh one just about needs to be a scientist but this is quite well explained
    thankyou j

  • I still have a stack of hand-written 5×8 cards from college days (BA, Fine Arts 1971). Wish all this info had been available back then! Good information for the potter who likes to mix from scratch. Break out your stubby No. 2 pencil and a pad of paper and have fun designing new glazes. Don’t forget to calculate for heat-work! Thanks.

  • I still have a stack of hand-written 5×8 cards from college days (BA, Fine Arts 1971). Wish all this info had been available back then! Good information for the potter who likes to mix from scratch. Break out your stubby No. 2 pencil and a pad of paper and have fun designing new glazes. Don’t forget to calculate for heat-work! Thanks.

  • good tables, it didn’t cover some things. Where’s the Gerstley Borate?

  • good tables, it didn’t cover some things. Where’s the Gerstley Borate?

  • A good potter is someone comfortable in the worlds of both art and science.

    Ever since college in the sixties I have collected all this written information, even though I never got a handle on unity to batch conversion, and can’t afford the software for it. But for me it’s more rewarding to go out and find clays and minerals in their natural setting. I like to pulverize, levigate, sift, mill, mix, and actually fire them, to learn their real character. Anyone can buy a little bag of someone else’s magic….I say, go dig up your own!

  • A good potter is someone comfortable in the worlds of both art and science.

    Ever since college in the sixties I have collected all this written information, even though I never got a handle on unity to batch conversion, and can’t afford the software for it. But for me it’s more rewarding to go out and find clays and minerals in their natural setting. I like to pulverize, levigate, sift, mill, mix, and actually fire them, to learn their real character. Anyone can buy a little bag of someone else’s magic….I say, go dig up your own!

  • Re: Gerstley Borate–My understanding is that this material is not consistent enough to provide technical data. That is, one batch might be different enough from another that a statement on its molecular formula wouldn’t really be valid. I remember a few years ago when the supply of it was in question, and many a potter/teacher was put in the position of finding a substitute–none of which, really, were predictable since the original material varied so much itself.

  • Re: Gerstley Borate–My understanding is that this material is not consistent enough to provide technical data. That is, one batch might be different enough from another that a statement on its molecular formula wouldn’t really be valid. I remember a few years ago when the supply of it was in question, and many a potter/teacher was put in the position of finding a substitute–none of which, really, were predictable since the original material varied so much itself.

  • That’s “Ghastly Borate” and that silly pun has been around as long as “Hairy Potter”. I heard the company that mines it has found a new vein, and the good stuff is available once again. Anyone worried about consistency should order a large quantity…it’s the same problem one has with wood ashes.

  • That’s “Ghastly Borate” and that silly pun has been around as long as “Hairy Potter”. I heard the company that mines it has found a new vein, and the good stuff is available once again. Anyone worried about consistency should order a large quantity…it’s the same problem one has with wood ashes.

  • Thank you for those usefull charts.I hope I can use them for my countrie’S clay types.

  • Thank you for those usefull charts.I hope I can use them for my countrie’S clay types.

  • What happens if I use 200 mesh silica? Thanks for the information on the charts and for the daily dose of ceramics!

  • What happens if I use 200 mesh silica? Thanks for the information on the charts and for the daily dose of ceramics!

  • Nothing “happens” as far as the chemistry goes, but 200 mesh material will settle out a lot more quickly than the finer mesh versions…..silica or otherwise.

  • Nothing “happens” as far as the chemistry goes, but 200 mesh material will settle out a lot more quickly than the finer mesh versions…..silica or otherwise.

  • Why claim to represent potters across the world if you only publish data for raw materials that are almost exclusively from the USA?

  • Why claim to represent potters across the world if you only publish data for raw materials that are almost exclusively from the USA?

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