There is an axiom in ceramic art that recipes don’t travel well. Why? Examining the success of celebrated 20th-century slipware potter Michael Cardew offers helpful answers.

Defining the Terms

Linear Shrinkage: Reduction in length of a sample divided by its original length, typically expressed as a percentage.

Mineral: A solid, normally crystalline, element or compound formed by geologic processes.

Primary Clay: A clay found where it was formed from its parent rock, usually by the weathering action of water.

Secondary Clay: A clay that has been transported by wind or water from where it was formed to a location where it settled into a minable deposit.

Specific Surface Area: A way of estimating particle size by measuring the amount of surface area of a powder per unit of weight of the sample, usually measured as square meters per gram.

Location, Location, Location

All the inorganic raw materials used to make ceramic art are composed of minerals. Quartz, feldspar, kaolinite, illite, smectite, and talc are but a few examples. As minerals, they are by definition composed of atoms of specific elements in specific proportions or over a specific range and crystallized in specific physical structures. Because of that they are the same wherever they are found on this planet, and even in the universe.

How nature has weathered, mixed, and deposited these minerals, however, is not the same (1). A clay deposit in Roseville, Ohio, or Leasburg, Missouri, or Igbo-Ora, Nigeria, or Iznik, Turkey, or Icheon, Korea can be very different, even though it may be composed of some, or even all, of the same minerals. What differs is how those minerals have been weathered and the geologic processes they have experienced over time.

1 Diagram of the fall line and coastal plain. Crystalline rocks in the mountains are eroded and sedimentary clay is deposited where the velocity of the flowing water slows, just above the fall line and on the coastal plain. Primary clay is found in the mountains where it formed, secondary clay at the fall line and on the coast. Images 1–3 from Science for the Potter by Linda Bloomfield. Published by The American Ceramic Society, 2017.

Granite in the hills near Jingdezhen, China, weathered to a fortuitous combination of kaolinite, feldspar, and quartz that fires to a fine porcelain. Granite in southwest England that was altered by water over many millennia has become an exceptionally white but rather non-plastic kaolin clay. Both of these are primary clays. They are found where they were formed.

Granite in the southeastern US weathered to primary kaolin on the east side of the southern Appalachian Mountains. On the west slope, however, similar kaolin was washed downstream to lakes where it settled in beds of secondary clay. The transport process ground the clay crystals into fine particles and produced very plastic ball clays.

Earthenware clays are typically secondary clays contaminated with significant amounts of some flux materials. Iron, which produces red earthenware clays, is the most common example of such a flux.

The point of all this is clays, as well as other ceramic materials, are naturally occurring. Their history is almost always long, complex, and unknown to us. Knowing something about the minerals they contain and how they vary is essential to understanding their properties when making ceramic art.

Cardew as a Model

Michael Cardew had been making stoneware and earthenware pots in England for almost 20 years before he left in 1942 to run a pottery in Africa. Once there, he ran into a common challenge: new materials created huge problems.

By the time Cardew’s book, Pioneer Pottery, was published in 1969, the celebrated slipware potter had met, and solved, many of those challenges. Chapter 1 of his book remains an excellent introduction to geology for the artist. The four following chapters are sound references to the science and properties of ceramic raw materials.

2 Close up of kaolinite in Jurassic sandstone, UK North Sea, clay type confirmed by X-ray diffraction. Width of image: 20 microns across (50 images side by side would measure 1 mm). Image: Evelyne Delbos, courtesy of The James Hutton Institute, reproduced from the ‘Images of Clay Archive’ of the Mineralogical Society of Great Britain and Ireland and The Clay Minerals Society.

To learn enough to make kiln-fired stoneware in Africa took Cardew years of study and testing. Unlike in England, where he was surrounded by a tradition of high firing, in Africa he found artists using local clays that for millennia had been handbuilt into porous cooking pots and water jars by bonfiring. Given the obstacles he faced, Cardew succeeded remarkably quickly. What was his approach? He studied hard and tested even more, always applying the science of ceramic materials to his problems.

Getting Started in a New Location

The following steps (as taken by Cardew) will help you make informed decisions about choosing new materials in a new location.

  • Assuming your new clay candidate is reasonably plastic, the most useful information about any new clay is a particle size analysis down to the finest fractions. As mined, plastic clays are mostly finer than 10 microns in diameter. Much of the material may be as small as 5 to 0.5 microns. For commercial clay a specific surface area analysis (SSA) is often available and offers an indirect indication of the size of the clay particles. It should be used with caution, however. A kaolin may have an SSA of 35 square meters per gram (M^2/gm) while a much more plastic ball clay can have a lower SSA. That’s usually because even though the ball clay crystals themselves are smaller, 10–15% of the ball clay is free quartz, which has a very low SSA for its weight. A complete chemical analysis of a clay is essential if it is to be substituted for another in an existing recipe. Size and chemical analysis are often available from vendors. Most tile and whitewares companies have their own test equipment for specific surface area, particle size distribution, methylene blue index, and sulfates, or they rely on their raw material suppliers. For X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) tests (2), people use universities/colleges—Alfred University is one such place that has these capabilities. Another option for mail-in analysis is Harrop Industries located in Columbus, Ohio, (www.harropusa.com). Harrop provides dilatometry, clay testing, and differential thermal analysis/thermogravimetric analysis (DTA/TGA) analyses.
3 Typical clay analysis. Note that the alumina to silica ratio of China clay is very near the theoretical ratio for pure kaolinite. The same ratio is much different for the ball, stoneware, and red clays. That’s because all these clays contain at least some free quartz. China clay has the least and it increases in the others so that red clay contains the most free quartz. The potash/soda and calcia/magnesia (oxides of calcium, magnesium, potassium, and sodium) are present as feldspars. All these clays are typically contaminated with at least some of it. Information from The Potter’s Dictionary of Materials and Techniques, by Frank and Janet Hamer, 2015.
  • The next, and simplest, test for any clay is to measure its shrinkage. Making six to eight test tiles from the same clay at the same time and in the same way assures the test will fairly represent the properties of the clay. Mark each tile with some identification number or label and scribe a fine line that is 10-units long on the tile surface. After drying and after each stage of firing, measuring how much shorter the line is gives a direct measure of linear shrinkage.
  • Keep detailed notes of the amount of shrinkage from wet to dry, from dry to low fired, and from low fired to peak firing temperature.
  • Cardew also advised making a subjective test of green and fired strength of the tiles by breaking one or more at each stage to get a sense of whether the material is weak, strong, or somewhere in between.
  • Questions for a new materials supplier will vary depending on how the clay will be used. Suitability for the intended process (tile, sculpture, casting, throwing, etc.) is most important. Peak firing temperature is also critical to know. The importance of fired color, chemical analysis if known (3), geology of the deposit, processing, etc. may also be important.
If a ceramic artist chooses to travel, the new things encountered will be new people, new places, new languages, new customs, but not new minerals. Once we understand the science, the challenge of travel is the changing practices and customs in new places and how they are applied to the same minerals found back home.

People are fundamentally the same everywhere. Clay minerals are a good metaphor for that. It’s not a bad thing to keep in mind as we look at clay, wherever we find it.

the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his Masters degree in Ceramic Engineering from Alfred University.

Topics: Glaze Chemistry

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