Understanding Plainsman Data Sheets

Our clay bodies are named using arbitrary numbers with prefixes and suffixes as follows:

  • P Prefix: Porcelain or Whiteware
  • H Prefix: High temperature
  • M Prefix: Medium temperature
  • L Prefix: Low temperature
  • G Suffix: Grogged
  • S Suffix: Manganese granular speckle added

Our boxes of clay are stamped with a code number underneath the name of each clay. The first four digits are a simple sequential number which is keyed to our lab testing records. The remaining digits encode the date on which the material was produced.

Each body data sheet has a boldface short description of the physical and firing properties of the material. This description is formatted the same for each and draws from the following:

  • Intended firing temperature: Low, Medium, High
  • Texture: Smooth, sandy, slight/medium/heavy, grogged, fine ground, fine grained
  • Plasticity: Plastic, medium plastic, low plasticity
  • Maturity: Vitreous, semi-vitreous, non-vitreous, refractory
  • Color: Described subjectively along with mention of any special effects
  • General purpose: Mentioned if applicable
  • Source: Native, refined, native refined material mix
  • Type: body, raw material
  • Firing: reduction, oxidation
  • Intended Use: Functional ware, sculpture, stoneware, earthenware, porcelain, whiteware, sculpture, terra cotta

There is no magic clay body that will fix every problem. You can dry, glaze and fire almost anything successfully if you do it in a way that is sensitive to the materials. Each body has a personality that you will only learn after using the material for a period of time.

When we call a clay 'balanced' we mean that its recipe contains materials which are almost complete in themselves and which have opposing properties that we can trade-off against each other to adjust the mix if changes occur in one of the materials. It also means diversity; many of our bodies have six or seven different types of clay in the recipe with no flint or feldspar (lots of competing bodies have only one or two with flint and feldspar).

In many cases the specific properties of our clay bodies are not due so much to the recipe, but to the choice of raw materials used for each item. Thus, if a recipe calls for a kaolin, it is the specific kaolin brand or blend of brands that is responsible for the properties of the body as a whole. We always attempt to introduce a variety of materials of a given class to add balance and make the recipe adjustable and resilient to shifts in individual materials.

Always test thoroughly before committing to using a large quantity of a clay body. If you do not know what properties to test for we can provide information on how to set up a testing program similar to ours. Never assume large scale production will work because a few test pieces look okay.

Working and Drying Characteristics

It is preferable to follow common sense drying procedures no matter what clay you are using. There are simple precautions you can take to vastly reduce drying losses due to cracking. For example:

  • Focus on 'even' drying, not on slow drying. If parts of the piece dry faster, then find a way to slow down these sections. Cover pieces with cloth and plastic if necessary (slow drying is not the key, but more even drying is often a byproduct of slower drying).
  • It is helpful to add an aggregate to the slip used for attaching elements together (i.e. molochite, grog). The particles tear at the opposing surfaces when the two pieces are compressed and moved laterally, this helps the surfaces heal together.
  • Use slip that has a minimal water content (don't use flocculated slip containing acid, vinegar, etc.) and apply as much pressure and lateral movement as possible when making the join.
  • Larger pieces of uneven cross sections, plates, or flat bowls require extra care in drying. Try to use even wall thicknesses wherever possible.
  • Avoid joins that meet at sharp angles; round off sharp edges and inside corners to deny cracks a convenient place to start.
  • Don't use water in any aspect of forming and joining unless it is absolutely necessary.
  • If you are handbuilding or modelling use a grogged clay if possible.
  • Don't proceed with firing until ware is thoroughly dried under heat (especially in electric kilns where there is no draft to take water vapor away).

Firing is sometimes assumed to be similar to cooking a roast in the oven. However this analogy is not appropriate because in firing a kiln you are overseeing very complex physical, mineral, and chemical processes. During firing clays go through a drying stage where the final pore water is removed, cristobalite inversion, water smoking (crystal bound water is removed), quartz inversion, sintering (where particles are cemented together), and finally glass formation and the complex mineral changes associated with the vitrification process.

A casual view of firing leads to oversights. The most significant of these is the use of automatic kiln controllers and shut-off devices without large witness cones in the kiln. Many also assume that shut-off devices are completely reliable and maintenance free. Most clays and glazes are optimal at a narrow range of temperatures and there is no way to be assured of firing correctly without the testimony of a free standing cone visible in a peep hole. You must manually intervene if necessary.

A similar problem many face is uneven firing of electric kilns. We recommend that you use large cones to assess which are the hot and cold spots and then stack your ware more densely in the hot spots or adjust switches to compensate. Also, you might try using half shelves and staggering these so that the kiln is less compartmentalized.

Going too fast in the early stages of firing is a very common problem. This leads to cracking and sometimes fracture of the ware. It is important to recognize that if you do not have a drier capable of sustaining +100C temperatures you have no way of completely drying the ware. Thus, the kiln is your final stage drier. Electric kilns have no airflow to remove water vapor as it is generated, so it is very important to fire the kiln slowly at this stage. Most people leave the kiln on low overnight so that in the morning it is at about 150C and ready to be fired normally.

Many users expect that their clay body be fully vitrified. However it is difficult to say at what temperature a clay is 'vitrified' because it depends on whether one judges this by fired strength, porosity, or surface appearance and feel. In addition, many clays will bloat and melt before they reach zero porosity, others appear vitrified yet are quite porous. In general we judge a clay to be fully vitrified after it has undergone the color change associated with glass development and the surface is a homogeneous color. Most bodies, however, are formulated to fire at a temperature slightly below this to provide a more interesting surface color and give a little margin for overfiring.

Because our clays contain some soluble calcium and magnesium sulphates and others employ plastic clays which contain some coal, you may detect a slight sulphur odor during bisque firing. This is not a concern because the amounts present are quite small.

When we recommend a clay for a certain temperature range or other use, we do not mean that it will not work for other things, simply that we do not test and maintain it for other uses. Also, each body in our lineup normally has another that is fairly similar but which offers a different set of advantages. It is usually best to stress working and drying properties if over fired appearance.

Clay Bodies Containing Barium Carbonate

Almost all raw clay materials (including kaolins) contain soluble impurities (i.e. iron stained calcium/magnesium sulphates). When a raw piece of clay is dried, these impurities move to the surface with the water as it evaporates. During firing the 'scum' left on the surface forms a glassy discoloration and in some cases can even create a glaze-like surface.

Soluble salts are highly prized in sculpture bodies because they highlight surface textures and contours. This is especially so if the salts are iron stained and create variations in coloration. However, in functional pottery and structural products these salts are usually a nuisance. These surface deposits often affect adherence to the clay and melting patterns of overlying glaze layers. Fused solubles on the surface can also stick ware together or to kiln shelves.

Clay body and material supply companies commonly add barium carbonate to clay bodies in small percentages (0.1-0.8%) to solve this problem. The barium chemically reacts with the sulphates to precipitate insoluble products. In the reaction, slightly soluble barium carbonate and soluble calcium sulphate convert to insoluble barium sulphate and calcium carbonate. Thus they both remain within the body and do not concentrate on the surface during drying. The ceramic industry has found this strategy to be extremely effective and has not yet found a viable substitute.

The practice of adding barium to clay bodies is also questioned by some as posing a leaching hazard in functional ware made from barium-containing clay. However one must consider some factors that indicate otherwise:

  • The barium is distributed within the clay matrix, only a very tiny fraction of that which is added actually presents itself at the surface.
  • The barium reacts with comparatively abundant fluxes and silica (and is a flux itself) in vitreous clay bodies to form insoluble silicate glasses within the body.
  • Functional ware is covered with glaze completely isolating the barium from food surfaces.
  • Recipes employing highly refined clays and minerals often do not require barium, however many are subjected to a variety of chemical processes that also have a bearing on functional safety.
  • The soluble portion of the barium converts to insoluble and harmless barium sulphate after the clay body is wetted during preparation.
  • Barium carbonate makes it possible to make bodies using native natural coarser-grained materials (which are inherently safer because they generate fewer of the minus 1 micron particles that are fine enough to lodge in the lungs).

Instead of getting involved in the trafficking of thousands of glaze recipes that never seem to work right, consider using one or two base glazes and create all your colors and surface effects by additives and blends. In this way you can perfect not only the base's fired properties, but also its slurry characteristics. For production work it is also very important to have a base glaze slurry that is both fluid and thixotropic, that is, it gels on the ware and does not run or drip.

Most glazes have adequate ball clay or kaolin (or some other clay) to suspend the slurry and harden the raw glaze layer so it is not necessary to use a gum or binder. However if the glaze has very little clay, you may need to add 2-5% bentonite to harden and suspend it (although this can really slow down drying time). You can also learn to reformulate the glaze to increase the clay content while maintaining the fired characteristics (Digitalfire INSIGHT software is excellent for this, there are videos on their website about how to do this).

You will find that the bisque ware of our fine porcelains or white stonewares is quite absorbant. Glaze slurries will stick well and dry quickly compared to some of our denser stonewares. However, the bisque surface is quite smooth and thus the glaze is not able to get as firm a foothold to hang on the way it can with bodies that have a coarser surface. For this reason your porcelain glazes should not have a high clay content which causes them to shrink excessively during drying. On the other hand, when you apply glaze to some of our coarser bodies like H440, it is able to get a very good foothold on the surface, so higher clay content can be tolerated. In addition, the bisquit ware of dense bodies like H443 and H440 will cause the glaze to pinhole as it dries. This does not happen with the finer bodies and occurs because larger particles create channels where water-displaced air escapes to the surface during glazing. These pinholes normally heal during firing.

You should pay attention to the specific gravity of your glaze. When it is working well, weigh an accurately measured volume and divide the weight by the volume of the container in cc's. This produces a measure in grams/cc that you can target on future mixes. Maintenance of this property will greatly assist in the consistency in your ware. If your glaze varies in viscosity even though you maintain its specific gravity, then consider using distilled water or adding a small amount of dispersant (i.e. sodium silicate, soda ash, Darvan) or flocculant (i.e. vinegar) to correct it.

A key aspect of glaze quality and function is 'fit'. However there are many clay users who do not know what this property is or how to test for it. Simply put, ceramic ware expands and contracts when heated and cooled. It is important that the glaze and clay have similar expansion:contraction characteristics or tension will be set up that either severely weakens the ware or manifests itself as 'crazing' (cracks in the glaze) or 'shivering' (the fired glaze flakes of the ware). These two conditions not only compromise your ware strength, but also its appearance and safety. We have plenty of information on this subject and can help you adjust your glazes to optimize their fit on your ware. Many potter's have doubled the strength of their ware with simple changes. The one simple test we recommend is that you quickly immerse a piece in boiling water and ice water in two minute cycles. The item should be able to survive 3 or 4 cycles before manifesting any problems.

We do not recommend 'penny-pinching' on glaze materials. Although some materials might be very cheap, they are usually also variable in chemistry and particle size. There are, of course, inexpensive ways to make glazes also, you might consider using our Alberta Slip and Ravenscrag Slip glaze recipes (more information below). Avoid carbonate or dioxide materials if possible (they generate gases during firing and can cause glaze bubbling and blistering). For example, if you are having trouble with glaze blistering consider using cobalt oxide instead of cobalt carbonate, wollastonite instead of calcium carbonate, a stain instead of manganese dioxide. In each case you must do the necessary calculations to make a substitution, it is not a 1:1 switch. Gerstley borate is known to be a variable material, we suggest you use a boron frit or our Boraq if possible.

Please consider the safety of your ware when making glazes. Don't use large amounts of barium carbonate (small amounts are likely OK if the glaze is well melted and has sufficient silica and alumina). 20% barium in any glaze is likely leachable, if it is simply there to produce a matte, try achieving the same effect with high calcium or high alumina instead. If it is used to achieve a certain color, try a stain instead. Watch out for glazes that have large amounts of heavy metal colors (like manganese) that can leach. Metallic glazes should only be used on non-functional ware. Glazes should be melted properly or they will almost certainly leach. You can test this by trying to make a mark on the glaze surface with a spoon; if you cannot clean it off the glaze needs a higher temperature. Watch out for flux saturated glazes (usually very high feldspar). These are over-melted and have little silica or alumina to produce a balanced ceramic glass, thus they are often leachable even though they may appear very glassy. If you want to understand glaze chemistry better, visit digitalfire.com.

If you produce dinnerware or porcelain, you might consider a ball milling your glazes. Ball mills not only grind down impurities, but also stabilize the slurry properties and intimately mix the ingredient particles to achieve a higher specific gravity without additives. Ball milled glazes invariably give a finer quality fired surface.

We recommend that you take bisque firings as high as possible while still retaining enough absorbency to glaze properly. This is because volatile organic materials in the clay must burn away during bisque, and if any of this remains it can disrupt the glaze melt, contributing to pinholes and blisters.

The use of engobes and slips (between glaze and body) is very common in industry, especially tile. If you wish to use them, carefully test the fit of the slip in drying and firing. Make tiles of the clay and cover them with a thick layer of the intended slip. If the slip flakes off during drying, increase its shrinkage with more fine plastic clay, if it shrinks off do the opposite. If the slip flakes off during firing, use materials of fine particle size to enhance fired shrinkage, if it shrinks off do the opposite. Ideally, a slip should be made from the same body as the ware with some added flux to help it form a glassy bond with the body. The clay slip can be ball milled if desired and stains added.

Glaze Recipes

We can supply base recipes for each major category of clay body if needed. Please visit the following websites for glazes:

If you would like to develop, adjust and fix your own glazes, you can get a free trial of the INSIGHT software at Digitalfire.com.

Dealing with Glaze Defects

These include things like crazing, shivering, pinholing, blistering, leaching, cutlery marking, slurry and drying issues, color problems, etc. Please visit the troubleshooting section at the Digitalfire Reference Library for help.

Physical Properties

Over the years Plainsman has developed a standard suite of lab tests that we perform on each run of clay (about one per day). Each Plainsman box has a code number and the first four digits of the number are keyed to our database of test results. We computerized our lab in 1980 and have complete historical test data starting from 1984 (there are hundreds of thousands of test results in the database). Our testing is performed using ordinary equipment that most potters and technicians have easy access to. Each test is thoroughly documented and is carried out step-by-step according to documented procedures. Our tests are procedures are outlined in the testing section at the Digitalfire Reference Database.

Data provided for each clay:

Each data sheet has a Physical Properties section that supplies the test results data. We monitor all of these properties on an ongoing basis.

  • Drying Shrinkage: This refers to the percentage by which a clay bar shrinks from wet to dry.
  • Dry Strength: This is a measure of the force it takes to break a bar by applying force at its centre while it sits on two supports at either end.
  • Water Content: A measure of the percentage of water in the wet clay.
  • Drying Factor: A value yielded by the FORESIGHT DFAC test. The theory behind this test is described in the book "The Magic of Fire".
  • Dry Density: A measure of the specific gravity of a dried clay bar.
  • Sieve Analysis (Tyler mesh): A set of numbers which show the amount of material left of each screen in a root-of-two series of screens through which a sample of the powdered clay is washed, then dried and weighed.
  • Fired Shrinkage: A percentage by which a clay bar shrinks from dry to fired. This test is usually conducted at a number of temperatures.
  • Fired Absorption: A percentage by which a clay by gains weight when it is boiled in water for 5 hours.
  • Chemical Analysis: The calculated chemistry of the body based on our data of the raw materials that make it up. Note that if a body contains grog, the chemistry of the grog is not taken into account in the analysis given.

Note that we are refining these data sheets on an ongoing basis. Where necessary bodies are fine tuned to line up with the data, or, the data is adjusted slightly to better reflect the body.

Logo Plainsman Clays Ltd.
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508
Email: plainsman@telus.net