Jonathan Kaplan overviews clay bodies, body materials and body types, how they are formulated and tested, how to protect yourself when buying prepared bodies, how to take responsibility.
So what's all this brouhaha over clay bodies? More Sturm und Drang over not getting the results that you expected? Whining to your colleagues that you are just not satisfied with your clay? One of the problems is that we don't know the right questions to ask. Obviously if it is in the catalog, and they wax ebulliently over its characteristics, it will work. Well, chances are that it may, and chances are that it may not. As Rocky Rococo said to Nick Danger "Maybe yes, maybe no!" Lets learn to ask some appropriate questions so that our wares are better because we have some basic clay information.
We assume that what comes out of the bag or out of the box is indeed what we have paid for (perhaps!) and will perform with all the attributes and accolades accorded to it. Again, maybe yes, maybe no. What we have purchased are materials that bear some resemblance to an idea of a fired product that we have seen or experienced, and chemically, it may be similar to its quantitative analysis furnished by the manufacturer. It may work well, but the final fired product may be problematic. Defects in color and texture, occurrences of foreign matter, glaze related problems, cracking, etc. only to mention a few could be encountered. We take for granted what we read. This is clearly a case of what we don't know will hurt us. More specifically, we produce inferior quality ware because we don't understand the nature of the material. Our frustration grows as the quantity of seconds gets tossed into the dumpster or trucked to the landfill. Or, sold as seconds.
Studio situations and often finances dictate that we do not beneficiate our raw materials. This is a first line of defense in an industrial situation where the raw materials are beneficiated, or, aptly translated, "made better." Our clay body producers take only the minimum necessary steps to insure that the finished body is comprised of ingredients that are free from contamination. And clay materials can be full off them. For example, screening low mesh and low grade fire clays to remove offending lime products. In the studio, this is a difficult, labor intensive, and huge financial undertaking.
While we could pass a few hundred pounds of 28 mesh material through a 50 mesh screen in about an hour, it is not something that we wish to do on a daily or weekly basis. If we truly want to have wonderful plastic clay, it involves first blending the clays and other additions, fillers, fluxes, etc., as a slip to achieve the maximum wetting of the clay materials. Next, the materials are passed through a vibrating sieve that removes larger particle sized materials, junk, etc., allowing the appropriately sized slurry to be pumped into a filter press to be dewatered. After the cakes are dewatered to the desired moisture content, they are pugged and deaired, ready for use. A good example to experience the differences between clay that is mixed as a slip and clays that are not is to mix about 100 pounds of your clay body as a slip, dry it on a plaster bat, wedge and throw it. Or take a 25 pound hunk of your prepared clay body. Dry it out, pulverize it as best you can, slake it and mix it into a slurry. Dry it out and throw it. Compare both to a conventionally mixed body.
This process yields clay of unparalleled quality, but the space considerations, the financial demands of such a system often place this method well beyond the reach of studio potters and even clay body producers. It is far easier and far less expensive to blend the weighed materials in a muller or pre-pugger with the necessary water, drop it into a deairing pug mill, and bag it and box it. It works. The clay quality is usually quite high and is very usable by a vast number of potters. But the materials are not wetted optimally, and the offending large size "stuff" is still in the mix. It may not haunt you now at this moment, but take my word, it most certainly will at some time in the future. While the body may appear plastic and have good workability, it is probably quite short.
No clay stays the same from lot to lot, and therefore characteristics that we have come to expect from our old trusty and reliable clay body will be noticeably different over time. As indicated above, studio potters cannot beneficiate materials on the level that is needed to insure reliable and consistent results. But we can have some better information. It doesn't matter if you are buying clay in a box, or mixing your own clay from bagged materials. The problems are the same.
While not intended as a primer on the formation of clays through geologic weathering of feldspathic materials, we know that what we now use in our studios and in our production has been formed a long time ago. It is precisely this fact that gives us the variation in our raw materials that is problematic. But it is not only the variation in clay makeup from pit to pit, it can be a function the machine operator removing the overburden, or the front end loader driver taking clay from the seam. Or how the clay is laid down in the shed, then subsequently removed to the crushers, dry pans, dryers, and mullers. After all, its nature's variations and then possible human error as well as miscalculation. How then can we make sure that what we end up using will work for us in the manner that we need?
Never assume that what is written about a clay body or material will mean that all these qualities will work for you. And also never assume that your supplier has on-going testing or even a testing lab. While the physical and chemical analysis of a clay does indeed provide specific guidelines, that's all they are. For example, how many times has the color of AP Green 28 mesh Missouri Fireclay been different? The chemical analysis is the same in the last lot. What does this mean to us in the studio? It means precisely that we need to exercise care in the use of this product, as well as many others in the formulation of our clay bodies. Is there something wrong with the product because the color has changed? Probably not, but the color is an indication that there have been changes in the material coming from the pit and this could adversely effect your clay body.
Here's a quick and dirty way to see exactly what I mean. Select some primary clays and some secondary clays from your studio. Lets use Redart, Old Mine 4, Foundry Hill, Yellow Banks, AP Green, EPK, and Tile 6 clay. Scoop a good amount of material and mix an appropriate quantity of water to make a loose slurry with your mixing device. Mix well. Sieve each mix through a 60 mesh screen, and look closely at what does not pass through. There will be clay material on the screen, but there will also be other "stuff." This is the garbage that directly impacts our wares. The screen residue will be very apparent in the Redart, and ball clays, and the least noticeable in the kaolin.
This other stuff is the LOI, or the loss on ignition. The stuff that burns out. One wonders how we have learned for so long to use products that have huge LOI factors. If we ask our clay body manufacturers what are the ingredients in a particular mix, most of the time they will act as if it is a high level top security request, request denied. Ask them about the LOI, what ball clays have been used, and if a DTA been run on this particular lot. (More on DTA testing later.) If they won't provide you with accurate information ( and I know that you really don't want quantities of each materials so you can go make the body in your studio, right?) take your business elsewhere and find a manufacturer that is willing to work with you and stand behind their products when the going gets tough, because at some point in your clay working career, it most certainly will. Trust me on this one.
Perhaps this particular seam of clay is higher in silica than the thousands of tons that came before it last month or so. Could lead to possible dunting problems in the formulated body, and later, related glaze problems also. Not to mention the associated lime problems with fireclay mining. 28 mesh material is fine for industrial fireclay
mortar or hard firebrick, but not an appropriate grind for studio use. Hawthorne clay is now offered in a 50 mesh grind, much more suited for pottery making as the chances of lime pop outs are significantly reduced. Other manufacturers of prepared bodies offer screened fireclay. What does this mean? What size screen? Is the resulting screened material retested for lime? What about the high silica content in fireclays?
Fireclays are exceedingly problematic and in my estimation, the weak link in all stoneware bodies. Our industry counterparts would look askance with dismay at even any notion of "fireclay" in a clay body, be it a plastic formula or one for casting. There are "really no good fireclays and more" is something that I have heard for many years since the last bag of Pine Lake 50 mesh went into my mixer. While in general this is true, it may be worthy to venture that fireclays are dirty, full of problems, their only benefit being the addition of a moderately plastic, more refractory, toothy addition to a body. In my book, not a necessary addition and one that I would eliminate completely from a stoneware body in my shop.
Most ceramic clays with the exception of coarse grained materials such as fireclays and bonding clays (28-50 mesh products) are airfloated. Use of 200 mesh air floated materials have long term benefits for any clay body in any temperature range. Typically, ball clays, some red clays, earthenware clays and stoneware clays are usually air floated and provide a distribution of various clay particle sizes that are necessary for optimum qualities in a body formulation. Be it a liquid casting slip, a plastic body for throwing, jiggering, or hydraulic pressing, all clay bodies benefit from a mix of different particle sized materials. Note, not mesh size, but particle size. All clay processors can provide a screen analysis for their products.
Never rely or even depend on one ball clay, one kaolin, or one stoneware or earthenware clay in the total formula. For instance, if a plastic formula suggests 25 parts of kaolin, use part EPK and part Tile 6 or Pioneer. A casting formula lists 20 parts of Tile 6? Use some EPK and Velvacast along with the Tile 6. Airfloated ball clays have a high lignin content and benefit from a diverse mix also. While the "Old Mine" of Old Mine 4 has changed over the years, it is still a reliable clay even though it is significantly high in iron. It can benefit from the addition of other ball clays such as XX Sagger, Thomas Ball, Spinks Blend, and others. If a light firing ball clay composition is required, use Starcast with perhaps some Foundry Hill Creme. By diversifying your materials there is a statistically wider base line as well as less of a chance of one singular material contributing to your clay body malaise.
Earthenware bodies in the 06-04 range can be made from ball clays, red clays, and fluxes such as Talc, Neph Sy, or some frit. Bodies in this range don't have much of a clay/glaze interface. A such, this temperature range exhibits noticeable glaze flaws as crazing and shivering in the ware. Also, at this temperature range, the body is not well glassified and is still quite punky, basically unvitrified. Suitable eutectics are difficult to achieve in this range also. With the addition of a frit to the body, proper vitrification can be achieved but they body has a very narrow range in the bisque before it becomes too tight so as not to readily accept a glaze coating. PV clay is a suitable addition at this range, and so is Cedar Heights Red Art. Various formulas of Red Art, Ball Clay, and Talc exist at this temperature range. Many "hobby bodies" for casting are various combinations of talc, ball clay, some PV clay, and deflocculation. An excellent addition at this range is Pyrophyllite, used in conjunction with some talc. Additions of silica at this range should be avoided. I remember using a clay body many years ago that was primarily Jordan Clay, (now long gone into clay heaven) and some spar. The blend was beautiful to throw on the wheel but horrible to stick up handles, spouts,etc. This formula needed some coarse grained kaolin for proper particle size distribution. At this temperature range, Red Art is a wonderful addition for terra cotta formulas. Care should be taken with this material as it does exhibit fluxing power and can over vitrify a body that contains other fluxes. Neph Sy can also be used at this range, but again, some caveats apply in forming powerful eutectics with a frit, talc, or what ever other flues you may use. EPK and other kaolins can be used in this range promoting better workability, better drying characteristics, and ultimately, better ware.
Casting bodies in this range are also easy to achieve using the above materials. Casting bodies are usually half plastics and half non plastics, with perhaps some variations over or under the 50% limit. Many commercial casting "hobby bodies" are half Talc, half Ball Clay, with perhaps some PV clay and of course deflocculation. Addition of pyrophyllite to the non-plastics side of any casting body can significantly improve vitrification as well as casting rate.
Mid range bodies in the cone 4-6 range can begin to use Silica, Feldspar, as well as Neph SY and Talc in combinations for the non-plastic part of a body formula. A good rule of thumb is a 70 plastic/30 non plastic body, with a combination of flues and Silica for proper vitrification. 80/20 bodies can work also.
Porcelain formulas in this range need more flux as the kaolins are quite refractory. A body can be as much as 40 parts silica and flux and 60 parts clay. And of course, there are other many formulas and combinations of materials that work.
At the higher temperature range of cone 8-11, the 70/30 and 80/20 arrangement works fine for stoneware bodies. Particular attention should be placed on including enough Silica in the body to produce a good glass within the structure. Most potters use stoneware formulas that are significantly low in Silica, and the resulting body is not quite vitrified. Can should be taken to include enough spar to melt the Silica without leaving any excess free silica.
Porcelain bodies at the cone 9-11 range are some variation on a classic equal parts kaolin, ball clay, silica and spar arrangement. Additions of ball clay in a porcelain body compromise translucency unless a very white or light burning ball clay is used. While porcelain bodies "throw like butta" they do pose forming problems in that they are quite non plastic. Even with the addition of plastic kaolins, one needs to include some Macaloid (a beneficiated Hectorite) or Bentonite to increase plasticity.
Grog in a clay body is a potential problem source. Grog is basically ground up firebrick seconds, or "culls." If these bricks are seconds other then out of dimensional tolerance, why put "second" quality materials into your first class clay body. Coarser grogs are quite problematic. The finer mesh materials just act as silica in the body.
There are two types grog materials that I would recommend. For stoneware bodies, Mulcoa, a domestically manufactured and formulated calcine is available in many sizes. White bodies can benefit from Molochite, a white calcine from England available in some fine mesh sizes. The down side to use of these products is an increase in the per pound cost of the body.
What do we need to ask clay body manufacturers so that we can get a clear picture of what we will use to produce our wares?
Are raw materials tested before they are mixed? A simple test with hydrochloric acid will show the presence of lime in fireclay. Are coarse fireclays sieved before they go into the muller or prepug? What size screen? What tests on the finished bodies are run? Are they fired in both oxidation and reduction atmospheres? How much reduction? Any standard here? How do you test for clay fit? How often do you run shrinkage and absorption testing? What about testing to determine if the clay body is oven proof? And very important, do you test for free silica? Do you run DTA tests to show those exothermic and endothermic peaks, indicating quartz inversions and potential problems? Are these graphs available and made part of the suppliers reference for customers?
DTA testing is overlooked and ignored in the studio. While such testing is not overly expensive, nonetheless studio potters ignore it. What DTA testing of your clay body provides is a precise footprint, so to speak, of the changes that happen upon heating and cooling. Specifically, the quartz related changes, or inversions, that can render a specific composition useless. I'm sure we are all familiar with the alpha beta inversion, but what of some of the other overlooked ones? Low temperature dunts in your bisque firings? Might be those highly silica laden ball clays cooling to quickly under 400 degrees F. DTA tests provide graphs of these peaks and valleys that can pin-point these very important changes in your clay bodies. It seems to me that if our livelihood depends on the correct formulation of balanced, well structured and formulated compositions, does it not seem to follow that to protect ourselves and our customers, every avenue needs to be followed, every test that can be done should and must be done? Why do we take for granted that all materials will give us predictable results? We need to take more responsibility and not depend on others.
So where do we go from here? Here are some basic guidelines for clay body formulation.
There are many formulas for clay bodies. Look critically at your formula and eliminate or decrease the fire clay content. Use only screened Fireclays, or screen them yourself. Hawthorne Fireclay is available in a 50 size. Avoid coarse fireclays. Use stoneware clays and ball clays that are airfloated. Diversify these additions with more than one Fireclay, stoneware clay, or earthenware clay. Use a combination clean ball clays. Use kaolins liberally. Excellent stoneware bodies can be made with Ball Clays, Kaolins, Silica, Spar, and Pyrophyllite. Color in these bodies can be achieved with a small percentage of Red Art or Iron Oxide. Tooth or texture can be easily obtained with Mulcoa or Molochite, or even a quality silica sand. Contact the producer of your clays and they will provide you with much useful information.
Buying "clay in a box" poses other issues. You can ask your supplier if the component clays are screen before they are wet mixed. Do they test the component clays? If so, how? Do they test the mixed bodies for shrinkage, absorption, at what is the date of the most current testing? How is their moisture content arrived at and what do they measure it with?
Most important, do they run a DTA test on each production run and can you see the resulting graph? If they won't give you a straight answer, find another supplier who will. Don't settle for excuses or lame assertions that "This clay works. Its worked for plenty of potters before you and will probably work for you as well as others in the future." Your art and craft depend on quality clay.
For prepared casting slips, is the slip sieved before shipping? What deflocculation system is use? Soda Ash and Sodium Silicate? A polyacrylate system? What is the recommended specific gravity and viscosity? Do they use a Brookfield viscosimeter or a Lehman? For dry casting slips, is soda ash mixed in? What percentage of water is required? Do they have published deflocculation curves for their casting bodies?
Once we become more proactive, we can and will find that our attitudes about our work are better because we have taken responsibility for our wares from the beginning.
A potter for over 25 years, Jonathan Kaplan holds a BFA from Rhode Island School of Design and an MFA from Southern Illinois University. A master potter and mold maker, Jonathan is the owner of Ceramic Design Group LTD, a ceramic manufacturing and design company. He is also an adjuct faculty member at Colorado Mountain College. When time permits, Jonathan enjoys skiing, inline skating, and cycling in the high country of Steamboat Springs Colorado.
In Bound Links
How can two potters have completely different opinions about the plasticity and workability characte...
Ceramic Design Group Ltd./Production Services
PO Box 775112
Steamboat Springs CO 80477