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Firing Clay According To Color

15 April 2017 - 12:15 PM

Firing Clay according to Color.


White Porcelain:  being white in color automatically concludes that much purer grades of materials where used in making this body. Grolleg, NZ Kaolin, and Super Standard kaolin have very low rates of impurities and virtually no carbon deposits. Sodium feldspars and standard silica also have little impurities, and contain no carbons.  Following the usual bisq and glaze firing schedules should work for this body. Test for absorption and raise peak glaze fire by ½ cone increments if absorption is more than 1%. Note: due to the composition of these bodies, the COE values are usually significant higher than standard porcelains.


White Stoneware:  like white porcelain, this type of body usually consists of cleaner clay varieties.  However, all fire and ball clay/s have some level of impurities and some level of carbons in them.  Part of the attraction of stoneware is the color/s produced by the presence of iron, magnesium, and titanium. The higher the levels of these three elements go: the deeper the toasty color of stoneware becomes.  Following standard bisq and glaze firings should  work for this body. Note: stoneware has a blend of coarse particle fire clay that must have a relational portion of sub-micron ball clay to minimize porosity. After absorption testing, raise peak temperature by ½ cone until absorption is below 2%.


Grey Porcelain:  this body color indicates a utility blend. It is designed to be inexpensive, plastic, and used in the widest range of applications.  Grey color indicates carbon, which are usually dealt with in the bisq fire.  If pin holes show in the glaze firing;  then adjust your bisq fire temperature up by 50-60F with a short 10-20 minute hold to help burn off carbons.  Grey Stoneware: can be a particular problem because of bloating. As carbons burn off, they create an impermeable barrier that traps off gassing feldspars that can be small to large in size. As bloating area increases: this also indicates higher levels of carbons. If bloating continues after several bisq or glaze temperature adjustments: it may be wiser to discontinue use. Bodies using higher carbon ball clays are tricky; if accompanying feldspar levels are too high: the problem is nearly impossible to remedy with firing schedules.


Brown/Dark Stoneware: brown/dark clay is most always indicative of high iron levels. With iron, comes higher magnesium and titanium levels: these two elements almost always proportionately increase as iron increases.  The combination of them creates that warm toasty brown that potters know and love. The problem then becomes, large particle feldspar minerals are always very common in these types of ball clay. Unlike finely ground feldspar in your glaze; these are coarse and hidden in the interior of the clay body where heat takes the longest to reach. Consequently, brown/dark stoneware is the most susceptible to pin holes and blistering because large particle feldspar in the body takes much longer to off-gas. This body is the one that requires a change in glaze firing schedules most often to resolve off-gassing issues. Rarely seen in cone 10 schedules, and often seen in cone 6 schedules.


The Remedy given is usually a higher bisq fire which will not work because you are not dealing with carbons; but rather feldspar particles. At bisq temps, there is simply not enough heat to fully resolve feldspar off-gassing issues. At 2190F, normally all feldspar are at the end of their useful cycle; unless they are large and inside a clay body. Some studies suggest it takes heat twice as long to penetrate a clay body as it does the glaze coating the surface.  This body then requires a higher peak temp at cone 6: 2230F. It also requires a short to extended hold pending the kiln size.


Of course if the goal is not vitrification; then trying to achieve optimum firing results really does not matter. However, some blended fixes may still be required to resolve bloating and pin hole issues. Primarily, higher bisq temps are utilized to burn off excess carbons, and higher glaze temps are used to remedy pin hole and crater issues. Bloating can be fixed or minimized mostly; but often cannot be resolved because of poor formulation of the body.



50 35m
Large particle feldpsar (20 mesh) inside a stoneware body. 200x


Glaze Additive/s

15 March 2017 - 06:36 PM

Need some input, point/s of reference. 


1. Do you add bentonite, EPK, or gums to your glazes?


2. In what amount do you add to the final (dry) batch weight?


3. Do you add more for high magnesium or sodium glazes?


4. Have you had problems with bentonite clumping?


5. Do you rely on additives to achieve viscosity?



Two Piece Molds

12 March 2017 - 05:34 PM

I would appreciate some links to technical information or videos showing the process of making two-piece molds. I have done many regular molds; so I am not unfamiliar with the process. To be more specific; making large two piece press molds.........very large. like 40" x 30" large. Building my own ram press is not an issue either.  



Stoneware Clay Properties

18 February 2017 - 10:34 AM

I have been studying plasticity in stoneware bodies, as most know. I am finding some results that are making me question the accepted belief that plasticity equates to ease of throwing. Plasticity in general comes from the electrostatic charges on the clay particles; which changes as the body ages. I am looking for articles that specifically review the relation of sub micron ball clays, to the ease of throwing. I am trying to determine/figure out how mass plays a role in throwing.


Ron Roy and I had this discussion at NCECA; what is the cut-off point for large and intermediate mesh sizes, before those additions create a denser mass: which makes the clay harder to push around on a wheel. It is very common for stoneware bodies to have 80% total clay content, there are some even higher than that. So I still find myself questioning if mass is playing the larger role in determining if a clay is easier to push around? Not sure if I am articulating my thoughts correctly, but hopefully I have made the question clear enough.


As a comparison: everyone knows how easily porcelain moves around when thrown. The most common analogy is that it throws like cream cheese. That is because porcelain in general has 25% silica, and 25% feldspar; which has much less mass than fire clay. One of the major differences is mass: stoneware has more clay content; and much larger particle sizes. I have tested this theory by adding V-gum and macaloid to high percentage formulas of fire clay/intermediate clay. These additions are not the norm; solely done to test if plasticity is the determining factor in ease of throwing.



Clay Buyers Guide

12 February 2017 - 11:23 PM

There have been many inquiries about clay bodies recently, so I thought I would put together a guide of sorts. General rules of thumb for trying to decipher what clay body to select. Always exceptions, but it should give you some sense of direction. The specs paint more of a picture than generally assumed: because they define clay characteristics.


Shrinkage: generally used by the potter for a guide to predict how much the body will shrink when fired to the recommended cone. However, shrinkage also gives an indication to how the body was formulated: what is in it. Shrinkage is a direct reflection of how much water is in the clay. The amount of water is determined by the type of clay used; determined by the WOPL (water of plasticity).WOPL is the measurement of an individual clay variety to hold water on its platelets; thus forming a pliable ball. A low plasticity clay will hold 26 to 30 grams of water per 100 grams of clay. A medium plasticity clay will hold 31 to 35 grams of water, and a high plasticity clay will hold 35 to 38. Plasticizers will hold 38 and upwards:such as bentonites, hectorites, macaloid, and V-gum T.


Application: The plasticity of a clay body is determined by the additions of fine particle ball clays or plasticizers. The more plastic a body is; the higher these types of additions are. However, high plasticity produced by ultra-fine ball clay or plasticizers also means higher shrinkage rates. As the shrinkage rates begin to go over 12%: plasticity generally increases as ball clay additions increase. So if you are looking at competing white stoneware bodies for instance; and one has higher shrinkage than the other: it will generally mean the higher rated is more plastic.


Absorption: This indicator is a little harder to determine formulation because so many formulating variables effect it. The hard set rule is: porcelain will always have lower absorption rates than stoneware due to composition. If a porcelain body has a higher absorption rate than competing stoneware; it would automatically be suspect for use. Porcelain in general produces a translucent appearance after firing (not always) which is in part due to higher flux levels, higher silica levels, and very low carbon clay varieties. The combination of these three produces a much higher glassy matrix (vitrification), which also results in nearly zero absorption rates.


Stoneware bodies typically consist of fire clays (large particles), ball clays; with lower levels of silica and feldspar. Stoneware body formulation can go all over the place: there are no set rules for mixing. It can have little or much fire clay, some or a lot of ball clay; silica can be added or omitted: and feldspars typically are a third less than found in porcelain. Again however, the lower the absorption rate falls: the higher the amount of plastic ball clay has been added. Highly plastic ball clays are typically sub micron particle sizes; which not only add plasticity, but also seal up the microscopic voids created by large particle fire clays. So the general rule of thumb for stoneware is to look more closely at absorption rather than shrinkage. Ball clay is used in all stoneware bodies, but lower absorption usually indicates that a sub-micron (highly plastic) ball clay has been used in lieu of a medium plasticity (larger particle) ball clay.


COE: Co-efficient of expansion is mostly defined by feldspar additions in porcelain; and by the percentages of fire clay verses ball clay: coupled with feldspar in stoneware bodies. As the amount of feldspar increases in any given formula, the COE increases. The amount of silica can lower it, as will the coarseness of the clay particles.Median COE values for porcelain typically run in the 5.75 up to 6.00, and stoneware typically runs from 5.50 up to 5.75. If a stoneware body falls below 5.50, it is a indication that more clay has been used in the formulation, couple with less feldspar. This would create problems, causing the absorption rating to climb. Porcelain bodies with COE values above 6.00 usually indicate higher feldspar levels: which generally produce a higher degree of translucency.


Color: The color of the clay body says much about the composition: it is an indicator of the specific types of clay varieties used in formulation. Kaolinitic ball clay has very low carbons and high alumina: and it generally runs from the light tan, light grey; to light buff in color: but it also tends to be a medium plasticity body. High plasticity ball clays are also higher in carbons: so they run brown, to dark greys. When ball clay are added to either a stoneware or porcelain body above 10%: its color will effect the final color of the clay. So porcelain that is white, light tan, and light grey either have used a plasticizer or a kaolinitic ball clay. On occasion, small amounts of bentonite are used: but those additions are so small they will not darken the body considerably.


Porcelain more so than stoneware is very revealing by its color. White porcelain uses grolleg, super standard porcelain, or air floated kaolins: which are all white. In addition, either plasticizers, plastic vitrox, or white kaolinitic ball clays have been added. These white porcelains tend to have higher percentages of fluxes as well: producing much higher degrees of translucency. If the body is buff, or light tan; then EPK, #6 Tile, or Om4 type ball clays have been added: which still produces a bright white body, but with less translucency. If a porcelain body is darker, or grey in color: then higher carbon ball clays have been added for plasticity reasons: which produce no translucency in general.


The darker a clay body becomes: the more high carbon ball clay has been added in general. In stoneware bodies, a darker body is a good thing. It means higher amounts of sub micron ball clay has been added. It can also mean higher amounts of fire-clay present: which can be a bad thing it not formulated correctly. Many stoneware bodies are not formulated for functional use: they are specifically formulated to with stand thermal shock from raku, salt, or wood firings. If you are looking for a stoneware body for functional use; look for a lighter tan or lighter grey color: which usually indicates higher levels of kaolinitic ball clays.


 Clay bodies that give a very broad range of firing cones, are typically unreliable for functional use. To vitrify at any given cone: a body has to have a set value of feldspar vs. silica/alumina to do so. Clay bodies intended for functional use should not give any more than a 2/value: cone 5-6 for example. A body that states for cone 04 to cone 6 is just flat misleading in regards to functional use. If the molar levels of flux required to vitrify a body at cone 04 were present in a cone 6 firing: the piece would slump.


As with all things pottery, there are exceptions to the these general guidelines. Hopefully these however will help you narrow down your shopping window.