|Monthly Tech-Tip |
Few people actually understood what AP Green fireclay really was (it is no longer available). By carefully ascertaining its physical properties we were able to formulate a substitute material mix.
AP Green Fireclay (APGFC) has been used for many years by potters to make stoneware clay bodies. It is a moderately refractory, light colored, slightly iron speckled, plastic clay. Its physical presence actually contrasts with the perception that most potters have of it. For example, it is not as plastic as many people assume. By itself, it is a little more plastic than an average clay body, thus it can exist in high percentages in the recipe. Its drying performance is better than expected given its plastic nature. In addition, unlike what many assume, its Cone 10 porosity is only a little higher than a typical stoneware. Likewise its fired shrinkage is quite high imparting a total shrinkage that is much more than a typical stoneware clay body. One misconception about AP Green fireclay relates to its particle size. While most people think of it as a coarse fireclay, it is actually a very fine particled clay material. It is perceived as being coarse because of the way its is powderized; the particles are agglomerates and slake readily in water to pass more than 90% through a 325 mesh screen. These particles however do not break down readily when being mixed into pugged bodies. This material also contains iron-bearing soluble salts that are left on the surface during drying. These impart a darker coloration to the clay surface. The fired color of APGFC bodies is therefore very much a product of maturity, iron content and surface solubles.
A buff burning stoneware clay body could be fashioned using 80% or more APGFC and a little feldspar to increase maturity and cut plasticity. The 'balance' and 'character' of this material compared to industrial minerals (like kaolin, ball clay, quartz and feldspar) are the reasons for its popularity.
However pottery uses are not a big segment of the APGFC market. Thus physical properties and consistency factors that are very important to potters are not well maintained. Impurities that are not a problem for other markets can be a headache for potters. The 'mystique' that has surrounded APGFC has been partly due to lack of pertinent information about its properties. Also, while the character imparted by the incomplete grinding of the material produces surface color variegation in dark bodies (lighter specks of fireclay in a darker dense clay matrix) the associated non-homogeneity is a source of fired weakness.
An exact match is impossible and we must stress that the key to a successful substitution is knowledge of the compromises made. This will enable you to take countermeasures. In many ways PFC is a superior material and ware quality should improve.
As noted, the first step was to describe the physical and fired properties of APGFC (we do not consider the chemistry to be as important as physical properties). We then compared these to our traditional Buff Fireclay material blend and made substantial changes to its recipe to harmonize the key properties. This produced Plainsman Fire Clay (PFC). We then selected a clay recipe containing 50% fireclay and mixed it using APGFC and PFC and compared. Differences that appeared were rationalized and compromises made so that PFC would work as similarly as possible to APGFC in clay body recipes.
PFC by itself is a little more refractory than APGFC (which has about 2% cone 10 porosity). Thus the PFC can produce a slightly higher porosity when used in a clay body. However more feldspar may not be needed since vitreous bodies can actually appear more vitreous with the PFC in spite of the higher porosity.
They have a similar reduction speckle character, however PFC bodies may exhibit a higher population of more evenly sized specks. APGFC bodies tend to have some large scattered specks and iron blotches and this inconsistency has been eliminated in our PFC.
The APFG exhibits some unpredictable behavior in this area. Some of the fired character of pure APGFC is a product of its iron bearing solubles. These deposit differently depending on the potter's process and natural variations in the raw material. PFC also has solubles that tend to flux the surface imparting a brownish color.
Although AP Green is understood to be a coarse grained material, the opposite is actually the case. Although it is mechanically ground to around 20 mesh the individual particles are agglomerates that easily break down in water to produce a material that feels like porcelain on the wheel. While PFC also contains a large fraction of naturally fine clay particles, it also contains a wide range of larger mineral particles lacking in the APGFC.
Plasticity of the two materials is similar. If anything, PFC produces a body that is a little more plastic with a slightly more ball-clay-like character (as opposed to kaolin-like). Thus it may be easier to keep pieces on center and to neck them in during throwing.
PFC has a slightly higher drying shrinkage. That means you may need to be a little more careful to dry pieces evenly in plastic bodies with substantial amounts of APGFC.
PFC contains 10% of a very fine white kaolinized sand. This additive gives us control of maturity and plasticity. Very sensitive throwers might feel this sand in high-fireclay bodies or notice it in the slip produced. This sand can provide a channel for water penetration into the plastic clay; if you experience water splitting during throwing (vertical cracks) then we recommend exercising more care to avoid leaving water on surfaces that are being stretched (e.g. bellies on vases). For a simple comparison test between the APGFC and PFC versions of your clay body, balance cigar shaped pieces of clay horizontally on your finger and put a few drops of water on top and compare the amount of time it takes for a split to begin.
Since PFC has a higher free silica content it will contribute a lower thermal expansion to bodies (crazing glazes will craze less, shivering glazes will shiver more). We see this as a big advantage of PFC since common high feldspar glazes typically craze on high APGFC bodies. However the higher expansion could increase dunting problems in bodies containing inadequate feldspar or recipes with high ball clay and silica additions.
Adjustability/Consistency: PFC is a blend of refractory materials (a ball clay, a stoneware, a kaolinized sand and a kaolin). Thus inconsistency in any one material will be dampened by a factor equal to its percentage in the mix. In addition, we will be able to make small adjustments in the mix to compensate for any inconsistencies that might arise and to fine-tune the properties to make it a closer match to APGFC. PFC is mixed and ground in a conscientious manner specifically for ceramic applications and each batch is thoroughly tested and compared with previous ones to reveal problems and isolate trends. Thus users will benefit from a material that is much more dependable than they have been used to.
We have incorporated non-refractory ingredients into our blend to match firing characteristics of APGFC below cone 12. Thus you will need to test PFC to determine its suitability for non-ceramic refractory applications.
Drying Shrinkage: 6.5-7.5% (APGF is 6.0-7.0)
PFC APGF +48 (300 microns): 0.1-0.5 0.0-0.5 48-65 (300-210): 0.5-1.5 0.0-0.5 65-100 (210-149): 1.0-3.0 0.0-0.5 100-150 (149-106): 2.0-4.0 0.0-0.5 150-200 (106-75): 4.0-6.0 0.0-1.0 200-325 (75-45): 7.0-10.0 0.0-1.0
PFC APGF Cone 6: 5.0-6.0% 6.5-7.5% Cone 8: 5.5-6.5 7.0-7.5 Cone 10: 6.5-7.5 7.5-8.5 Cone 10R: 6.0-7.0 7.5-8.5
PFC APGF Cone 6: 5.0-6.0% 2.5-3.5 Cone 8: 4.0-5.0 2.0-3.0 Cone 10: 3.0-4.0 1.5-2.5
Plainsman Fireclay (PFC) differs from Plainsman Buff Fireclay (BFC) as follows: PFC is a little smoother (less sand), not quite as plastic and not quite as refractory. PFC is actually BFC with some ball clay and sand replaced with a smooth stoneware clay.
APG Missouri Fireclay
About Plainsman Clays
|By Tony Hansen|
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