continved from page J AP Green Fireclay has been used for many years by potlers to make stoneware clay bodies. It is a moderately refractory, light colored, slightly iron speckied, 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 lithe more plastic than an average clay body, thus it can exist in high percentages in the recipe. lis 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 thitt is much more than atypical stoneware clay body. One misconception about AP Green fireclay relates to its particle size, While most people think of it as acoarse fireclay, Techno Tip it is actually a very fine particled clay material, It is perceived as being coarse because of the way it is powderized, The particles are actually agglomerates and slake readily in water to pass more than 90% through a 325 mesh screen. These particles however do not break down read- ily when being mixed inte pugeed bodies. This material also contains iron-bearing soluble salts that are left on the surface during drying. These impart a darker col- orution to the clay surface, The fired color of AP Green Fireclay bodlies 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 AP Green Fireclay 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 important reasons for its popularity. So can you just use any fireclay in its place? If someone is offering an alternative then ask him or her how its properties compare to AP Green Fireclay. Is it more or less plastic’? Is itcoarser or finer grained? Does ithawe more or bess solubles? It is more or less refractory? Does it contnbate more or less iron speckle’? Is its mineralogy simi- lar? How does its fired colour compare? | am guessing most suppliers won't be able to answer any of these questions: they will tell you totry itto seeifit works. Hmummm. That opens another can of worms, How does one test a clay body to note subtle or even obvious changes in physical and fired properties? Techno Tip is generously supparted by Plains- man Clays Lanied in Medicine Har, Albena avul iy local affiliates, Greenbarn Potters Sip- ly in Surrey and Veacerver friand Pottery Werehoure inc in Narurime. erving BC potters for over 30 years 95486 192 Street Surrey, BC. V4N 3A9 Phone: 604.888.3411 Fax: 604.888.4247 GREENBARN POTTERS SUPPLY Monday - Friday 9-5 Saturday 9-1 Closed long weekends email: greenbarn @telus.net Be Potters Guild of British Columbia Newsletter April 2002