PORCELAIN CLAY BODIES FOR POTTERS:RECIPES, TESTING AND MIXING by David Beumée I remember my introduction to while day. I was standing by my favorite Randall wheel one day, near the end of my art school career at Montana State, when I was presented with a ball of white stoneware by a woman who said, “Here, try this!" | Uhrew a turnbler that T deco- rated with a black slip, later covering the plece with a clear glaze, When | saw the contrast on the fired piece I was in- stanily hooked, Thave been working with porcelain ever since, fring to cone 10 and 11] reduction and experimenting increasingly with clays and glazes. | am forever fascinated by the clarity of glaze color achieved over porcelain, and 1 have tested every clay beady recipe [ could find. The following porcelain clay body recipes are the best [ have found for throwing on the wheel. Notes on clay bodies and explanations of materials follow the recipes. 1. Dave Comell's Porcelain AlO-11] Grolleg 55 Custer Feldspar 20 200 Mesh Silica 13 Pyrophyilite _12 100 Add: 2% Veegum T, (Note; An excellent porcelain fer func- tional ware and ovenware, as well as for working large in porcelain, such as large Platters or tall forms. If necessary, you may lower the expansion of your glazes to match this low expansion clay body by adding silica to 5% increments.) Grofleg A blended English china clay, combining good plasticity. low Utanta content and relatively high flux content, making it excellent for throwing and giving translucency in the fired result. Grolleg ls expensive because It is Lm- ported. Custer Feldspar, an excellent choice for use in porcelains. producing the highest viscosity glass of any feldspar [ tested, thus reducing warping. Pyrophytlite (Pyraxy: Decreases thermal expansion of the clay, making ovenware less likely to crack, improves throwing characteristics and decreases warping. Clay bodies which contain 30% feldapar have reduced warping and slumping problems as compared with similar translucent porcelains. 200 mesh Silica: Flint. In my experience 200 mesh is best for all porcelains, adding teoth and thixotropy missing with use of 325 mesh silica. The wider particle size provided by 200 mesh silica alsodecreases cracking; the higher green shrinkage of the clay with use of 325 mesh allica may increase cracking. Veegum T: {also known as Macalodd). A superb plasticizer and flocculant, Veegum T contributes none of the grayness to the body given by Bentolite and Bentonite. It has a very high (alka- line) pH of 5.5, and yet acts as a floceulant, contributing to the workabil- ity of the porcelain. [| recommend using 2% Veegum, [Available from R.T.Vanderbilt Co., 30 Winfleld St., Norwalk. CT 06055.) 2. Peter Pinnell’s Porcelain AlO-11 Grolleg 55 Custer Feldspar 20 200 meah Silica 24 325 mesh Alumina Hydrate _2 100 Add: 2 % Veegum T. Alumina Hydrate 325 mesh: reduces shrinkage, Increases thixotropy, and stiffens the clay at stoneware tempera- ture, minimizing warping and slump- Ing. Alumina benefits glaze fit and Page 4 strengthens the fired porcelain consid- erably. Disadvantages are that alumina adds weight to the fired result and re- duces translucency. | use C-39 325 mesh alumina hydrate.(Available from ALCOA, 150] ALCOA Bldg, Pittsburgh, PA 15219. Tel 413-553-4001) 3. Jeff Zamek's Original J Body AlO-1 1 Grolieg 50 200 mesh Silica 30 G200 Feldspar 20 $25 mesh Alumina Oxide a Bentollte | Macaloid _Z 108 Note: | have used this porcelain suc- cessfully for years and believe [t to be an excellent all-arcund body. G-200 Feldspar, A predominantly pot- ash feldspar, melting slightly faster and producing amore translucent glass than Custer feldspar. Potash feldspars are less prone to solubility than soda-based feldspars and produce a highly viscous glass. Thus, warping is less likely, mak- ing potash feldspars the best choice for use in clay bodies. Alumina Oxide: 3% calcined alumina oxide is equal to S or 6% alumina hy- drate, further helping to stiffen the clay and decrease warping at stoneware temperatures. 4. Jeff Zamek'sJ Body Revised A10-1] Grolleg 49 200 mesh silica 29 G200 feldspar 19 325 mesh Alumina Oxide _ 3 100 Add: 2% Veegum T-. December, 191]