In the last article we talked about glazes that ‘powder’ onto your hands and create dust during handling. We identified the problem as lack of binding and talked about what makes clay particles glue to- gether. Clay particles have a surface chem- istry that has an ‘affinity’ for water that produces plasticity in lower water mixes and suspension in high water systems. Conceptually, other mineral powders are just dead microscopic rocks, . However | did not discuss the mechanism of the bonding that takes place during drying. Clay particles are flat in shape and exceedingly fine. As interparticle water is removed during drying they pack together in a dense fashion and the inevitable result is shrinkage. Large particle clays (like kaolins) shrink 5% or less from plastic to dry whereas fine partichked clays might shrink 25% or more, However mere parti- cle proximity does not in itself create a bond. The chemistry on the surface results in the migration of some chernical species across the boundary. While this creates a very weak bond, the fact that there are billions of particles bonded together in such a fashion creates what we perceive to be a fairly hard product. However clay bodies and glazes also con- tain all kinds of other particles in the mix that don't bond, they reduce the number of clay-to-clay bonds (which is bad) bur also reduce the drying shrinkage (which is good). Therefore a dried matrix, whether clay body or glaze layer, is a bunch of rock particles held together in a house-of-cards structure of weakly bonded clay particles. Now, the question is, What bonds a dry glaze layer to a piece of bisque ware? Well there is no obvious dry adhesion mechanism or boundary chemical reac- tion that glues glaze particles to the bisque wall. The mechanism of the bond relates to the sticky nature of the wet glaze and the microscopically rough surface of the bisque ware. During hardening the glaze layer loses its adhesion and subsequently hangs on to minute surface cracks and pores. As you can imagine, this bond is weak at best. Since all glazes shrink during drying, it is not clear how the weak bond with the bea Techno Tip Drying Cracks in Glazes bisque is able to withstand the pulling forces associated with that shrinkage. The answer is that they don’t. Micro-cracks do develop to relieve the stress and when there is too much shrinkage they become visible cracks. With even more stress the glaze cracks to form islands with curled up edges (like a dried up lake bowtoem), Try mixing pure kaolin or ball clay and paint- ing this onto a piece of bisque pottery and you will see the effect clearly. AS you Can sec, we want a glaze to have enough clay so that it forms a hard dry layer but not so much clay that it shrinks excessively and cracks off the bisque, There are # number of strategies you can employ if your glaze is cracking off. «If your glaze uses a relatively plastic kaolin (like #6 tile), try switching to a bess plastic one like EPK or Pioneer. This will not affect glaze chemistry much. A similar switch ifthe recipe employs ball clay is not as likely to work since pretty well all common ball clays are very plastic. * Ifthe glaze contains bentonite, remove it. Some people put bentonite in every glaze without realizing its purpose. * Add CMC gum. Like bentonite, it needs to be added during dry mixing. Gum is very sticky and it hardens, using it is a way of ‘gluing’ a glaze on the ware. Gum burns away so it has no effect on glaze chemis- try. One problem: gummed glazes dry slower and drip-drip-drip after glaze pullout. Experiment with the amount, try 0.4% to start. * Use kaolin instead of ball clay. Todo this you may need to resort to the dreaded ceramic chemistry, since the chemustry of kaolin and ball clay differ. You need to juggle the recipe to source less alumina from feldspars (since kaolin contains much more than ball clay). * Check the specific gravity of your glaze {its weight perce). If this is too low (below 1.4) then there is too much water in the slurry, Perhaps your water contains clec- trolytes that are flocculating the mix; try using distilled water instead or add a small amount of deflocculant (like Darvan or Sodium Silicate) so that less water is nec- essary. Don't go overboard onthis.or you'll end up with casting slip instead of glaze! Potters Guild of British Columbia Newsletter There you have it. The moral of the story is still the same, you need to understand the purpose of each material in the glaze so you can juggle the recipe to optimize it to your circumstances. Tony Hansen The first part of this article, riled Powdering Glazes, appeared in the Novermber‘December newsletter, page 4, Plainaman Clays [ad in Medicine Hat, Alberta, aid its affiliates Greenbarn Potters Supply in Sarrey and Varncou- ver Jsland Porrery Warehouse inc on Vancowver [sland cenerously support this colurnan. Table of Contents Sally Michener Techno Tip by T Hansen Made of Clay President's Report Project Empty Bow! Review of Jim Stamper Ex Gallery of BC Ceramics Call for Entry 5 Exhibitions 5 Canadian Clay Symposium Exhibitions 6 Lectures Workshops NW Ceramic Foundation Maureen Wright Funds Mug Fundraiser Dig in to Your Archives ECLAD Visiting Artists Lectures Clay Week Getaway Peter King in Washington State MISSA Malcolm Davis Wkshop Les Manning Workshop Millennium Plate Project Fraser Valley Potters Guild Ex Clay 2002 ia Classified 12 How to Contact the Guild 12 am Bie wew whe Ow so co oo oo oo oe —_ — March Newslerter Deadline Wednesday February 13 Send to Guild office, see page 12, or email editor: February 2002