[Editor's note: print this document with a monospaced font (Monaco or Courier on a Macintosh) to have the graphs and tables appear in their proper format.] THREE WAYS OF LOOKING AT A GLAZE Notes on Glaze Formulation (glz1a.txt) Robert Fromme We have three ways of looking at a glaze, (1.) the aesthetic and functional contribution it makes to the design of the ceramic form, (2.) its batch weight recipe of raw materials which will provide the oxides for the chemical action which results in the quality of its fired composition and (3.) the oxide composition of the fired glaze. The oxide composition is the primary tool to analyze problems and positive results from a firing. The batch weight recipe of raw materials is of value while we are involved with the cost, sources, weighing, physical behavior and use of the glaze slurry materials. The aesthetic and functional value of the glaze is the bottom line for the clay craftsman. The success of the product depends upon its design the quality of surface is a critical aspect of its formal composition. Most of us are sensitive observers and can quickly sense if an object is poorly designed and we usually share an appreciation for those objects which are extremely successful in their sensitivity and unity of execution. Within the world of creative expression and formal organization of ceramic objects, concerns for the elements and principles of design usually boil down to the nature of the clay bodies which are chosen, the specific three- dimensional form of the object, and the characteristics of the finished clay or glaze surface on the object. We do not need to understand ceramic processes or the technical aspects of making ceramics in order to gain a basic appreciation of these objects. However, when we want to move from the experience of one who simply appreciated the object to a person who is expressively involved in the creation of these objects, we must look beyond matters of fired clay surfaces, glaze color and other surface characteristics. Soon after we begin to discover some of the techniques for creating ceramics, we are confronted with batch weight recipes for glazes. In the clay studio classroom, we begin to think of glazes in terms of the batch weight recipes, as well as, their aesthetic and functional properties. The glaze batch recipe is created from powdered ceramic minerals measured to a specific relationship in order to get the mixture to melt into a glassy coating when the desired temperature in the kiln has been achieved. In the melt, the materials are reduced to their basic chemistry in oxide form. As the objects in the kiln cool, the melted glaze materials begin to form a glass surface. The glaze coating results from a structure of oxide molecules provided by the original glaze materials. When the glaze materials have melted they can never again be returned the recipe materials which were used in the raw batch recipe. Since most batch material sources contribute two or more oxides to the melt, an understanding of the basic oxide chemistry of the melt is the only reasonable source of useful information for the control and evaluation of glazes. We will be unable to move beyond mystery and confusion if we are unable to look deeper then the raw materials of the batch weight recipes of glazes. In other words, the fired (final) properties of the glaze need to be managed and evaluated and we can begin to accomplish this control when we learn to concentrate on the oxide makeup of the fired glaze. All kinds of fired properties, such as surface, color response, expansion, and melting temperature, can be predicted when you develop an understanding of the unique characteristics of the oxides used in glaze compositions. These oxide formulas routinely draw from fifteen or fewer oxides. The most common oxides found in ceramic base glazes are: SiO2 - Silicon Dioxide, Silica Al2O3 - Aluminum Oxide, Alumina B2O3 - Boric Oxide BaO - Barium Oxide, Baria CaO - Calcia, Calcium Oxide K2O - Potassium Oxide Li2O - Lithium Oxide, Lithia MgO - Magnesium Oxide, Magnesia Na2O - Sodium Oxide, Soda PbO - Lead Oxide SnO2 - Stannous Oxide, Tin SrO - Strontium Oxide, Strontia ZnO2 - Zinc Oxide TiO2 - Titanium Dioxide, Titania ZrO2 - Zirconium Oxide Each of these oxides has its own "personality" in the melt and provides unique contributions to the final fired glaze properties. The batch weight recipe can come from hundreds of materials and each of these raw materials can provide an assortment of oxides for the chemistry in the melt. Thus, the picture takes on an overwhelming complexity if we try to focus on the glaze in its batch weight recipe of raw materials. If we want to understand the glaze we must turn our attention to the oxides the materials contribute to the fired melt. THE SOURCE OF COLOR IN GLAZES Color in glazes results from the chemical interaction between the base glaze and additional metal oxides which are usually added or tested after a functional base glaze has proven to melt at the desired temperature with the desired expansion, surface qualities, etc. Some of the most common coloring metals are: Ag - Silver Au - Gold Cd - Cadmium Co - Cobalt Cr - Chromium CuO - Copper Fe - Iron Mn - Manganese Mo - Molybdenum Ni - Nickel Pr - Praseodymium Sb - Antimony Se - Selenium U - Uranium V - Vanadium REVIEW 1. In addition to considering the aesthetic and functional concerns for a glaze, we can look at a glaze in its batch weight recipe and its oxide form. 2. You mix and apply the glaze recipe (made from powdered ceramic minerals) to the clay object before the firing. 3. The kiln temperature decomposes the raw materials to liberate their basic oxides. In the molten glaze almost all of the oxides are emancipated and fluid, but the melt solidifies to a new oxide glass structure as the kiln cools. 4. The glaze has changed in the kiln and emerges as an object composed of oxide molecules. 5. Each of the glaze oxides has its own "personality" in the melt and provides unique contributions to the final fired glaze properties. 6. Color in glazes results from the inclusion additional metal oxides. ______________________________________________________________ (c)1994 Robert Alexander Fromme NOTE: for educational purposes only. You may contact Robert Fromme at <rfromme@tenet.edu>