[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.] glz2c.txt Clay and Glaze Formulation Robert Fromme ----------------------------------------------------------------- NOTES ON THE THREE PARTS OF A GLAZE At this point in our course of study, we need to turn back to the first lesson and the discussion of the three ways of looking at a glaze. You will remember that most batch material sources contribute two or more oxides to the melt. We decided that 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 powerless to look deeper than 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 - Tin Oxide, Stannous Oxide SrO - Strontium Oxide, Strontia ZnO - 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. If we want to understand the glaze we must turn our attention to the oxides in the fired melt. I would like you to look over this list of oxides and notice that they come in several forms. If we make (R) stand for the chemical symbol and O stand for Oxygen, some of these will match the form of RO or R2O. Some will match the form of R2O3. Others will match the form RO2. When we group the oxides in our list according to this relationship, we get SiO2 (Silicon Dioxide, Silica), SnO2 (Tin Oxide), TiO2 (Titanium Dioxide) and ZrO2 (Zirconium Oxide) in the RO2 group. We get Al2O3 (Aluminum Oxide, Alumina) and B2O3 (Boric Oxide) in the R2O3 group. The remainder of our list will fit in the RO, R2O group. In reality, glaze-forming oxides can be divided into these three major categories. The majority of our list, those that fall into the RO, R2O group are called the base or fluxing oxides. Among other things, all of these oxides do help to lower the melting point of silica, alumina and other refractories in the glaze. The oxides that fit in the R2O3 group are called the neutrals (refractory) oxides which help to control the viscosity (flow) of the glaze. On occasion, you will also hear these R2O3 oxides referred to as the viscous agents and the crystal retardants because crystals do not form well with the viscosity which these ingredients contribute. Silicon, or our RO2 oxide is called the acid oxide or the glass former. (For those of you who need to know what the R in the formulae stands for, I have been told that it represents the root element (radical). 'O', of course, represents oxygen. You will also remember that the number behind (usually below when possible) the element tells the number of atoms of the element that are present in the molecule. If there is no number, that means only one atom is present. When the number is on line and ahead of the oxide symbol,it means that there are that many of the particular oxide relationships in the formula.) Thus, the base (flux) symbol RO indicates that one atom of an element is combined with one atom of oxygen to form one RO molecule. R2O means that two atoms of the root ingredient have linked with a single atom of oxygen to pattern one molecule of R2O. Still, the base group contains both RO and R20 oxides, it is commonly referred to as the RO group. The neutral (refractory or viscous) classification R2O3 includes all oxides that have two root atoms in association with three oxygen atoms. The acid (glass-former) group. RO2 contains those oxides having one root atom united with two oxygen atoms. RO/R2O = The Fluxing or Base Oxides The fluxes or base oxides stimulate a glaze to liquefy, melt or flow. Each one of this oxide group is somewhat active as a flux ingredient. Some of these bases will melt early and burn out as the temperature increases, others will melt early and last throughout the firing. Some will not get involved until the firing is well underway. Yet, each one of these fluxes or bases will bring its own characteristics or qualities of texture, color response, viscosity, and other unique constitution to the glaze. If we take a minute to think about the previous paragraph, we will come to the conclusion that, while in theory, it is needless to include more than one base oxide in a glaze, doing so generally gives a more ample firing range and a elevated potential for diversity in the texture, color qualities and glaze effects. R2O3 = Neutral, Viscous, Crystal Retardant or Refractory Oxides The neutral oxides give the glaze strength, durability or body. As you have discovered, the principal neutral oxide is alumina (Al2O3). In the melt, alumina enhances the glaze viscosity. In turn, this inhibits the tendency of the glaze to flow off the surface of the clay, and it slows devitrification or the retards the formation of crystals in the cooling mixture. Although boron (B2O3) is also considered neutral, it cannot replace the full amount of alumina needed except at very low temperatures. In some ways, boron is like the class clown, it refuses to fit and act like it should according to its formula. As an example, it can form a kind of soft glaze without the presences of silicon at very low temperatures. We usually associate the RO2 oxide or silica as the glass former in most glazes. Boron is also a very active flux throughout the full temperature range of glazes so it acts like a RO or R2O oxide, as well. We will talk more about boron later. RO2 = The Acid or Glass-Forming Oxides The RO2 or Acid oxides are the genuine glass formers for most glazes. Of course, the principal acid or glass-forming (RO2) oxide in a glaze is (SiO2) silica, which usually can not be replaced with any other oxide. Zirconium (ZrO2), tin (SnO2). and titanium (TiO2) also are acids, however, they behave as opacifiers in a glaze and are usually added later, as are colorants. Opacifiers are refractory ingredients which are added to make the glaze opaque. White glazes are created through the use of these elements. We will discuss their use in a later lecture. Here are ceramic oxides as they function in a glaze: -------------------------------------------------------------- RO, R2O Base or Fluxing Oxides 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 SrO - Strontium Oxide, Strontia ZnO - Zinc Oxide -------------------------------------------------------------- R2O3 Neutral, Viscous or Refractory oxides Al2O3 - Alumina B2O3 - Boron (acts at times like a base, neutral, and/or acid) -------------------------------------------------------------- RO2 - Acid or Glass former SiO2 - Silicon -------------------------------------------------------------- RO2 Acid Oxides that serve as Opacifiers SnO - Tin Oxide, Stannous Oxide TiO2 - Titanium Dioxide, Titania ZrO2 - Zirconium Oxide ______________________________________________________________ -------------------------------------------------------------- (c) 1994 Robert Fromme For educational use, only.