[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.] ------------------------------------------------------------------- NOTES ON SIMPLE GLAZE CHEMISTRY by Robert Fromme The chemical compositions which make up clays, glazes and glass are the result of atomic and molecular combinations. In fact, all materials in our universe are made up of atomic elements, the building blocks which can be built or coupled together to form each of the various materials around us. There are at least 109 different elements but only 40 are commonly encountered in ceramics. These are chemically expressed in the form of symbols (Al for alumina, Si for silicon, O for oxygen, Pb for lead, H for hydrogen, etc.). The clay artist will discover that these symbols keep showing up in the written material about clays and glazes. It is a good idea to try to put the most common symbols to memory. Atoms The atom consists of parts (electrons) in orbit about a central core (nucleus) forming from one to seven "layers." The structure of the nucleus and the electron layers surrounding it combine to determine the atom's atomic number and atomic weight. The nucleus is made up of protons and neutrons. Within the atom, the electrons are very light compared to the protons and neutrons. Atomic Weight The nucleus makes up most of the atomic weight, however, the total number of protons, neutrons, and electrons in orbit determine what the complete atomic weight of the atom will be. Lets look at a few examples. The atom which has an atomic weight of 1 is hydrogen. Here we find one proton and no neutrons. Now, compare the simple atom of hydrogen with something which is a little more complicated. Oxygen, with eight protons and eight neutrons in orbit, has an atomic weight of 16. The number of electron layers in a particular atom helps to determine the precise relationship between it and other atoms in a chemical compound. A compound is a chemical combination of two or more atoms in some definite proportion by weight, forming a molecule. The Molecule The molecule is the smallest particle of matter that can exist with all of the same chemical properties of a particular compound. It is made up of two or more atoms united by their unique electron relationships. When we think in terms of clays, glazes and glass, we are primarily concerned with oxide molecules, which are derived from the combination of various elements with oxygen. Molecular Weight As we mentioned, when atoms of different elements combine together, a new material, a 'compound' is formed. Again, the smallest possible portion of a compound is a molecule, this molecule being made up of a combination of one or more atoms of one or more elements. The molecular weight of a compound is the total of its component atomic weights. Each atom combines with oxygen in a manner agreeable with its electron structure. For example, two atoms of hydrogen plus one atom of oxygen can combine to form the compound water. This can be expressed chemically as H + H + O = H2O (water). The grouping together of the chemical element symbols to denote the compound is referred to as the formula. All compounds can be chemically expressed in this way. Here, an example can be the molecular formula for alumina (aluminum oxide), Al2O3. (We should point out that the numbers which follow the symbols are traditionally dropped below the normal line of text and we will have to overlook the limitations of the ascii text editors and accept the formulae with the number on the same line as the symbols). The symbol of alumina, Al2O3, means that two atoms of aluminum are combined with three oxygen atoms. The atomic weight of aluminum is 27.0 and the atomic weight of oxygen is 16.0. The molecular weight of the ceramic material, alumina, is 102 : (2 X 27.0 + 3 X 16.0 = 102). Water (H20) has a molecular weight of 1 +1 + 16 = 18, if we use the system on our earlier example. Hydrogen has the atomic weight of one and Oxygen has the atomic weight of 16. Arrangement in glazes and glass depends upon the structure of component atoms, in combination with oxygen. The combinations form molecules of a specific oxides. The substance of our ceramic materials are formed when the molecules are alone or in combination with other molecules. When the materials are fired in the kiln, the heat and atmosphere (oxidation vs. reduction) encourage the combination of particular materials in specific relationships to form glazes or glasses. Solubility and Solutions With just about any general consideration of the properties of clays, glazes and glasses it is important to review the basic principles of solubility and solutions. Solutions One may remember that a 'solution' can be defined as a molecular combination of two or more substances. So, if we go to the kitchen and stir a glass of water and sugar until the sugar is totally dissolved in water, the separate particles in the solution remain molecules of sugar and water. This solution will remain indefinitely unless the water is evaporated away. Then the sugar will reform crystals from its molecules. Suspensions If we take our discussion to the beach, however, and look at the mixture of sand and water in the tide, we see a 'suspension' and not a 'solution'. With the suspension of the water and sand at the edge of the tide the individual grains of the material mixed with the liquid are not reduced in size down to the constituent molecules. They remain floating there in the liquid for a time, without being changed, and then they gradually settle out because their weight is more than that of the liquid. Our instance suggests that a mixture of glaze and water in the studio is, for the most part, a suspension. The bulk of the glaze particles are insoluble in water. Most of them remain quite static and do not break down into their constituent molecules or dissolve while in the glaze bucket. (Think of the sand and water in the tide.) Solubility and Temperature In a solution the substance present in the larger amount, the means for dissolving, is usually alluded to as the solvent and the other medium which is dissolved in the solvent is designated as the solute. We soon determine that regardless of the substance, at any temperature, there is a explicit ratio of solubility in the solvent. With few exceptions, the more elevated the temperature of the liquid or solvent, the more solid it will dissolve. With gas, the contrary is true. When gas is dissolved in a liquid, the more heated the liquid, the less the quantity of gas it will be able to keep dissolved. Saturation When a solvent at a fixed temperature will not dissolve any more solute, the solution is expressed as being 'saturated'. In this condition no further solute can be dissolved. Additional materials will remain unaltered unless the temperature of the solvent is increased. However, if a saturated solution of a solid in a liquid is cooled to a lower temperature the solvent becomes incapable of holding so much solute in solution. Then we get a result showing that some of the solute will separate out of the solution in the form of the crystals of the original solute. Crystallization Remembering our first lesson about the origins of ceramics materials, we will realize that the process of crystallization resulting from a saturated solution is responsible for the formation of many of the earth's minerals. As the earth cooled, these materials have crystallized out of molten rock-developing materials. As we think of over saturation and cooling temperatures, we realize that this is also the phenomenon causing the devitrification (crystallization) of transparent pottery glazes and the formation of crystals necessary in the formation of matt and some opaque glazes and glasses. We should note that the formation of crystals will only take place as long as the solution is sufficiently fluid to allow the molecules to orient themselves to form the crystal lattice. In the kiln, glazes and glasses are normally comparatively viscous when molten. Because of the viscosity of the melt, the formation of crystals of the various minerals dissolved in the glaze takes place with difficulty. Glazes and Glass Glazes and glass are inorganic materials that develop in a molten state and then solidify when cooled. One may think of water in its liquid form at room temperature being changed to a solid by cooling the temperature to below freezing. With both the cooled ceramic glaze (or glass) and with ice we have a solid which has been created by cooling materials which were once in a fluid state. Ice is, however, crystalline in nature, but glass or glaze has a random, non-crystalline arrangement of molecules. Glazes are therefore commonly referred to as 'solid solutions' since the random arrangement of the atoms and molecules is similar at room temperature to the random arrangement when the glaze was a molten liquid. Glazes and glasses can be defined as 'supercooled liquids of great viscosity at our ordinary temperatures'. They are not definite chemical compounds but are mixtures of complex silicates and borates. Because of high viscosity (resistance to flow) from the random (amorphous or helter-skelter) arrangement of its atoms and because it is not overly saturated, glass is not free to move into a crystalline structure when it solidifies. Being non-crystalline, glass remains in its most placid (unstructured) condition. Because of this amorphous structure, glass has varied points of weakness and it softens gradually rather than at a specific melting point, as do crystalline structures. Glass will flow if heated enough. In most cases the viscosity of glass is determined by the amount of alumina present. The principal factor distinguishing glaze from glass is the amount of alumina in a glaze. Since glaze must be more viscous than glass in order to stay on the clay surface and not flow with gravity onto the kiln shelf, it requires much larger amounts of alumina to provide the needed viscosity. While there are exceptions, most glazes are a mixture of finely ground silica, alumina, and various oxides for melting or fluxing the more refractory ingredients. The mixture, when subjected to intense heat will fuse to form a glass-like surface on any ceramic object. As your course of study continues, you will realize that the ceramist must know every oxide's influence on a glaze in order to choose the best one for a desired effect. --------------------------------------------------------------------- (c) 1994 Robert Fromme / For educational use, only. You may contact Robert Fromme at <rfromme@tenet.edu>