[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.] glz2b.txt Clay and Glaze Formulation Robert Fromme ---------------------------------------------------------------- SOME NOTES ON GLAZES IN THE KILN Most of us have fired several types of kilns for a variety of different processes and we have a working knowledge of the changes which are going on in the clays and glazes as they are heated and cooled. We know that the glaze goes through a series of physical changes which are similar no matter if it is a Raku firing or a stoneware firing. Steam (1.) At first the pots begin to give off steam and we can tell that water is moving out of the walls of the clay and the glaze by evaporation. Kindling Temperature (2.) If wax was used in the glazing process to protect the foot or for wax resist decoration, that eventually reaches kindling temperature and starts to smoke or burst into flame. Other organic materials will also begin to burn out of the clay such as paper used to support hand built forms, string, leaves, nuts, and other objects used for surface decoration. Sintering (3.) By the time the kiln reaches a black-red heat we may begin to see the earliest signs that a few of the lowest melting components of the glaze start to 'sinter'. Although one seldom sees the sintering process, the glazes often show a few isolated, tiny, shiny spots. Sintering is a valuable process in the early stages of firing. In this step of the melt, heat converts powder into a cohesive mass without developing a glassy phase. What is really happening is the corners and contact surfaces of the particles soften in the heat and the particles begin to stick to each other at those points. If you have ever had to stop a firing just prior to the fusion stage and then studied the sintered but unfused glaze, you will understand that the temperature at which sintering begins is well below the level of heat which is required to melt most of the pure substances in the glazed surfaces, their 'fusion temperatures'. Let's keep in mind that the materials in the glaze mixtures are usually very finely ground. In the heating kiln, the glazed surfaces have relatively large surfaces and therefore high surface energy with relatively low melting points for most of the components. All of this helps to promote the sintering mechanism. Fusion (4.) With additional heat, some of the lower fusing materials begin to show signs that they are trying to enter the mix as more plentiful melting is underway. With the rising temperature, the lattice structures of the crystals of glaze materials begin to loosen and new stresses are set up in the materials because of dissimilar thermal expansions of those elements. As you would expect, little cracks develop in the stressed crystals and particle size is reduced in the heat. With the additional activity in the glaze layer more particle surfaces are exposed for fusion while the corners and edges of the crystal lattices are exposed for chemical activity through abrasion. As the particles of crystal continue to break up, a greater number of smaller ions and atoms are able to enter the chemical activity, scatter, and diffuse into the melt. In the elevated temperature, many of the glaze raw materials begin to disintegrate into two or more substances. Eutectics As the crystals of the raw materials continue to regress the formation of eutectics begin to develop in the liquid phase. Eutectics are mixtures of ceramic elements which have, together, a fusion temperate which is lower than that of any of their individuals components. (The usual example is sodium silicate which melts at a lower point then either silicon or sodium.) In the complicated melt of the glaze the more refractory particles are eventually surrounded and then gradually taken into the molten solution of the liquid glaze. Gasses Boiling and Bubbling (5.) As this fusion gets underway, we will see a very active or violent period where the glazes will be boiling or bubbling. We have mentioned that in the heat, many of the raw glaze materials disintegrate into two or more substances. Gasses like sulfur, oxygen, and carbon are produced as the ceramic raw materials are separated. Other gasses are liberated by the bisque body under the glazes. Some gasses are absorbed from the kiln atmosphere, by the liberation of gases assimilated on the surface of some of the raw glaze particles and particularly by the unbinding of air present in the voids between the glaze parts. The gasses pass into and through the fusing layer of glaze producing a productive stirring action which helps to make the glaze more uniform or homogeneous. The boiling of gasses in the glaze is very slow in the early stages when the viscosity of the fusing glaze is greater, but as the temperate elevates and as the glaze becomes more fluid the activity is increased and vigorous. (6.) As time and temperature continue to work on the clay and glaze, the active dislocation of gas from the mixtures gives way to smooth, liquefied, but viscous, glass. We have reached the maturation temperature for the glaze. Viscosity We need to keep in mind that the viscosity of the melt at high temperatures is an exceedingly important attribute. (a.) Viscosity in the glaze can be a consequence of the temperature and (b.) the time for which the glaze was subjected to the heat. (c.) Viscosity also depends upon the nature of the materials used in the composition of the glaze. The rate and uniformity of the fusion process depends on those glazes which begin to fuse early and encompass, permeate, and dissolve the more refractory raw glaze materials. Of course the uniformity within and the quality of thickness in a mature glaze depends upon the viscosity of that mixture at maturity. Any bubbles which are not released from the glaze may create pinholes or surfaces that look like orange-peel (dimpled) surfaces. Because of this potential problem, most clay artists prefer to 'soak' the kiln before it is turned off and allowed to cool. The additional time in the elevated temperature helps to let the last of the gasses move on out of the liquid and the surface tension can help it regain its uniform surface. In drastic cases the viscous glaze may create crawling as the gas bubbles are liberated and the surface pulls the glaze out and away from where the bubbles moved though the molten mix exposing the raw clay body beneath. We can not leave our examination of viscosity without mentioning that the viscosity of a glaze has a repressing influence which limits or prevents the formation of visible crystals in the cooling glaze. Without this constraint, nearly all glazes would devitrify or recrystallize during the cooling. Sequentially, transparent glazes would be extremely difficult to create. Matt Glazes (7.) At this point, even those glazes that will have a (true) matt surface will appear glossy and liquid at the maturation temperature, however, as the glaze begins to cool, some glazes will begin to devitrify. A thin layer of crystals, often too tiny to see, are beginning to grow just below the surface of the matt glaze. The coating of miniature crystals are often invisible but if they have developed, the light will not be reflected from the surface in a uniform or consistent manner and the object will appear to be dull or matt. ( We should note that there are some surfaces in ceramics which are called matt but they result from an excess of non-fusible materials which will not dissolve in the molten glaze. There are other surfaces which are simply dry immature or under-fired glazes which are often called matt glazes.) True matt glazes, as well as other crystalline glazes such as those opaque glazes which result from crystal growth, must be cooled slow enough over their molten phase so that there is enough time for the crystallization to take place. When the cooling cycle is too rapid for these kinds of glazes, opaque glazes may remain transparent and clear while matt glazes remain glossy. -------------------------------------------------------------------- (c) 1994 Robert Fromme For Educational use, only.