Making a glaze melt so completely that it is a liquid at the peak firing temperature, but is still not runny, can be tricky. Glaze thickness and chemistry are critical variables, but not the only ones.
Defining the Terms
Frit: Ceramic materials that have been melted, cooled, and then ground to fine powders for use as ingredients in glazes and sometimes in clay bodies.
Refractory: Resistant to being modified by heat.
Viscosity: The measure of the resistance of a fluid to flow at a given rate.
Time and Temperature
Both the duration and temperature of a glaze firing work to increase or inhibit glaze flow. These two firing variables affect the interaction of the glaze and clay body. The interaction is also influenced by glaze thickness and glaze and clay body compositions.
With so many variables, where does one begin in order to control glaze flow? A simplifying assumption is to think about an ideal glaze, one formed from a fully melted glass. In such a glaze, by the peak firing temperature all frits, minerals, and crystals in the glaze have dissolved into a liquid glass. The fluidity of an ideal glaze is a function of chemistry, the time that the glaze is fully molten, and the temperature to which the glaze is fired.
The amounts and proportions of glass formers, modifiers, and fluxes are chemical influences on the flow of an ideal glaze. The principle glass formers in glazes are, of course, the oxides of silicon and boron. Alumina, an oxide of aluminum, modifies the glass by making it more viscous. Fluxes, the oxides of mainly sodium, potassium and calcium, but also in lesser amounts lithium, magnesium, strontium, and barium control glaze melting temperature.
As the ratio of alumina (Al2O3) to silica (SiO2) in a glaze recipe exceeds 1:10, the lessening proportion of alumina allows the glaze to flow readily. Glazes using zinc to form crystals have Al2O3:SiO2 ratios as high or higher than 1:25 and are excessively runny. Glazes with an Al2O3:SiO2 ratio between 1:5 and 1:10 tend not to be runny.
The higher the proportion of flux elements to glass formers, the more fluid a glaze will be. For mid-range firings, around cone 5, the ratio of fluxes to glass formers is usually between 1:3.5 and 1:5. At low-fire temperatures, say cone 04, the ratio is lower than at mid-range temperatures. At cone 10, the ratio is higher. As these ratios indicate, the appropriate amount of fluxes in proportion to glass formers is a function of firing temperature.
Even though boron is a glass-forming element, rather than a flux, adding boron also makes glazes more fluid. This is because both the frit and naturally-occurring (mineral) sources of boron melt at relatively low temperatures in the kiln. The boron-rich liquid formed speeds up the melting of other materials in a glaze. The sooner a glaze melts, the more time it has to flow during the firing.
1 Dehydrate a fluid glaze on a dry plaster slab. 2 Scrape the glaze up with a plastic rib, then roll it into small balls. 3 Each ball should weigh between 9–10 grams. 4 Label each glaze ball with its name and a number for record keeping. 5 Place each ball on a separate tile. Label the tile with same name and numbers stamped into the glaze ball. The diameter of the molten glaze circle on a flat ceramic tile after a glaze firing is a direct indication of how fluid a glaze may become in a firing of your ceramic work. 6 A vertical flow tester is a more accurate way to test new glaze recipes before putting them on ware and into your kiln. 7 Two glazes are compared by placing dried glaze balls into the reservoirs at the top and firing to the desired temperature (with a tile below to catch any glaze that may run off the end). During the firing, the glazes flow down the runway according to melt fluidity. These two particular glaze test balls were fired to cone 4. The fritted glaze on the left is completely over-melted, while the one on the right did not move at all. Photos courtesy of Tony Hansen and Plainsman Clays Limited. To learn more about the glaze flow tester pictured above, visit https://digitalfire.com.
Fritted glazes melt readily. Glazes made from minerals, such as quartz, clay, and feldspar, require more energy to become molten. These raw glazes either need a slower firing schedule or a hotter firing temperature to become fully melted.
A glaze bonds to a clay body when a small amount of the body’s surface is dissolved by the glaze. This typically adds silica, alumina, and a small amount of flux to the glaze. The clay body is necessarily more refractory because it contains mainly silica and alumina, so the additions make the glaze more viscous (less fluid).
The thicker the glaze coat, however, the less the dissolved body affects the glaze chemistry. As a result, a thicker application of glaze causes a smaller clay body effect on the flow of the glaze.
For any given peak firing temperature, adding more fluxes to a glaze recipe will increase glaze flow. Adding boron will also increase glaze flow. Additions of alumina and silica will have the opposite effect. Clay, added in small increments, is a convenient way to add alumina and silica to a glaze.
Sodium and potassium are fluxes that will increase glaze fluidity the most. However, they also increase glaze expansion, which can create crazing issues. Lithium and magnesium are two fluxes that reduce glaze expansion. Frits are the most reliable and controllable sources of boron.
There are very basic ways to test how fluid a glaze is. Perhaps the simplest is to pour some of the glaze on plaster (1) to absorb enough of the water that the remaining damp glaze can be formed into a small ball (2). Weigh the ball and adjust it to the desired test weight (3, 4), then place it at the center of a bisque-fired tile of bare clay and fire. The diameter of the molten glaze circle on the tile after the firing is a direct indication of how fluid the glaze became in the firing (5). This is a quick and simple way to compare glazes and test the effect of new materials and recipes on glaze flow. Knowing how a material melts is important to knowing how to use it in your recipes, your glazing process, and your firings.
A more elaborate and more precise way of measuring glaze flow requires using an industrial technique. A glaze flow testing device is cast from a mold (6). Ideally the casting is from the same composition as the clay body used to make the ware you plan to glaze. The slope of the tester is less than a 45° angle. At the top of the tester are two cavities, which can hold samples of the glazes to be compared. To compare the flow properties of the samples it is important that the samples are of the same weight. Tip: Before firing the test, place a glaze catch at the bottom of the tester to prevent damage to your kiln shelves. Attention to testing details and taking good notes permits comparing samples from different firings (7).
the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.