Overview of Glazing

HPG glazes are high fired, cone 10, “stoneware”, durable glazes v No exotic toxic chemicals are used, no lead, no bariumv Kiln uses natural gas and is a downdraft reduction kilnv Glaze composition and terminology explained

Glazes are glass-making chemicals mixed for special effects which work well in particular firing conditions. HPG uses cone 10 clay exclusively, so the glazes must match this clay. The HPG kiln is a gas kiln which is usually fired in a reduction atmosphere. This means that the inside of the kiln during a certain stage of firing is starved of oxygen. The fire needs some oxygen to burn, so oxygen remaining in the clay and glazes is pulled out, leaving the various metals and other compounds free to interact producing characteristic colors and effects.

“Cone 10” simply means the triangular shape of a clay stick used to indicate firing temperature in the kiln. HPG uses cones 7 8 9 10 and 11 during firing to monitor conditions. Cone 08 (1600 F) is added to indicate when to start of reduction conditions. Cones are made of special clays which melt at certain temperatures. As the firing proceeds, the cones “melt” down at their particular melting temperature. Cone 10 melts at 2381° F, 1305° C. At that point most HPG glazes have reached maturation which is the proper melting point to fit the pot and develop the desired finish for that particular glaze. Glazes tend to expand and move on the pot during firing.

Glazes are made of finely ground rocks of particular chemical composition. Three basic components are required: Flux, Alumina, and Silica. A flux is a potassium or sodium feldspar acting as a melting agent which helps to melt the glaze when heated and enable a good fit to the clay surface. Examples are Custer feldspar and G-200, but also bone ash or wood ash act as fluxes. These are potassium (K) fluxes best for our high fire glazes. There are other minerals which work in certain glazes such as borates, lithium, calcium. Aluminastabilizes the glaze to prevent over melting, or “running”. Kaolin (EPK) is often used as a source of alumina, and some clays are used because of their alumina content. Alumina adds durability and strength to the glaze surface.

Silica (flint) is the glass forming part of the glaze. Other compounds may be added for color or other effects.......iron for brown, cobalt for blue, copper for green or red, manganese for black, chrome for green, tin for a non-shiny, matte finish and color development. Whiting (limestone) is a good flux which helps keep the other fluxes in control and provides a good glaze finish.

Glaze formulae vary widely with weird sounding components named after the type of rock or geology or places the compound comes from. A particular glaze effect requires that the percentage of each ingredient in the recipe be weighed

exactly. Proportions are designed to interact chemically for the expected effect, and the components tend to settle in their vats. For this reason thorough mixing of glazes is essential before glazing a pot. Otherwise junk happens.

A glaze with a high alumina content will flux, or melt, at a higher temperature and sometimes give a matte finish. Low concentrations of alumina will cause early melting below HPG’s cone 10 conditions, and might drip off the pot. The ratio of alumina and silica for a particular glaze is optimized for finish and color, but this means that some
glazes will be more or less stable during the firing, more or less chance of running or over-melting. A best effort is made to place pots in the kiln according to expected temperature.

There is no kiln in the world which fires exactly the same throughout the kiln, so many unexpected results occur due to hot or cold spots which can show up in different parts of the kiln. HPG has a “community kiln”. The firing atmosphere is affected by the variety of glazes used on HPG pots due to volatile glaze salts and metals, the weather, wet or windy or dry type of loading, and the time of reduction firing. Art is not an exact science, and the unexpected results can be ordinary or extraordinary.