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Ceramic materials melt under specific conditions. As the temperature in the kiln increases, greater melting action occurs in both the clay and glaze. Additionally, the use of a longer heating time to reach clay body and glaze maturity can also increase the melting action.
Small kilns with less thermal mass can heat and cool fast, resulting in immature clay bodies and glazes. Conversely, larger kilns expose the ceramic materials to longer durations of heat and promote greater melting.
The particle sizes of the raw materials found in clays and glazes can affect melting. Smaller particle sizes have greater surface areas and are exposed to heat more effectively than larger particles.
The kiln atmosphere can also affect melting. Reduction atmospheres (more fuel than air present during combustion) cause carbon monoxide, which is oxygen hungry. Removing an oxygen molecule from many ceramic oxides found in clay bodies and glazes will flux or melt them to a greater degree than when they’re fired in an oxidation atmosphere.
Eutectics can play a part when two or more ceramic materials are combined, causing them to melt at the lowest possible temperature. This condition is most often observed when two glazes overlap to create a third glaze combination that can run, blister, pinhole, or show other defects of a lower melting point.
Glaze thickness can also influence the melting action of the glaze. Thinner glaze applications can melt to a greater degree than thicker depths because the heatwork created in the kiln firing needs to go through less ceramic material to achieve a complete melt.
Depending on the oxide, metallic coloring oxides can also increase or decrease the melting action of clay body or glaze formulas. Iron oxide can be considered a strong flux, which can bring other materials into an aggressive melt. Being very refractory, chrome oxide can inhibit the melting action of clays or glazes. To varying degrees, other metallic oxides can be classified as either refractory or fluxing in their capacity to influence a clay body or glaze glass formation.
The clay body itself can also influence the melting character of a glaze. Some clay bodies can leach out the fluxing components of a glaze during the kiln firing, while other clay bodies can promote greater fluxing action in the glaze.
Methods of Heat Transfer
Heat transfer within the kiln can also react with ceramic materials in the firing process. Heat is transferred by several methods, which, once understood, help insure a better knowledge regarding how to fire kilns.
Convection: heat is transferred through the air. We have all experienced this by opening a hot oven and feeling the wave of heat on our bodies.
Radiation: heat is transferred through the kiln by energy waves. When the kiln is heated, it creates thermal mass in the kiln bricks, shelves, posts and pots. When the heat source is turned off, radiant heat still affects all objects in the kiln. Frequently, a loosely stacked kiln will result in immature glazes due to the lack of thermal mass in the kiln.
Conduction: heat is transferred through solids. When the kiln is fired, heat is moved to pots resting on the kiln shelves. Some materials, such as hard bricks and posts, transfer heat more efficiently, while others, such as soft bricks, transfer heat to a much lesser degree.
It is only through understanding the heating process and the raw materials contained in the kiln that potters can acquire the ability to achieve consistently good glaze firings.