Firing Clay Basics
Working with ceramics requires creativity for the most part and scientific knowledge. Firing is an essential step in the ceramic process. Firing is the process of applying controlled and sustained heat to the clay in a kiln. This process subjects the clay to several physical and chemical changes. It’s important to understand the firing process to you get better results. Here are some of the basic you need to know.
There are many types of firing and each produces a different result. Keeping things simple, lets discuss bisque firing and glaze firing in an electric kiln. Bisque or biscuit firing is a first stage firing to dry the clay from physical and chemical water. This firing transforms the clay into permanent pottery. The bisque firing normally reaches temperature between 900 degrees and 1100 degrees Celsius.
A bisque firing is a very slow gradual firing, generally firing no more than 100 C per hour. If the climb or the ramp rate of firing is too fast in the early stages of bisque, the work may crack or explode. At 100 C the water in the clay turns into steam and expands massively, more reason to dry your object well before putting them in the kiln. At temperatures between 90C and 300C here the chemical changes start to happen. Any organic matter present in the clay is burnt away. If the surface of the clay vitrifies before the impurities in the clay are burnt, then bloating can happen.
As the kiln heats up gradually from 300C to 700C, the molecular water is driven off and ceramic change begins to happen. The ceramic particles fuse at above 500C and the clay body becomes soft ceramic. At this temperature also the silica crystals in the clay are converted to Crystobalite and mullite. This process is referred to as quartz inversion and expands the size of the clay by 3%. If the firing is to past during this process, you will get cracks and dunting, more reason to plan a slow bisque firing.
The final temperature of the bisque firing will depend on the degree of porosity required. The higher the temperature the lower the porosity and vise versa. Above 900C the clay is fairly hard and strong, but still porous enough to accept gaze readily. If the final glazing temperature is going to be high, then a low bisque firing is preferred as the clay will absorb the glaze and during glazing the clay body can mature. For earthenware clay bodies, its best to do a high temperature bisque to ensure the clay body is strong and then the glaze can be fired to lower then maturity and you can get the required color without over firing.
The second type of firing is a glaze firing, that is normally between 1200 degrees and 1400 degrees Celsius for stoneware clays and glazes. During the glaze firing, the glaze fuses to the ceramic object. Glaze firing can normally proceed at a faster rate than bisque firing. As the kiln reaches the required temperature, its preferred to slow down the ramp to allow the glaze to melt and mature at the high temperatures.
A glaze is a combination of ceramic materials designed to melt and stick as a glassy coating to a fired ceramic body. Glazes can be translucent, glossy, foamy, matte or opaque. A glaze is made from 3 main materials: glass former, flux and stabilizer. During a glaze firing, the materials in the glaze have transformed from separate particles into a molten skin. The heat causes a chemical transformation to the molecular level. The crystalline structure of the glaze materials breaks down and move into the shapeless random structure found in glass. When kiln cools, the formless structure remains and hardens into a glassy veil and vitrifies.
Packing a bisque kiln is very different then packing for a glaze firing. For a bisque firing, you can place objects near each other, or rim to rim, or footring to footring. In a bisque firing you can also stack objects and nest different shapes together. Differently, during a glaze firing, it is important that objects do not touch. It is also important that there is no glaze at the bottom of you object. As the glaze melts it turns into a flowing substance, it bubbles and expands. If the objects are to close to each other, the glaze can stick and as the kiln cools, the glaze will solidify and attached two objects together. Similarly, any glaze on the base will stick the object to the kiln shelf.
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