Heat Work in Pottery

A Confluence of Temperature, Time, and Atmosphere

When firing pottery and ceramic objects, we tend to think in terms of temperature. That is only one factor, however, involved in maturing a clay body and glazes. Heat work is the real objective...the transfer of heat from the kiln's interior atmosphere to the pottery within it.

  • 01 of 04

    Limitations of Temperature Alone

    Firing ceramic work at different temperatures
    Firing ceramic work at different temperatures. Getty

    Did you know that as a glaze melts, its temperature will not go above its melting temperature until all the solid particles have become liquefied? No matter how much hotter the kiln's atmosphere gets, until all the particles have melted, the glaze itself cannot go any higher than that melting point.

    This means that the difference between atmosphere temperature and the pot's temperature really only effects how quickly the glaze particles melt. As you can see, raw temperature (of the kiln's atmosphere) is not the best way to judge whether a load is done or not. Temperature is a huge factor, but not the only one.

    Newer kilns offer increasing amounts of control over your firings and you can programme your firings to be as efficient as possible, retaining the heat of high temperatures and cooling the kiln slowly. As you get to know your kiln this process will get easier and easier. These types of electric kilns are best suited, as we discuss below, to oxidation firing, not reduction. 

  • 02 of 04


    Firing times and temperatures and soak times
    Leaving a soak time for your ceramics. Getty

    In some firing schedules, the ramp is slow enough that a separate soak is not needed. As we have seen, however, a kiln's interior may reach the desired temperature before the pottery does. Because of this, many potters add a soak period to the end of their firing schedules. This period of maintaining temperature allows the glazes to fully melt and mature.

    A soak period, which can also be known as a hold or dwell period is when the temperature in the kiln is held for a fixed amount of time. This temperature can be programmed at any time during the firing, but it's most commonly programmed at the top temperature of the firing. A benefit of soak firing is that it gives glazes the chance to melt properly and evenly, lessening the chance of any imperfections. If you're after a flawless glaze then add in a soak period to you firing. There is no one temperature in which a soak period works best, as different glazes melt at different temperatures. Research your glaze properly before you start your firing, so you can plan in a soak time. As well as a soak time you can introduce slow cooling to your kiln, to reduce the appearance of pin holing. 

    Clay bodies have a wider maturation range. Because of this, a soaking period is not as important for a clay body to mature, but in some cases it can have an effect. Generally, this is seen when a clay body is soaked at the upper range of its maturation range. Such an occurrence may cause the clay to blister, warp, and slump.

  • 03 of 04

    Kiln Atmosphere

    What is kiln atmosphere? And how it affects your firing
    What is kiln atmosphere? And how it affects your firing. Getty

    Kiln atmosphere can have a strong effect on how quickly glazes and clay bodies mature, especially those containing iron. As the oxygen-deprived air pulls oxygen from the clay and glazes, the metal oxides are converted as they lose oxygen atoms. This makes many of them into stronger fluxes.

    On the converse, reduction is an inefficient firing method, as far as temperature is concerned. It does not burn the fuel completely, and heavy reduction cycles can actually decrease the interior temperature of a kiln considerably.

  • 04 of 04

    What Is Reduction Firing?

    The difference between reduction and oxidation firing
    The difference between reduction and oxidation firing. Getty

    Basically, a kiln atmosphere is how much available oxygen there is in the kiln's atmosphere. When lots of oxygen is present, equally lots of fuel to burn, this is called an oxidation atmosphere and in contrast, when there is limited oxygen, it's called a reduction firing. So, why do a reduction firing? Well, it can actually produce some really great and quite unique results. In a reduction firing gases like carbon, hydrogen and CO are more present. This change in gases means a vast change in your glaze colors. For example, in reduction copper burns red and in oxidation it burns green.