Heat Work and Cones

Heat Work

Many potters see heatwork as some sort of semi-mystical property of kiln firing, whereas it is simply a function of temperature and time, reflecting the time taken for heat to diffuse from the surface of a pot into the centre of its body by conduction - this is proportional to the square of its thickness, so if the wall is twice as thick, it will take four times longer for the centre to reach the same temperature.

Thermal Lag Time

Key to understanding heat work is the thermal lag time, calculated as:
(t_o - t_i)/R
where t_o and t{SUB())i{SUB} are the external and internal temperatures in °C, and R is the heating rate (ramp) in °C/hr
This is constant for a given object - varying with its thickness and physical properties.

The lag time explains why it is necessary to either have a hold time at the top of the firing, or overshoot the required kiln temperature to let the clay in the middle of the pot reach maturity. Also, why a faster firing needs a longer soak or needs to overshoot the maturing temperature more.

Cones

Cones are a means of measuring heat work. They are tall pyramids in shape, at a slight angle off the vertical, made of a glaze-like material. As the material softens in the firing, the tip slowly bends over, indicating the level of heat work achieved.

Note that as Orton cones are the only ones still available, I will only be referring to them unless I state otherwise.

History of Cones

The earliest instance of cones have been found in kilns at Shaanxi in China during the Northern Song dynasty (960 - 1127CE). They were then reinvented by Josiah Wedgewood in 1782. But the first common standard for cones was developed by Hermann Seger in 1886, and called Seger cones - these became standard throughout Europe, and other areas such as Japan. They were made exclusively by the Prussian government owned Königliche Porzellanmanufaktur (Royal Porcelain Works) at Charlottenburg, but in time a number of alternative cone systems developed. In 1896, Edward J. Orton developed the Orton cone series in the USA, made by the Edward Orton Jr. Ceramic Foundation, In the UK, the North Staffordshire College developed Staffordshire cones which went into production in 1915, presumably driven by the impossibility of importing Seger cones from Germany during the First World War. Other cone series include Cramer and Simonis.

Initially, cones were used as a measurement system in their own right. It wasn't until the 1920s that measurements were made of the temperature at which the cone is fully down, at fixed heating rates. And it is only now that we are able to properly model what happens inside a cone.

Although several cone series were developed by a variety of companies, Orton cones are the only survivors - Seger having ceased manufacturing in 2001.

How they Work

Cones are tipped over at an angle of 8°, which means that there is a natural tendency for them to bend. When solid, the force is much too small to have any effect, but as they heat up and the cone material slowly becomes increasing liquid, a point will come when the cone will begin to bend. This happens at the top first, as the cone is at its thinnest, and so it will heat up through its thickness more quickly. There is a band of about 200°C (depending on cone composition) between when the cone composition stops being fully solid, and when it is fully liquid, and it is within this and that cone will bend.

The angle of the cone can be used to measure the temperature more accurately, which is useful as the difference between one cone and the next can exceed 50°C in some cases. Orton's web site shows a template for measuring the cone angle, and they also issue tables of angle versus temperature on request.

Cones are calibrated to a number of different ramp rates, and changing the ramp rate from 60 to 150°C/hr may change the maturing temperature by anything between 15 and 25°C.

Orton's data on soak times is a clear indication of the time needed for heat to conduct in to the centre of the pot: a soak of 1 - 2 hours is needed to go up by one cone, increasing to 4 - 6 hours for 2 cones, and 16 - 20 hours for the third cone.

Using Cones

Cones may be either self-supporting (so they sit on the kiln shelf) or regular (so they sit in a support).

The cone support can be made of wet clay when the kiln is loaded, so long as you ensure that the cones are at the right angle (by pushing them right through the wet clay onto a flat surface), and the clay comes up the cone side to leave the right amount of cone sticking up (2" or 51mm for large cones, ¹⁵/₁₆" or 24mm for small); also, lots of holes in the clay ensure it doesn't explode. I prefer to assemble these out of the kiln, and let them dry to leather hard. If you anticipate firing to well above the kiln temperature, put them onto a disposable bisqued saucer, to catch the liquid as the cone goes into meltdown.

Often cones are mounted as sets of three: a guide cone one below the target, the firing or target cone, and a guard cone one above to show over firing. All cones should face the same way, with each falling away from the guard cone so the y don't interfere with each other as they bend.

For the chart of cone number versus temperature, go to Cones vs Temperature.

Cone Problems

As with any other measuring device, if not used correctly cones can give incorrect results. Here are the issues that may occur in an electric kiln:

• Incorrect mounting. If the cone is at the wrong angle, or the support in the base is the wrong height, it won't bend correctly. For self-supporting cones, this means putting the cone onto a flat surface. For other cones, if making your own clay support
• Damaged cones. If the cone is damaged, it won't bend in the same way, so will give inaccurate results. Most commonly, as cones are supplied in pairs that need breaking apart, if you are clumsy you can break the tip off when separating them. Just bin the damaged cones.
• Bloating. If fired in reduction, or the temperature rise is too fast, the organic matter in the binder and the raw materials can burn out, causing a black core and bloating, which affects the maturing behaviour of the cone. The surface of the cone will have a bobbly finish, and if broken will display a grey-black interior.
• Hard shell. If fired in reduction or in a sulfurous atmosphere, the outside of the cone develops a hard layer whilst the inside remains soft. The cone only bends at the base, rather than throughout its length, and bends at a higher than normal temperature. This primarily affects cones made with red iron oxide, between 010 and 3 - they will often show a green to black colour after the firing. This can be avoided by using the iron-free cones in this range.
• Surface glaze. This may be produced by volatile materials in the kiln, such as lead, vanadium, antimony or boron, affecting the cone bending behaviour
• Radiation. Strangely, Orton say that exposure to radiation affects the cone bending temperature, yet at most firing temperatures it is impossible to avoid radiated heat. Perhaps they mean excess exposure, e.g. placing the cones in line of sight of the kiln elements, so they are exposed to radiated heat throughout the firing.