The slow cooling phase in the ceramic firing process is a crucial technique for creating enhanced surface effects, particularly the formation of microcrystals in glazes. This article explores the principles behind this process, its effects on glaze outcomes, and how to implement it effectively in your firing schedules.
Understanding Slow Cooling
Slow cooling is a controlled firing schedule that allows the kiln to decrease in temperature at a slower rate than it naturally would. This phase usually occurs after the peak temperature has been reached and can significantly affect the visual and physical properties of glazes.
The Science Behind Microcrystals
During the slow cooling phase, the molten glaze starts to solidify. The rate at which this happens can influence the size and quantity of crystals formed within the glaze. Faster cooling rates can prevent crystal growth, resulting in a smooth, glass-like surface. Conversely, slower cooling rates allow for the nucleation and growth of microcrystals, which can impart a range of visual effects on the glaze surface.
Microcrystals form when certain ingredients in the glaze, often those that act as fluxes or colorants, become supersaturated in the glassy matrix of the glaze. As the glaze cools, these ingredients can precipitate out of the solution and crystallize. The specific temperature at which this happens, and the size and shape of the crystals, depend on the glaze's composition and the cooling rate.
Impact on Glaze Characteristics
The formation of microcrystals can affect the glaze's appearance in several ways:
- Surface Texture: Microcrystals can create a range of textures, from a subtle matt finish to a heavily crystalline surface.
- Color: The color of the glaze can be altered by the presence of crystals, often deepening or changing hue.
- Visual Depth: Crystals can add depth and complexity to a glaze, making the surface appear more varied and interesting.
Implementing Slow Cooling
To achieve effective slow cooling, it's essential to have a programmable kiln that allows you to control the cooling rate. A general approach is to slow the cooling rate to 100°F (about 55°C) per hour or slower, starting from the peak temperature down to around 1400°F (760°C). However, the optimal cooling curve can vary widely based on the glaze composition and the desired effects.
Examples and Case Studies
Many potters and ceramic artists have experimented with slow cooling to achieve unique glaze results. For instance, crystalline glazes, which are known for their large, distinct crystals, rely heavily on carefully controlled slow cooling cycles. Other effects, such as variegated surfaces and enhanced depth of color, can also be achieved with less dramatic cooling schedules.
As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases.
Conclusions
Slow cooling is a powerful tool in the ceramic artist's arsenal, offering the potential to unlock a range of visual effects through the controlled formation of microcrystals. By understanding the principles behind this process and experimenting with different cooling rates and glaze compositions, potters can significantly expand the palette of surfaces available to them.