How can I make my pieces frostproof?

There are three key factors in making a pot that can withstand cold weather conditions:-

• Firstly, the clay body should have a low absorption rate. This means firing the clay at the upper end of its recommended firing range to make it as vitreous as possible (further information about vitrification of clays can be found below)
• Secondly, clays with a higher grog content tend to perform better outdoors. Very fine, smooth clays generally lack the strength of coarser clay bodies. Although coarser clays are more open in structure and may seem more likely to absorb water, the added grog provides greater strength and helps the pot resist frost damage.
• Thirdly, the shape and construction of the pot are important. Pots should have an even wall thickness wherever possible, with rounded forms rather than sharp angles. Smaller bases can also help minimise water absorption.

WHAT HAPPENS TO CERAMIC PIECES IN FREEZING CONDITIONS?

Pottery is damaged when moisture that has been absorbed into the clay freezes. Water expands as it freezes and the force of this expansion can cause a pot to crack or the surface to shale off. 

VITRIFCATION OF CLAYS

The vitrification point of a clay body is the highest level of heatwork — the combination of temperature and firing time — that can be achieved without the clay deforming.

As clay is fired to higher temperatures, the feldspathic materials within the body begin to flux and melt. This molten material flows into many of the spaces between the clay particles, bonding them together in a glass-like structure. However, not all of these spaces are filled. Some porosity remains, which is important for the strength and stability of the finished piece. Complete vitrification would produce a material closer to glass, which could deform during firing and would be more brittle and prone to breaking.

For this reason, even fully vitrified clay bodies retain a degree of porosity. Porcelain bodies fired to their maximum temperature may still have around 0.1% porosity, while more open, heavily grogged clay bodies may retain 2% porosity or more.

Where pottery is intended to hold liquids, we recommend applying a suitable glaze to ensure the finished piece is watertight.

We specify a vitrification range for most of our clay bodies, and a clay is considered mature when fired within this range. Generally, the higher the firing temperature within the recommended range, the greater the degree of vitrification and the lower the resulting porosity.