With the weather taking a further turn for the worst and a cold snap predicted over the coming week we thought it might be useful to give some tips and advice on how to protect your clay from the elements, and in worst case scenario how to restore your clay so that you can use it.

Clay storage

The best advice is to store your clay in a warm environment during winter weather to avoid it freezing, although this can sometimes be difficult especially if you have large amounts to store, so some useful tips for storing clay in a cold environment are:

  • To place pallets around the clay and then cover with tarpaulin, this will provide an air gap between the tarpaulin and the clay
  • To cover the clay in old blankets

If your clay still freezes despite the above then you can still restore it back to its original condition.

Frozen clay

Moist clay is made up of clay particles that are layered together. Clay bodies contain water which allows clay platelets to glide back and forth on each other. These water molecules surround each clay platelet. It is also called water of plasticity as it lets the clay stretch and move and keeps your pots in one piece. Water makes up between 30-40% of the total weight in a bag of moist clay, depending on the particular clay body.

So when the clay freezes the water within the clay body expands to become a solid crystalline structure known as ice. The expansion of the physical water pushes the clay platelets apart and since it is the water that binds the clay together once the clay has frozen and then thaws these particles become separated from the clay body so that it has more of a crumbly consistency and looks rock solid. At this point some potters may throw away the clay under the impression that it is no good, the clay has not been damaged it has simply gone through a change that it has experienced in nature countless times. For many potters the clay once frozen can often be just as good if not better than before.  Remember that clay materials have gone through millions of years of erosion, freezing and thawing.

How to restore your clay once frozen

If your clay has frozen all you need to do is to ensure the clay platelets are mixed together again. To do this you will need to wait for the clay to have thoroughly thawed, then remove the mushy crumbly part that will be covering the clay, the core of the clay may still be in a usable state, this is dependent on whether the surface was frozen or the entire body. The mushy part can be spread on a plaster batt, rotated or flipped over to ensure even removal of excess water until the clay is of an even plastic consistency. Then using the remainder and what was the mushy part that had the excess water removed can then be wedge (or pugged)* together and needed.

*Wedging (hand version of pugging) is when you layer sections of the clay one on top of each other, alternating layers of softer and stiffer clay together, you then cut the clay and layer again and repeat this process until you have an even homogenous mass. This can then be kneaded by a bulls head (rams head) or spiral (shell) method. This process removes the air from the clay and prepares it ready for making.

Freezing conditions to consider with fired clay

It is also worth considering how freezing conditions effect ceramic objects when placed outside. As some potters have already discovered, glazed or unglazed ceramic forms can fracture and spall (chip due to internal stress) when placed into freezing/thawing conditions. Think about when plant pots crack during the winter weather. Not all clay bodies can withstand these conditions irrelevant of the temperature that they were fired to.

You have to consider that most materials shrink when frozen, however, as mentioned previously water expands to form ice crystals. Due to the open pore structure of fired clay moisture can be trapped in the form of rain, snow and humidity by capillary action. When the pot freezes this moisture then expands into ice crystals causing the pot to crack or chip.

Testing the Clay

Any clay body that is to be placed outdoors in the conditions mentioned above should be tested, this can be done in accordance with Lucideon (previously Ceram) or the American Society for Testing Materials.

According to Lucideon with the testing of ceramic objects to withstand freezing condition the main problem is when moisture penetrates a ceramic item, expands as it freezes leading to micro-cracks developing, which cause crazing and spalling to occur: i.e. what we would call frost damage. A good illustration of water expanding when it freezes is the formation of ice on the top of ponds and not at the bottom (water expands when frozen, becoming less dense). A good test for frost resistance for the type of products produced from your body is the ISO 10545-12 freeze thaw test for ceramic tiles.

For further information about the freeze thaw testing or if you would like the test to be carried out we would recommend that you contact Lucideon on 01782 764428.