What can I do if my clay has frozen?
Clay that has been "lightly-frosted" (that is to say, the freezing has been limited to the surface) is unlikely to suffer adverse effects and once thawed will return to a useable state on its own with time.
Freezing to a depth of about 1” (when considering the dimensions of a standard 12.5kg bag of clay) will require the clay once thawed to be thoroughly wedged before use, but should return to useable condition.
Freezing beyond this depth may require substantial work in recovery/redistributing the water in the clay.
There's no set timescale for recovery once the clay has thawed, but in our own experience with lightly-frozen clays it normally occurs within 4-6 weeks provided the clay is stored in a cool (above freezing) area. The faster the clay is thawed, the more sticky it becomes so avoid storing it in too warm an environment.
Prevention is better than cure, so make provisions for storing your clay in a suitable location during the cold weather to prevent any risk of freezing. If clay can be stacked in a large ‘block’ as if on a pallet then freezing is likely to affect only a few bags even in the severest of winters. Covering with any kind of insulating material and a tarpaulin and will provide even more protection.
The Science
When clay freezes, the water molecules become detached from the clay particles and the result upon thawing can be a sloppy bag of clay. Clay types vary in constituents and fineness:
Coarse clays such as crank and raku types contain typically up to 50% non-plastic material (grog) which can be frozen without adverse effect. The remainder of the mix is normally clay mineral typically composed of fireclay with some ball clay and/or china clay dependent upon individual formulations. Fireclays have a larger particle size of around 5 microns than china clays at 2 microns and ball clays at 1 micron. Consequently, whilst they are affected by freezing the effect is less damaging and most fireclay-based stoneware clay bodies will thaw and return to a useable consistency with little intervention.
Finer stonewares, such as white stoneware or clays designed for throwing, generally contain much lower proportions of fireclay and rely more heavily on ball clays. During manufacture the processing of the clay typically by sliphousing is carefully controlled to prevent the clay being broken down too finely, a problem known as over-blunging. This creates a clay which is often more plastic and sticky to handle. Freezing has the same effect as over-blunging by breaking apart the agglomerated clay particles into smaller ones. This then requires more preparation by wedging before it can be returned to a useable condition.
Earthenware clays typically contain similar clay contents to coarsely grogged bodies, in the order of 50%, but unlike the grogged stoneware bodies the clay minerals are ball clay and china clay and so they react to freezing more like the finer stonewares.
Porcelain clay types are generally the finest types of clay produced and contain high proportions of highly plastic ball clays and china clays, often with other plasticisers. These bodies quite often show the most dramatic effects to freezing, requiring much work to be put into recovery.
Further reading (Ceramic Arts Network)>
What About Frozen Casting Slip?
Frozen casting slip rarely returns to a good condition after thawing. Casting times are typically much longer and cast items more difficult to handle.
And Liquid Glazes?
Dipping glazes normally suffer no damage from freezing but if they have a high clay content they may need some additional water to give the correct glaze thickness. Brush-on glazes do not always return to a good state after freezing because the binders and extenders which are added to enable the glaze to be brushed easily are adversely affected by the ice crystals.
