Guidelines for collecting, reprocessing, testing and adjusting scrap recycle clay in a pottery or ceramics studio or production facility.
In a production situation, procedures and equipment are generally in place to incorporate a percentage of scrap into fresh clay mixes or to use it for certain types of production. Let us then take on the other challenge: the art center or studio.
Mix the biggest possible batch, this makes it worthwhile to test and decide how to use it.
Decide if you want to be strict, with separate collection containers and rules for the types of clay allowed into each. Or, since people do not tend to follow rules anyway, throw everything into one. Of course, grog clays will need a separate container.
A powerful mixer is important to be able to thoroughly mix batches into a thick and smooth slurry (so less water needs to be removed and they can be screened).
Take measures to assure that plaster bits or chunks do not get into any batches. If someone mechanical is at your disposal consider making a vibratory screen (then contamination becomes a non-issue).
Mixing porcelains and stonewares in scrap is not a problem. Or buff, white and brown bodies. The type of body you get is intuitive.
Testing the Batch
Test the batch: Endeavour to describe what the scrap clay is (e.g. 'Fires medium to light tan, vitreous cone 5-6; very slick and smooth; plasticity a little low.'). Fire a small sample tile (if possible in hot, cold and normal spots in the kiln).
Dry shrinkage and plasticity: Often it will be lower than the source bodies. Make a mug and pay attention to throwing to note if it is less plastic than usual. Attach a handle and watch how it performs in typical drying (look for cracks around handle joins or on the base). If it dries well and is lacking plasticity it can tolerate an addition of a 50:50 mix of bentonite:ball clay (often only 5% is needed). Shake the bentonite and ball clay together in a plastic bag to thoroughly mix them and sprinkle that onto the top of the slurry bucket and let soak before you power-mix it. Or, slurry the bentonite:ball clay and dewater it on a plaster bat and wedge it together with the scrap (beware, it can take days to dewater and is incredibly sticky). Since the percentage required is low it will have minimal impact on fired properties.
Glaze fit: Try several glazes and stress-test tiles using an ice water/boiling water test to reveal any crazing or shivering. If glazes craze incorporate some silica in the scrap clay batch (e.g. 5%). This can be done by slurrying a 50:50 mix of silica:ball clay, dewatering and wedging together with the scrap. This will decrease maturity somewhat for high temperature bodies (more so for medium) but at only 5-10% the effect should be minimal.
Maturity: If the scrap is a mix of medium and high fire stonewares and porcelains, then it could be too vitreous at cone 10 or too immature at cone 6. You will note this if the fired test pieces warp or bubble (too mature) or are weak and porous (immature). Reduce maturity by powder-sprinkling or wedging in a 50:50 mix of silica:ball clay or just fireclay (start with about 10%). Increase maturity by powder-sprinkling or wedging in a 50:50 mix of feldspar:ball clay (start with about 10%).
Sculpture clay: If plasticity of the scrap is high it may tolerate grog or paper. Wedge in 10-20% grog (e.g. 20-40 mesh) or incorporate paper fiber. If that cuts plasticity too much mix in some of the 50:50 bentonite:ball clay.
Color is not red enough: Wedge in a 95:5 mix of Redart:Bentonite. Or add 1% iron oxide.
Soluble salts on the fired surface: Add about 0.3% barium carbonate to the slurry.
If you have more than one scrap batch on hand and their descriptions show that they have complementary pluses and minuses in their properties, you can mix them.
Dewatering the Batch
This is not simple. For large batches a large plaster table is required. Having the thickest possible slurry reduces the demand on the table. Another method is to pour the slurry onto a canvas stretched over a wooden or metal frame and cover it with another canvas.
Dealing with scrap can be a dustry process so wear a mask when needed. A large mixer can be dangerous to use, be careful.
Supercharge the plasticity of cone 6 reclaimed clay
If your reclaim is short and non-plastic you can make it better-than-new by using an additive of 50% ball clay and 50% bentonite. While only a few percent bentonite supercharges the plasticity of any clay body it is almost impossible to get it to mix into a wet slurry or plastic clay. But thoroughly shaking it together with ball clay (in a plastic bag) separates the super-tiny particles of bentonite between the almost-as-tiny particles of ball clay, that new powder will easily mix with water. And it fires to a tan-buff stoneware at cone 6 so it won't change the fired appearance of most buff or brown cone 6 stoneware bodies. There is one downside: I can leave a scum on your plaster batt if your bentonite is high in soluble salts, so test on a small bat first. Or dewater by another method. Or use a dedicated batt whose surface you can scrape periodically.