Recylcing Scrap Clay

Description

Guidelines for collecting, reprocessing, testing and adjusting scrap recycle clay in a pottery or ceramics studio or production facility.

Article

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.

Collecting Scrap

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.

Safety

Dealing with scrap can be a dustry process so wear a mask when needed. A large mixer can be dangerous to use, be careful.

Related Information

Supercharge the plasticity of reclaimed clay using bentonite

If your reclaim is short (non-plastic) restore it by mixing in some bentonite - as little as 2 parts per 100 scrap (in dry form) is often plenty. Note however that bentonite is hyperplastic and won’t mix with water as a pure material. The preferred method is thus to dry out the scrap, crush it as much as possible, then mix in the bentonite as thoroughly as possible. Finally, slake it, propeller mix as a slurry, sieve if needed and then dewater on a plaster table.
In a wet climate, the previous method might not be practical. An alternative is to make a 50:50 ball clay:bentonite powder mix, as demonstrated in the video. Thoroughly shake them together in a plastic bag (to separate the super-tiny bentonite particles), slurry it up and dewater to plastic form for wedging into plastic reclaim. There are issues, however. If the bentonite is high in soluble salts the slurry can leave a scum on your plaster batt (fixable using a sandscreen). Blender mixing is best but will demand much more water than typical body mixes. A third option is the use of pure ball clay, but 5-10% might be needed. Ball clay can be blender mixed using much less water but it will still take a long time to dewater and is super sticky in plastic form. And the percentage needed will likely impact body fired maturity, making it fire a little more porous.

Links

Glossary	Propeller Mixer In ceramic studios, labs and classrooms, a good propeller mixer is essential for mixing glaze and body slurries.
Glossary	Plaster table Essential in a pottery studio for dewatering reclaim clay, stiffening clay that is too soft or making your own clay bodies.
URLs	https://www.youtube.com/watch?v=0zXfghDNxUg ShowerShelf.com vibratory screen that fits the top of a five-gallon bucket The tank, sieve and vibration assembly are all in one piece that fits into the top of the bucket. This unit can handle 1.8SG slurry.
URLs	https://www.amazon.com/Vibration-3600RPM-Industrial-Asynchronous-Vibrator/dp/B07P5CLJLP/ref=sr_1_12?qid=1559306388 Industrial vibration motor at Amazon.com

By Tony Hansen
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