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Recylcing Scrap Clay

Section: Clay Bodies, Subsection: General


Guidelines for collecting, testing, reprocessing scrap clay in a multi-person ceramic studio.

Article Text

In any studio or production facility scrap clay accumulates quickly. In a large art center, for example, smaller amounts of a dozen different bodies may be in use. In a production situation it is more likely that just one kind of clay is employed and the major challenge is dealing with scrap volumes. Typically procedures and equipment are 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.

Everyone likes to have clay with consistent properties but you can forget about that when using bodies prepared from scrap. The trick is gathering some information about the working and fired properties so you can either choose a suitable use for each scrap batch or adjust and fine-tune it to a specific use. You thus need a way to test this unique new clay body and evaluate it for its basic properties. Unlike typical clay formulation efforts you cannot choose to remove something from the recipe, you can only add things. If you are instructing in a school studio and see an opportunity to turn buckets of scrap clay into an educational opportunity you are not alone.

Some Guidelines on Collecting Scrap

-Mix the biggest possible batch, testing effort on small ones is pointless.

-Decide how many collection containers you need and rules for the types of clay allowed into each. The stricter the rules the more likely you can predict what sort of concoction each is. On the other hand, the better your ability to evaluate and adjust a batch the more relaxed the rules will need to be. Actually since people do not tend to follow rules anyway it is best to treat every batch of scrap as if it could have anything in it (including previous batches of scrap).

-Employ large plastic containers of water as collecting points for scrap. Use a large jiffy mixer or similar and a powerful drill to thoroughly mix batches into a thick slurry so that you can take a 500 gram (half pound) sample that is representative of the whole. Pour this sample onto plaster to dewater it for testing.

-Take measures to assure that plaster bits or chunks do not get into any batches.

Testing the Batch

This testing can quite simple or complex. A testing area on this site has step-by-step for all common tests done in ceramics (hundreds of them). Remember that your goal is to describe what the scrap clay is in a clear and concise way (in engineering circles they call this characterization). Why do I say this? Because it is likely you will have more than one scrap mix around or more than one person be will using it. Thus each batch needs a unique code number label, a fired sample or two and a card describing what it is. Following is what a description might look like.

'Fires medium to light tan, vitreous cone 5-6; very slick and smooth; lower than normal plasticity.'

Now consider the properties that are likely to vary from your regular bodies. As I mention these I am assuming of course that you would know the properties for a typical body you use. If not, take the time to measure the dry shrinkage, for example, on your regular clay in your circumstances.

-Fired color: This is a product of both iron content and firing temperature; fire a sample to see (if possible at several cones since progression of color is often a good indicator of firing temperature).

-Dry shrinkage: If this is too high the batch will be more plastic and therefore more likely to crack during drying (if too low vice versa). For a variety of reasons the dry shrinkage can be higher or lower than expected. To measure dry shrinkage roll a bar (using a consistent technique each time) and make 10 cm apart marks to measure after drying (100 minus the span after drying is the dry shrinkage).

-Texture: lf you have experience this should be obvious in wet texture and on the fired surface. Also examine fired and glaze samples for signs of iron speckle or tendency toward pinholing or blistering.

-Firing temperature: Try to fire samples at various temperatures. As a second choice, fire at the most likely maturing temperature and extrapolate a likely target from the result.

-Glaze fit: Try several glazes and stress the test tiles using an ice water/boiling water test to reveal any crazing or shivering.

Adjusting the Batch

Do not try to adjust every batch to make it perfect. Sometimes it is better just to describe well what you have so people can adapt to it. But since you have a slurry it is easy to dump in some extra powder to condition the mix. Obviously you need to know the total weight of dry material in the batch to be able to add a percentage of something to it. Determine this by drying out a small known volume to find the water content and extrapolating total solids weight from the total known volume.

Following are some possible addition candidates to condition your batch (if you want to know more about any of these materials visit materials area on this site). Keep in mind that for most you need to add at least 5% to see an effect (except for bentonite, iron, barium). Also, if you have bags of long unused materials in your glaze room watch for opportunities to put them into scrap batches. If possible, test again after you make an addition to develop a body of knowledge about the effects of additions.

-Ball clay: Add if the batch lacks plasticity and it is a little vitreous.

-Bentonite: Add in 1-3% amounts to impart plasticity with minimal effect on other properties. It is very difficult to mix this in so add a 50:50 bentonite:kaolin mix (ignore the kaolin, e.g. in a 100 lb batch add 2 lbs of the 50:50 mix for a 1% addition of bentonite).

-Silica: Add if the body is too vitreous and especially if glazes craze. Add bentonite also if needed since adding silica cuts plasticity.

-Feldspar: Add if the batch is too refractory. Add bentonite also if needed.

-Non-plastic kaolin (e.g. EPK): Add to whiten a batch that is a little too plastic and vitreous or for which a lower plasticity is tolerable.

-Plastic kaolin: Add to whiten a batch that is too vitreous.
-White talc: Add to a batch of low-flre clay to whiten it or improve drying properties. Add if glazes are crazing on a low-fire batch.

-Redart (or other red burning clay): Add to redden a low fire terra cotta batch (some plasticity will be lost, add 1 bentonite for each 20% redart added). Add less to redden a medium fire batch that is not vitreous enough.

-Fireclay: Add to non-white stonewares if they are to vitreous or need some earthy character.

-Iron oxide: Add 1% amounts to darken fired color. Yellow iron is cleaner to work with than red.

-Grog: To create a sculpture clay add as much as it will take and still be workable. Add extra ball clay to make room for even more grog.

-Barium carbonate: Add 0.1 to 0.5% if the body fires with soluble salts on the surface.

-Paper: Add to make paper clay.

For many of the above you could run a small test on 500 g of the batch for extra insurance. If you are a teacher you could have each student appraise the scrap batch and suggest what could be done to condition it into something useful.

What if the batch goes bad and appears unusable? This is highly unlikely, almost any clay can be used for something. In extreme cases consider compensating on the opposite property extreme on the next batch and wedge them together.

Dewatering the Batch

This is not simple. There is no plaster table large enough to remove the amount of water typically required so one effective way is to mix it to the thickest possible slurry so it does not settle out and then pour small amounts on a plaster table to dewater as needed. Needless to say you need a plaster table or large portable plaster concave bats. In drier climates another method is to pour the slurry onto a canvas stretched over a wooden or metal frame and cover it with another canvas. This will take some time to dewater by evaporation and you will have to fold the edges in as they dry faster, but this method can be effective.

If you are suspicious that the batch is contaminated by chunks of plaster then put the slurry through a 30 mesh screen as you pour it out onto the plaster table.


Dealing with scrap is generally a wet process, but if you need to deal with larger quantities of dry material and generate dust, then beware. Try searching the hazards area of this site for 'safety' or 'dust', there is a good article there.

Out Bound Links

By Tony Hansen

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