These are behind the art center. A dirty secret. Or an opportunity!

When many clays are in use there is good reason to collect like this - you will see why in a moment. Each barrel is a unique product, a mix of random clays used by students. Each could be thus considered a wild clay, inconsistent throughout and with impurities. As such, each needs to be characterized, as a whole. For example, consider the front barrel: What does it look like when fired at various temperatures? At what temperature does it mature? What is the drying shrinkage and how plastic is it? Does it fit the glazes we use? This information not only describes the clay but also points to what needs to be added to make it useful (e.g. bentonite or feldspar). An account at Insight-Live.com is a way to organize the testing and post the results to a group (the SHAB test, for example, is perfect to describe the plastic, drying and firing properties). Once the clay in a barrel becomes predictable then it becomes useful.
To do the testing it is necessary to get a representative sample from each barrel. The method depends on how processing will be done. In a wet climate, the wet contents are likely to be thrown into a pugmill (with impurities), recycled until mixed and then bagged - a sample can be taken when complete. But the dry climate option is much better: Dewater each barrel, do quartering to get a representative sample and then characterize it. Finally, slake, slurry up batches using a propeller mixer, screen in a vibrating sieve and then dewater (e.g. on a plaster table).

Related Pictures

Testing your own native clays is easier than you think

Some simple equipment is all you need. You can do practical tests to characterize a local clay in your own studio or workshop (e.g. our SHAB test, DFAC test, SIEV test, LDW test). You need a gram scale (preferably accurate to 0.01g) and a set of callipers (check Amazon.com). Some metal sieves (search "Tyler Sieves" on Ebay). A stamp to mark samples with code and specimen numbers. A plaster table or slab. A propeller mixer. And, of course, a test kiln. And you need a place to put all the measurement data collected and learn from it (e.g. an account at insight-live.com).

How to decide what temperature to fire this clay at?

Test bars of a terra cotta clay fired from cone 06-8

This is an extreme example of firing a clay at many temperatures to get wide-angle view of it. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware warps during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.

The clay aging myth: Prove it wrong by slurry mixing

Do you really need to age clay when you make your own? No. In ancient Japan they did not have power blenders and propeller mixers. We do. To illustrate: I just sieved out the +80 mesh and +200 mesh particles from this raw clay (from one of our stockpiles) and then propeller-mixed it as a slurry. That wetted the particles very well and made it easy to sieve. Then I poured the slurry on to a plaster table and thirty minutes later it was ready-to-use. Slurry mixing is just as good as deairing in a pugmill. No wait! Particles wet even better. The plasticity of this clay is wonderful, and, it will not get any better with aging. Ancient Japanese potters used non-plastic, coarse particled clays so they needed to squeeze every last bit of plasticity out of them. Today, fine particled plastic clay materials are readily available. And we have something else the ancients did not: Micro-fine bentonite. A few percent of that and any clay can be made super-plastic (provided you have a good mixer to wet and separate all the particles to release their full power).

Make your own vibrating sieve

Being more independent is now cool again. Actually, it is being forced upon us by necessity because of supply chain issues and skyrocketing prices of convenience glazes, bodies, engobes, etc. Independence involves using sieves. True, it is no problem for a potter or lab tech to manually coax a glaze slurry through a small 80# sieve. But real independence is about sieving in volume - clay bodies and casting slips. About making your own porcelains and sieving out agglomerates. The ultimate in independence: Sieving particulates from your own native clay slurries. And doing it at 100, 140 and even 200 mesh. That requires a vibrating sieve. This one cost us less than $100 to make. Of course, a Tyler sieve (or similar) is needed, these can be purchased on Ebay or Amazon. And a vibration motor, some metal and hardware and a friend with metal fabrication tools.

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.

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