Tn any studio or production facility scrap
clay accumulates quickly. In a large ari
center, for example, smaller amounts of a
dozen different bodies may be in use. In a
production situation it is 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 take the
other challenge: the art center or studio.

Everyone likes to have clay with consist-
ent properties, but you can forget about
that when using bodies prepared from
scrap. The trick is gathering some infor-
mation about the working and fired prop-
erties so that you can either choose a
suitable use for each scrap hatch or adjust
and fine tune it to a specific use, You'll
need a way to test this unique new clay
body and evaluate it for its basic proper-
ties. Unlike typical clay formulations, you
cannot choose to remove something from
the recipe; you can only add things. If you
are instructing in a school studio and sec
an opportunity to tum 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 pointicss.

* Decide how many collection contamers
you need and rules for the types of clay
allowed into cach. 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 treal every
batch of scrap as if it could have anything
in it (including previous batches of scrap).

* Employ large plastic containers of water
#s Collecting points for scrap. Use a large
jiffy moxer or similar tool and a powerful
drill to thoroughly mix batches into a thick
slurry so that you can take a 40 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.

NovembenDecember 2005

Techno Tip
Recycling Clays
by Tony Hansen
Testing the Batch
This testing can be quite simple or com-
plex. At www-ceramicmatenals.info, a
esting section is under construction and
will eventually have step-by-step for all
common tests done in ceramics (humireds
of them). Remember that your goal is to
descnbe what the scrap clay is in a clear
and concise way (in engineering circles
they call this characterization). Why do I
say thes? It is likely you will have more
than one scrap mux or more than one per-
son Will be using it. Thus. cach batch needs
aunique code number label. a fired sample
or two and a card describing what it is.
Following is a sample description:
Fires medium to light tan, vitreous come 5-
6) very slick and smooth; lower than nar-
mal plasticity.

Now consider the properties that are likely
to vary from your regular bodies. As |
mention these | arm assuming of course
that you would know the properties for a
typical body you use. If mot, take the time
tomeasure the dry shrinkage, forexample,
on your regular clay in your circumstances.

* Fired colour: This is a product of both
iron content and firing temperature: fire a
sample to see (if possible at seweral cones
since progression of colour is offen 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). Por 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
cach time) and make 10cm apart marks to
measure after drying (100 minus the span
after drying is the dry shrinkage).

* Texture: If you have experience this
should be obviows 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 sam-
ples at vanous temperatures, As a second
choice, fire at the most likely maturing
temperature and extrapolate a likely target
from the result

Potters Guild of British Columbia Newsletter

+ Craze fit: Try several glazes and stress
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 de-
scribe well what you have so people can
adapt to it. But since you have a slurry itis
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
waler content and cxtrapolating total sol-
ids weight from the total known volume,

Following are some possible addivons to
condition your hatch. (if you want to know
mor: about any of these materials visit
www.ceramicmilerials.info), Keep in
mund that for mast, you need to add at least
5% to see an effect (except for bentonite,
iron, barium). Also, if you have hags of
long unused materials in your glaze room,
watch for opportunitics to put them into
scrap batches. If possible, test again after
you make an addition to develop a bedy of
knowledge about the effects of adiditions.

* Ball clay: Add if the batch lacks plastic-
ity and it is a little vitrecus.

* Bentonite: Add in 1-3% amounts to
impart plasticity with minimal effect on
other properties. [tis very difficult to mix
this in so add a 50:50 bentonite: kaolin mix
(ignore the kaolin, c.g. in a 100 Ib hatch
add 2 Ibs 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 ben-
tonite also if needed since adding silica
cuts plasticity.

* Feldspar: Add if the batch is too refrac-
tory. Add bentonite also if needed,

= Non-plastic kaolin (¢.¢. EPK): Add to
whiten a batch that is 4 little too plastic and
vitreous or for which a lower plasticity is
tolerable.

* Plastic kaolin: Add to whiten a batch
that is too vitreous,

Techno Tip continued on next page

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