vince pitelka on wed 11 dec 02
> What is your take on the use of Epsom Salts instead of bentonite to keep
> glazes from compacting?
Les -
Both work, and I suppose it depends on how bad the settling problem is. In
the worst situations, I have done both. Of course they are completely
different processes and effects. You probably know this, but bentonite
clays come from the long-term breakdown of airfloated volcanic ash, and of
all the clays they have by far the finest particle size. Only a 1% to 3%
addition makes a big difference. With such small amounts, the bentonite is
pretty much chemically neutral in terms of the glaze melt - it just makes a
creamier mixture which reduces the settling of non-plastic particles.
If you want to see something impressive, take a handful of dry bentonite,
add a little water, and work it into a paste, and form it into a small ball.
In working it, you will be amazed how slippery and jelly-like it is.
Plasticity in clays is partially due to the fineness of particle sizes, and
the texture of wet bentonite really shows that. Leave the ball of wet
bentonite out to dry completely. Once it is dry, place it in a dish and add
water. Observe how much it expands as the bentonite absorbs its full
capacity of water. As you know, the expansion and contraction of any clay
when it is wetted and dried has to do with the number of water layers
between the particles. With such fine particle size, bentonite has more
water layers, and therefore more expansion on wetting and contraction on
drying.
One of the tests you can do to measure the potential plasticity in a found
clay is the "water of plasticity" test, where you weigh a dry sample, then
add water and work the clay to plastic consistency, and then weigh it again.
The percentage increase is the "water of plasticity" measurement. The
higher that percentage, the more the water absorbed to reach plasticity, and
thus the more the water layers, the finer the particle size, and the more
plastic the clay will be.
Epsom salts are of course just a flocculant, and they will introduce
opposite electrical charges to the particles, causing them to "floc"
together, which thickens the suspension noticeably, reducing the settling of
nonplastics. This is a curious phenomenon, because in a very thin
suspension, like a terra sigillata, adding Epsom salts would cause all of
the suspended particles to floc together and sink to the bottom quickly,
which would of course ruin your terra sig. In a thicker suspension like a
glaze, the thickening effect of the flocculant creates the opposite effect.
It is important to remember that after bentonite or a flocculant is added to
a glaze, it will behave slightly differently in application. Usually that
is not a problem, because the glaze actually goes on more evenly. The
important thing is just to be prepared for a glaze that behaves slightly
differently.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://iweb.tntech.edu/wpitelka/
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