Dan c Tarro on thu 23 jan 97
Lets try this once more, having problems with the system.
Sandy,
You probably will get lots of help on this one.
I also have gone through this one with tea pots mugs... . I hate to see a
customer holding a tea pot at my studio door a week after I sold it, it
only means one thing. I have solved my problem with this and maybe you
may wish to consider my findings.
I mix my clays myself so I can keep an eye on the qualities of the clay.
In doing this I find that there is more than just the workability of the
clay when raw that has to be considered. What happens when the clay is
fired and that wonderful thing that goes on inside that makes it almost
glass has to be understood.
Simply- Clay, like glaze, is made up of three types of material, alumina
(refractory), Silica(glass former), and flux (lowers the melting temp of
the glass former). In glazes you can see what is going on most of the
time just by looking at the surface of the item, with clay it is a
little more difficult. There is a term used called vitrification. This
is a measurement as to how much the of the silica has actually been
converted to glass by the fluxes within the clay body. This can be
tested by a water soaking process which is discussed in any good
ceramics book. If you can't find any info I can help if needed. But the
key words here are silica and flux. Silica(free silica) is an odd item,
when it is still in its original state and heated will react very
unstably. Atomically, if can be described that way, it jumps.(If there
are chemists out there maybe they can describe what goes on better.)
But, when this same silica combined with a flux and converted to
glass(no longer free silica) becomes more stable and looses it's desire
to dimensionally jump. A lot of potters have historically added
feldspars to their clay bodies to do this and get away from this
cracking. I have been adding about 8%, and am firing ^10-12. I have
heard concentrations up to 11% used by others. Too much may tend to warp
your work at higher temps(slump). It will help some of your drying,
reduce your plasticity to some extent, and make your green ware slightly
more brittle. Warpage during drying is lessened and additions at leather
hard is improved. Lots of pros and cons. Best of all your items become
harder, crack less, and last longer. Happier customers if your producing
to sell.
Other things to consider, Iron content of your clay should be kept below
2% if possible, especially if you are reducing. Iron does weird gassy
things in the body in reduction and also makes the body very brittle.
Avoid the addition of earthenware clays for darker clay bodies at
stoneware temps. Too much junk in the clay and it also makes for a
brittle body. Watch your cooling at about 1000 deg. and 500, this is
where the silica does its jumping the most, something called quarts
inversion, go slowly past these temps..
I will stand back now after all this and see what other response we can
get.
What do others think? Am I all wet?
Lets help Sandy out.
Best of luck,
Dan Tarro
Oak Tree Stoneware
Ham Lake, Minnesota
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Clothes make the man, naked men have little influence on society. Mark
Twain
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