Harrington on sat 22 feb 03
Hi all, and thanks to all of you who helped me get a clue of what
porcelain exactly is. Now I need help using it.
I bought a box of cone 6-9 Byrne Porcelain, threw 11 vases, and bisqued
to cone 04. Almost all of them came out with the same flaw: fine cracks
that sort of wrap around the bottoms, above the feet (which were
flat-bottomed). Now I am just a baby potter, but I have never seen this
with earthenware or stoneware. My theory is that porcelain needs to be
thrown a certain way, and that I suck at it. I know from what I've read
that porcelain needs to be thrown pretty dry compared to other bodies,
so I tried real hard to be dry, using about a half cup of water for a
five or six pound blob of clay.
Maybe I was too dry and created too much torque on the bases of the pots
and the clay got twisted? Or maybe my clay was too hard when I started
out? Maybe I should have wedged instead of using it straight from the
bag? Any ideas? Please help me not do this again.
Also, since the cracks are pretty fine, does anyone think I could maybe
still glaze them and at least use them as glaze test pieces, or would
the stress of going to cone 6 or 9 make them blow up or something?
Thanks yet again!
Lisa Harrington
Not Really a Potter
Merrie Boerner on sat 22 feb 03
Good problem solving questions, Lisa.
I have little experience....or should I say, the same bad experience....
once or twice ; ) Always wedge clay from a bag. Try to throw with an even
thickness. When you throw, compress....when you trim, compress.....when you
handbuild, compress.........Compress. ( I coach myself with that word when
working with clay). And, sorry to say...but, the cracks will get larger
with higher firing.
When I ruined 12 bowls, first I blamed the clay....then, I got real
and blamed myself for poor throwing, trimming, and for lack of experience
with that type of clay. Don't expect to pick up a new clay (especially
porcelain) and have the exact same results as your usual clay.
Good luck, and watch for good advice on Clayart.
Merrie
John Rodgers on sun 23 feb 03
As my porcelain of choice I use Laguna Cone 6 Grolleg. There are others,
but I just happen to like the grollegs. Yes it is more expensive, but it
is also wonderful to work with, once you get into it. The stuff just
stretches and stretches. Before throwing, I ALWAYS make a slurry to
throw the porcelains with. I NEVER throw porcelain without it. This one
thing will allow you to throw easily throw porcelain and yet keep the
water content relatively low. Porcelain loves tht water but it creates
all kinds of problems. The slurry really helps your hands slide along
yet have reduced water content. Also, I always throw porcelain with a
heat gun handy. It helps to stiffen the porcelain when it starts to get
to wet.
Good luck,
John Rodgers
Birmingham, AL
Jeff Tsai on sun 23 feb 03
Hi,
When you switch clays, sometimes you need to switch the way you do things,
and porcelain is definitely held to such a rule.
Stonewares and many strong earthenwares can resist a lot of the normal wear
and tear that pieces go through during forming, drying and firing, but
porcelain is one of those picky eaters where you hate going out with them
because you have to accomodate them constantly.
Anyway, I'd suggest several things.
Wedge the clay extremely well. I've usually found my bought porcelains to be
extremely hard until I wedge, and suddenly they turn to play-doh. Wedge it
well and you'll get a more even body to start with.
I don't think heavy wetting is too big a deal, though, porcelain, by it's
nature doesn't require as much water to work with, but I think keeping things
even during throwing and drying is key. You don't want to collect excessive
amounts of water in any one area as you throw, nor do you want to let areas
dry exceedingly more quickly than other. Use plastic to facilitate more even
drying time.
Work the clay well. cracks can form from leaving parts of the clay unworked,
for example, the bottom of a pot. after pulling out the walls from the
center, work the bottom of your pot by creating a slight dimple with your
finger in the middle of the bottom and slowly spiraling outward and inward as
you spin the wheel. This just helps align the clay particles so they will
shrink together rather than pull away from one another unevenly. After you've
made several such passes, you can compress the bottom of the piece to get a
flat, unspiralled bottom.
These simple methods have helped me throw porcelain in the past when I
started out with similar problems, hope they help you as well.
-jeff Tsai
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