kruzewski on wed 19 sep 01
Dear Louise,
I don't know how you would stop your existing bisque ware from bloating, but a
simple way to find out whether the problem is the clay or you (sounds like the
clay).
I was at a Nic Collins demonstration day this weekend. He is a well known English
potter who uses locally dug clay, as dug - lumps, stones and all, and fires in an
Anogama kiln.
He got to this point in his career partly because he had an order of clay that
seemed a bit "different". Threw it, fired it - it bloated like yours, he got lime
popping too. He got on to the supplier who said it was him - something he was doing
- their clay hadn't changed, it was OK etc.
He fired several kilns full - trying to find the solution to what he had supposedly
been doing wrong, lost the lot. Then had a brainwave. He got some of the old, good
clay - recycled probably - and made a few things then fired them right alongside
pots made from the new clay. The old clay was fine, the new clay still bloated.
He never got the clay replaced, but he knew where the problem lay and didn't waste
time and firings trying to figure out what he was doing wrong.
He said that inadvertently the clay company did him a favour because he started
looking around for an alternative supply, began making his own and then used this
locally dug clay which changed his work entirely.
Jacqui, North Wales
>
LJTCW@AOL.COM on sat 22 sep 01
I am having a problem with my clay body, it is bloating in the glaze firing.
The clay as it comes from the bag seems a bit different from the previous
batches as it seems to " tear" instead of just break apart when I am weighing
it out. There are visible pieces of white material ( fire clay?)when
trimming the bottoms.
I bisque at bar 05. I have an electric kiln with 3 switches and use all at
low for 2 hours, medium for 2 and then turn it up. I have a kilnvent which
draws air from the top to the exhaust at the bottom. I do not crack the kiln
open at the beginning.
My clay body is a brown color( I do not want to say which supplier or kind
for obvious resons). I know it uses Hawthorn fire clay and Gold art. The
rest is a mystery to me.
I have alot more pieces of bisque ware to glaze. Does anyone have a
suggestion to avoid the bloat on these pots?
Louise
Louise Jenks
Turtle Clayworks
Cincinnati OH USA
Cindy Strnad on sat 22 sep 01
Hi, Louise.
I don't know why you say you don't want to mention
the manufacturer of the clay you're using. You'd
mention the name if it was good clay, wouldn't
you?
I suspect that this particular batch of clay is
"short", in that it hasn't been wetted as
thoroughly as the clay you received earlier. That
would account for the "tearing", and will improve
with aging. The little white specks sound like
grog. Grog's okay, as long as it doesn't bother
you. It helps the clay to shrink less and to be
more resistant to cracks. Too much of it can also
"shorten" the clay.
It's also possible (and likely) that the
manufacturer has changed the batch recipe. They do
that from time to time (without telling,
unfortunately) for various reasons. A material may
no longer be available or economical, for example.
I'm assuming this clay didn't bloat for you during
your normal glaze firing cycle previously. A
couple of things to check:
Is your kiln firing normally? Are you
double-checking the firing with free-standing
physical cones? Sometimes a kiln sitter will need
calibrating, or a thermocouple will wear out, or a
computer control will need calibrating.
If your elements are getting old, firing could
take a longer time, therefore if you're firing to
a specific temperature (set on a computer
controller), it may take you longer to get there.
A longer firing time will produce more heat work.
You can overfire your kiln this way without
exceeding the temperature you were aiming for.
Did you order precisely the same clay as you
had before? Was the correct clay shipped to you?
If none of these things turns out to be your
problem, ask your supplier if the recipe has been
changed and tell him about the bloating. Chances
are he wants his clay to be satisfactory, so he
should work with you. Of course, folks aren't
always reasonable, so you never know what the
reaction will be until you talk to him.
The clay I use bloats at a full ^6, despite
the fact that it's supposed to be a ^6 clay. I
just fire it slightly below ^6, as this works fine
for me with my glazes. Clay isn't always exactly
what the box claims it to be.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Ron Roy on sun 23 sep 01
Hi Louise,
Could be you have fired your bisque too fast and the carbon has reduced the
iron in the body. Don't turn your switches on high till you are over 900C.
I know that will result in a long firing but it will tell you if the bisque
firing is part of the problem.
Carbon fluctuations in our raw materials - especially clays is quite normal
- and it can result in bloating if kiln firings don't take into account
worst case scenarios. The problem is most common in iron bearing clay
bodies - the more iron the more likely it is to happen.
Your clay may be a mistake - best to call your supplier and ask if anyone
else is having a problem - it would be helpful if they are testing their
batches - you and they will know for sure if the clay is not mixed right.
Let us know how your supplier responds - we have had examples of suppliers
being more reasonable when they know they are dealing with a ClayArter.
RR
>I am having a problem with my clay body, it is bloating in the glaze firing.
>The clay as it comes from the bag seems a bit different from the previous
>batches as it seems to " tear" instead of just break apart when I am weighing
>it out. There are visible pieces of white material ( fire clay?)when
>trimming the bottoms.
>
>I bisque at bar 05. I have an electric kiln with 3 switches and use all at
>low for 2 hours, medium for 2 and then turn it up. I have a kilnvent which
>draws air from the top to the exhaust at the bottom. I do not crack the kiln
>open at the beginning.
>My clay body is a brown color( I do not want to say which supplier or kind
>for obvious resons). I know it uses Hawthorn fire clay and Gold art. The
>rest is a mystery to me.
>
>I have alot more pieces of bisque ware to glaze. Does anyone have a
>suggestion to avoid the bloat on these pots?
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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