jonpjon@ibm.net on wed 24 jul 96
Hi,
Now that typhoon season has set in again and the humidity pushes
up to 100% for days at a time I am faced with the problem of
getting my pots to dry. Rainy season here in the Philippines runs
from now all the way to December, which unfortunately for me is
always the busiest time of year.
When the humidity gets this high it's virtually impossible to dry
things out completely, even if the pots look dry they still
feel a little cold if pressed against the cheek, meaning that
there is probably a slight water content. The obvious solution
is PREHEATING, but in spite of all precautions and remedies I
find bisque firing in these conditions to be a risky business.
In dry season it's not all that unusual to unload my bisque and
find only 1 or 2 cracked pots out of the 200 or so I usually load,
sometimes I even find no cracks at all (happy day). But once the
typhoons and monsoon rains that are born somewhere around Guam and
Midway and head up through Luzon on their way to China and Japan
begin, I sometimes find that 10%, 20% or even 30% have cracked.
I have made a couple of observations about this and I'm wondering
if anybody else out there, working in similar conditions, would
agree. Even if pots are artificially dried, say in a dry box or
by preheating before the bisque, I think that in extreme humidity,
if they are allowed to contact the ambient air they may reabsorb
enough water to cause an expansion crack, like when you dip a dry
pot in a bucket of water. I have noticed, for example, that if I
leave the top 2-3 rows of bricks off the door during preheating I
sometimes find cracked pieces on the top shelf with the cracks
facing the door. I seem to have better luck when I preheat with
only the top peep open. I'm also wondering if it's possible for
enough moisture to enter the kiln through the gas burners at the
early stages of firing to cause cracking problems.
Any advice or ideas would be appreciated. Thanks
Jon Pettyjohn Manila jonpjon@ibm.net
Bob Pulley on thu 25 jul 96
John's problem reminds me of one of my own that is probably related. I
have an outdoor, but well roofed, kiln. I do large,3'-4', handbuilt
stoneware sculpture. I have found that if I bis fire in a cold kiln I
always get little spider web cracks near the base of the sculptures. These
become more evident in the glaze.
My fix has been to always pair a bis with a glaze first. I unload the
kiln while it is pretty hot and load it right back up with a bis and brick
the door. The hard brick will hold enough heat overnight that it works
whether I unload in the evening or the morning.
Robert Pulley
pulley@mail.cpbx.net
http://www.cpbx.net/PBXonline/Arts/listings/bpulley/pul.html
Robert Pulley
pulley@mail.cpbx.net
John Blossom Pottery on sat 27 jul 96
Slow even heating should dry pots adequately no matter what the outside
humidity is, but try to get your thickest part of your pots off the shelves
somehow. I would bisque them upside down, or slip a shard under one end of
the pot so gasses can circulate under the pot.
I have the opposite climatic conditions here where the average humidity is
about 16%. I dry everything in the sun with no cracks ever. Most of my
pots are thrown in the morning and into the bisque by afternoon. I have
come to learn that cracks generally occur when the middle of a piece dries
last. Instead of fighting the sun, I use it: I dry the middle first by
turning my pots upside down outside as soon as possible and using plastic
if necessary to delay the drying of the rim and the outside of the trimmed
foot). If you follow the rule of making sure that drying always starts in
the middle of your pieces and moves evenly outward, you will avoid cracks -
no matter how fast or by what method your pots are dried. This also is
true when drying in a kiln - hence my suggestion to get the thick middle
part of your pots into the circulating gasses by whatever method you can.
By the way, this rule works with drying tiles as well. I sneak Saran Wrap
under and over all four edges of my tile exposing only the center to the
air (or the sun). Tiles dry flat even if I've beaten them up and bent them
like crazy in the forming process.
John Blossom
LINHARES on mon 29 jul 96
Hi Jon
I have a studio in the basement of a house with a sandstone foundation
which seems to be able to hold moisture for ever. I don't know what
the actual humidity is but I know it's high. Sometimes when I'm in a
hurry to get some pots bisque fired I put them in my electric kiln with
the bottoms are still dark. The day before the firing begins I turn it
on low for 45 miniutes and then let it sit overnight completly closed
except for the top peep hole. The next morning I do the same thing
again and let it sit until about 7 in the evening when I start the
firing. I haven't lost a pot yet( knock on wood ) using this method.
It might be adaptable to your kiln. Good luck! Keep us posted on your
progress.
Paul in Ohio where I'm getting a new kiln today
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