John Baymore on sat 10 apr 99
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(snip)
I took an order for 4 large platters (18x24) having never made anything
this large before. Stupid, huh? ................... After reading about
all the dunting problems I'm scared.
What do I need to do to assure these platters won't crack? OKOKOK, what
precautions can I take to minimize the chances of these platters
breaking and/or warping? ................... Any other
help/suggestions/precautions, please advise=21
(clip)
Tena,
Hi. Just a couple of thoughts from my personal perspective. I'm sure
you'll get lots more from smarter people than me.
You didn't mention what type of kiln you are firing in, but from the cone
and the glaze you mentioned, I'd bet you are firing in a hobby-type
electric (as opposed to a less common more industrial style electric kiln).
If that is true, one consideration I'd make is FIRING DOWN the kiln on the
cooling cycle. Most of these 2 1/2 to 3 inch thick firebrick electric
kilns are WOEFULLY underinsulated and have a high outer surface area
relative to the interior volume and thermal mass. So they cool rapidly
from heat loss thru the walls, and air infiltration/leakage from all the
spy ports and ring joints.
Differential cooling on such a large piece can set up stresses that can
cause cracking. If the rim is cooler than the intertior (sitting on the
thermal heat sink of the kiln shelf) then the rim is contracting when the
center is not. Once the glaze has =22set=22 ....frozen..... whatever you =
want
to call it......... the even cooling issue is pretty much true all through
the cooling cycle to minimize stresses on the piece.
This stuff is particularly critical at 1063F... alpha / beta quartz
inversion point. So with variations in the actual temperature in various
points in the chamber compared to the thermocouple insertion point along
with the terrible accuracy of most potters cheap type K pyrometers....
figure 1100F to 900F to be quite critical. Ditto for the alpha / beta
cristobalite issue at 439F. So make 375F to 500F another slow range.
Cone 6 bodies are not prone to huge cristobalie issues.... but there can be
some particularly for a =22dry=22 body without sufficient fluxes and a long
firing cycle.
Another consideration is the fit between body and glaze. If the glaze is
in slight compression (which is generally desireable) then the clay body is
under a little =22squeeze=22 from the glaze. In smaller forms, more closed
forms, and evenly glazes forms this is usually not an issue. On large
forms if the compression the body is under is getting toward the cusp of
breaking the body AND the cooling ALSO gets uneven, the combined stresses
can cause things to crack. Add to this glazing a large expanse on only one
side and you have potential for problems.
If you have a programmable controller this firing down is a piece of cake.
Just program the cooling cycle as part of the overall firing cycle, makeing
sure that the cone in the kiln sitter is HIGHER than the peak end point
temperature of the firing so the sitter doesn't shut off the cycle. (You'll
need to know the appropriate temp and time rates to program to peak for
your body and glazes in this case.) Just program the entire cooling cycle
as another series of ramps. If you ar doing it by hand, you'll have to
play with the switches and watch the pyrometer.
The kiln and controller doesen't care if it is controlling the kiln to go
UP or DOWN. It just knows to compare the thermocouple temp to the
programmed set point and alter the ON/OFF durations to make sure it doean't
overshoot or undershoot it by too much. So it will fire it down as easily
as up.
I'd also fire UP slower than normal. Again particularly through the A/B
quartz inversion. The bisque is pretty forgiving to internal stresses, but
why take chances? The edges of these plattes will be quite close to the
elements and the centers will be far away. The close part will recieve
more radiant energy than the far away part and hence tend to heat faster.
These pieces are much thicker than the average you make, I bet. So the
heat will take longer to be conducted through the walls themselves. They
also have a lot of thermal mass, so will heat slower than the more typical
light walled ware (thermal mass lag). Gases being evolved in the body will
take longer to get out thru the thicker section. All in all, this too
will dictate a slower UP cycle than your typical one.
As to the one side being glazed and the other not...... why not glaze the
bottom? Put a good foot on the pieces and glaze the interior of the footed
area. The only unglazed part is the foot ring itself. Or if it is
individual smaller feet.... wax only the feet. This reduces the
differential stresses completely. I do this on large slab built plates,
large thrown platters (18 inch dia.) , and large thrown bowls (24 inch
dia). Works fine.
If they are too hard to add feet to, maybe make little =22feet=22 that =
aren't
really attached. Then wax the places these feet need to be, glaze the
whole piece, and place the feet on the piece with glue (over the waxed
area) and place plate and attached feet in the kiln. The feet will support
the piece (with glaze on all areas EXCEPT where the feet are) in the kiln
for firing, and then fall off when you lift it out. These unglazed areas
become decorative elements on the bottom.
I would place the peices on some light silica sand sprinkled on the shelf.
Particularly if the bottoms are NOT footed. If you glaze the bottom of the
piece (with feet) make sure that the layer of sane is really thin and
doesn't pile up to contact the glazed area. Watch sprinkling the sand
around.... it falls on the pieces below. Sliding a footed piece around
into position on a layer of sand can cause it to pile up snagged sand and
cause it to hit the glazed area on the bottom.... be careful.
If the shelves are flat the fired pieces will have the best chance of being
flat.
So.... hope this has been of some use to you.
Best,
.....................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Tom Wirt on sun 11 apr 99
Hi
Along with the notes by John Baymore, before you even get near the kiln,
the drying stages are critically important. We do quite a few 14"-24"
Platters. And they will typical take 2 weeks from wet to the Bisque kiln.
I throw them, wire them free from the bat, and set on a shelf. As soon as
the rims are leather hard, I cover for a day to let moisture even out. Then
uncover, let dry a bit again, recover, etc.. The goal is to get the center
to about the same moisture as the rim, without letting the rim dry. As soon
as the center loses wetness (soft leather hard), re wire the base loose.
Keep the wire extremely tight in doing this.
As soon as the bottom is hard enough to support itself, flip the platter
over sandwiching a bat on top and flip. Take what was the throwing bat off.
Let her dry open to trimming stage.
Trim, then put on a shelf, rim down and loosely covered until the clay
starts to lighten up. Then you can uncover and finish off.
It's a bit of a pain, but it gets them through about 100%.
Tom Wirt
Bonnie Staffel on mon 12 apr 99
April 10, 1999
I have approached the execution of large platters in a different way. I
use oiled masonite bats, 1/2" thick. I then make a slab for the bottom
of the platter. Place it on the wheel, add a little water and spin the
wheel to compress with my fingers. I cut a perfect circle on the wheel,
then extrude a 1-1/2" coil 36" to 38" long, cut the ends on the
diagonal, and with no water compress the ends together to make a coiled
circle. Place this on the now non-wet slab on the wheel, apply to the
base as a coiled pot, then center this coil and proceed to throw a wide
rim for the platter. Before removing from the wheel, I then use water
wax and apply it to the rim both top and bottom. Take it from the wheel
and allow to become leather hard. The oiled bat does not require wiring
off. It releases by itself. The wax prevents premature drying of the
platter. I then flip over on the trimming wheel and clean up the
bottom, and depending upon the thickness of your original clab, trim a
foot. I then return it to the drying rack which is open wired where the
top and bottom can dry at the same time.
To fire, I place bisque shards under the platter in three places.
Matching the placement of these shards, I can stack several platters on
top with shards between for heat circulation. So far in many, many
years, I have not had any breakage and no warpage. I fire in a square
L&L electric kiln to Cone 10. Previously I used a Skutt hexagon kiln to
Cone 9.
Try this method. It is easy on the muscles and for us potters who are
over 70, we can still make big pots.
Bonnie Staffel bstaffel@freeway.net
Judy Weeden on mon 12 apr 99
I'd like to add my 2 cents worth to Tom Wirt's description of the drying
process necessary for drying large platters. Yes, two weeks minimum for
drying. In addition I have found that contolling the drying such that the
centre of the platter dries somewhat ahead of the rim works like majic. I
do this by cutting a round hole in the plastic about 3-4 inches in
diameter over the centre of the platter. For me this allows the platter to
dry slowly enough so that I don't have to take plastic off and on. I can
just forget about it for 2-3 weeks.
Judy Weeden
weeden@saltspring.com
Mimi Stadler on tue 13 apr 99
Hi Bonnie-
Your method of throwing large platters interests me. But what is
oiled masonite?
Mimi in NJ
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