Jeff Tsai on sun 7 apr 02
Vince Wrote:
<advise against that. It does allow the bottom to move, but it insulates it
from the heat, which can cause far more serious problems. Just fire the
piece on wadding. Instead of small round wads like we use in salt kilns,
for large pots I use a starburst pattern of 3/8-inch coils.>>
Vince,
I was wondering. I use a small bed of sand and grog for large flat pieces all
the time, with few cracks occuring (mostly student work). I don't lay it too
heavy, actually, it tends to only be a single layer of grog and sand at most.
I figure this doesn't insulate but probably still provides movement...does
this sound like a problem in any way. (well, it works anyway, was just
wondering).
-jeff
Bill Karaffa on sun 7 apr 02
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Jeff Tsai
Sent: Sunday, April 07, 2002 5:29 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: firing on a bed of sand
Vince Wrote:
<strongly
advise against that.
On large pieces I do use a little silica sand, they tend to act as small
ball bearings. However to apply the sand I only press the waxed footring
into a VERY SPARSE layer of sand spread out on a kiln shelf which is outside
of the kiln. If the piece is a large platter that will have to span 2 kiln
shelves I will wipe the sand off the bottom of the piece where the shelf gap
will be. Nothing I hate more than opening a kiln to find little pieces of
sand have worked their way down to land on some plates or bowls lower in the
kiln as often will happen when sand or grog is just sprinkled on a shelf.
Bill Karaffa
Firemouth Pottery and Gallery
http://fp1.centurytel.net/karaffa
Jim Bob Salazar on sun 7 apr 02
i am teaching a class on extruded ceramics. we are making large baking
dishes by using a sidewall extrusion with a slab bottom. when i load the
highfire kiln i am putting down silica sand to set to work on to keep it
level and to allow for movement. i don't but a "bed" of sand down but i
pour it on the shelf in the general shape of the dish. so far we have not
lost a single piece.
jim bob
vince pitelka on sun 7 apr 02
> I was wondering. I use a small bed of sand and grog for large flat pieces
all
> the time, with few cracks occuring (mostly student work). I don't lay it
too
> heavy, actually, it tends to only be a single layer of grog and sand at
most.
> I figure this doesn't insulate but probably still provides movement...does
> this sound like a problem in any way. (well, it works anyway, was just
> wondering).
Jeff -
It probably has a lot to do with the claybody. Some are more sensitive than
others.
As a general rule, I avoid using sand or grog under flat things. Any large
flat object is at risk unless it can be heated and cooled evenly overall,
and if it is flat against the shelf, the edges will tend to heat up before
the center, and this can of course result in cracking, usually from the
outer rim to the center. For large vases or jars, and for very large
platters and bowls, I use the sunburst pattern of coils as mentioned before.
Elevating the piece off the shelf allows gases to circulate evenly beneath
the piece. For large flat slab forms I usually support the piece on a
series of parallel coils (3/8" dia.) spaced three or four inches apart.
This allows great circulation beneath the piece, and allows it to move
easily as it shrinks.
As someone else suggested, for VERY large work, it is an excellent idea to
make a thick slab of the same claybody as the work, and fire the piece on
that. Then the work and the slab shrink together.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Ron Roy on mon 8 apr 02
The probelm is not in the heating but in the cooling - the edge cools
before the inside - goes through the quartz inversion - tries to get
smaller - can't and the typical quartz dunting crack apears - open at that
point - as the inside catches up and goes through the quartz inversion
(gets smaller on cooling) the crack closes.
In other words - If the crack happens on the way down the edges of the
crack will fit perfectly together.
If it happened on the way up the crack would remain a bit open and usually
a bit offset because of the clay starting to sinter (melt) a bit.
If the words "cool down" are substituted for "heat up in the following
statement then I agree.
RR
> Any large
>flat object is at risk unless it can be heated and cooled evenly overall,
>and if it is flat against the shelf, the edges will tend to heat up before
>the center, and this can of course result in cracking, usually from the
>outer rim to the center.
Ron Roy
RR #4
15084 Little Lake Road
Brighton
Ontario
Canada - K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Steve Mills on mon 8 apr 02
I have a box of 2 inch squares of ceramic fibre which I use to put under
large pieces, or between stacks of bowls in threes in the bottom of each
bowl, up to 15 high in a biscuit kiln. I've had the same bits of fibre
for a lot of years (lost count), using them time and time again. with
some of the really big planters I used to make I could stack them 6 high
and never had a crack or a break. The clay I was using then used to give
me a lot of problems gripping the shelves and then splitting when it
shrunk.
--
Steve Mills
Bath
UK
cookie davis on tue 9 apr 02
here's my 2 cents, for what it's worth:
we fired pieces at my undergrad school with sand on the shelf if we were
worried that the glaze would run off. helped it to not stick to the shelf.
thanks for letting me put my input,
cookie
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