bivaletz ginny on fri 24 nov 00
there was a post recently about the soft bricks
cracking in a new electric kiln. i meant to follow up
on this thread but was off list for awhile.
the experience i have with kilns is limited and i
was wondering if i could get interest up in a
discussion about cracked bricks in electric or gas
kilns.
is it normal to have cracks appear in the soft
brick of kiln walls?
what is the cause of the cracking and how can it
be prevented from happening?
what should one do once they appear? should they
be patched or left alone?
i've heard conflicting opinions about this subject and
was hoping to clarify some of my questions. my thanks
to anyone wishing to discuss this.
=====
ginny from orcas island, washington.
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Marni Turkel on sat 25 nov 00
Ginny asked about soft brick kiln walls cracking. I currently have 6 Skutt
kilns purchased at different times over the last 15 years. All have some
amount of cracking, not necessarily in proportion to age or number of
firings.
Some of it I would attribute to not getting the stand properly leveled
after shifting a kiln to work on it or the wiring behind it. That shows up
most in the bottom, but the walls are stressed by this also. One kiln had a
top metal jacket that wasn't tightened enough, slipped down 1/2" causing
the lid to applying uneven pressure on the wall and cause cracking near the
top of the wall.
I am always trying to get the kilns cooled off fast so I can get one load
out and the next in. The kiln lids especially show wear from that. Since I
bought bottom vents for the kilns, a lot of that is eliminated. The last
kiln I bought has always been fired with a vent and the lid shows almost no
damage after a year and a half.
But one kiln in particular purchased about 3 years ago just seems to have
low quality brick. It started cracking immediately, looks much worse than a
kiln that is now 6 years old, even though it is fired less often. Before I
put in vents, I used a block of soft brick to prop the lids open. This 3
year old kiln has worn away about 1/2" on the top of the wall were I prop
it. I fire it 2-3 times a week. This spot is more worn down than on a 15
year old kiln that has been fired an average of 4-5 times a week for the
last 9 or 10 years. The kiln with the bad brick also cools a little faster
than 2 others of the same size, it seems as if there is just more air and
less brick. It won't last as long as the others, but I still like it.
Marni
Marni Turkel
Stony Point Ceramic Design
2080 Llano Rd 1B
Santa Rosa, CA 95407
Pottery: 707-579-5567
Office: 707-579-9511
Fax: 707-579-1116
Tom Wirt/Betsy Price on sat 25 nov 00
We have a Skutt 1027 that was fired (accidentally) to about cone 14
(yes Virginia, they'll do that). Anyway, I think the operative factor
here is that yes, the bricks can and do crack. Every time they are
fired the hot face expands significantly. And on cooling shrinks.
This will be most apparent where two separate bricks meet and the
joint will slowly open on the hot face...up to a point.
Should you repair these? We never repaired the Skutt that had both
wide joints and some cracks. Everytime you heat the kiln, those
bricks re-expand and fill the cracks. If you religiously stuff them
full of grout, they'll just crack somewhere else.
Our ConeArts, which haven't been overfired, also have cracks (a few)
in the field bricks. If these were affecting the insulating value of
the kiln significantly, I'd do something about it.
All 3 used to get fired to cone 10, 2 times a week with bisque
firings in between. Now they are retired with just a couple of
bisques firings a week each.
As to the gas kiln, we've had more cracking than most...although Betsy
fires every 3 days...maybe 100 firings a year. The first iteration
used some 2" k28 softbrick laid dry and that was a disaster. After
rebuilding, I used 3" on the bottom half and 2-1/2" on the top half
all laid wet on the advice of Donavan Palmquist. This time much less
cracking, although there is still some. I suspect the biggest part of
the issue is that the 2" brick 1) denser and more brittle and 2)was
weaker because of less cross section.
On the other hand, I've seen Mel's (and other kilns) that are laid up
dry and have few or no cracks after years of use. (The first question
you have to ask is not how old is this kiln, but how many times has it
been fired).
We patch only when it seems there may be a structural issue or a
spalling issue where a chunk might fall into a pot.
There's little science here, only experience and what, for me, is
common sense. How can I worry about a kiln when, with age I've got a
few cracks here and there too.
Incidentally, the only filler I've found (having tried Greenpatch and
several others) is ITC's filler (I forget the number). The others all
spalled over time.
Tom Wirt
One thing that wasn't brought up in regard to soft brick electric kiln
floors is that I had always heard and Skutt confirmed, you should put
a full shelf in the bottom supported on 1" posts Lots of them. That's
what saved us from losing the bottom in the overfire, plus it gives
you a level bottom surface.
Subject: kiln cracking - normal?
> there was a post recently about the soft bricks
> cracking in a new electric kiln. i meant to follow up
> on this thread but was off list for awhile.
> the experience i have with kilns is limited and i
> was wondering if i could get interest up in a
> discussion about cracked bricks in electric or gas
> kilns.
> is it normal to have cracks appear in the soft
> brick of kiln walls?
> what is the cause of the cracking and how can it
> be prevented from happening?
> what should one do once they appear? should they
> be patched or left alone?
>
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