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hot spots and other firing questions

updated wed 7 jul 99


Lyla Kaplan on thu 1 jul 99

hi all,

after numerous great firings or firings where we (Lois and I) could figure
out the problem, this time we are unsure of what went wrong and were hoping
for some insight. kiln - gas reduction downdraft, approx 40 cubic ft, well
insulated, two powerful marc ward forced air burners. problem - immature
glazes and tiny blisters.

we packed it tighter than we thought possible, it was the first time firing
in low barometric pressure, and first time to start reduction actually
before ^8 started bending, therefore reducing a bit longer than normal
(75min). we shut her down when ^10 just started a bend on bottom and top,
^11 flat bend in the middle. thought we had a good firing, but opened her
up yesterday to find almost the entire bottom didn't reach ^8, and all over
in spots there were immature glazes or tiny blisters (copper-red-blue,
pete's red w/titanium, shaner oribe, amber celedon, all very reliable
glazes). The only ting we can come up with is that we started reduction
just too damn early and didn't let the kiln go long enough for glazes to
settle, despite the cones indicating that it was time.

we would like to know:

- anyone ever heard of hot spots, and if so, what can cause them? it seems
that even where ^11 went down, glazes nearby weren't mature.
- maybe the hotspot is related to tight packing? could that trap the heat
at the front of the kiln?
- any idea how low barometric pressure can effect the firing?
- my understanding of "soak" is that by taking the kiln out of reduction
around ^10 and preventing the temp from rising, the glazes will have time
to settle and brighten up. what tends to be effective - 5 minutes or 50?

always learning, never bored,

durham, north carolina

ps. i think one of the greatest things about firing is that at some level
it is up to the kiln gods. i dig the life of the fire. i think there is a
fine balance between knowledge/prediction and letting go of control.

David Woodin on fri 2 jul 99

You mentioned a tighter load than usual, I suppose it is possible to restrict
the flow of gases if packed too tightly. You didn't mention temperatures
but 75 minutes indicates around 80 to 90 deg per hour which seems quite fast.
My best results are when the kiln slows down to 25 or 35 deg per hour when
getting to cone 9 and 10. Many people reduce all the way from body
reduction and others reduce from 2010F to cone 9 or 10 and soak in oxidation
for 20 to 30 minutes and let the temperature start falling so that you don't
overfire at the end.

Cameron on tue 6 jul 99

Hi all,

The question of problems in kilns resulting in hot spots,
reduction problems, heavily loaded kiln unevenness, etc. come up
so often I have decided to make it the topic of my next ezine. I
would have responded on the list, but the comments are, frankly,
to long for this list.

There have been a number of very good responses, but I think that
I can put it into overall perspective for you.


Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine:
THE place for total kiln and drier support