Barb & Ray Sapergia on sun 21 jul 96
Help!
We've been having a terrible time with one of our production glazes.
We've used it for several years without a problem, but after moving a
year ago we started getting major blisters. Some firings are ok, then
we'll have a terrible load even tho we fire the same schedule each time.
We thot it might be the new water (it's softer, so we wondered if it
might have more sodium), but tried the 'old' water, got the same
blisters, altho not as bad (for a load or 2).
We typically fire (propane) to ^10 tip touching, in about 11-12
hours) start reduction at ^012 at the coolest spot in the kiln. We take
an hour from ^9 to ^10, then usually leave the damper open for an hour to
quick cool before we close it. We tried closing it immediately for a
slow cool, hoping to smooth out the glaze, but still had blisters and
had lost all our great blues. The temp is within 1/2 cone throughout the
kiln, and reduction seems fairly even.
The other thing is that it isn't every pot in that glaze. On one shelf,
the pots on one side might be blistered and the other side fine, and one
shelf up or down the opposite side is affected. It seems worse when the
glaze is thin. Sometimes a re-fire helps smooth them out.
The formula is as follows:
talc 9.84
whiting 17.97
custer spar 28.28
alberta slip 12.5
kaolin 14.38
flint 17.03
add: rutile 7.34
Our 2 claybodies are Plainsman H443 (brown) and H551 (white). Could it be
the clay? We have some blisters in other glazes too, altho not as bad.
The only common ingredient in custer spar.
I hope I've given you enough info to help me. It's been a year of
seconds, losing about 20%.
TIA
Barb
in beautiful BC
Jonathan Kaplan on mon 22 jul 96
>----------------------------Original message----------------------------
>Help!
>
>We've been having a terrible time with one of our production glazes.
>We've used it for several years without a problem, but after moving a
>year ago we started getting major blisters. Some firings are ok, then
>we'll have a terrible load even tho we fire the same schedule each time.
>We thot it might be the new water (it's softer, so we wondered if it
>might have more sodium), but tried the 'old' water, got the same
>blisters, altho not as bad (for a load or 2).
>
>We typically fire (propane) to ^10 tip touching, in about 11-12
>hours) start reduction at ^012 at the coolest spot in the kiln. We take
>an hour from ^9 to ^10, then usually leave the damper open for an hour to
>quick cool before we close it. We tried closing it immediately for a
>slow cool, hoping to smooth out the glaze, but still had blisters and
>had lost all our great blues. The temp is within 1/2 cone throughout the
>kiln, and reduction seems fairly even.
>
>The other thing is that it isn't every pot in that glaze. On one shelf,
>the pots on one side might be blistered and the other side fine, and one
>shelf up or down the opposite side is affected. It seems worse when the
>glaze is thin. Sometimes a re-fire helps smooth them out.
>
>The formula is as follows:
>talc 9.84
>whiting 17.97
>custer spar 28.28
>alberta slip 12.5
>kaolin 14.38
>flint 17.03
>add: rutile 7.34
>
>Our 2 claybodies are Plainsman H443 (brown) and H551 (white). Could it be
>the clay? We have some blisters in other glazes too, altho not as bad.
>The only common ingredient in custer spar.
>
>I hope I've given you enough info to help me. It's been a year of
>seconds, losing about 20%.
>
>TIA
>Barb
>in beautiful BC
There could be many things contributing to your glaze problems.
I am not familiar with the Plainsman clays, but with just a quick glance at
the glaze, I would identify the rutile as a potential problem area, as well
as perhaps the outgassing of the carbonate from the whiting. It also could
be that you may be extending the useful firing range of the glaze at cone
10 tipping and hence cauing the glaze to boil, rather than melt completely.
If you have an oxygen probe, it might be useful to follow the numbers that
the atmosphere reading provides, especially in the oxydation range of
firing.
What kind of blisters? Custers? Big zits, or what? Are they sharp if you
rund your finger over them? Do they make a slight cracking noise when you
do the same? To me, the points to the rutile and it also could be that
there are some eutectic problems with the rutile and the Alberta slip.
These would be themain culprits.
Hope this may shed some light on a rather vexing and demoralizing problem.
Jonathan
Jonathan Kaplan
Ceramic Design GroupLtd./Production Services
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139 voice and fax
call before faxing
"Show up, pay attention, tell the truth, don't be attatched to the results!"
Karl P. Platt on mon 22 jul 96
>>We typically fire (propane) to ^10 tip touching, in about 11-12
hours) start reduction at ^012 at the coolest spot in the kiln... We tried ... a
slow cool, hoping to smooth out the glaze, but still had blisters and
had lost all our great blues. The temp is within 1/2 cone throughout the
kiln, and reduction seems fairly even.<<
The body is outgassing at high temperature. This suggests that the reduction is
started too early -- on principle I'd say it is. The fix is simple: ensure
oxidation early in the firing. Since this problem is sporadic, it would seem
that the kiln has poor control(s) on the combustion system. Upgrading these to
give more certain results would have a pay-back period of a single firing.
John Blossom Pottery on mon 22 jul 96
>----------------------------Original message----------------------------
>Help!
>Sounds to me like you might be boiling your glaze in hotter parts of your
>kiln. Try firing a half cone to a cone lower. (Talc is typically
>inconsistent from batch to batch and may be fluxing your glaze down a bit
>too much.) An alternative would be to up your silica a bit to raise the
>boiling point of the glaze. A slower and higher bisque firing might also
>help if you suspect the culprit is the clay - but I doubt that is the
>problem because you said it is only with one particular glaze.
Cathy Nelson Hartman on mon 22 jul 96
I have listened to the replies about blisters. I would suggest a
different, easily checked solution before you reinvent your kiln or buy a
lot of equipment. A common contaminant of clay is iron pyrite,FeS, which
is a compound of iron and sulphur-sulphur being volatile at the cone 10
range. That could be the source of your gassing which is leaving the
blisters/craters/ bloats. Put your glazes on another cone 10 clay body
and you should know if its your clay body causing the problem. The clay
is the most variable of the elements in the process. Cheap to try this
experiment. The test will not by itself tell you what is gassing(fool's
gold is only one candidate).
Hope it works. walter hartman
Dave Murphy on mon 22 jul 96
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From: Barb & Ray Sapergia[SMTP:sapergia@island.net]
Sent: July 21, 1996 1:53 PM
To: Multiple recipients of list CLAYART
Subject: glaze blisters
----------------------------Original message----------------------------
Help!
We've been having a terrible time with one of our production glazes.
We've used it for several years without a problem, but after moving a
year ago we started getting major blisters. Some firings are ok, then
we'll have a terrible load even tho we fire the same schedule each time.
We thot it might be the new water (it's softer, so we wondered if it
might have more sodium), but tried the 'old' water, got the same
blisters, altho not as bad (for a load or 2).
We typically fire (propane) to ^10 tip touching, in about 11-12
hours) start reduction at ^012 at the coolest spot in the kiln. We take
an hour from ^9 to ^10, then usually leave the damper open for an hour to
quick cool before we close it. We tried closing it immediately for a
slow cool, hoping to smooth out the glaze, but still had blisters and
had lost all our great blues. The temp is within 1/2 cone throughout the
kiln, and reduction seems fairly even.
The other thing is that it isn't every pot in that glaze. On one shelf,
the pots on one side might be blistered and the other side fine, and one
shelf up or down the opposite side is affected. It seems worse when the
glaze is thin. Sometimes a re-fire helps smooth them out.
The formula is as follows:
talc 9.84
whiting 17.97
custer spar 28.28
alberta slip 12.5
kaolin 14.38
flint 17.03
add: rutile 7.34
Our 2 claybodies are Plainsman H443 (brown) and H551 (white). Could it be
the clay? We have some blisters in other glazes too, altho not as bad.
The only common ingredient in custer spar.
I hope I've given you enough info to help me. It's been a year of
seconds, losing about 20%.
TIA
Barb
in beautiful BC
Have had same trouble with a celadon glaze I have been using for years. Guess
what clay I have been using it on. H443 by Plainsman. I noticed that the clay
itself has gotten lighter brown over the last year (before firing). I have
stopped using the clay until I can work it out.
Barbara
in beautiful Ont.
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Tony Hansen on thu 25 jul 96
Re: Glaze Blisters on Plainsman H443
I am pretty sure I know the cause of your blisters (glaze I mean!).
Blisters mean that large volumes of gases had to bubble up through the
glaze. H443 is a mix of natural stoneware clays mined from deposits in
Montana, Manitoba and Saskatchewan ground. It is ground to 42 mesh.
Thus, unlike most prepared bodies made from 200 mesh and finer
industrial minerals, it has a fairly open structure. This has a lot of
advantages, not the least of which is a fired color to die for. It has
some disadvantages also. Because of the coarser particle size, gas
expulsion from the body tends to be channeled to fewer sites of larger
volume, a factor that comes into play in this situation.
H443 has a moderate reduction speckle from an iron pyritic ball clay
used in the mix however I have never seen a blister in 25 years that I
could attribute to the small pyrite particles.
Here is what I would suggest:
1) Bodies of this type require a thorough bisque fire and should be
bisqued as high as possible in a kiln with good ventilation. Cone 06 is
normal, but if your glazes will work with less absorbency, fire even
higher. In fact, bisquing as high as possible is good practice with any
body and is no problem if your glazes are thixotrophic.
2) The bisque firing must be oxidizing. Tightly packed electric kilns
with no ventilation usually reduce your bisque. You can tell by noting
the different colors of ware out of the bisque kiln. Gas kilns are best
for bisque firing. If you must use electric, get a kiln vent, pack less
densely, or fire slower.
3) If you start your reduction one or two cones higher than the bisque
temperature, this period of oxidation in the glaze kiln can oxidize any
remaining potential 'blister producing' volatiles that the bisque did
not take care of. H443 will reduce just fine if you start reduction at
cone 04 (I've started as high as cone 02 with no problems).
4) Make sure that stage 1 or your glaze fire is actually oxidizing.
Watch the kiln to make sure there is plenty of oxygen present at all
times.
5) Although it is not usually necessary to soak a reduction firing, it
is good practice to fire the final approach to cone 10R a little slower
to give the glaze a change to heal and imperfections.
If your bisque practice is the source of the trouble, that explains why
some pieces are blistered and others not. The blistered ones are the
ones that had the poorest bisque.
--
-----------------------------ooO-(_)-Ooo---------------------------
Tony Hansen, Plainsman Clays, 134 UPLAND DRIVE, Medicine Hat, Alberta
T1A 3N7 Canada Phone:403-527-2826 FAX:527-7441 BBS:527-6074
email: thansen@mlc.awinc.com web: http://digitalfire.com/imc.html
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