Phil Rogers on fri 17 oct 08
There are probably many more experienced wood firers on this list than I.
However, I have a theory based on 25 years of salt glazing and having had
the same problem in my wood kiln.
I believe that your problem is caused by heating the kiln too fast in the
early stages. Edges and handles are particularly prone to fast temperature
rise. I think what happens is that those areas begin to vitrify before all
the burning out is done and the layers become unstable. By layers I mean
either glaze on body, slip on body or just the very outer layer of the body
itself. Hence, later in the firing blisters or bubbles appear. I have seen
this happen in a salt kiln too - it is often associated with dark, rough
patches that, to me at least, indicate an oxidised area created because of
premature scorching. When you think about it, this problem of blistering can
be associated with pots that haven't been bisqued cleanly or slowly enough.
I'm not saying this is the case with yours however, trapped gases are the
root cause however they become trapped.
I think the same problem appears when a slip or a glaze contains low melt
materials like Spodumene or Amblygonite both Lithium based fluxes. Glazes
that melt early in the firing cycle seem prone. I know that certain flashing
slips that contain high %'s of Spodumene are prone to blistering on edges
and rims - sometimes over the whole pot or at least the fireside of the pot.
Here is the clue I think. It is often the fireside of a pot where the
problem exists. It stands to reason that this side of the pot will get
hotter quicker than the lea side. The rims of tall bottles high in the arch
of the kiln are in the direct path of that early, often carbon rich flame
and when I look through a very high spy in my arch I can see those rims
getting a dull red heat before the rest of the kiln.
Take the early stages very slowly. Warm up overnight with a gas burner (
just one!) and try to keep the kiln even throughout. Try firing the very
early part - upto 600C - with the dampers well in. That tends to keep the
temperature even. There's plenty of time to burn out extraneous carbon
later.
Hope that helps and I'll be interested to read others remarks as I might be
totally wrong!
Phil Rogers
I wood fire in a train kiln and am having a problem with the rims and edges
of handles blistering. Many of these blisters can be removed with my finger
nail. Not a good thing when it's functional ware. I have had this problem
with slipped, glazed and unglazed pots although less often with unglazed. My
glazed pots have only a very small amount of Malcolm Davis' Carbon Trap
Shino. Most often this happens on the top shelf. I am aware that others are
having the same problem. I suspect it may be due to the flames travelling
too quickly across the top of the pots but if any of you have an answer to
my problem it would be much appreciated.
Thanks, Maeva
Phil and Lynne Rogers,
Marston Pottery,
Lower Cefnfaes,
RHAYADER,
Powys. LD6 5LT.
phil@philrogerspottery.com
Please visit www.philrogerspottery.com
tony clennell on fri 17 oct 08
Maeva: I think Phil has nailed the problem with early off gassing
during a quick hot preheat. I used to think it was not soaking long
enuff at the end of the firing but subsequently took the pots and
refired them with a long soak and could not get the bubbles to settle
down. I had to grind the bubbles off and add glaze over but it was
always apparent and not satisfactory.
I always start slowly with the train believing in the "heat goes where
heat is" theory and wanting a even kiln.
If you put a shino glazed pot near a burner you will find these
bubbles which I think points to Phil's advice about the pot getting
hot quickly.
Bon feu!
Tony
On Fri, Oct 17, 2008 at 4:22 AM, Phil Rogers
wrote:
> There are probably many more experienced wood firers on this list than I.
> However, I have a theory based on 25 years of salt glazing and having had
> the same problem in my wood kiln.
>
> I believe that your problem is caused by heating the kiln too fast in the
> early stages. Edges and handles are particularly prone to fast temperature
> rise. I think what happens is that those areas begin to vitrify before all
> the burning out is done and the layers become unstable. By layers I mean
> either glaze on body, slip on body or just the very outer layer of the body
> itself. Hence, later in the firing blisters or bubbles appear. I have seen
> this happen in a salt kiln too - it is often associated with dark, rough
> patches that, to me at least, indicate an oxidised area created because of
> premature scorching. When you think about it, this problem of blistering can
> be associated with pots that haven't been bisqued cleanly or slowly enough.
> I'm not saying this is the case with yours however, trapped gases are the
> root cause however they become trapped.
>
> I think the same problem appears when a slip or a glaze contains low melt
> materials like Spodumene or Amblygonite both Lithium based fluxes. Glazes
> that melt early in the firing cycle seem prone. I know that certain flashing
> slips that contain high %'s of Spodumene are prone to blistering on edges
> and rims - sometimes over the whole pot or at least the fireside of the pot.
>
> Here is the clue I think. It is often the fireside of a pot where the
> problem exists. It stands to reason that this side of the pot will get
> hotter quicker than the lea side. The rims of tall bottles high in the arch
> of the kiln are in the direct path of that early, often carbon rich flame
> and when I look through a very high spy in my arch I can see those rims
> getting a dull red heat before the rest of the kiln.
>
> Take the early stages very slowly. Warm up overnight with a gas burner (
> just one!) and try to keep the kiln even throughout. Try firing the very
> early part - upto 600C - with the dampers well in. That tends to keep the
> temperature even. There's plenty of time to burn out extraneous carbon
> later.
>
> Hope that helps and I'll be interested to read others remarks as I might be
> totally wrong!
>
> Phil Rogers
>
>
>
>
>
> I wood fire in a train kiln and am having a problem with the rims and edges
> of handles blistering. Many of these blisters can be removed with my finger
> nail. Not a good thing when it's functional ware. I have had this problem
> with slipped, glazed and unglazed pots although less often with unglazed. My
> glazed pots have only a very small amount of Malcolm Davis' Carbon Trap
> Shino. Most often this happens on the top shelf. I am aware that others are
> having the same problem. I suspect it may be due to the flames travelling
> too quickly across the top of the pots but if any of you have an answer to
> my problem it would be much appreciated.
>
> Thanks, Maeva
>
>
> Phil and Lynne Rogers,
> Marston Pottery,
> Lower Cefnfaes,
> RHAYADER,
> Powys. LD6 5LT.
>
> phil@philrogerspottery.com
>
> Please visit www.philrogerspottery.com
>
--
http://sourcherrypottery.com
http://smokieclennell.blogspot.com
Luke Nealey on fri 17 oct 08
Maeva (& Phil)
I have had blistering problems in my fast fire wood kiln as well, with both
flashing slips and glazes. They occurred most often on the top shelf and
with the sides of pieces that where in/adjacent to the inlet flues. I was
also getting some black coring with this as well. I just start a fire in
one of the two boxes and go, the second box gets lit after an hour or two
when the first has a good coal bed. I have concluded that my problem was
reducing too early. I now have the damper most of the way open at the start
and all the way open when I bring the second firebox up. I get little or no
black smoke on stokes early on and both the blistering and the black coring
have dissappeared (last firing 11 touching on top shelf witness in 10 hrs).
Also: Thanks for post on flues Nils, I blocked an eighth or so of my outlet
and reduced top to bottom variation from 1.5 cones to 0.5 cones on a firing
last week.
Regards,
Luke Nealey
Rankin Co. MS
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