Tony Ferguson on thu 16 oct 08
Is this ash scale?
Tony Ferguson
Maeva Collins wrote: 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
Take Care,
Tony Ferguson
Artist...Clay, Web, Photo, Video
...where the sky meets the lake...
http://www.tonyferguson.net
Maeva Collins on thu 16 oct 08
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
jonathan byler on fri 17 oct 08
maybe you are over reducing?
jon byler
3-D Building Coordinator
Art Department
Auburn University, AL 36849
On Oct 16, 2008, at 8:12 PM, Maeva Collins wrote:
> 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
Dave Pike on fri 17 oct 08
Maeva Collins wrote:
> 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
>
>
>
Hello Maeva,
I have a blistering problem if I have any part of the ware extending out
beyond the footprint of the shelves. That is to say if one were to look
down from the top at the line the stack of kiln shelves makes and see
any part of ware, that part would blister. That may be unclear, if so I
am apologize. I take care of the problem by making sure all parts of the
work is completely shielded by the shelf.
Dave
http://togeii.wordpress.com/
Maeva Collins on fri 17 oct 08
Thank you all for your suggestions. I will, of course, consider them all
and am happy to have more. Just to add to the info. you already have, I
will add a little more detail and answer some of your questions. I think it
is more than ash scale although that can be a problem as well; seems the ash
does not melt as well as we would like. It is almost as though the glaze
and slip have lifted off into a blister although it does happen with the raw
clay so in that case I guess it is the melted ash that is lifting up
Re. the fast heating, what is too fast? We fired this past weekend and lit
the fire at 3:30 p.m. and at 3:00 a.m. the temperature was 620 degrees C.
What you think the temperature should be after 12 hours or 24 would be
helpful. The firing concluded after 32 hours. As far as the bisque, I
learned the hard way when I electric fired that a slow bisque got rid of
pinholing and blistering, therefore, I always bisque (computer kiln) at 40 %
slow with a 30 second soak. I do no reduction during the firing except a
light one in the cooling. My glaze has no Spodumene and the slip has
27.67%.
The pots that were blistered included the whole of 2 top shelves except, and
maybe this is the best clue, eight pots of another potters who had used
terra sigillatta instead of a glaze or slip.
Thanks again, Maeva
----- Original Message -----
From: "Tony Ferguson"
To:
Sent: Thursday, October 16, 2008 8:00 PM
Subject: Re: wood firing problem
> Is this ash scale?
>
> Tony Ferguson
>
> Maeva Collins wrote: 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
>
>
>
> Take Care,
>
>
>
> Tony Ferguson
> Artist...Clay, Web, Photo, Video
>
> ...where the sky meets the lake...
>
> http://www.tonyferguson.net
Lee Love on sat 18 oct 08
On Sat, Oct 18, 2008 at 1:07 PM, Bill Merrill wrote:
> saturated with the soda ash solution, that could cause blistering
> especially when the ash goes over the lip that is mostly the soda
> solution. Also try thickening the lips if appropriate to the form of
> your pots.
The clay body has an effect too. In the past, I have used a soda ash
dip. Always works well on a light clay, but if it has a lot of iron,
you sometimes get a burnt look to the surface.
--
Lee Love in Minneapolis
http://heartclay.blogspot.com/
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Bill Merrill on sat 18 oct 08
The blistering maybe caused when the glaze is not thick enough. When the
lops are thin they don't get a full coating of glaze. Malcombs Shino
glazes usually has a lot of soda ash in them and if the lip is getting
saturated with the soda ash solution, that could cause blistering
especially when the ash goes over the lip that is mostly the soda
solution. Also try thickening the lips if appropriate to the form of
your pots.
Bill Merrill
bill@pcadmin.ctc.edu
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Tony
Ferguson
Sent: Thursday, October 16, 2008 8:00 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: wood firing problem
Is this ash scale?
Tony Ferguson
Maeva Collins wrote: 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
Take Care,
Tony Ferguson
Artist...Clay, Web, Photo, Video
...where the sky meets the lake...
http://www.tonyferguson.net
Phil Rogers on sun 19 oct 08
I think that Luke Nealey's post is very interesting and kind of backs up
what I was trying to say. If the early stages of the firing - up to 600C
let's say - are done in a medium to strong reducing atmosphere then it is
hardly the conditions to encourage the clean burning out of gaseous material
in the clay. Couple this with the propensity for edges and handles to get
hot quite quickly and the resulting vitrification and therefore sealing of
the pots surface and you have a prefect recipe for blistering, peeling and
bloating.
In my last firing I had some peeling of an otherwise totally friendly,
benign white slip that I have used, trouble free, for twenty years. I put
this down to the rapid escape of steam dislodging the interface between slip
and body. Like Luke, it happened only on the top shelf at the back of the
kiln. However, I do think that it is the top shelves, that warm quickly at
first even though, ironically, by the end of the firing this particular
shelf remains the coolest spot in the kiln. This, to me at least, is proof
that rapid temperature rise in the early stages is to be avoided and in my
next firing I will be taking Luke's advice re the reduction atmosphere as
the kiln begins to climb. Wood firing is a balance. Too oxidising at the
beginning with the dampers out will encourage a hot flame through the lower
part of the kiln thus creating a possible new problem.
This has been a particularly interesting and, for me at least, very useful
thread. Thank you Maeva for raising the issue and to Tony for your
contribution. I hope anyone else with observations will write their
findings.
Phil Rogers
Phil and Lynne Rogers,
Marston Pottery,
Lower Cefnfaes,
RHAYADER,
Powys. LD6 5LT.
phil@philrogerspottery.com
Please visit www.philrogerspottery.com
Maeva Collins on tue 21 oct 08
Phil thanks for your second post. I like your theory regarding the top
shelves becoming too hot in the beginning as particularly with the train
kiln the flames are fairly direct. How slow would you suggest I go; what
would be a more approprate rise of temperature say for the first twelve
hours?
I'm very curious about the few pots that belonged to another potter that
were scattered among mine but were very smooth and shinny; they were sprayed
with terra sigillatta. Why did they avoid the blistering? Her clay body is
different, less iron.
Not so sure early reduction is the problem as there is only a small amount
of smoke when stoking but if a slow cool doesn't work that will be something
I'd want to consider as well. Thanks for your help.
Maeva
----- Original Message -----
From: "Phil Rogers"
To:
Sent: Sunday, October 19, 2008 2:39 PM
Subject: Re: Wood firing problem
>I think that Luke Nealey's post is very interesting and kind of backs up
> what I was trying to say. If the early stages of the firing - up to 600C
> let's say - are done in a medium to strong reducing atmosphere then it is
> hardly the conditions to encourage the clean burning out of gaseous
> material
> in the clay. Couple this with the propensity for edges and handles to get
> hot quite quickly and the resulting vitrification and therefore sealing of
> the pots surface and you have a prefect recipe for blistering, peeling and
> bloating.
>
> In my last firing I had some peeling of an otherwise totally friendly,
> benign white slip that I have used, trouble free, for twenty years. I put
> this down to the rapid escape of steam dislodging the interface between
> slip
> and body. Like Luke, it happened only on the top shelf at the back of the
> kiln. However, I do think that it is the top shelves, that warm quickly at
> first even though, ironically, by the end of the firing this particular
> shelf remains the coolest spot in the kiln. This, to me at least, is proof
> that rapid temperature rise in the early stages is to be avoided and in my
> next firing I will be taking Luke's advice re the reduction atmosphere as
> the kiln begins to climb. Wood firing is a balance. Too oxidising at the
> beginning with the dampers out will encourage a hot flame through the
> lower
> part of the kiln thus creating a possible new problem.
>
> This has been a particularly interesting and, for me at least, very useful
> thread. Thank you Maeva for raising the issue and to Tony for your
> contribution. I hope anyone else with observations will write their
> findings.
>
>
> Phil Rogers
>
> Phil and Lynne Rogers,
> Marston Pottery,
> Lower Cefnfaes,
> RHAYADER,
> Powys. LD6 5LT.
>
> phil@philrogerspottery.com
>
> Please visit www.philrogerspottery.com
Paul Herman on wed 22 oct 08
Hi Tony C,
Would explain how smoke could be the enemy of reduction?
And I think you mean that smoke is just plain elemental carbon (C),
not carbon dioxide (CO2), right?
Carbon monoxide (colorless CO) will reduce the pots and glazes I
believe.
Best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Oct 22, 2008, at 9:17 AM, tony clennell wrote:
> Maeva wrote:
>> Not so sure early reduction is the problem as there is only a
>> small amount
>> of smoke when stoking but if a slow cool doesn't work that will be
>> something
>> I'd want to consider as well. Thanks for your help.
>>
>
> Maeva: You can still have heavy reduction without smoke. In fact smoke
> could be the enemy of reduction. Generally reduction is considered
> to be the result of starving the fuel of oxygen during the firing
> creating carbon monoxide and hydrocarbon gases. I understood that
> smoke (carbon dioxide) has a negative role in the creation of
> reduction.
> I can have a smokeless train firing and still have heavy reduction. I
> find using hard woods give me a smokeless firing. If I use pine or
> cedar look out for the fire dept.- there is a lots of smoke.
> my 2 cents,
> tony
Lee Love on wed 22 oct 08
On Tue, Oct 21, 2008 at 11:24 PM, Maeva Collins wrote:
> I'm very curious about the few pots that belonged to another potter that
> were scattered among mine but were very smooth and shinny; they were sprayed
> with terra sigillatta. Why did they avoid the blistering? Her clay body is
> different, less iron.
>
Iron will contribute to blistering.
--
Lee Love in Minneapolis
http://togeika.multiply.com/journal
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
tony clennell on wed 22 oct 08
Maeva wrote:
> Not so sure early reduction is the problem as there is only a small amount
> of smoke when stoking but if a slow cool doesn't work that will be something
> I'd want to consider as well. Thanks for your help.
>
Maeva: You can still have heavy reduction without smoke. In fact smoke
could be the enemy of reduction. Generally reduction is considered
to be the result of starving the fuel of oxygen during the firing
creating carbon monoxide and hydrocarbon gases. I understood that
smoke (carbon dioxide) has a negative role in the creation of
reduction.
I can have a smokeless train firing and still have heavy reduction. I
find using hard woods give me a smokeless firing. If I use pine or
cedar look out for the fire dept.- there is a lots of smoke.
my 2 cents,
tony
jonathan byler on wed 22 oct 08
smoke results from a cold fire starved of oxygen. CO2 and not much
CO indicates complete combustion. smoke is very definitely due to
incomplete combustion and is an indicator of a reduction atmosphere
in the kiln. Smoke is unburned particulate and ash, which does not =
CO2. Lack of smoke does not indicate that there is no reduction, but
it does mean you are wasting a lot less fuel as unburned particulate
polluting the atmosphere.
jon byler
3-D Building Coordinator
Art Department
Auburn University, AL 36849
On Oct 22, 2008, at 11:17 AM, tony clennell wrote:
> Maeva wrote:
>> Not so sure early reduction is the problem as there is only a
>> small amount
>> of smoke when stoking but if a slow cool doesn't work that will be
>> something
>> I'd want to consider as well. Thanks for your help.
>>
>
> Maeva: You can still have heavy reduction without smoke. In fact smoke
> could be the enemy of reduction. Generally reduction is considered
> to be the result of starving the fuel of oxygen during the firing
> creating carbon monoxide and hydrocarbon gases. I understood that
> smoke (carbon dioxide) has a negative role in the creation of
> reduction.
> I can have a smokeless train firing and still have heavy reduction. I
> find using hard woods give me a smokeless firing. If I use pine or
> cedar look out for the fire dept.- there is a lots of smoke.
> my 2 cents,
> tony
Phil Rogers on thu 23 oct 08
Maeva,
I think that your temperature rise is probably OK. I usually reach 600C in
approx 6 hours. That isn't a fast temperature rise. The problem is, in my
estimation anyway, that in a wood firing, because of the very nature of the
flame and the way a wood kiln draws it through, heat is channelled to very
specific areas especially early in the firing when there is little back
pressure to force it into those areas away from the obvious route. Hence,
things like bottle lips, jug handles etc. that protrude from the main
stacking area either at the top or at the sides get too hot too quick. I
don't think it is a case of slowing down more than you already are - to
spend 12 hours up to Cone 09 would be adequate if a little long as it is
only after Cone 04 that one begins to see the real benefits of wood firing
as the clay begins to become pyroplastic. I tend to think that 12 hours up
to the point of beginning reduction proper would be adequate. I put the kiln
into reduction at 09.
My feeling is to warm up for a longer period with a gas torch perhaps. That
would eradicate steam expulsion problems. Then, take the fire slowly but
having made sure that, as much as possible, bits of pots don't protrude
beyond the main stack. That's difficult at the top I know. That won't cure
the problem completely ( if at all!) Think of flame, vapour and gas as water
flowing through a stream. It goes around rocks and stones in exactly the
same way - always. That's how it is in a kiln too. Flame will always find
the quickest and easiest way out.
Regarding the Terra sigillatta pieces. There was no glaze involved. That may
be one reason. As soon as a glaze sinters it seals the body off from the
effects of reduction. It also seals in any gaseous material that should have
been burnt out. So, later in the firing when the body or the glaze is soft
enough those gases try to escape - can't, but form bubbles or bloats
instead. The unglazed clay has plenty of opportunity to burn out because
there is nothing to prevent it well into the firing. I don't think that iron
in the body is a factor. Iron in reduction is a flux but I'm not sure that
that has anything to do with the problem we are dealing with. I have one
body that is VERY iron rich. I use it for a Kohiki effect. That particular
body did experience some slip peeling and blistering on the top shelf at the
back. HOWEVER, the same body, same slip, same glaze was perfectly OK on the
next shelf down. It is the extremities of the kiln where the problem seems
at its greatest.
Tony is right. Lack of smoke in the early stages is not a true indication of
reduction or lack of it. It is difficult to fire a wood kiln in anything
other than reduction particularly after a stoke. There will be carbon in
plenty floating through your kiln. For the first few hundred degrees it
won't matter but as tips of pots, handles, spouts etc. get hot it is then
that the problem begins to lay itself down and manifests itself later as
blisters. I find that those areas are prone to a grey discolouration as well
as roughness and, sometimes, blistering. Try to burn the fire as clean as
possible early on. Don't have the dampers fully out though. In my experience
this encourages a hot flame through the bottom of the kiln and you simply
move the problem from one area to another. Let's face it - it's wood and
there are always going to be areas that don't work as well as others. That's
the nature of it and part of the reason why we are attracted to it. I looked
at your website and you make fine pots that appear very well fired. Try
these measures and see what happens.
Just on another point briefly. Are you aware of Dave Conrad's website -
http://www.woodfiredceramics.net/ It really is an excellent site and I
would recommend you visit, register and take advantage of it.
Phil Rogers
Phil thanks for your second post. I like your theory regarding the top
shelves becoming too hot in the beginning as particularly with the train
kiln the flames are fairly direct. How slow would you suggest I go; what
would be a more approprate rise of temperature say for the first twelve
hours?
I'm very curious about the few pots that belonged to another potter that
were scattered among mine but were very smooth and shinny; they were sprayed
with terra sigillatta. Why did they avoid the blistering? Her clay body is
different, less iron.
Not so sure early reduction is the problem as there is only a small amount
of smoke when stoking but if a slow cool doesn't work that will be something
I'd want to consider as well. Thanks for your help.
Maeva
----- Original Message -----
From: "Phil Rogers"
To:
Sent: Sunday, October 19, 2008 2:39 PM
Subject: Re: Wood firing problem
>I think that Luke Nealey's post is very interesting and kind of backs
>up what I was trying to say. If the early stages of the firing - up to
>600C let's say - are done in a medium to strong reducing atmosphere
>then it is hardly the conditions to encourage the clean burning out of
>gaseous material in the clay. Couple this with the propensity for
>edges and handles to get hot quite quickly and the resulting
>vitrification and therefore sealing of the pots surface and you have a
>prefect recipe for blistering, peeling and bloating.
>
> In my last firing I had some peeling of an otherwise totally friendly,
> benign white slip that I have used, trouble free, for twenty years. I
> put this down to the rapid escape of steam dislodging the interface
> between slip and body. Like Luke, it happened only on the top shelf at
> the back of the kiln. However, I do think that it is the top shelves,
> that warm quickly at first even though, ironically, by the end of the
> firing this particular shelf remains the coolest spot in the kiln.
> This, to me at least, is proof that rapid temperature rise in the
> early stages is to be avoided and in my next firing I will be taking
> Luke's advice re the reduction atmosphere as the kiln begins to climb.
> Wood firing is a balance. Too oxidising at the beginning with the
> dampers out will encourage a hot flame through the lower part of the
> kiln thus creating a possible new problem.
>
> This has been a particularly interesting and, for me at least, very
> useful thread. Thank you Maeva for raising the issue and to Tony for
> your contribution. I hope anyone else with observations will write
> their findings.
>
>
> Phil Rogers
>
> Phil and Lynne Rogers,
> Marston Pottery,
> Lower Cefnfaes,
> RHAYADER,
> Powys. LD6 5LT.
>
> phil@philrogerspottery.com
>
> Please visit
www.philrogerspottery.com
> < http://www.philrogerspottery.com/>
Luke Nealey on thu 23 oct 08
Hey all,
Combustion of wood is pretty complicated and as Jonathan indictated has a
lot to do with the temperature of combustion. I found a good review article
out of the forest products lab on line:
http://www.fpl.fs.fed.us/documnts/fplr/fplr2136.pdf . Unlike like a pure
small hydrocarbon molecule (natural gas(mostly methane) or propane) that
immediately can combust(react with oxygen in the air) the primary
thing(there is lots of other stuff) burning in wood is cellulose a long,
long polymer of thousands and tens of thousands of glucose units. Anyway,
the way wood burns (simplified, see article for better analysis) is that it
first pyrolyzes, i.e. heat starts to break down, react, however you want to
think about it, the large molecules/polymers in the wood into combustable
compounds. This soup of pyrolysis products then burns (or doesn't) depending
on amount of O2, temperature etc. Bottom-line, if it's hot enough, mixed
well enough, and there is enough O2 available everything but inorganics
should make it to CO2 and water before it leaves the stack. As Tony
mentions, softwoods have a lot morre resinous matl that pyrolyze into tars
that don't burn well, these will go out stack as smoke more readily than
other things that combust better. Further bottom-line, in our kilns CO is
the reducing agent, it will be produced if there is not enough O2 to combust
our fuel completely. Although we may have a reducing atmosphere with no
smoke, I think all smoky atmospheres would be reducing(there is stuff left
to burn). One has to remember as well that you can't put a pot in a box of
CO on your desk and think it will reduce, activation energies, pressure etc.
all are important here.
Regards,
Luke Nealey
Rankin Co. MS
Steve Slatin on thu 23 oct 08
Paul --
I suspect that the idea is C + CO2 = 2 (CO). Raw
carbon soot and CO2 won't reduce. CO will. I don't
fire in reduction and can't give experiential
information, but the chemistry seems apparent.
Steve S
--- On Wed, 10/22/08, Paul Herman wrote:
> From: Paul Herman
> Subject: Re: Wood firing problem
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Wednesday, October 22, 2008, 4:21 PM
> Hi Tony C,
>
> Would explain how smoke could be the enemy of reduction?
>
> And I think you mean that smoke is just plain elemental
> carbon (C),
> not carbon dioxide (CO2), right?
>
> Carbon monoxide (colorless CO) will reduce the pots and
> glazes I
> believe.
>
> Best wishes,
>
> Paul Herman
>
> Great Basin Pottery
> Doyle, California US
> www.greatbasinpottery.com/
>
>
>
>
> On Oct 22, 2008, at 9:17 AM, tony clennell wrote:
>
> > Maeva wrote:
> >> Not so sure early reduction is the problem as
> there is only a
> >> small amount
> >> of smoke when stoking but if a slow cool
> doesn't work that will be
> >> something
> >> I'd want to consider as well. Thanks for your
> help.
> >>
> >
> > Maeva: You can still have heavy reduction without
> smoke. In fact smoke
> > could be the enemy of reduction. Generally reduction
> is considered
> > to be the result of starving the fuel of oxygen during
> the firing
> > creating carbon monoxide and hydrocarbon gases. I
> understood that
> > smoke (carbon dioxide) has a negative role in the
> creation of
> > reduction.
> > I can have a smokeless train firing and still have
> heavy reduction. I
> > find using hard woods give me a smokeless firing. If I
> use pine or
> > cedar look out for the fire dept.- there is a lots of
> smoke.
> > my 2 cents,
> > tony
Paul Haigh on fri 24 oct 08
As for pine/softwood vs hardwood, the pine burns much faster (pounds of wood per unit time, not just volume) and so it consumes /scavenges more oxygen- all other things being the same. The leftover wood after the oxygen has been accounted for is more likely to become smoke/particles. That's why new EPA woodstoves have secondary air introduced through tubes at the top of the firebox- reburning smoke (I burn pine indoors with no issue of creosote). Likewise, my kiln collection box (as with a sutema I guess) with passive dampers pulled seems to do a good job of reburning smoke.
The presence of smoke means some incomplete combustion. Unburned smoke particles, however, are very reactive with any excess oxygen in the air... maybe less so with glazes and clays that the particles are too big to penetrate like a gas (hydrogen from water reduction, or CO and simple hydrocarbons). That mobility/penetration could account for better reduction in a less smoky fire, but I don't have enough experience with that to say (just with chemistry and lighting fires- LOL)
I have really taken to firing with just pallets- they are (in my less than expert opinion) the definition of good kiln wood: kiln dry, thin split, a mix of soft and hardwoods for combined coal production and FAST heat liberation, free for the taking, an act of reuse/recycling, and they happen to be the right length for my firebox (normally). I find that I have to watch the fire a bit before my reduction cycle at ^010 because they burn so fast that I can easily get early reduction when I'm single-firing. They are a joy to fire with, and when I want reduction, I just load up the fire box with some pallet pieces (nails and all) for some heat and reduction, and whatever junk wet wood I have around to get rid of for water reduction etc. and to slow that puppy up a bit. Last firing I threw in diseased garden plants (with bugs... what a wet summah), overgrown squash, invasive vines... lots of fun cleaning up the yard a bit.
Paul Haigh
Londonderry, NH
Maeva Collins on sun 26 oct 08
Thank you for your help; I appreciate you taking time to share your
knowledge. I plan to fire again at the end of Nov. or beginning of Dec. and
am anxious to try your suggestions. Thank you also for Dave Conrad's
website address. I was not aware of it but when I get a chance will visit
it and register as you suggested.
Maeva
----- Original Message -----
From: "Phil Rogers"
To:
Sent: Thursday, October 23, 2008 12:23 AM
Subject: Re: wood firing problem
> Maeva,
>
> I think that your temperature rise is probably OK. I usually reach 600C in
> approx 6 hours. That isn't a fast temperature rise. The problem is, in my
> estimation anyway, that in a wood firing, because of the very nature of
> the
> flame and the way a wood kiln draws it through, heat is channelled to very
> specific areas especially early in the firing when there is little back
> pressure to force it into those areas away from the obvious route. Hence,
> things like bottle lips, jug handles etc. that protrude from the main
> stacking area either at the top or at the sides get too hot too quick. I
> don't think it is a case of slowing down more than you already are - to
> spend 12 hours up to Cone 09 would be adequate if a little long as it is
> only after Cone 04 that one begins to see the real benefits of wood firing
> as the clay begins to become pyroplastic. I tend to think that 12 hours up
> to the point of beginning reduction proper would be adequate. I put the
> kiln
> into reduction at 09.
>
> My feeling is to warm up for a longer period with a gas torch perhaps.
> That
> would eradicate steam expulsion problems. Then, take the fire slowly but
> having made sure that, as much as possible, bits of pots don't protrude
> beyond the main stack. That's difficult at the top I know. That won't cure
> the problem completely ( if at all!) Think of flame, vapour and gas as
> water
> flowing through a stream. It goes around rocks and stones in exactly the
> same way - always. That's how it is in a kiln too. Flame will always find
> the quickest and easiest way out.
>
> Regarding the Terra sigillatta pieces. There was no glaze involved. That
> may
> be one reason. As soon as a glaze sinters it seals the body off from the
> effects of reduction. It also seals in any gaseous material that should
> have
> been burnt out. So, later in the firing when the body or the glaze is soft
> enough those gases try to escape - can't, but form bubbles or bloats
> instead. The unglazed clay has plenty of opportunity to burn out because
> there is nothing to prevent it well into the firing. I don't think that
> iron
> in the body is a factor. Iron in reduction is a flux but I'm not sure that
> that has anything to do with the problem we are dealing with. I have one
> body that is VERY iron rich. I use it for a Kohiki effect. That particular
> body did experience some slip peeling and blistering on the top shelf at
> the
> back. HOWEVER, the same body, same slip, same glaze was perfectly OK on
> the
> next shelf down. It is the extremities of the kiln where the problem seems
> at its greatest.
>
> Tony is right. Lack of smoke in the early stages is not a true indication
> of
> reduction or lack of it. It is difficult to fire a wood kiln in anything
> other than reduction particularly after a stoke. There will be carbon in
> plenty floating through your kiln. For the first few hundred degrees it
> won't matter but as tips of pots, handles, spouts etc. get hot it is then
> that the problem begins to lay itself down and manifests itself later as
> blisters. I find that those areas are prone to a grey discolouration as
> well
> as roughness and, sometimes, blistering. Try to burn the fire as clean as
> possible early on. Don't have the dampers fully out though. In my
> experience
> this encourages a hot flame through the bottom of the kiln and you simply
> move the problem from one area to another. Let's face it - it's wood and
> there are always going to be areas that don't work as well as others.
> That's
> the nature of it and part of the reason why we are attracted to it. I
> looked
> at your website and you make fine pots that appear very well fired. Try
> these measures and see what happens.
>
> Just on another point briefly. Are you aware of Dave Conrad's website -
> http://www.woodfiredceramics.net/ It really is an excellent site and I
> would recommend you visit, register and take advantage of it.
>
>
>
> Phil Rogers
>
>
>
>
>
> Phil thanks for your second post. I like your theory regarding the top
> shelves becoming too hot in the beginning as particularly with the train
> kiln the flames are fairly direct. How slow would you suggest I go; what
> would be a more approprate rise of temperature say for the first twelve
> hours?
>
> I'm very curious about the few pots that belonged to another potter that
> were scattered among mine but were very smooth and shinny; they were
> sprayed
> with terra sigillatta. Why did they avoid the blistering? Her clay body is
> different, less iron.
>
> Not so sure early reduction is the problem as there is only a small amount
> of smoke when stoking but if a slow cool doesn't work that will be
> something
> I'd want to consider as well. Thanks for your help.
>
> Maeva
>
> ----- Original Message -----
>
> From: "Phil Rogers"
>
> To:
>
> Sent: Sunday, October 19, 2008 2:39 PM
>
> Subject: Re: Wood firing problem
>
>
>
>>I think that Luke Nealey's post is very interesting and kind of backs
>
>>up what I was trying to say. If the early stages of the firing - up to
>
>>600C let's say - are done in a medium to strong reducing atmosphere
>
>>then it is hardly the conditions to encourage the clean burning out of
>
>>gaseous material in the clay. Couple this with the propensity for
>
>>edges and handles to get hot quite quickly and the resulting
>
>>vitrification and therefore sealing of the pots surface and you have a
>
>>prefect recipe for blistering, peeling and bloating.
>
>>
>
>> In my last firing I had some peeling of an otherwise totally friendly,
>
>> benign white slip that I have used, trouble free, for twenty years. I
>
>> put this down to the rapid escape of steam dislodging the interface
>
>> between slip and body. Like Luke, it happened only on the top shelf at
>
>> the back of the kiln. However, I do think that it is the top shelves,
>
>> that warm quickly at first even though, ironically, by the end of the
>
>> firing this particular shelf remains the coolest spot in the kiln.
>
>> This, to me at least, is proof that rapid temperature rise in the
>
>> early stages is to be avoided and in my next firing I will be taking
>
>> Luke's advice re the reduction atmosphere as the kiln begins to climb.
>
>> Wood firing is a balance. Too oxidising at the beginning with the
>
>> dampers out will encourage a hot flame through the lower part of the
>
>> kiln thus creating a possible new problem.
>
>>
>
>> This has been a particularly interesting and, for me at least, very
>
>> useful thread. Thank you Maeva for raising the issue and to Tony for
>
>> your contribution. I hope anyone else with observations will write
>
>> their findings.
>
>>
>
>>
>
>> Phil Rogers
>
>>
>
>> Phil and Lynne Rogers,
>
>> Marston Pottery,
>
>> Lower Cefnfaes,
>
>> RHAYADER,
>
>> Powys. LD6 5LT.
>
>>
>
>> phil@philrogerspottery.com
>
>>
>
>> Please visit
> www.philrogerspottery.com
>
>> < http://www.philrogerspottery.com/>
| |
|
