Dave Finkelnburg on thu 18 apr 02
Hi Tony!
Your comment on carbon trapping and early reduction caught my eye.
Carbon trapping is simply reducing so heavily in a kiln that the
hydrocarbon fuel source (wood, gas, propane, oil, coal, even corn) is
reduced to pure carbon before the glaze melts.
The carbon deposits on surfaces in the kiln which are cooler than the
kiln gas. During firing when temperature is climbing, essentially all ware
is cooler than the kiln gas, so that's where the carbon goes.
The trapping occurs only IF
1. the carbon stays, isn't reoxidized and burned away again, and
2. the glaze melts over the carbon and seals it in, so it can't be
easily reoxidized. All the carbon not under the glaze burns off as soon as
you go into oxidation, like at the end of the firing if not before.
I think most of us have learned this, in my case the hard way, with a
reduction kiln.
Some things to consider.
It's easier to reduce to pure carbon at lower temperatures.
If this reduction is done too early when single-firing you will be
guaranteed to have black coring. :-(
Once the glaze melts, you can oxidize all you want and the carbon will
remained trapped and show in the glaze color.
Just remember, carbon trapping requires heavy reduction BEFORE the glaze
melts.
I try to keep this in mind as I fire, so I can understand and control,
or avoid, the carbon trapping.
Good firing!
Dave Finkelnburg, watching a lonesome snowflake waft down onto the
spring-green lawn in Idaho, and contemplating a warmer weekend in L.A. (very
short-notice trip,) and wondering, if I have only one stop (or maybe two
:-) where should I go to see functional pots?
Brandon Phillips on thu 18 apr 02
When I want carbon I begin reducing at 010 or sometimes 012. Any later than
that and it doesn't work for me. I had a conversation with Warren Mackenzie
back in december and he told me that shinos tend to melt really early on in
the firing. Told me a story about Ken Ferguson's shino tea bowls, he used
to refire them in bisque firings to cone 08. He thought that since it was
such a low temp. that he could just stack them. They all stuck together, at
CONE 08!! Just thought it was an interesting story.
Brandon Phillips
> Your comment on carbon trapping and early reduction caught my eye.
> Carbon trapping is simply reducing so heavily in a kiln that the
>hydrocarbon fuel source (wood, gas, propane, oil, coal, even corn) is
>reduced to pure carbon before the glaze melts.
> The carbon deposits on surfaces in the kiln which are cooler than the
>kiln gas. During firing when temperature is climbing, essentially all ware
>is cooler than the kiln gas, so that's where the carbon goes.
> The trapping occurs only IF
> 1. the carbon stays, isn't reoxidized and burned away again, and
> 2. the glaze melts over the carbon and seals it in, so it can't
>be
>easily reoxidized. All the carbon not under the glaze burns off as soon as
>you go into oxidation, like at the end of the firing if not before.
> I think most of us have learned this, in my case the hard way, with a
>reduction kiln.
> Some things to consider.
> It's easier to reduce to pure carbon at lower temperatures.
> If this reduction is done too early when single-firing you will be
>guaranteed to have black coring. :-(
> Once the glaze melts, you can oxidize all you want and the carbon will
>remained trapped and show in the glaze color.
> Just remember, carbon trapping requires heavy reduction BEFORE the
>glaze
>melts.
> I try to keep this in mind as I fire, so I can understand and control,
>or avoid, the carbon trapping.
> Good firing!
> Dave Finkelnburg, watching a lonesome snowflake waft down onto the
>spring-green lawn in Idaho, and contemplating a warmer weekend in L.A.
>(very
>short-notice trip,) and wondering, if I have only one stop (or maybe two
>:-) where should I go to see functional pots?
>
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John Christie on sun 21 apr 02
Dave Finkelnburg wrote on the 18th April:
>Carbon trapping is simply reducing so heavily in a kiln that the
>hydrocarbon fuel source (wood, gas, propane, oil, coal, even corn) is
>reduced to pure carbon before the glaze melts.
>The carbon deposits on surfaces in the kiln which are cooler than the
>kiln gas. During firing when temperature is climbing, essentially all ware
>is cooler than the kiln gas, so that's where the carbon goes.
>The trapping occurs only IF
>1. the carbon stays, isn't reoxidized and burned away again, and
>2. the glaze melts over the carbon and seals it in, so it can't be
>easily reoxidized. All the carbon not under the glaze burns off as soon as
>you go into oxidation, like at the end of the firing if not before.
>---------------------------------------------------------------------------
-----
>Some things to consider.
>It's easier to reduce to pure carbon at lower temperatures.
Dear Dave,
In a kiln, the fuel is the reducing agent - it cannot (generally speaking)
itself be reduced. The carbon that is deposited on the pots in the early
stages of the firing has not been reduced but occurs as a result of
incomplete combustion (which is not at all the same thing as reduction). In
a wood (or corn) kiln, this incomplete combustion is due (as you hint) to an
activation barrier in the reaction of carbon with oxygen which requires heat
to overcome it and occurs in oxidising as well as reducing conditions in the
early stages of the firing. (Thus if you old a cold spoon over a burning
candle, soot will be deposited although there is no shortage of oxygen
around the candle flame). In an (efficient) gas kiln, it will be necessary
to limit the primary air supply to achieve the deposit of carbon on the
pots, but this will happen in corn (or other solid fuel) kilns in normal
oxidising conditions. Smoke emerging from the chimney of a wood kiln in the
early stages of the firing does not necessarily point to a shortage of
oxygen.
It is, as you pointed out, necessary to limit the oxygen entering the kiln
after the ignition temperature of carbon has been reached if you wish to
prevent its oxidation.
Sorry to be disagreeable!,
John Christie.
Gail Dapogny on tue 23 apr 02
Have a question.....What is the range of glaze melting? What are the
earliest and latest temps (ballpark), and are these temperatures affected
by the type and amount of metal oxides in the mix? Also, since whiting and
talc and ...--I forget-- dolomite (?) produce more gases than other
substances, are these gases affected by when the melt occurs and how fast
you are firing at that time?
---Gail
>> Once the glaze melts, you can oxidize all you want and the carbon
will
>>remained trapped and show in the glaze color.
>> Just remember, carbon trapping requires heavy reduction BEFORE the
>>glaze melts.
>> I try to keep this in mind as I fire, so I can understand and
control,
>>or avoid, the carbon trapping.
>> Good firing!
>> Dave Finkelnburg
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
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