James Dapogny on sat 8 nov 97
Help, all you firing wizards!
I have some firing issues on my mind and am having trouble sorting them out
into clear questions. But I need to try. (Many of these issues arise
because our 70 cub foot gas kiln underwent MAJOR repair in August and is
now much tighter. And different.)
1) CONES: Am I right in assuming that "firing to cone 9" means that cone 9
is touching BEFORE you close up the kiln and not merely at that point when
the kiln is opened up? (At that time the cone of course will have slumped
more from residual heat.) I feel that some of the glow is missing because
we're firing a little too cool. ???
2) FIRING DURATION, SOAK: When I'm involved in firing, I keep trying
valiantly to slow the kiln down, especially after body reduction and get
7-8 hours by cone 9 over. ( You have to understand: this is a Guild, i.e. a
group effort!). I've been trying for a 13 hour firing from turn-on of main
burners --ca. 800f-- to cone 9 over,(especially after reading and
re-reading Cushing and Zamek),including a soak as close to an hour as
possible, and reasonable heat. (Unfortunately, I like a bit more heat than
others in our guild so I am having to curb my cone 10 desires, although
cone 10 does get over in a few parts of the kiln.)
Anyway... other people don't seem particularly interested in firing length,
a healthy soak, etc. As long as cone 9 is down when the kiln is opened
(not when it is shut off), and cone 10 merely soft, and the temp has
reached about 2280f, plus adequate reduction,... however that's
accomplished, then that's all that some people feel is important.
(Incidentally, I notice that the kiln jumps up in temperature about a
hundred degrees faster than it used to when the main burners are lit
--something like 300-350 in the first hour--which creates additional speed
problems.)
I feel--and everything I've read suggests this-- that we are losing body
integrity and glaze quality when we avoid a slow firing and a good soak.
What do you think?
3) REDUCTION (another problem that is tough when you are firing in a group
situation): How can you have healthy reduction, get the copper and iron
reds, shinos, etc., and avoid black coring which we have had some of
lately? I like to lighten up after body reduction, then advocate staying
neutral (light reduction) to the end.
Some people like to blast it with reduction near the end, but I've noticed
that that seems to dim some of the colors. Aren't colors more or less set
by about cone 1-2, and isn't the purpose of the leisurely-fired end
(including the soak) to smooth things out and just set the color and
texture (sort of like melted cheese)??
Other people like to keep our reduction fairly strong even after body
reduction is finished (our body red. begins at cone 012--about 1680f or
so). Doesn't this often result in black-coring?
Is body reduction over after an hour or at a particular temperature?
4) PITTING: We often have pitting, even with the 13 hour firing that I got
last time (that firing by the way did not have black-coring). We bisque to
cone 06, pots aren't dusty, etc.
6) BLACK-CORING: My understanding of things is that this results in lower
thermal strength. In other words it's a symptom of something undesireable:
carbons trapped inside eventually affecting strength. Correct?
I figured that my clayart buddies would be interested in wrangling over
this sort of stuff. Sigh...
---Gail Dapogny in Ann Arbor (jdapogny@umich.edu)
Jennifer Rhinesmith on tue 11 nov 97
James, let me first say that in no way am I an expert on this stuff and
that there are plenty of more knowledgeable people on the list. But I may
be able to answer some of the questions that you have.
1) CONES: A hard nine would be the cone touching when you shut the kiln
down, not when you open it. A soft nine would be when the cone is only
down on the tip.
2)REDUCTION: When we fire we would set body reduction at about 1500
degrees, for a hour and then keep it in slight reduction all the way
through. Ending with it in full reduction again. Also, we always tried
to keep the bottom of the kiln hotter then the top. You can always push
the heat up, but not so easy to push the heat down.
3)BLACK-CORING: From everything that I understand about it, you are right
in your assumptions.
I hope this has helped in some way, Jennifer in Alpine, TX
Where it is cold enough to see your breath and have car problems.
On Sat, 8 Nov 1997, James Dapogny wrote:
> ----------------------------Original message----------------------------
> Help, all you firing wizards!
> I have some firing issues on my mind and am having trouble sorting them out
> into clear questions. But I need to try. (Many of these issues arise
> because our 70 cub foot gas kiln underwent MAJOR repair in August and is
> now much tighter. And different.)
>
> 1) CONES: Am I right in assuming that "firing to cone 9" means that cone 9
> is touching BEFORE you close up the kiln and not merely at that point when
> the kiln is opened up? (At that time the cone of course will have slumped
> more from residual heat.) I feel that some of the glow is missing because
> we're firing a little too cool. ???
>
> 2) FIRING DURATION, SOAK: When I'm involved in firing, I keep trying
> valiantly to slow the kiln down, especially after body reduction and get
> 7-8 hours by cone 9 over. ( You have to understand: this is a Guild, i.e. a
> group effort!). I've been trying for a 13 hour firing from turn-on of main
> burners --ca. 800f-- to cone 9 over,(especially after reading and
> re-reading Cushing and Zamek),including a soak as close to an hour as
> possible, and reasonable heat. (Unfortunately, I like a bit more heat than
> others in our guild so I am having to curb my cone 10 desires, although
> cone 10 does get over in a few parts of the kiln.)
> Anyway... other people don't seem particularly interested in firing length,
> a healthy soak, etc. As long as cone 9 is down when the kiln is opened
> (not when it is shut off), and cone 10 merely soft, and the temp has
> reached about 2280f, plus adequate reduction,... however that's
> accomplished, then that's all that some people feel is important.
>
> (Incidentally, I notice that the kiln jumps up in temperature about a
> hundred degrees faster than it used to when the main burners are lit
> --something like 300-350 in the first hour--which creates additional speed
> problems.)
>
> I feel--and everything I've read suggests this-- that we are losing body
> integrity and glaze quality when we avoid a slow firing and a good soak.
> What do you think?
>
> 3) REDUCTION (another problem that is tough when you are firing in a group
> situation): How can you have healthy reduction, get the copper and iron
> reds, shinos, etc., and avoid black coring which we have had some of
> lately? I like to lighten up after body reduction, then advocate staying
> neutral (light reduction) to the end.
>
> Some people like to blast it with reduction near the end, but I've noticed
> that that seems to dim some of the colors. Aren't colors more or less set
> by about cone 1-2, and isn't the purpose of the leisurely-fired end
> (including the soak) to smooth things out and just set the color and
> texture (sort of like melted cheese)??
>
> Other people like to keep our reduction fairly strong even after body
> reduction is finished (our body red. begins at cone 012--about 1680f or
> so). Doesn't this often result in black-coring?
>
> Is body reduction over after an hour or at a particular temperature?
>
> 4) PITTING: We often have pitting, even with the 13 hour firing that I got
> last time (that firing by the way did not have black-coring). We bisque to
> cone 06, pots aren't dusty, etc.
>
>
> 6) BLACK-CORING: My understanding of things is that this results in lower
> thermal strength. In other words it's a symptom of something undesireable:
> carbons trapped inside eventually affecting strength. Correct?
>
> I figured that my clayart buddies would be interested in wrangling over
> this sort of stuff. Sigh...
> ---Gail Dapogny in Ann Arbor (jdapogny@umich.edu)
>
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