Candice Roeder on sun 3 may 98
Hello all,
I recently attended a Funcitonal Ceramics workshop in Wooster, Ohio
(which was great). Pete Pinnell was one of the presenters and spoke
about how electric kilns cool too quickly, and the effect this has on
glazes. He mentioned "firing down". Controlling the rate of cooling.
I have an electric kiln w/a controller, so I am able to program a
cooling phase, but I do not know where to start for rate of
cooling...250 F. per hour? 150 F. per hour? Cool at what rate, and
until what temperature?
If anyone can give me an idea of where to start, I'd appreciate it.
Thanks,
Candice Roeder
Janet H Walker on tue 5 may 98
CRoeder asked:
...Cool at what rate, and until what temperature?
This is a hard question to get solid numeric answers to!! But here
is where I am after about two years of reading in search of numbers.
I fire to ^6 electric with a controller. In order to get anything
but browns out of my iron glazes, I started firing down. If you
read Hamer under , you will
find that much of the depth and color in glazes develops in the
temperature range of 700C to 900C (sorry I don't do those nasty four
digit temperatures ending in F). So you want your glazes to spend
some time in that temperature range.
How much time? I chose 4 hours, with a 50C/hr temperature drop.
Val Cushing writes somewhere in his handbook that a big gas kiln
cools at about the rate of 1C/minute at most at high temps. So this
would be 60C/hr in the crucial temp range.
What about between your top temp (e.g. 1210C for ^6, fired slow) and
900C? After watching the pyrometer and experiencing a lot of
pinholes, I decided to control the ramp down between top and 900C.
I don't let that go faster than 150C/hr. (Unrestrained, it drops
250C/hr in my Skutt 1027 in the first hour after the firing. Much,
much too fast unless your goal is just glassy clears.)
One very interesting thing to realize -- you don't REALLY have to
fire down carefully if it doesn't suit all your pieces. Take the
pieces that need to "cool slowly" and put them in the next bisque.
They will travel that crucial 700-900 temp range TWICE during the
bisque firing (it doesn't matter whether it is heating or cooling
just so long as those are the temperatures, says Hamer). Then those
pieces will be nice and "developed" as a result. [So that Ron Roy
doesn't have to say it, I'll remind people that not all pieces can
stand the heat work of extra firing. Depends how much cristabolite
got developed in the first firing. Most things I've tried that were
structurally OK to start with come out OK fired again.]
So try this kind of thing and see what happens. There are temp
conversion charts in lots of the ref books if you like the ones
ending in F better than the ones ending in C. Just don't "estimate"
the conversion!!
REgards,
Jan WAlker
Cambridge MA USA
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