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cone 6 electric kiln reduction with propane (long)

updated thu 8 jul 99


John Post on wed 7 jul 99

Hey kids, throw out your silicon carbide. You won't need it for reduction
firing in your electric kilns anymore. I just tried Nils Lou's method that
he outlines in his must have book "The art of Firing"....and it worked
great! The idea is to create a reduction atmosphere in the electric kiln
using a small propane burner. The kiln fires to temperature using the
elements and the atmosphere is controlled by the burner.

The results from my first firing are great. I used many of my cone 6
oxidation glazes on small pots and test tiles in various locations in the
firing. I got nice body reduction and wonderful glaze surfaces as well.
The glazes have a depth and richness that they don't get in the oxidation
fire. There is a much more interaction between the glaze and clay body
than I normally get. The iron spotting is wonderful too. (I list a couple
of the glaze recipes below for those looking for cone 6 reduction glazes.)

If you want to try this in your studio here is a brief overview of what you
need to get and what you need to do.....

Make sure your studio has good ventilation. My studio is in a garage
behind my house. I had the big roll up door open and a window. I also
installed an exhaust fan on the wall directly behind the kiln. I got my
exhaust fan from Grainger for about $100 US.

I cut a 1 1/2 inch hole in the top and bottom of the kiln.

I sprayed the interior of my kiln with ITC100. I coated the floor, walls
and elements with it. I DID NOT spray the kiln sitter. I masked that off
with tape when I sprayed. The ITC is what keeps the burner elements from
being destroyed in the firings. The ITC cost me $53.00 (1-904-285-0200).

I purchased a small propane burner form Frey Scientific (1-888-222-1332).
It is item number 07311 Burner, Tirrill, Cylinder Gas. It cost $17.95 plus
shipping. It's a little table top Bunsen burner that has separate controls
for flame temperature and length.

Then I purchased a Raku burner system assembly (less burner and valve) from
Marc Ward (1-423-397-2914). The propane burner fits snugly into this hose
assembly and is secured with a band clamp. The burner system is
essentially used to regulate the pressure from the propane bottle to the

The kiln that I tried this on is just a little one. It's inside chamber is
13 inches deep and it fits 13 inch round shelves. This is my test kiln.
It's a 20 year old Cress but it had less than 25 low temp firings on it
before I got it 4 years ago. It easily hits cone 8 in a reasonable amount
of time, 7-9 hours.

For the reduction firing, I fired up to cone 06 on just the elements alone.
Then I lit my burner and slid it under the burner port in the bottom of
the kiln. The propane pressure was set at 2 1/2 pounds from a 40# tank. I
adjusted the flame from the exit flue so that a 6-10 inch orange flame
danced out the top of it. A 4 inch orange flame came out of the top peep
hole plug when I opened it. There was none from the bottom peephole, so I
slid a damper brick one sixth of the way over the exit flue and I got a 2-3
inch orange flame to come out of the bottom peep hole. I didn't change
these settings for the whole firing.

I was monitoring the temp on my pyrometer. When it hit 2100 degrees
Fahrenheit I checked on the cones and they were all over, even the guard
cone. I had hit cone 7, probably 8. I shut of the elements first and when
I did this the flame out of the flue changed from orange to a very lazy
yellow. I had no idea the elements were effecting the firing as much as
they were. I originally was going to slow cool the kiln using the burner,
but then decided to use the elements instead. So I cooled the kiln off by
gradually backing the switches down at the rate of 50-100 degrees F an
hour. I slow cooled from cone 7 all way down to 1450 degrees F over 8
hours. The burner was not on for any of the cool down.

The next morning I opened the kiln and was pleased as punch at the results.
I got 7 rich, tiny pots, 2 duds and one that looks great but is too dry and
rough to touch. I also had 30 test tiles in the firing. 1/2 of them look
good enough to try on pots.

It was so gratifying to actually have FIRE be part of the
firing......especially in the middle of a subdivision. This little kiln
fires so quietly that the only person who knew it was firing was me (...and
my wife who I had to keep telling how much fun I was having.)

I'm going to keep running test firings this summer in the little kiln and
then if it seems to be holding up well, I'm going to try this in my larger
kiln with it's automatic controller (this should make the cool downs much
easier to control).

*** A word of caution here. If you try this, do not leave the firing
unattended. There is no safety shutoff for the gas if the burner goes out
or has some other problem. (The burner was only on for 2 hours and 20
minutes of the firing, so you don't have to spend the entire day watching
the firing.) BUT, it does need to be attended during the whole time the
burner is on.

I'm sure I left off some of the details about this process, so if you have
questions post them to the list so that everyone who wants to try this can
follow the discussion.

The successful glazes follow...

+++ *Jade East +++
Cone 6 oxidation and reduction
This is from Robert Piepenburg's book The Spirit of Clay.
A variegated grayed-green satin matte. It is a brighter green over light
colored slips and clay bodies. The PMSP-4 Ron Roy Variation looks great
over this one.

Range: 6-10
Firing type: Ox. or Red.
Glaze type: Stoneware

Nepheline syenite 3040 38 %
Kentucky OM #4 1920 24 %
Whiting 1600 20 %
Flint 1040 13 %
Gerstley borate 400 5 %
-------- ------
8000 100 %

Copper carbonate 240 3 % Hazard!
Titanium dioxide 400 5 %

+++ Butterscotch +++
Cone 6 oxidation and reduction
This one was posted on clayart a short while ago.
A white and tan mottled satin glaze at cone 6 in oxidation. In reduction
it has a softer sheen and the iron spots from the claybody look great
breaking through it.

Nepheline syenite 1784 22.3%
Whiting 1520 19 %
EPK 1512 18.9% Hazard!
Flint 2328 29.1% Hazard!
Gerstley borate 856 10.7%
-------- ------
8000 100 %

Superpax 400 5 %
Rutile 464 5.8%

+++ PMSP-4 Ron Roy variation +++
Cone 6 oxidation and reduction
This is my gloss liner white glaze. I use it inside of functional ware.
In both oxidation and reduction it is a glossy white. Breaks into
wonderful mottled green colors over Jade East.

Frit 3134 1680 21 %
Nepheline syenite 1520 19 %
Wollastonite 1600 20 %
EPK 1280 16 %
Flint 1920 24 %
-------- ------
8000 100 %

Superpax 1200 15 %

+++ *Mackenzie White +++
Cone 6 oxidation and reduction
A fat white glaze. In oxidation it is a brighter white than in reduction
where it is more gray. I added some coarse iron filings from the bottom of
a rusty file cabinet to this glaze. This helps it to look somewhat like a
reduction glaze in the oxidation fire.

Kentucky OM #4 415.17 5.2%
Dolomite 550.9 6.9%
Gerstley borate 990.02 12.4%
Talc 1101.8 13.8%
Custer feldspar 3297.41 41.2%
Flint 1644.71 20.6%
-------- ------
8000 100 %

That's it for now...

I'm off to formulate some cone 6 chuns and copper reds...


John Post
Sterling Heights, Michigan