John Post on fri 8 jan 99
Hi Chris,
>I'm finally working on the iron reds I've been collecting recipes
>for, and need some help. I fired to cone 5, Skutt 1027 with
>controller, "Slow" speed, Envirovent on the whole time.
Was the kiln only firing slowly as it heated up or did you fire it down
with a controlled cooling. I have found that iron reds at cone 6 need a
controlled cool down to develop. Most electric kilns cool down much too
fast to let the iron precipitate to the surface of the glaze. If your
glazes are mostly very glossy, you're probably cooling the kiln too fast.
When I am trying to get iron reds I make sure that the ware is densely
packed. A densely packed kiln cools more slowly. Try and keep the shelf
that is above your iron red pots as close to the pots as possible. The
shelves retain a lot of heat and the closer it is to your pots, the slower
the cool down will be. I even take kiln posts and place them in between
iron red pots to help hold more heat on the shelf.
>I also tried
>a batch to cone 5 using a much slower schedule previously posted by
>Janet Walker, with extended time in the 700-900 range.
I have the same question here. Was the extended time in the 700-900 on the
way up or on the way down? The extended time needs to be on the cool down.
Another thing is that Jan Walker thinks and fires her kiln in degrees
centigrade. If you fire your kiln using the Fahrenheit scale the
controlled cool down would convert to 1292 to 1652 degrees F.
This is just a small excerpt from Hamer's The Potter's Dictionary...
"Saturation with iron oxide. During firing iron oxide is dissolved in the
glaze. During cooling, some of the iron is precipitated.
The molten glass can absorb more iron oxide in active state, that is,
involved in fusion, than can be eventually incorporated in combined state
in the cooled glaze. The difference between these two amounts is isolated
during cooling and forms its own crystals. This precipitation starts at
the surface, which is the first part to cool, and spreads into the glaze
partially to embrace other iron oxide as it is isolated.
It is this saturation amount that is responsible for some very beautiful
colorings from iron. The best known is the stoneware kaki (rust)...
....It is impossible to predict exactly what the two amounts of oxide will
be: the amount that will be absorbed when hot and the amount when cold.
Each glaze is different and the factors affecting the two saturation
amounts are:
(a) the alkali:acid ratio of the glaze;
(b) the iron oxide state; and
(c) the top temperature linked with the type of clay body."
I would add a (d) and that would be the rate of cooling.
>So, what else do you suggest?
Here are some results from my experiments with an iron red I developed...
8% iron oxide - controlled cool down - nice kaki rust results
8% iron oxide - kiln cools on its own, quickly - ugly brown glaze results
16% iron oxide - controlled cool down - dry matt red brown glaze results
16% iron oxide - kiln cools on its own, quickly - nice kaki rust results
As you can see the 8% iron glaze with the controlled cool down let just
enough iron precipitate to the surface. On a quick cool down it took 16%
iron in the glaze to achieve the same type of result. You will need to
determine how much iron you need to use so that just enough precipitates to
the surface as your kiln cools.
So if I was you, I would experiment with a controlled cool down of your
kiln. I would also do a line blend with these glazes. I would increase
the amount of iron in them in 2% increments from 6% up to 20% to find the
amount of iron that it takes to achieve an iron red in your kiln based on
the rate of cooling you are using. The cooling is everything as this is
when the iron precipitates to the surface. The amount of iron and the rate
at which your kiln cools are the two factors which are influencing the
development of the iron red colors. Experiment with those two things and
you'll eventually get some nice iron reds.
This is the firing schedule I use for iron reds and other matt glazes.
You'll notice that my controlled cool down is a little slower than Jan
Walker's. Instead of slow cooling from 1652F to 1292F, I slow cool from
1800F to 1292F.
heat up at 300 degrees per hour up to 2215F
soak at 2215F for 20 minutes
cool down at 250 degrees per hour to 1800F
cool down at 75 degrees per hour to 1292F
let the kiln cool on its own from here...
As for our prior discussion about finding a vitrified cone 6 commercial
clay body, I like Tuckers Mid White. Just tested some with my glazes and
it works nicely. The glazes look good, the body is vitrified and it throws
well. It's not too sticky, it has nice tooth.
Hope this helps, let me know if you have more questions.
John Post
Sterling Heights, Michigan USA
rp1mrvl@moa.net
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