Corinne Null on fri 22 nov 96
Awhile back I solicited firing advice and got wonderfully informative
advice. However, I've still come up with another question!
During the bisque firing there is the initial slow fire expelling
atmospheric moisture between 350-400 F, then the phase when chemically
combined water is resolved and shrinkage occurs at 950-1200 F, and during
those periods, one fires slowly.
When glaze firing, do you still have to proceed slowly through those
temperatures, or can you fast fire up to 1750 F? What is fast fire - 200 F,
300 F ? What is happening above 1750 F?
Hope you're not losing patience with me! Geesh, there's so much to learn!
TIA
Corinne Null
Bedford, NH
cnull@mv.mv.com
Ron Roy on sat 23 nov 96
Corinne asked
>----------------------------Original message----------------------------
>Awhile back I solicited firing advice and got wonderfully informative
>advice. However, I've still come up with another question!
>
>During the bisque firing there is the initial slow fire expelling
>atmospheric moisture between 350-400 F, then the phase when chemically
>combined water is resolved and shrinkage occurs at 950-1200 F, and during
>those periods, one fires slowly.
>
>When glaze firing, do you still have to proceed slowly through those
>temperatures, or can you fast fire up to 1750 F? What is fast fire - 200 F,
>300 F ? What is happening above 1750 F?
First of all the atmospheric water still in clay leaves from the beginning
of heat upwards. It is at 212F that it begins to turn into super heated
steam and that is when most blowups occur. No text I have read agrees about
when the chemically combined H2O leaves. Some say it starts at 350C (660F).
No shrinkage takes place at this stage - trust me here I have done the
experiments)
Speed of glaze firings. Some glazes will need a slower rate of rise for
certain glazes (wet glaze = crawling) until sintering starts. Going slow
through the quartz inversion period at 573C (1060F) is wise if the ware is
large, has lots of free silica, and or edges are close to elements or
flame. If cracking happens on the way up the cracks will open and the edges
of the cracks will be softened by the melted glaze. If the cracking happens
on the way down the edges of the cracks will be cutting sharp.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Eleanora Eden on sun 24 nov 96
Hi Corrine and All,
The firing schedule of the glaze kiln for me depends on what is in it.
If there are any platters then the same restrictions seem to apply
as for the bisc firing.
Eleanora
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@maple.sover.net
[the address fga@world.std.com is temporary. My mailbox at
eden@maple.sover.net still works -- do not change address books]
Evan Dresel on mon 25 nov 96
At 10:18 AM 11-23-96 EST, you wrote:
>----------------------------Original message----------------------------
>Corinne asked
>>----------------------------Original message----------------------------
>>Awhile back I solicited firing advice and got wonderfully informative
>>advice. However, I've still come up with another question!
>>
>>During the bisque firing there is the initial slow fire expelling
>>atmospheric moisture between 350-400 F, then the phase when chemically
>>combined water is resolved and shrinkage occurs at 950-1200 F, and during
>>those periods, one fires slowly.
>>
>>When glaze firing, do you still have to proceed slowly through those
>>temperatures, or can you fast fire up to 1750 F? What is fast fire - 200 F,
>>300 F ? What is happening above 1750 F?
>
>First of all the atmospheric water still in clay leaves from the beginning
>of heat upwards. It is at 212F that it begins to turn into super heated
>steam and that is when most blowups occur. No text I have read agrees about
>when the chemically combined H2O leaves. Some say it starts at 350C (660F).
>No shrinkage takes place at this stage - trust me here I have done the
>experiments)
>
>Ron Roy
>Toronto, Canada
>Evenings, call 416 439 2621
>Fax, 416 438 7849
>
The temperature that chemically combined water leaves will depend on the
clay minerals present. If you really want to know, you could do a
differential thermal analysis. A friend in college built a DTA with a
muffle furnace hooked up to a motor to ramp up the temperature at a steady
rate. If you have a kiln with a controller, that should be a piece of cake.
You then use a separate thermocouple inbedded in your sample to record the
temperature rise of the sample. My friend used dry clay packed around the
thermocouple in a porcelain "boat". The loss of water, both atmospheric
moisture and chemically combined water will show up as a slower temperature
increase in the sample than in the kiln. If I remember my thermodynamics
correctly, the formation of new minerals shows up as an exothermic reaction
so the temperature of the sample rises faster than the kiln. I imagine you
would have to experiment with sample size and rate of temperature increase.
This is one way to identify the clay minerals present. Sounds like a good
project for a liberal arts student.
-- Evan Dresel in W. Richland Washington, who didn't get his fence finished
before the snow came.
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