Rimas VisGirda on wed 5 oct 11
=3D0A=3D0AClay has two types of water, physical and chemical (Al2O3.2SiO2.2=
H2O)=3D
.XH2O=3D0A=3D0AThe physical (XH2O) is gone at 212F. If not,=3DA0 -BOOM!=3D0=
A=3D0A=3D0AT=3D
he chemical (2H2O) gets driven off around red heat (1000F) and is not insta=
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ntaneous, as liquid water turning to steam at 212F... The water molecule co=
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mes off the clay molecule (Al2O3.2SiO2.2H2O) a little at a time throughout =
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a range of 300 or so degrees and as the material is still porous the steam =
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is not concentrated and works its way out of the material leaving bisquewar=
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e as the product...=3D0A=3D0AIf you stop a firing before the chemical water=
is =3D
released, say 700-800F, the clay can still be slaked and made back into pla=
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stic clay.=3D0AOnce the chemical water is released it is no longer clay and=
w=3D
on't slake back into a plastic material.=3D0A=3D0AIn some industrial proces=
s it=3D
is possible to perform a firing with damp clay in a matter of hours. It is=
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done by pressurizing the kiln and thereby raising the boiling point (where=
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the liquid turns to gas) of water. A piece of clay will normally "explode"=
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if still damp at 212F because the liquid (water) inside the clay turns to =
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a gas (steam) at atmospheric pressure and that state (gas) has a tremendous=
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amount of energy accompanied with a sizeable increase in volume (as a gas)=
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and wants to get out of the dense environment it finds itself in so blows =
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out the sidewall or whatever is in its way... But if the kiln is pressurize=
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d, the boiling point increases and the clay can be heated way above the sea=
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level boiling point. Once the high temperature is achieved the atmosphere =
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can be bled off at a constant rate and we find the clay at a high temperatu=
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re and dry. This can take place in a matter of hours.=3D0A=3D0AWe used to d=
o a =3D
public service event at the last university I taught. We invited the univer=
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sity community as well as the public to come to the studio and make things =
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out of clay for a morning and afternoon session. We then loaded the wet war=
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e into an electric kiln, with a kiln sitter with a cone 05, along with some=
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open containers of water. Closed the lid, put in all the peeps and turned =
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all elements on high and went home. The next morning everything was bisqued=
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and the makers came back and slopped some raku glaze on the bisqued pieces=
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which we raku fired. So in 2 days people could come in and make something =
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and walk away with a finished glaze fired piece... The mechanism that kept =
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everything from blowing up was generating enough steam early on to pressuri=
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ze the kiln and increase the temperature under pressure, the cracks and gap=
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between the lid and the kiln allowed the pressure to bleed off at a consta=
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nt rate; as all the water was siphoned off the pots
were dry and continued to their final temperature. One drawback was that i=
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f you put in pieces that were too wet, the steam would penetrate into the p=
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iece and actually make it wetter enough to start slaking... A faculty wife,=
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Joyce Kavanaugh, who worked in the studio became interested in controlling=
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the process and came up with a regimen after numerous experiments and tria=
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ls; it's buries somewhere in my files. If anyone is interested I can try di=
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gging through my teaching stuff to see if it's still there...=3D0A=3D0ARega=
rds,=3D
-Rimas=3D0A
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