John Baymore on sat 15 jul 00
The air contained in an enclosed volume expands when it is heated. I
believe, though someone may correct me, the equation is PV=3D3DRT and tha=
t
the volume increases by a fraction of 1/273 for each incremental rise of=
1
deg Kelvin. So the volume wishes to more than double. But in a contained
space this cannot happen, so the pressure increases by the same factor. =
Now what was the modulus of rupture for dry clay?
Ivor,
Good science.
Other thoughts:
During a good portion of the heating process, the clay body is pretty gas=
permeable. Easily well up into the bisque range for the body.... and a b=
it
further. So a significant portion of the total volume change is probably=
negated due to the migration of air through the porous clay walls. To kn=
ow
how much force was generated you'd have to know the porosity / rsistance =
to
air flow of the particular body at any given pressure differentail across=
the clay layer (thickness will be a factor to flow also in this case) and=
also the point at which no more air was escaping from the enclosure....if=
the body ever actually reaches that point.
You also have the slight factor that the interior volume is decreasing as=
the clay vitrifies.... but that is a somewhat insignificant change compar=
ed
to the gas volume change .
This is an interesting area. I know sealed enclosed forms are possible t=
o
do, cause I and many others have done them.
One other thought is that if some of the original air volume that was
contained has escaped due to the pressure increase on the interior, and t=
he
body has slowly been becoming non-gas permeable,..... then on the cooling=
cycle is there actually a negative pressure (relative to origial "day mad=
e"
atmospheric) on the interior of a totally enclosed form? This would tend=
to place the "sphere" in a tad of compression from the atmospheric
pressure, and actually make it a little stronger.
Totally absurd stuff to be considering.....I should just be making
pots....but it is interesting .
Best,
.....................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000"
| |
|
