firestones

["daedelus lanthanien" <daedeluslanthanien@zzzzzzzzzzz>]

I propose an experiment, everybody please throw your local geology to the
fire and post what it does. twice dry, twice soaked 2 days, thrown to the
eye of a well pitched hardwood fire for 2 or 3 hours.

Tomas I must ask, since your improvised hearth was a carved basin I assume
that it is most likely a carving grade limestone. Is it a Oolitic type
Limestone like Indiana's stone? If so could we attribute its resiliency to
its high level of air entrainment? Basalt is highly air entrained also, much
like a Coal Clinker or Pumice. Entrainment may be the wrong word to use
since the voids in these stones tend to be a interconnected lattice. The
lattice eleviates interior gaseous pressure by venting it effectively when
the stone is wet and heated possibly? Or when dry would the voids provide
just enough insulating while still allowing sufficient thermal entrophy
within the stone to avoid shearing and erratic exspansion?

I know that our local Silurian epoch limestone is horribly explosive when it
is heated well and it is not thoroughly dry, so are the erratic glacial
granites here. The Silurian Limestone is very tightly stratified with little
if any air inclusion. The granite is tighter yet ranging from a fine to a
massive crystalline structure. I have also noticed that these stones tend to
get damaged thermally. Even when they are kept dry they tend to break after
just a few hot firings, although with much less explosive force. The granite
does last longer than the limestone but it will succumb to direct heat as
well in time.

Since I have little experience with soapstone I questioned my colleagues
about this. Dave mentioned that it has great thermal conductivity and he has
seen some woodburning stoves built almost exclusively with it. So why
soapstone and not non-oolitic limestone and granite? I am begining to
suspect that soapstone has a low coefficient of expansion and high thermal
conductivity as far as stone goes. Also suspect is soapstones poor
permiability, thus avoiding heat induced degassing of the material which is
arguably the direct cause of trapped pressure in heated
permiable tight stones.

Sandstone is highly permiable with a well defined voided lattice-like
silicate bonded structure. I would suspect that it degasses quite
efficiently and would be fine when fired wet. Sandstone as it is well
entrained must have a fairly robust insulating quality lacking much thermal
conductivity. This lack of thermal conductivity in silicates sounds absurd
but I would suggest that this deficiency must be attributed to poor
siliciclastic sedimentary bond coupling. Lacking a fully fused structure
which is also voided leads me to suspect that sandstone slowly degrades due
to high temperature gradient within the material. The immediate surface gets
extremely hot while the sub-surface does not. This effect sets up a thermal
shear point just under the surface leading to a slight degradation upon each
firing due to surface expansion.

I am surprised that Tomas reports little degradation of his limestone basin,
maybe the heat moves better through the tighter calcite bonds which help
eleviate thermal shearing? Oolitic limestone voids are also much smaller
than those seen in sandstone. I am very discriminatory of sandstone, I do
not like to build with it. It stands up to hard weather poorly and makes for
moist walls.

Excellent exposed Pre-Cambrian sandstone may be found in Wisconsin around
the city of Madison. If you do swing in to see it please endevour not to
miss the supurb vien of red granite found 20 miles southwest of Madison in
the small town of Montello, it has been established that it is the hardest
granite known to man. Civil war generals Sherman and Lee both chose this
granite for their tombs. A added bonus to visiting this area is a visit to
Frank Lloyd Wrights architectural school and the bizarre "House on the
Rock". Madison is our capitol and it is a fun, vibrant college town with
loads of fine architecture. Among these beauties stand a few done by Louis
Sullivan and other architectural legends.

I hope some of you can perform this experiment safely with little trouble. I
am eager to define safe fire stone properties and types so they may compiled
in the archives. I will run the proposed test on stone available to me using
the proposed method to ensure a sound consistant benchmark. JPGs would be
pretty cool too. Is there possibly a chemist with a strong geology
background or vice versa here?

           Have a good day, Daedeluslanthanien