Harry
Depending on the size of the stone I will go as much as 5 hours at
around 550F and then turn off the kiln and let it cool with the door
closed. I do it to maximize color. The stone will be brittle after
heating that is true. Most of the stuff I get is such and ugly gray
until heated, I could not imagine messing with the stuff were it not
heated.
I never grind dry, I don’t even begin grinding until the wheel is
evenly wet and the same with my sanding. I have had no problems with
cracking though, how I noticed that it was more brittle was in the
course grinding, it would chip more than a non heated stone.
I don’t know what you mean ‘without regard to the integrity of the
stone’. Under 350F the color does not come out, or it goes brown,
550F seems to give the best color change for me, so if you want the
color, that is the temperature you use, and the stone will be come
brittle. Higher temperatures, and there is enough moisture in the
stone they will break apart. If they have pockets they will break
anyhow, but the crystals do something really unique in the way of
coloration. As far as the integrity of the stone, to me it is like
any other stone you work with, what you make from it has to be in
balance with its characteristics. It would not do well to make a very
thin disk of turquoise and then not back it, it would break or
crumble very easy. If you know the stone is brittle, or has a very
strong cleavage, your make it thicker to allow for it, or you
mechanically protect it. Simply cutting the stone alters it integrity
from what it was. I have a green stone I really like to work with,
but don’t cut it in advance, the slices “dry” and will split, if you
cut it and then immediately work it to shape and finish, it does fine
and I have had no pieces crack even after several years, it is just
the characteristic of that stone.
Another thing is most stones will contain water, and they will dry
and crack, it is not only Opal that is prone to this. Usually
finishing them slows the evaporation or seals the surface that I have
only seen one stone that cracked after completion. Wait till you get
a chance to work on really fine grained dense Jade, that one you have
to make shellac wheels to work with. It was the most expensive 12
hours I have ever had in my life short of surgery.
Don’t know if that answered your question, but it is what I have
observed in the cutting I have done. Also, you might want to start a
notebook on the stones you work, what you notice and how they react
to your efforts, it will help in the long run and keeps down the
learning curve. There is a world of differences in the same stone but
from different locations on the planet.
Terry