Working with Obsidian

[Don Dougan <dondougan@xxxxxxxx>]

Arthur;

In reply to your query:  "Has anyone worked with obsidian, and have some
ideas of how to carve it etc."

I have worked with it, and the information in the other replies you have
received are correct.   Use of an organic cartridge respirator is
mandatory  for safety, as is eye protection.  Gloves will prevent
molecule-thin edges of chips from slicing up your hands.

The pieces I have done were all relatively small - less than eight inches
in the largest dimension when finished.  They were all jet-black obsidian
 from Idaho that my father and I picked up in the late 50's (I was a child
rockhound) and managed to keep through many moves around the country.  My
father, an engineer and amateur archer,  experimented and taught himself
to flake arrowheads with some of the obsidian.  I became a
stonecarver/sculptor and although most of my work is in marble, I can't
resist trying to 'carve' most types of rock-like things that come out of
the earth.  In the case of the obsidian, I have worked it several times
and probably will again.  I still have two or three more chunks of the
same obsidian in my stonepile.

As far as shaping the obsidian, I used both pneumatic bushhammer
'chisels' and powered abrasive grinders to shape the forms.  The
abrasives used were diamond and/or silicon carbide, and I used them dry.
If I had access to water-fed tools several years ago when I was last
working on the obsidian I would have used them, but I didn't, so I had to
rely on even more patience than is needed with other types of stone.

You must proceed with patience if shaping the obsidian dry, because the
heat generated by the grinding can cause abrupt fractures of the surface
material.  Little chips will fracture and spall off the main body of the
work due to the unequal expansion rates between the surface and the
interior.  To prevent this:  apply only a minimal degree of pressure and
keep the edge of the grinding wheel moving over the entire form - do not
focus all your work in small areas and thus allow that area to heat too
rapidly.

When I used the bushhammer chisels, I was working towards a rounded and
compactly shaped form.  After the form was shaped, I wanted to retain
some of the texture left by the teeth of tool, but I also wanted a bit
less 'rawness' to the finished surface.  I used a light application of a
sandblaster to soften the surface yet keep some evidence of the tool.
For contrast I polished a very crisply defined area of the obsidian.
This polished area became like a translucent window into the interior
depths of the form.   After using all the grits of silicon carbide
wet-or-dry paper (wet), the final polish was attained using cerium oxide
powder .  The powder is mixed with water into a slurry and applied as you
would tin oxide on marble.  Cerium oxide is used for polishing glass, and
supply houses that serve the flat glass and automotive glass repair
industries will carry it.

For smoothing larger broad surfaces the use of random-orbit sanders with
silicon carbide paper works very well.  The random-orbit pattern of the
abrasive cut  is cooler and less likely to create problems than using a
regular circular-orbiting type sander.  Relatively coarse silicon carbide
sanding discs used on a regular circular-orbiting sander will work dry
without generating too much heat.  But when finer grits are used (say
 from 120 grit and finer) in a common circular-orbiting tool, one tends to
work in small areas for focused control.  But this generates excess
friction heat and causes mini-fractures on the surface, which sets back
your progress until you learn to keep it moving rather than focusing your
work in any one area.

If you are serious about working hard 'stones' like obsidian (and your
budget allows it) then water-fed power tools & accessories would
definitely be recommended over dry working abrasive tools.  The
water-feed cools the material and allows a more normal degree of focused
work without running the risk of setback due to friction heat.  Water-fed
tools are available from stone industry suppliers like Gran Quartz , VIC
Industrial, or Eastern Marble Supply.

I have experimented with using a pneumatic sandblaster to 'carve'
obsidian, and it works as long as you pay attention and allow the design
of your form to follow the nature and limitations of what the tool can
do.  Rubber masking materials will allow crisp definitions in surface
etchings, but become less useful as the depth of cut increases.  A
pressurized-tank type sandblaster will be needed to 'carve'  - though a
siphon type sandblaster will work for surface etching of minimal depth.

Good luck, and don't forget the respirator!