polishing basalt and other materials

[John Twilley <jtwilley@zzzzzzzzzzz>]

Robert,

This is a very good point and sculptors can take an example from the
experience of scientists making metal (metallographic) cross sections.
If you want a uniform polish, not only do you have to remove scratches
but you have to avoid creating surface relief in the process of getting
the scratches out.  This becomes harder the greater the difference in
hardness between the minerals is in a stone of mixed mineralogy.  For
example in a "granite" with black flecks the black is usually biotite
mica which is very soft and also prone to flaking.  The clear grains
consist of quartz and feldspar that are much harder.  Both the abrasives
and the abraded particles of the quartz and feldspar scour out the soft
mica, so that the longer one works at getting the scratches out, the
more the mica grains turn into small depressions.  By the time the
scratches are gone from the harder minerals, the surface is dull due to
the unevenness of the eroded mica.

The solution to this is the same as we use for making a cross section of
two metals of differing hardness like a solder joint between two pieces
of steel:  Use an abrasive of very high hardness like diamond (so that
the differences between the minerals being polished becomes minimal
relative to the hardness of the abrasive); proceed through gradual steps
of abrasive fineness (so that big gnarly particles from the earlier
stages don't dish out soft material deeper than the next abrasive can
smooth off); CLEAN THE SURFACE between stages (there's no reason to go
to a finer abrasive if you are scrubbing large particles around from the
previous stage); don't dwell too long with any given abrasive; and use
low nap wheel on a more rigid support.  In other words, work quickly
and  make rapid changeovers.

Consider the difference between a linen bedsheet, flannel and corduroy:
The linen has practically no "nap".  The flannel has an even, low nap
and the corduroy has a high nap that can "reach" into depressions.
When  working with materials of differing hardness you want the abrasive
to mow down the high spots without reaching into depressions and dishing
them out further.  It's counterproductive to use a nappy wheel or soft
support in this situation because the fibers will push the abrasive into
depressions, deepening them ahead of the harder areas.  You'll get rid
of scratches, but when you do, the surface will have an orangepeel
texture if the grains are coarse, or a frosted appearance if the grains
are fine.  The solution is to keep the abrasive moving in a plane on a
support that doesn't conform to the surface relief.

John Twilley

Robert Houghtaling wrote: