From:
Bill Brayman <blb@zzzzzzzzzzzzzz>
Date:
Tue, 23 Sep 2003 12:18:04 -0700
Subject:
How do granite carving techniques differ from softer stones?
On Tuesday, September 23, 2003, at 07:42 AM, Tim Bunton wrote:
Quoted text begins.Greetings to all,
I am ready to take a break from carving softer stones and take the
plunge to granite. Are the basic techniques of marble transferable?
End of quote.
I started learning stone sculpture where there was abundant granite.
Recently I began working marble from canada and california and a little
from italy. I like granite for outdoor sculpture. I mean, how many big
sculptures will your family let you keep in your house!
The one main difference between marble and granite is mineral hardness,
as you probably know. Since granite has quartz in its matrix, steel
won't scratch or cut it. Secondly, granite granularity is different
from marble because of the combination of quartz, feldspar, and mica
which seems to create little nodules. So, some of the operations you
are used to won't work on granite.
Imagine you have two blocks of stone, one granite and one a hard
marble. When you start out you will find that pitching big chunks off
with a pitching chisel is pretty much the same for the two. In fact
some marble will be tougher than some granite. So, you observe the
matrix toughness is comparable between the two materials.
Then you try a diamond saw blade which will be pretty similar between
the two. Both are pretty tough material. You knock off fins between
parallel cuts the same in both materials. But don't bother with
non-diamond blades in granite because the quartz will destroy the saw
blade. And don't bother with bonded diamond blades (made for soft
stone). Granite will eat them for lunch. You must use sintered diamond
blades. Use them dry for convenience, or wet to keep the dust down or
to use large blades.
Then you try the point chisel. Quickly you learn that pointing in
granite is at best a limited operation. With marble, the point actually
cuts into the marble. In granite, the point just knocks some surface
material loose and quickly wears a steel point out. So, you try a
carbide tipped chisel on the granite. Now the point lasts much longer,
but you have to work hard to make very deep chisel marks. And when you
try to create a long groove with the point, you find it won't go
straight, but glances off side to side. That's when you discover
granite has a lot of structure, like little nodules that deflect your
point off to the side one time or pop out cavities another time.
You try the tooth chisel on granite. Then throw it away. Useless. Then
you try a flat chisel on the granite. It smoothes out the bumps okay,
but doesn't cut in much.
As far as chiseling goes, this is where bushing comes in. Since you
can't chisel deeply and accurately in granite, instead you wear away
the surface by bushing. Its actually kind of a fun process, just
wearing away the surface in a guided manner. The trick here is to work
tangent to the curve you are creating. Think of pointing the chisel
direction towards the center of the circle (curve) you are making. The
bushing chisel then always strikes the material at the tangent of the
circle (or curve). Now instead of the big saw blade that limits your
curves to big 4 or 5 inch radius curves, you can make small radius
curves (at least on the outside of the piece where there is room to
maneuver the air hammer and chisel).
By now you are wondering how are you ever going to get any fine detail
or interior spaces in your piece!
Now you realize, on no!, another expensive tool I got to have, a die
grinder with sintered diamond grinding points. Yikes! this is getting
to be an expensive operation. But, if you want fine detail in granite,
you are going to have to grind it in. Well, actually, small very sharp
chisels may work for you, if you are determined, methodical, and
patient and don't have to work interior spaces in the stone.
Eventually, if not soon, you discover the diamond cup grinding wheel.
They are expensive, but they last for months, if not years.
Well, the story goes on of course. But, some of the main facts have
been brought to light.
Matrix toughness is similar between marble and granite. Some of each is
soft and some is very hard. Mineral hardness is much, much, greater in
granite, and the quartz in granite will wear steel tools out very
quickly. Sintered diamond saws and grinding wheels are your main tools.
You won't use chisels nearly as much, but carbide tipped chisels are
worth their cost and don't forget to get a couple of good pitching
chisels.
Granularity in granite is more mixed, in marble it is constant.
However, marble, being a layered material can be flaky and the grain
very pronounced. Granite grain is more constant.
There are some wicked hard granites, usually the dark ones, black, red,
or brown. They will not be as satisfying to carve as the softer gray
granites (in this region anyway).
Enjoy,
Bill
- Follow-ups
- message 00132: How do granite carving techniques differ from softer stones? - John VanCamp (23 Sep 2003)
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- message 00131: How do granite carving techniques differ from softer stones? - Tim Bunton (23 Sep 2003)
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