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Man has been creating art from stone
ever since he found he could shape it by striking a softer stone with a
harder one. Over the millennia, a few types of stone have become popular
with sculptors. Here are the most common ones used for carving from the
three different types of rock: igneous, sedimentary, and metamorphic.
Igneous:
The characteristics of igneous rocks result from the way in which they
were formed. Deep in the earth, under the intense heat of volcanic action,
magma was forced up through the older solid rock. It then cooled, forming
granite, basalt, and diorite. Granite is made of quartz crystals and
feldspar which contain silica. Prolonged breathing of silica can cause
lung damage. Wear a good respirator when around granite dust.
Granite
is very hard stone and difficult to carve. In fact 'carving' might not be
the right word, since you are not pushing the chisel through the material
like you would with the limestone or marble, but are pulverizing the
stone, trying to break off chips of crystals (which can be as sharp as
glass). Your carving tools must be carbide-tipped to stand up to the wear
and tear of working granite. Fine detailed carving is better left to other
types of stone.
Diamond saws and cup grinders cut through granite quite
easily and can speed up the carving and finishing process. Granite comes
in a wide variety of colors, and the crystal size can vary from large and
coarse to very fine and dense. Granite takes a high polish and holds up
very well outdoors.
Sedimentary:
As rocks began to erode from wind, rain, and sun, the particles were
washed into low lying areas where the sediment accumulated. In the sea,
small plant and animal forms died and drifted to the bottom, adding to the
sediment. Over thousands of years, the pressure of these layers cemented
the sediment onto limestone or sandstone.
Limestone formed on the sea floor from
sediment and the bodies of primitive sea creatures. As you are working,
you can find small fossilized creatures like crynoids and braciopods in
the stone. Limestone is composed primarily of the mineral calcite, or
calcium carbonate.
Limestone is easy to carve and will hold small detail
work, but is also strong enough to support undercutting. While it does not
have the innate beauty of the more colorful stones, a wide variety of
textures can be created that can really bring the stone to life. There is
a definite 'grain' or bed (formed as the layers of sediment stacked upon
each other on the sea floor) in limestone. The stone breaks fairly
predictably along the bed lines, but less cooperatively going across the
bed. (Think of opening a phone book with the pages, compared to trying to
tear it across the pages.) Limestone can be polished, but the polish will
not last long outdoors. Limestone seems to survive acid rain better than
marble, so is a good choice for outdoor sculpture.
Sandstone is formed from sedimentary sand
held together by silica or calcium carbonate. Sandstone also has a
distinct bed direction, and large chips can be broken off when going with
the bed. Sandstone tends to wear out your tools
quickly. Sandstone contains silica, so a respirator must be worn.
Metamorphic:
Metamorphic rocks are formed when a sedimentary layer is exposed to heat
and pressure and undergoes a chemical change which forms a new crystalline
material. After metamorphism, limestone becomes marble.
Marble has been the most preferred stone
for carving since the time of the ancient Greeks. Marble is moderately
hard to work. It will hold very fine detail. Marbles from the United
States comes in over 250 colors. When brought to a high polish, its
crystalline structure sparkles. It does not hold up well outdoors. Acid
rain begins to deteriorate the surface within a few years.
Alabaster is a very soft stone for carving
and tends to flake and split along hidden cracks in the stone. It will
take a high polish, which brings out its incredible colors and patterns.
In fact the stone is so beautiful that the viewer may overlook your
sculptural forms and only admire the stone. The dust from alabaster may
cause an allergic reaction in some people, so wear a respirator.
Soapstone, or steatite, is soft enough to
carve with a knife. It is composed of talc and has a slippery, soapy feel.
It will take a polish and hold fine texture detail. It is a good choice
for your first stone carving. Prolonged exposure to talc dust can cause
respiratory problems, so wear a respirator. |
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