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, diorite, and others. 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 into limestone or sandstone.
Limestone formed
on the sea floor from sediment and primitive sea creatures. As you are working, you can
find small fossilize 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 take small detail work, but
is also strong enough to support undercutting. While it does not have the innate beauty of
other stones, a wide variety of textures can be created that can really bring the stone to
life. There is a definite 'grain' or bed in limestone. The stone breaks fairly predictably
along the bed lines, but less cooperatively against the bed. Limestone can be polished,
but the polish will not last 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 taken 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 Greeks. Marble is moderately
hard to work. It will hold very fine detail. Marbles from the United States come in over
250 colors. When brought to a high polish, it's crystalline structure sparkles. It does
not hold up well outdoors and acid rain can begin deteriorating the surface in 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.
