The Sculpture Stone Company
About Sculpture Stone
All stone can be identified by the chemical composition of the mineral (a pure, solid crystalline substance) or the combination of minerals and non-crystalline substances of which it is made up. The study rocks is based on a science that uses many properties for identification and classification. Understanding these properties will not only help sculptors understand the materials they are using, but will help them in the selection and carving of stone. Not all properties are relevant to sculptors, those that are will be described in detail here:
This describes a stones ability to resist abrasion or scratching, or the ability of one substance to scratch another. Geologists use the Moh's Hardness Scale based on 10 minerals as an arbitrary way to test this property:
Minerals that can be scratched by a fingernail (H 2.5) are called "very soft", and those that can be scratched by a copper penny (H 3.5) are called "soft". Those that can be scratched by a pocket knife (H 5.5) are called "semi-hard" and those that cannot be scratched by a pocketknife "hard" and those that cannot be scratched by a steel file are called "very hard". Those stones that are hardness above 7 (cannot be scratched by Quartz) are sometimes also known as "gemstone hardness". Some minerals will also exhibit different levels of hardness when attempting to scratch in different directions, depending on the direction of the stones grain or bond.
Toughness and Brittleness
These properties are often confused with the hardness of a mineral. A "fragile" mineral is one that is easily split or broken into pieces. The term "brittleness" is another term that is applied to a mineral that rather than pulverize tends to shatter. This could be a soft mineral such as a piece of Talc (hardness = 1) or a it could be a piece of Quartz (hardness = 7). A piece of Jade, on the other hand although having a hardness less than or equal to a piece of glass may not break even when hit with a hammer. The term for this is called "toughness".
A stone that is not brittle is often difficult to split when driving a wedge or chisel into the stone. Rather than allowing the tool to drive into the stone to split it apart, these stones will tend to pulverize around the tool leaving just small chips and powder. The quality of brittleness is also important when carving. When carving a brittle stone it may be easier to crack or shatter the stone if hit too hard. Alternatively a stone that is not brittle that is hit too hard will crush that section of the stone leaving what is called a "bruise". Bruises must be eliminated by carving or sanding deeper to remove it, or will show up on the finished piece. The effect of bruising or leaving tool marks in a stone is also an effective way to contrast one area of the sculpture with another area that is highly polished. This technique is often called "texturing" the stone.
Cleavage / Fracture
This is another indicator of how a stone breaks, also described using the term "layering" or "grain" (similar to the term used for wood). Those minerals with a strong layering will break in a predictable manner, called cleavage, giving a generally flat plane according to the crystal structure. Although many stones do not have strong cleavage almost all will exhibit some of this property on close inspection. The surface of those that do not break in a predictable manner are called fracture. Some brittle stones fracture in with patterns of curved ripples similar to the pattern and shape of a clam shell. This phenomenon is called conchoidal fracturing.
The density of a stone is typically measured as the mass of a stone per unit of volume (grams per cubic centimeter), known as the specific gravity. The specific gravity measures how many more times the stone will weigh compared to an equal volume of water which weighs 1 gram per cubic cm or one thousand kg per cubic meter.. For example a mineral with a specific gravity of 5 will have 5 times the weight of a similar volume of water. A very light mineral will have a specific gravity that is less than 2, and a heavy mineral will have a specific gravity greater than 10. Remember all the movies where the bad guys took off with large quantities of gold bars? Highly unlikely given that one cubic foot of gold would weigh 1000 pounds or more. That being said... what is heavier, a pound of Styrofoam or a pound of gold?
For a sculptor a more commonly used measure of the density is pounds per cubic foot, of which water weighs about 62.5 pounds. Some examples of common stones are:
Calcite (Marble) SG = 2.71 (170 pounds)
Gypsum (Alabaster) SG = 2.35 (147 pounds)
Some pure minerals occur mainly in one distinctive color, others may occur in many different colors. For the purpose of sculpture the purity of stone is not really a factor assuming that the impurities do not affect the ability of the stone to be carved. Because most stones are often a mixture of minerals it is possible to find the same types of stone in a never ending variety of colors and patterns.
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