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On Stone - Spectrums of Hardness

The solid crust of the earth's surface consists of large rocks called stones when they are fragmented.  Each rock is the result of a random combination of minerals, so that it is practically impossible to find two rocks exactly alike.  Even two rocks formed of the same minerals may have been subject to various chemical, magnetic or eroding influences that make them different. 



Gneiss, Sechuan Province, China



Given the infinite variety of these combinations, geologists distinguish between the essential minerals that make up rock (the most abundant), accessory elements and accidental elements (the remainder).  The sculptor is interested in knowing about the essential minerals of the stone he is carving, as any difficulty in carving is due to those minerals.  Hard but brilliant silica and malleable limestone are the most abundant of the statuary stones.  There may be other minerals mixed in with them that provide texture and colour.   For example marble is primarily limestone (essential), but it may also contain arabesques of dolomite or graphite (accessory) and fossil remains (accidental) or be tinged with iron oxide.

Based on the essential mineral, we classify the stones in the table below into two large groups, silicates and carbonates, according to their composition.  This will help us to understand their similarities. 



The stone sculptor and stone craftsman is primarily concerned with the following characteristics:

Specific weight: Often calculated kg/m2. A simple list of stones ranging from lightest to heaviest would be: basalt, granite, marble, limestone, sandstone. 

Hardness: knowing this is important so that you can use the most suitable tools.  The conventional and unscientific Mohs' scale does not help us very much because all statutory stones would fall between 3 and 5 on this scale.  Stones that contain quartz, the most compact stones, heavy stones, and fine-grain stones are the hardest.

Resilience or impact resistance: Hard stones tend to be resilient and soft stones to be fragile but this is not always the case.  The same stone may turn out to be resilient when hit in a direction opposite to its growth pattern and fragile when its layers, visible in the form of seams, are separated.

Workability or ability to be carved and polished: Soft, compact, and fine- and uniform-grain stones such as alabaster or marble are most generous to the sculptor.  All recently quarried stones are worked more easily because they become harder later as they lose moisture.

But, always with a view to carving, classification according to essential minerals is not enough because sandstone behaves more like limestone than granite.   In addition, the characteristics which we have mentioned are also the result of the rock formation process and it is thus advisable to know the origin of the rock.

(Selected text and table from 'Sculpture in Stone' by Josepmaria Teixido i Cami & Jacinto Chicharro Santamera, Barrons, 2001)

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