The Stone for the Adelaide Hills International Sculpture Symposium

A variety of stone is used by the sculptors; they are described below.

‘Stone’ is a quarryman’s term for what a geologist would call ‘rock’.  The natural fractures and joints in any rock aid the quarrying process as the rocks are removed from a quarry ready for a myriad of uses. The natural features of the rock also influence how the sculptors ‘carve their dreams in stone’ from the huge blocks of rock that they begin with.

Granite

Granite is the signature rock of the continents. More than that, granite is the signature rock of the planet Earth itself. The other rocky planets - Mercury, Venus and Mars - are covered with basalt, as is the ocean floor on Earth. Only Earth has this beautiful and interesting granite rock type in abundance.

Granite forms when magma (molten rock) cools and solidifies very slowly beneath the earth’s surface. It is an igneous rock; the name comes from ‘ignis’ which is the Latin word for fire. The word ‘granite’ also comes from a Latin word, ‘granum’, a grain, because granite has a grainy texture that is made up of a mixture of minerals including quartz and feldspar, plus a range of other accessory minerals.

Granite is related to other igneous rocks; some, like basalt, are formed when the magma erupts as lava from volcanoes.

Granite comes in different colours, usually pink to grey, but sometimes black; classic granite has a ‘salt-and-pepper look’ caused by the light coloured quartz and feldspar which is speckled with darker accessory minerals such as black biotite mica and the black amphibole hornblende.

Granite is a very hard rock.  It is hard because some very hard minerals grew as it slowly cooled and crystallised from magma underground. These include quartz which is a common mineral in granite and has a hardness of 7 on a scale of 1 - 10 (Moh’s Scale of hardness).  On this same scale the mineral Talc (think about talcum powder which is very soft) has a hardness of 1; Calcite, similar to the calcium in your fingernails and bones has a hardness of 3 and Diamond, just about the hardest mineral of all, has a hardness of 10.

Granite does not have the layers that are found in sedimentary rocks like limestone, or the strong cleavage fractures that are found in metamorphic slate. Granite is so hard that people use it as building stone or for statues if they want them to last a long time. There are granite slabs edging the eastern side of the South Australian Parliament building in Adelaide; people often sit on them whilst waiting for buses, they are not likely to ever wear out!

The black granite the sculptors are using in this Symposium is actually “norite” because it does not contain enough quartz to be properly called granite and has a much higher percentage of dark coloured minerals. From a sculptor’s perspective this is a softer stone to carve because whilst it has less quartz it is still a strong rock that can be cut.

All granites can be polished to a smooth surface with a shine. This is the best way to see the composition and colour variations of the granite, because all the different coloured minerals show up well on the polished surface. To get some idea of what the rough stone will look like when it is polished, try wetting it – geologists and quarrymen used to spit on them to see the detail!

Marble

Marble is a hard, crystalline metamorphic rock made up of calcite or dolomite crystals. It starts its life deposited as limestone sediment in the world’s seas and during the process of mountain building (or orogenesis) it is subjected to heat and pressure over millions of years. This process is called metamorphosis and it changes the limestone to marble – it’s a bit like the process that happens when you bake a cake in the oven, it begins as a soft wet mixture and ends up as harder, drier cake. In much the same way the limestone is ‘cooked and squeezed’ into marble, as crystals of calcite grow into the tiny spaces in the rock. The length of time the marble takes to cool determines the size of the crystals.

Marble is a marvellous rock for sculptors to work with as it takes a high, lustrous polish and is relatively easy to carve as it is much softer than granite. It has been sought after for use by sculptors over 1000’s of years as a material for statues and monuments; it is also used as a facing stone in buildings and residences, for pillars, colonnades and floor tiles.

Marbles range in colour from snow-white to grey and black; many varieties have shades of red, yellow, pink, green or buff.  The colours, which are caused by the presence of impurities mixed in with the calcite, are frequently arranged in bands or patches which add to the beauty of the stone when it is cut and polished.

Marble was used extensively by the ancient Greeks, for example in the Parthenon, and has been favoured as a sculpting material throughout history, especially during the Renaissance.  Michelangelo's famous sculptures ‘David’ and the ‘Venus de’ Medici’ are made of marble.

Among the famous marbles of Italy are the Carrara and Siena marbles of Tuscany.  Several of the sculptors participating in the 2012 Sculpture Symposium spent considerable time in Carrara, which has become the mecca for stone sculpting and learning related to it.

Some of the marble being used in the Sculpture Symposium this year (2014) is from a now disused quarry at Paris Creek in Battunga Country, in South Australia. Famous for its grey and cream marble, Paris Creek marble was used for the State War Memorial on North Terrace in Adelaide.  Macclesfield, also in Battunga Country, was an important source of pink marble.  Marbles are quarried in all parts of the world.  When freshly quarried it is slightly “soft” and a little more flexible, therefore easier to work in finer detail.

Sculptors carving tools are often made of steel or iron which has a general hardness of about 5 (on Moh’s Scale of hardness from 1 – 10), making it easier to carve marble.  Those who carve the granite need to use diamond tipped tools.

Mt Gambier Limestone

The Mt Gambier Limestone is being used in the workshops that are being run as part of the 2014 Adelaide Hills International Sculpture Symposium.

Limestone is a sedimentary rock made of calcium carbonate in the form of the mineral calcite. It forms in clear, warm, shallow seas, like the waters around Pacific islands, Indonesia and the Bahamas today, where calcium carbonate is precipitated from the water.  Limestones may also include skeletal fragments of shells, coral, foraminifera or other marine organisms which form fossils or fossil fragments within the rock. The sediment is deposited and builds up on the sea floor in sedimentary layers.

Limestone is used as a rubble base for roads and railways, as a pigment or filler in toothpaste and cosmetics and as an aggregate in concrete.  It is a relatively soft rock which is easily cut for use as a building stone; many of the buildings in Mt Gambier are made from this limestone. The pyramids in Egypt are built from blocks of limestone quarried there.

Limestone is a porous rock which has very small spaces between the tiny grains of calcium carbonate. This means that rainwater, turning to groundwater, can seep through it, much like coffee does through a percolator.  Over time the action of groundwater can erode large underground cave systems, like those at the Naracoorte World Heritage Site not far from Mt Gambier.

Limestone has mixed fortunes as a rock for carving by sculptors: the process is relatively easy as the rock is fairly soft (about 3 on Moh’s Scale of hardness from 1 - 10), but it is very susceptible to weathering and erosion so statues would not retain their detailed features or last long in a wet environment.

Stone used in 2014 Sculpture Symposium: all the marble and granite used in the 2014 Sculpture Symposium is South Australian. The marbles and granites were donated by Barossa Quarries, Chesini House, Melocco Stone, Southern Quarries, Graham Arthy, current owner of the disused Paris Creek Quarry and Bert Davis from Macclesfield. The limestone was supplied by Mt Gambier Limestone.

Stone used in the 2012 Sculpture Symposium: all the stone used in the 2012 Sculpture Symposium was South Australian and was donated by Barossa Quarries, Melocco Stone and Graham Arthy, current owner of the disused Paris Creek Quarry.

The rock descriptions and information were written by Emeritus Professor Jane James BSc (Hons) PhD, Director of JayTalk Consulting.

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