Be Inspired: Materials Under the Microscope - Marble

And we’re back to stone! In the fourth edition of our series on sculptural materials, William (now writing from Oxford) takes us on a written journey to the Oxford University Natural History Museum (and the British Museum) – and in particular, to the geology and sculptural exhibits. Informed by his visits, he explains the chemical composition of marble, how it’s formed, and reflects on why it’s history’s favourite sculptural material.

When the layperson hears the word ‘sculpture’, the first images evoked are inevitably of marble: the large monumental sculptures we see in art history monographs, in our high school history textbooks, and in museums and galleries worldwide. We think of Michelangelo’s David, the Parthenon Marbles sculpted by Phideas in the 5th century BC (now controversially locaed in the British Museum), of the Venus di Milo, or of any other number of classical sculptures. We think of the famous quarries in Carrara, Italy (currently active; and where our favourite marble sculptor, Usama Alnassar, is from), and historically from Paros and Athens. It has become a worldwide symbol of the traditional and of fine taste.

So why is marble such a popular sculpting tool, and where does it come from? Once again, we have to go back to chemistry!

Learning about rocks and metamorphosis – visiting the Oxford Natural History Museum

During my first weeks in Oxford, while the town bereft of students and academics (undergraduate students are forced to vacate in the Christmas holidays), I took advantage of the many (free!) museums and galleries on offer in this historic city. First on my list was the Oxford Natural History Museum!

The Museum was established in the mid-19th century to draw together and showcase the scientific work which was being done around the University of Oxford. It is most famous for being the venue of ‘The Great Debate’ between Darwinian advocate Thomas Huxley and Bishop of Oxford Samuel Wilberforce, concerning the recent publication of On The Origin of Species. But what interested me more as I entered was some of sculptural features of the interior.

Firstly, on the periphery of the atrium are 28 full-body sculptures or busts of great men in science (and only men – the inclusion of some of the great women of science appears an ongoing project…). The sculptures are almost all in Caen limestone sourced from Normandy in France, sculpted by various famous Victorian sculptors. And second, the outer columns that surround the atrium (126 in total) are each made for a different decorative British rock (various kinds of marble, limestone, granite, alabaster, etc.) – all polished and labelled. Truly beautiful! You can see the limestone sculpture and stone columns if you look carefully on the left-hand side of the interior photograph below. If you’re interested, you can read more about the architecture of the Museum on their website.

Then I began to peruse the exhibits – the dinosaur skeletons, the slightly comical ‘Australian mammals’ exhibition…until I found what I was looking for: the rock/stone exhibits! Sure enough, on the very outskirts of the main atrium was an exhibition that explained, in an astonishing level of detail, the aetiology of hundreds of different kinds of rocks, complete with illustrations that explained the different types of rocks and how they form.

In the previous edition on soapstone, I explained that stone, as we understand it in sculpture, is made up of a mixture of minerals – or crystals that have a regular chemical composition and structure. We learnt that soapstone was primarily comprised of the mineral talc (or Mg3Si4O10(OH)2), and Christine, in her article on limestone, explained that limestone, on the other hand, is primarily composed of the mineral calcite, or calcium carbonate (or CaCO3). But what we didn’t learn about is how these minerals form in the first place, or how different combinations of minerals come to mix together!

There are three main types of rocks, named after the processes that lead to their formation:

1.    Sedimentary rock – formed by the accumulations of mineral or organic matter (such as that from skeletons, fossils, or shells), cemented together via chemical processes;

2.    Igneous (or volcanic) rock – formed when magma or lava (from below the Earth’s crust) cools and solidifies (either above or below the Earth’s surface); and

3.    Metamorphic rock – formed where existing rocks change their chemical composition or structure (or mix) as a result of intense heat and pressure in the Earth’s crust.

Limestone, as Christine explained, is an example of sedimentary rock, often formed by the cementation of particulate matter from the decay of sea creature skeleton or shells. Examples of metamorphic rock encountered in sculpture include granite and obsidian. Finally, both soapstone and marble are examples of metamorphic rock – formed by the conversion of existing minerals/rocks into new ones via the alteration of both the rock’s chemistry and the minerals’ crystal structures. As we saw in the article on plaster, slight variation in a compound’s chemistry can cause radical changes in its properties).

The various forms of rock can be converted into one another via processes that are summarised in what is often called ‘The Rock Cycle’ (see below).

Marble – metamorphosised limestone

So back to marble! It may surprise you to know that marble is nothing more than metamorphosised limestone – that is, under intense heat and pressure, the original calcium carbonate crystals (CaCO3; formed via sedimentation and not particularly tightly packed) are dissolved and recrystallised in a far more tightly packed arrangement of interlocking carbonate crystals. The simple process transforms a softer rock (2-3 on the Moh scale) to a harder rock (3-4 on the Moh scale).

Marble isn’t the only well-known stone that is formed by the metamorphosis of well-known existing rocks. Granite, an igneous rock, can be transformed into gneiss; shale into slate; various kinds of magnesium and iron rich igngeous rock into serpentine; and sandstone into quartzite.

The desirable properties of marble

But enough about chemistry! Let’s talk about the rock. Why was it so popular?

Well, for one, it was hard enough (but not too hard) to be carved by the old masters in a controlled way (with a hammer and chisel). And this hardness (and relative chemical inertness) also makes it durable and resilient (although bronze and granite are obviously more durable if one is sculpting for the outdoors!). On the other hand, marble is susceptible to decay via acid (as the calcium carbonate decomposes to form carbon dioxide), and also absorbs natural oils from human skin, leading to discolouration (hence all of the ‘DO NOT TOUCH’ signs!).

But it’s really marble’s characteristic colour, and its skin-like translucency, which has made it a material that has stood the test of time – in particular for sculptors attempting to represent the human form.

Pure kinds of marble are ordinarily waxy white. The ‘veins’ or dark intrusions in the rock are usually due to impurities in the antecedent limestone, such as silicates, iron oxide or silt, or sometimes even other kinds of rocks (such as serpentine). This characteristic waxiness is due to the carbonate crystals’ low refractive index, meaning that light is able to penetrate a significant distance into the stone before scattering (resulting in opaqueness).

Working with marble

Those students at the Tom Bass Sculpture Studio School who have completed one of our famous summer workshops in marble carving (with Alnassar) will be familiar with the 4-step traditional process of creating a marble sculpture with hand tools. Of course, power tools are always an option!

First, the sculptor ‘roughs out’ the basic shape of the sculpture by removing large unwanted portions of stone with a hammer and point chisel (sometimes called ‘pitching out’). Chisels work effectively by transferring the force of a hammer strike into a single point in the stone. Sculptors will often draw on the stone with charcoal or coloured chalk to assist them in marking out stone for excavation, or to help them ‘see’ their sculpture in their chosen piece of stone.

Second, the sculptor ‘carves’ their figure in the stone using shallower strokes and a variety of chisels. For example, tooth chisels (which carve parallel lines in the stone) are particularly effective at creating textures.

Third, rasps and rifflers are used to smooth out surfaces, or to create detail in the stone.

And finally, the stone is polished if desired (usually by using sandpaper, in combination with water and/or chemical polishes) to accentuate its natural sheen.

It’s a time consuming process, and causes one to reflect on just how long it must have taken some of the classical masters to complete their monumental works.

…but sculpture, like all arts, is a labour of love.

Next time: As the final article in our series, we take a look at alabaster, and how it is that it comes in so many different colours and textures!

Images

(1)  L: Michelangelo, David (early 16th century) (source: Wikimedia commons); R: The Parthenon (or Elgin) Marbles on exhibition in the specially built Duveen Gallery at the British Museum, London (source: Wikimedia commons);

(2)  L: The Oxford Natural History Museum exterior, on Parks Road in Oxford (source: Wikimedia commons); R: an interior photograph of the Museum’s main atrium (source: Wikimedia commons);

(3)  A simplified version of ‘The Rock Cycle’ (source: Wikimedia commons);

(4)  Some common metamorphic transformations (source: Wikimedia commons);

(5)  A piece of marble with clearly visible black veins (source: Wikimedia commons);

(6)  A typical carving hammer and a variety of chisel types (tooth chisel on the left, and point chisels on the right) (source: Wikimedia commons).

Words by William Jackson