We recently visited the Dorset Jurassic coast, so named because it features layers of Jurassic limestone: 145 million year old Portland stone and Purbeck stone, as well as layers of Wealden clay, Greensand and Cretaceous chalk cliffs. Some of these layers contain fossil sea urchins and shells, and there is a even a fossil forest, with the remains of cypress tree stumps. The layers, or strata, have been uplifted and are now at a steep angle, some even vertical. The hard limestone has been eroded less than the clay, resulting in some unusual features like Durdle Door, a natural sea arch. Lulworth Cove is a perfect circle, where a river has cut through the limestone and the sea has come in and washed away the softer clay.
We collected some of the clay to make into a pinch pot. It was quite sandy clay and may contain too much calcium to fire above earthenware temperatures.
The layers of limestone are very thin because they were deposited in a shallow lagoon which frequently evaporated and dried up. The chalk cliffs are much thicker because they were deposited when the sea was relatively deep. The pebbles on the beach are the flints which dropped out of the chalk cliffs as they were eroded by the sea. This took tens of millions of years, the pebbles gradually getting smaller until they became shingle and then sand. The chalk on the seabed reflects the light, making the sea look turquoise here even on a cloudy day.
Earthenware glazes are made mostly of frit, which is a kind of man-made feldspar. Borax, soda and other water soluble materials can be made insoluble by heating together with silica. The resulting glass is ground up and used together with clay and silica to make low temperature earthenware glazes. Frits are also used to lower the melting temperature of mid-range glazes. The raw mineral borates are found in dried up lake basins in Turkey and California.
Opacifiers can be added to make the glaze opaque. Tin oxide is the most effective opacifier but zirconium silicate is less expensive.
A respirator mask should be worn when weighing dry materials. The dry powders are added to water and left to slake before sieving.
Stoneware and porcelain glazes are made up of at least four ingredients: silica, feldspar, whiting and clay. This is what they look like in their raw mineral forms. Silica is ground flint or quartz, usually from sand or sandstone. These are large pieces of milky quartz from a vein in igneous rock.
Silica has a high melting temperature. In order to melt in a kiln, it needs a flux. The main flux in stoneware and porcelain glazes is feldspar, found in granite, an igneous rock composed of the minerals feldspar, quartz and mica. This piece of granite from Devon has both muscovite (silver, sparkly) and biotite (black, crystalline) mica. The dark vein is mafic rock, high iron and magnesium. Granite forms when molten magma cools and solidifies. The darker minerals solidify first, then the feldspar and finally the quartz, often in large veins running through the rock. Slow cooling, deep in the earth’s crust results in large crystal size.
Clay contains both alumina and silica and is added to increase viscosity to prevent the glaze from running off the pot when molten in the kiln. Clay also helps to keep the heavier quartz and feldspar suspended in water in the glaze bucket.
Calcium carbonate added in the form of whiting is an extra flux which helps to stabilise the glaze and can also produce a matt glaze surface. Whiting is ground up chalk or limestone.
Clay is an amazing material. You can shape it and it holds its form, then you can fire it and it turns into stone. Under the microscope, clay is made up of tiny stacked hexagonal crystals, which are able to slide over each other when lubricated by water. The kaolinite image below is reproduced from the Images of clay archive of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society.
Each clay crystal is made up of thousands of layers of silica tetrahedra and alumina octahedra. These can build up in alternating layers as in kaolin, or in three layers; silica-alumina-silica, as in bentonite. Water can get between the crystals, which enables them to slide easily over each other, allowing the clay to be moulded into shape. Kaolin is less plastic than bentonite as it has a larger particle size. A third type of clay, illite, is derived from mica, and is a constituent of red earthenware clays.
I am currently writing a book on Science for Potters for the American Ceramics Society. An extract on Geology for Potters has been published in Ceramics Monthly November 2015. Many potters use locally sourced clays and rocks in their clay bodies, slips and glazes. While researching for my book, I have become interested in geology and spent part of my recent holiday in Cornwall visiting clay mines and searching for rocks. My father used to take me on rock collecting expeditions to Aust Cliff under the old Severn suspension bridge, where we found quartz crystals and fossils. Aust Cliff is composed of layers of red mudstone and limestone. You can see in the geological map below, it is part of the Mercia mudstone red clay shale deposit which stretches from the north of England coal beds down to the south coast. I recently visited an exhibition in National Museum Cardiff on the geological maps of William Smith, who first realised that layers of rocks were laid down in a sequence according to the fossils embedded in them.