Posted on

Glaze materials

chalk and limesone, glaze material

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.

Quartz

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.

Granite: feldspar (white), quartz (grey) and mica (black)

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.

Clay, dried and powdered

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.

Whiting: chalk or limestone

 

Posted on

Geology for potters

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.

You can read my article here:

Bloomfield_Nov2015

OLYMPUS DIGITAL CAMERA
Aust Cliff, photo Adrian Pingstone 2002
Map of clay deposits in UK
Map of commercial clay deposits in UK by Henry Bloomfield