For most of my practice to date, my making has focussed on low-fire clays (local terracotta wild clay in particular). Not only does this reflect the colours of the local geology that I find so inspiring, but also because firing earthenware clay demands less energy than stoneware or porcelain. Low firing also maximises my opportunity to use locally-sourced wild clays.
The problem however with lower firing is often the lower choice of more interesting glaze recipes when compared to the plethora of glaze types and effects that can be applied to higher firing stoneware clays. Ever since I have been glazing my work, I have always wanted to find recipes that go beyond the usual shiny bright solid rainbow glaze palette geared toward use within a school or a beginner’s evening class environment.
As part of my early explorations into making glazes, I stumbled across the amazing rock glazes created by UK-based ceramic artist Matthew Blakely. Although I initially resigned myself to thinking that rock glazes would only be feasible for high-fire work, I researched rock glazes further and wondered if my local geology could directly contribute to a new earthenware glaze recipe.
Bristol Pennant sandstone dating from the Carboniferous period was quarried and used extensively during the expansion of the city of Bristol during the 19th century. There were around 16 sandstone quarries in North East Bristol associated with local coalfields although none survive today. Local pennant sandstone with subtle variations in its natural grey background colour with greens, ochres, reds, browns, pinks and plums, make very attractive walls.
Being a sedimentary rock with a very high amount of quartz (hence the name sandstone) I was tempted to take a basic low-fire glaze recipe consisting largely of borax frit, and replace the quartz element with sandstone readily available in massive flat slabs located just about two feet below my flower beds.
The freshly exposed faces of the rock obtained from the garden have a pleasant reddish-plum colour. Luckily, breaking up lumps of this rock is not too arduous a task. Once broken-down enough, the rock fragments can be sieved with a standard mesh kitchen sieve. The next step in the process, which is pretty laborious (as I do not have a ball mill in my possession) is the crushing of the stone fragments further with a mortar and pestle into a powder that can pass through a 60-mesh glaze sieve.
Once this is done, the glaze is made up of 50% borax frit (a lead-free flux necessary to bring the glaze mixture down to earthenware glaze firing temperatures), 30% sandstone and 20% potash feldspar. A surprisingly low amount of water (approx 50% by volume) is required to give a glaze mix to the required milky consistency. Maybe this is due the the fact that the sandstone element of the glaze is largely insoluble.
To date, I have had some pretty eventful experiences (and not often in a good way) when experimenting with earthenware glaze mixes. However my first test firings revealed a beautiful, slightly speckled plum-coloured satin-finish glaze that reflects the freshly-cut surface of the rock perfectly. My initial test firings of the glaze into which I dipped a couple of basic pinch pots made from local wild terracotta clay, have resulted in nice clay-glaze colour combination.
My decision to experiment with this local pennant sandstone has been a pretty lucky one for me, as apart from its largely quartz composition, I have absolutely no idea what other elements are present in this rock, which may be at play in delivering the final result. I’d really love to get a sample of this powdered rock X-ray analysed in order to determine its chemical composition.
After obtaining my results, I could not resist having a quick peek at the glaze close-up under a low-powered binocular microscope.
I am wondering if the red patches could be iron (which I know the local stone and soils above are rich in). I have no idea about the darker elements though.
As well as applying this glaze to additional work composed of differing clay bodies, I am now looking to identify other rocks that can also be easily broken down and incorporated into low-fire glazes. Watch this space.