Last Tuesday I fired up the small gas kiln at Bath-Spa Sion Hill Campus, with very mixed results. It is a tricky kiln to fire, with the single gas burner positioned right beneath the thermocouple on the right wall of the kiln and a rather temperamental gas knob. The firing results were fairly bizarre, in that the top shelf reached cone 9-10, but the bottom barely reached cone 7, with only cone 06 fully down.

However, I made some useful observations. First of all, the Tenmoku glaze Rich Winfield introduced me to, fired very well across all shelves, and after a discussion with Rich, it may well be that the high iron content helped flux the glaze even at low temperatures in reduction (see the rows of mugs below). The beach clay Tenmoku, which has so far been a real success, came out almost metallic, and there was some crawling – similar to some of the pots I fired in my electric kiln over the weekend. I have an idea as to why the crawling takes place. The crawling seems to only occur where the glaze dips join, for example on the rim and where there is no real overlap. There seems to be an issue with discontinuity with this glaze – and it seems to work best when the glaze is applied either all at once, or when the glaze poured in / out on the inside, is then immediately followed with a dip for the outside (i.e. – when the glaze is still damp – which may help the glaze join?). There may also be an issue where this glaze is too heavy / thick.

Beach clay tenmoku crawl…..

The beach clay glaze from west Wales has yielded some really great results in oxidation and reduction (cones 9-10). The beach the clay comes from took a hammering from several very heavy storms over the winter and the clay can be collected in buckets by hand with no invasive use of spade / trowel etc. – which is important in fitting with my low impact commitments to sourcing natural materials. A friend actually shared her dismay at hearing I had taken clay from the beach, but I have no worry in collecting something that will only be washed away in the next storm and where I make no visual impact. Indeed the cliff from which the clay comes is perhaps now quite unstable and it seems likely that more will fall soon. Finally of course, the quantities that I am taking are minute – a bucket or two every 2-3 months.

I have been adding the blue glaze and a copper glaze to see what the results might offer in oxidation. Some lovely, encouraging results – see below.

Beach clay with blue glaze splashes

Beach clay glaze with copper glaze splashes

White zirconium glaze over cobalt / manganese dioxide

I have also been experimenting with a white zirconium glaze (from Stephen Murfitt’s  The Glaze Book, with under and over glaze. My best results are when the glaze is used thinly and with strong blue / blue black glazes or oxides. The problems come when the glaze is applied too thickly and then when it runs, it picks up the under / overglaze and the colour washes downwards.

Following the trip to Clee Hill, I have since dried, broken up, slaked and sieved the raw clay (through a 60 sieve). The result was a beautiful pond of yellow slip (see below), but the results were incredibly tacky and whilst highly plastic, impossible to work with for building / throwing. I then re-sieved the residue through a wire cooking sieve to remove the larger stones and organic debris and added the coarser slip to the earlier sieved clay (c 30%-70%). This made a highly workable clay, which threw well – and two thrown test cylinders are going through a biscuit firing as I write. I will test the slips capacity as a glaze material this weekend in my next glaze firing.

Clee Hill clay as a slip drying in the sun

Wheel thrown Clee Hill clay (fired to 1000 degrees)