All melted

Glad to see the snow and ice go but not quite as chuffed to see my lovely plastic Devonian freshwater deposited clays come out of the kiln like this.....

Cone ten is clearly pushing them too far - and thus continues my poor success in finding local clays that can withstand a temperature high enough to allow a glaze to be fired on them. Strangely clay 3, which was the black one that bloated severely and seemed very dense at 1000C, has actually done the best and could probably be used as a slip. Clay 6 looks unpleasantly like cat shit and the others are puddles of varying volcanicity. Time for some lateral thinking in what I am actually going to be able to make.

Crushing on ice

This week I have (mostly) been doing the most laborious part of the process of making my geological pots - a series of quick 700C firings to calcine small rocks and then...

....banging away in my steel mortar and pestle to get them down to 2mm or less grit. Most have been OK but the limestone I collected from the Malvern Hills and a Cumbrian Rhyolite were INCREDIBLY hard. My arms and shoulders are still aching.

This image shows a finished crush of the oldest rock that I have collected so far - a PreCambrian partially metamorphosed granite that is around 800 million years old- ready for the ball mill.

I'm finally ready to mix up my glazes and start glazing pieces for the firing. It's going to be longer than usual (three days instead of two) so I am using my stiffest (most refractory) mixes to allow for the extra ash and heat work. I also did a test firing today with new Leicester materials and firing those Worcester Carboniferous clays to cone 10 to see if any of them can withstand it!

Never judge a clay by its colour

....or, revealing the iron content of clay by the magic of bisque.

These are the clay samples fired relatively quickly to 1000C and I have to say I'm torn. There are some really gorgeous rich red browns there and a lovely range of colour... but I was hoping for something rather paler. Deep red means higher iron content and therefore less chance of it being a stoneware clay (especially in reduction). Surprisingly the darkest red clay (number 4) was the palest clay in its natural state. Where did that come from? The most promising clay is number 1, which was a green/blue in its natural state. lots of sulphur given off during the firing, so the iron probably present as iron sulphide. I was hopeful of the black clay (number 3) and it is relatively pale, however the test bar has swollen dramatically indicating a failure to burn out all the impurities (such as carbon) from the inside. A slower biscuit firing will solve that but the problem is the vitrification of the clay. A stoneware clay will be very open and porous at this temp. but the black clay has become very dense, suggesting it is approaching its maximum useable temperature. Now for some higher firings with the test rings at first to see....How far can I go?

Colours of the Carboniferous

And by that I mean the Carboniferous era - not the results of my last attempt to cook potatoes in a bonfire.

I collected these seven clays yesterday from the trenches of the geological dig. They were all laid down in the Carboniferous era and as such are relatively unusual. by far the largest carboniferous deposits in Britain are the massive layers of limestone that stretch across the country. The limestones were deposited in shallow oceans, but here, where these thin clays are, the land was above sea level and the deposition was in freshwater lakes or ponds. The range of colours is quite beautiful. It seems that at least a couple of them may be lowish in iron (it's difficult to tell with the carbonaceous one) and I'm hoping that as they were freshwater deposits they are low in lime. Feeling optimistic - now to dry and fire!

Black clay, green clay, red clay...

I had a fantastic day near Worcester today, being shown around a geological dig by Natalie from Herefordshire and Worcestershire Earth Heritage Trust, who was extremely generous with her time and knowledge. This image shows the site of the dig, where trenches have been cut into a field to expose the geology below. I was expecting clay as the surface is so sticky but it was like a potters (or geologists) Aladdin's cave down there.

At the centre of the dig is a dome of the Precambrian Malverns complex - igneous intrusions of granite and dolorite that have been variably metamorphosed. In layers away from this are Cambrian sandstones, various Carboniferous sediments (including the clays in this image), Silurian mudstones and Triassic deposits. The clay seams are quite thin and interlain with sands and sandstone and have incredibly varied colours - red, purple, green and black. They were deposited in freshwater and some are quite pale, so I'm hopeful they may be refractory enough to glaze fire (eternal optimism of the potter!)

The area is a goldmine for me as there is such a varied local geology. This sandstone outcrop is overlooking the excavation. It is beautiful red Triassic sandstone. There are also limestone ridges nearby littered with beautiful old overgrown quarries (such as in the last image). These are Silurian nodular limestones, interbedded with shales and (beleive it or not) bentonite! It certainly helps having a geologist to show me around.