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.

Good times

The workshop and kilns seem to be floating on a completely saturated sponge of mud and grass (or is that pond weed?) and a White Christmas seems a long way further north but the festive spirit is building - helped in no small way by my latest glaze test results.

Some may feel that glaze tests are not particularly festive in themselves but when they come out as well as these I feel myself reaching for the mulled wine and a rum soaked mince pie. Happy Christmas everyone and I hope 2013 is a truly great year!

Winter solstice

Ignoring the Mayan prediction and hopes of an alien spacecraft erupting from a French mountain, I decided to look to the future and celebrate the now lengthening hours of daylight by preparing for a test firing.

Much weighing, mixing and syringing later and the test kiln is packed and ready to go tomorrow for the last firing of 2012. These are lineblends of various Leicestershire Cambrian and Pre-Cambrian rocks - variations on previous tests and using some new ash that I collected from the Beacon Hill National Park rangers from wood that grew in the park. Fingers crossed for some good results (the last lot of ash was too full of earth) and no doubt this time tomorrow I will be toasting the kiln with a festive bottle of Fullers ESB.

Festive firing

No post for a while, no Caribbean holiday, just long cold hours in the workshop trying to prevent my feet freezing to the floor. I am also researching a couple of new places to study + went to Worcester University last week to discuss the geology of the Malvern Hills and begin the long process of getting permissions to collect samples. It is a fascinating area with the oldest rocks in England (at a cool 800-600 million years old)plus a huge variety of other sedimentary rocks.

This image shows two of the shale samples I obtained from the geology department fired in reduction to 1270C. Just goes to show that it's not only the clays of the East or the Triassic mudstones of the Midlands that melt at stoneware. The one on the left is a Silurian era red mudstone and the other a grey Carboniferous mudstone. Other than that it's making and remaking for the next wood firing.

It's always a long process making new forms. I liked the later pieces I made so much more than the precursors, so I reconstituted them and carried on making. Time for one more firing before christmas.