Friday, 8 April 2011
Clays of Cambridgeshire
So.....I've sorted through the various deposits I sampled at the archeological dig site and made shrinkage bars and test rings. All of the deposits were quite fine - very little grit at all, except for the glacial silt layer at the top. The Oxford clay (Jurassic period) was the only sample that needed work (crushing and slaking) before is became usable, but is a nice plastic, though not too sticky, clay. The river deposits were mostly usable as found, though two are very high in organic material and so unusable. These clays have varying amounts of fine silt in them and are generally highly sticky with a very high dry shrinkage.
I dried the samples, treated them with hydrochloric acid, washed them and dried them agian - to ascertain the calcium carbonate content. Oxford clay is high at around 20%. All the river deposits have some calcium content, though that for the 'buttery clay' and the 'rodden silt' seems lowish at 3 - 5%.
This image shows the tests fired to 1000C. As you can see, a few of the samples have developed such bad fire cracks that they have fallen apart - these are not the ones with the high organic content - but a few have held together and will form the basis for my next test mixtures.
I will fire these clays to 800C, 1100C and 1200C to see how they cope, colour and porosity. I will also be using these tests to help the archeologists ascertain which clays were used by the bronze age and iron age settlers here.
All this plus more biaxials and lineblends for a cone 11 glaze firing - I can hardly control my excitement!