A few weeks back I posted three videos (see them below) showing how I process and test hand dug local clay. Here is an update of the results.
Here is a pint sized mug that was thrown from clay that I dug from a trench just a mile from my house. This was straight dug clay without any additives. It threw well (see video) and fired pretty well too. I fired it to cone 04 (earthenware temps) first. At that temp. it was underfired and quite porous. I believe that was because of the abundant fine silica sand in the clay. I fired it again to cone 6 with good results. It fired to a dark red-brown with 3% absorption and 10% shrinkage. It is glazed with a slip and ash glaze that consists of 50% local slip clay and 50% wood ash from the woodstove. The inside has a pebbly, almost salt glazed look. I think this is because the soils around the Red River are high in salts and are quite alkaline. I think the salts in the clay interacted with the ash glaze.
I have been using the mug in the shop daily for the past two months. It has been through the dishwasher many times with no ill effect and has been used for everything from rootbeer to cider. This clay should work well with a bit of flux (PV clay), a little ball clay and some grog.
The test bar was 5" long when we started and is now 4 1/2" . That is a 10% shrinkage. It was fired with each end resting on a piece of broken kiln shelf. It did not sag at all. After firing it was weighed then soaked in water overnight then weighed again. It had absorbed 3% water. This is acceptable to me as a mid-fired pot. With a a bit of flux this absorption will be even less.
Clay Processing Videos#1