|Monthly Tech-Tip |
Casey Larson, our shipper and a pottery enthusiast, found this while breaking lumps on a stockpile. This is a very unusual find. The vast majority of fossils we find are preserved in iron stone concretions in our A1 raw clay, the top layer. Layers below that are highly plastic, as their lumps weather (many of which arrive the size of microwave ovens) they shrink and break down into smaller and smaller sizes. But our 3D material (the majority ingredient in Ravenscrag Slip and the lowest layer we mine) is less plastic so the lumps shrink much less as they dry. This keeps larger ones intact and has preserved this beautiful fossil imprint. This lump has been bisque fired to make the impression durable, thus the lighter color.
Plainsman Clays extracts 6 different sedimentary clays from this quarry (Mel knows where the layers separate). The dried test bars on the right show them (top to bottom). The range of properties exhibited is astounding. The top-most layer is the most plastic and has the most iron concretion particles (used in our most speckled reduction bodies). The bottom one is the least plastic and most silty (the base for Ravenscrag Slip). The middle two are complete buff stonewares made by mother nature (e.g. M340 and H550). A2, the second one down, is a ball clay (similar to commercial products like OM#4, Bell). A2 is refractory and the base for Plainsman Fireclay. The second from the bottom fires the whitest and is the most refractory (it is the base for H441G).
This 50 lb lump is from a quarry where we are mining the Whitemud Formation in southern Saskatchewan. This layer is extracted from the top of a hill at the bottom of a valley, putting it more than 50 meters below the prairie surface. The lumps are extremely dense and very heavy. They are also quite damp, about 12% by weight fossil water. They exhibit this horizontal layering, a clear indication of the sedimentary nature of the deposit. The clay is exceedingly fine-particled and the silica present exists in rounded grains finer than about 150 mesh. There are flecks of high-carbon material and some tiny iron particles. When lumps like this dry out when exposed to the sun they break down into thousands of pure-white pieces. These dry lumps slake quickly in water to create a creamy smooth slurry from which I can easily sieve out the carbon and iron particles to produce the hyper-smooth natural porcelain.