|Monthly Tech-Tip |
Since the 1970s, Plainsman Clays has been importing thousands of tons of refined materials from all over the US (and even other parts of the world) to make porcelain bodies. However, for us, the incredible rise in cost of shipping is forcing a reassessment.
It is important to note that porcelains are such, not just because they fire white, but because they fire vitreous and strong. Where pieces are being covered with opaque glazes, the color of the underlying porcelain is not an issue. The Saskatchewan clays I am studying contain the natural feldspar, quartz and a variety of clay minerals, all blended by nature, to produce porcelains, that although not white-burning, rival or exceed the strength and durability that can be achieved by using industrial minerals. Some of these clays can be fired well past the point of maximum density, developing a more and more glassy surface, yet resisting warping, even of very thin-walled ware. Mineral-mix porcelains always require the addition of often costly bentonites to achieve enough plasticity to make them workable, but these Saskatchewan clays have natural plasticities. Many areas around the world have similar materials, some are much whiter that this.
3B is the smoothest material Plainsman mines in the Whitemuds (no sand detectable to the touch). And the whitest in the raw state. It fires vitreous at cone 6-8 (2200F). It is at the middle of the formation that we mine, about 1 metre thick. We depend on this material for many clay bodies and the depletion of 3B is the primary motivator in doing new minings. Recently we have discovered that if slurried and sieved to 325 mesh, quartz particles are revealed. When removed, the resultant material is a plastic ready-to-use porcelain for cone 2-4. One that fires to steel-like strength. It is not even possible to make a body like this using traditional mixes of industrial minerals!
Written by W. C. Worcester. He had a lab equipped with clay processing and testing equipment that many would admire today! He outlines clay geology in general, then the geological history of the province of Saskatchewan in that context. He describes the technology of ceramic materials, the major clays used in industry and the equivalent materials in the province. He submits hundreds of samples with physical test data clearly describing them and their locations (using extensive maps and diagrams). His work inspired Luke Lindoe, who continued it during the 1950s to 1970s. That inspired us to develop the testing methods used at Plainsman Clays to this day. And it gave us several clay quarries that have served the company for 50 years.
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.
Plainsman Clays did 6 weeks of mining in June-July 2018 in Ravenscrag, Saskatchewan. We extracted marine sediment layers of the late Cretaceous period. The center portion of the B layer is so fine that it must have wind-transported (impossibly smooth, like a body that is pure terra sigillata)! The feldspar and silica are built-in, producing the glassiest surface I have ever seen at cone 6 (2200F). Despite this, pieces are not warping in the firings! I have not glazed the outside of this mug for demo purposes. I got away with it this time because the Ravenscrag clear glaze is very compatible (similar thermal expansion). But with other less compatible glazes they cracked when I poured in hot coffee. This mug was the beginning of an exciting project the sieve out +325 mesh particles any make many more pieces.
Fired at cone 6. It is impossibly vitreous, the surface is smooth like a glaze. And it has not warped. In fact, other pieces made from it having walls as thin as 2mm did not warp either!. This comes from a two-foot-thick section of the 3B layer from a Plainsman Clays quarry near Ravenscrag Saskatchewan, Canada. A cretaceous dust storm! It is plastic and feels impossibly smooth. Smoother than any commercial porcelain. It does not fire white because mother nature did include a little iron oxide. It accepts glaze like a porcelain.
This is made from 100% of a natural clay (3B) from the Whitemud formation in Ravenscrag, Saskatchewan. To make this body, which I call MNP, I slake and slurry up the raw clay lumps, sieve it to 200 mesh and then dewater on a plaster table. I rolled the plastic clay into a thin layer, cut it into a cross-shape using a 3D printed cookie-cutter, drape-molded it over a plaster form and then slip-joined the seams. It fires very dense and strong (to zero porosity like glass!). It holds together well and joins well with its own slip. Although not super plastic, it is smooth and fine-grained like a commercial porcelain body. I add 1-2% bentonite to make it more plastic when needed. It has the ability to be rolled extremely thin and yet does not warp in the firing! This mug has a weight-to-volume ratio of 2.08 (the weight of water it will hold compared to its own weight).
This natural porcelain is so vitreous that no glaze is needed for a functional surface. Fired at cone 6. Clear glazed inside is GA6-B Ravenscrag Slip. There is a small crack on the lower handle join, I am continuing to learn how to dry these pieces crack-free. By Tony Hansen.
Slab-built using my 'pie crust' technique. Cone 6 C6DHSC slow-cool firing schedule. The glaze is GA6-C Alberta Slip rutile blue. The raw porcelain surface exhibits a stunning deep-blue color (although not visible since this piece is glazed). However the blue does bleed up into covering glazes, making them more vibrant. And it highlight contours since the thinner glaze layer shows more of the underlying blue. Mug by By Tony Hansen
|By Tony Hansen|
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