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Technical Tips Blog
The world's largest T. Rex could have walked on your clay!
Eastend, Saskatchewan hosts the incredible T. Rex Discovery Centre. The centre hosts Scotty, the world's largest T. Rex. The building is an architectural marvel and the exhibits are on par or better than anything else where. The building is actually embedded into the hillside in the same geological layers in which the fossilized skeleton was found, the Battle formation (just above the Whitemuds that we mine). If you ever get a chance to drive from Ravenscrag to Eastend we recommend the south road for the most dramatic views. Do it in the spring when the grass is green. The sheer scale of the valley, the land formations and the countless outcrops of the whitemuds will amaze. At Eastend, have a meal of Jack's Cafe, you won't be disappointed! Then go and see Scotty.
Thursday 17th June 2021
Clay in "dinosaur country" of southern Saskatchewan
This is a "badlands" slope in the Frenchman river valley. The valley exposes the "Whitemud Formation" in many places (clearly visible here half way down on the left). Two surface mines of Plainsman Clays are nearby (over the top and down the other side), in a place where lower-lying rolling hills leave much less over-burden to remove. These materials were laid down as marine sediments during the Cretaceous period. The skeleton of the world's largest T-Rex, dubbed "Scotty", was found 50km east of here (in the layers just above the Whitemuds). Where are the layers of Scotties ancestors from the Jurassic period? Straight down until you hit the bed rock!
Context: Ravenscrag Slip is Born, Mother Nature's Porcelain - From a Cretaceous Dust Storm!, These Saskatchewan farmers are growing their crops in M340!, The world's largest T. Rex could have walked on your clay!, T.Rex Discovery Centre, Medalta Potteries, Plainsman Clays
Thursday 17th June 2021
Outcrops of the Whitemud formation in the Eastend river valley - 2021
In the spring this is what you will see to the north (near Ravenscrag on the south road to Eastend). The whitemud clays are clearly visible. The amount of overburden would be impossible to remove at this site, so Plainsman mines on the opposite side where gentle rolling hills fall to the valley bottom. But on the other side there are no outcrops, these white layers are largely hidden and can only be discovered by exploratory digging. But on this side they are easily accessed and can readily be sampled. This is the exact location where the whitemud clays were first discovered for pottery in the early 1900s, there are actually some mineable sections on the front hills to left. I-XL brick mined clay here for many years. This is also the site of a mine for the former Medalta Potteries.
Thursday 17th June 2021
Cone 10R dolomite matte effect at cone 6 oxidation
This clay, L4115J3S, a Plainsman 3D-based experimental body, fires vitreous and dense (it contains 0.2% granular manganese). These glazes are very durable and functional. The outside glaze on both is G2934W (adds 10% zircopax). In our C6DHSC firings this produces as matte a surface as is possible without having excessive staining problems. Left mug inside glaze: An 85:15 mix of G2934 matte (without zircopax) and G2926B clear glossy. Right mug inside: G2926B clear glossy ball-milled, over this body it produces a striking visual surface. These mugs look as close to cone 10R dolomite-glazed ware as we have ever seen!
Context: Dolomite Matte
Sunday 6th June 2021
Calculating a substitute for Minspar
Why do this? We did not have it in stock and customers needed to mix recipes. When the chemistries of the two feldspars are very similar substitution is often not a problem, especially when a recipe only calls for 5 or 10%. However, when a recipe calls for a significant percentage the situation becomes much trickier (in our cone 6 test recipe, "Perfect Clear", 40% Minspar is needed). Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward 'glazedom'. In this project I calculated a mix of materials, in my Insight-live.com account, that sources the same chemistry as Minspar. I made a cone 6 GLFL test comparing the Minspar and Minspar substitute (left) and comparing the Perfect Clear glaze with each feldspar (right). As you can see, the similarity in melt flow is stunning! This is a real demonstration of just how practical and valuable glaze chemistry calculation can be.
Sunday 6th June 2021
Are manganese speckled clay bodies a toxicity hazard?
Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.
Wednesday 2nd June 2021
Serious cracking in a crystalline-glazed P700 Grolleg porcelain. Why?
The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.
Sunday 16th May 2021
By the magic of delflocculation, all this powder will mix into that water
Casting slips require a minimum of water. Amazingly, it is possible to get 3000g of M370 powder into 1100g of water! And the fluid slurry produced, 2250cc, still fits in the container. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to electrolytically repel each other! An awareness of “the magic” can help give you the determination to master deflocculation, the key enabler of the slip casting process. Determination? Yes, the process is fragile, must must develop the ability to “discover” the right amount of Darvan for your clay mix and water supply. And the ability to recognize what is wrong with a slurry that is not working (too much or little water, too much or little deflocculant).
Sunday 9th May 2021
Two cone 10R bamboo glazes. One stains, one does not. Why?
These mugs are Plainsman H443. The bamboo glaze on the left (A) has 3.5% rutile and 10% zircopax added to the base G2571A dolomite matte. The one on the right (B) has the same addition but in a base having slightly less MgO and slightly more KNaO. B stains badly (as can be seen from the felt marker residue that could not be removed using lacquer thinner). Why does A stain only slightly? It has an additional 4% Gerstley Borate (GB). GB is a powerful flux that develops the glass better, making the surface more silky. The differences in the recipe provide another advantage: (A) has a much lower thermal expansion and is less likely to craze.
Tuesday 27th April 2021
Should you expect to vitrify terra cotta?
These were cast by Anna Lisovskaya, they are fired at cone 03. They are supposed to fit into hexagonal welded frames, but during firing many of them warp enough to fit poorly. Why? The color differences are most obvious here. With that color associates a firing shrinkage difference, the darker ones shrink significantly more. Something less obvious: the sides against the elements receive direct radiant heat, so they shrink more, turning a perfect hexagon into an imperfect one. Terra cotta clays are volatile, that means their approach to maximum density during heat-up, accompanied by shrinkage, happens across a narrow temperature range. Accurate and even firing are paramount. In a radiant-heat electric kiln this can be very difficult. Two approaches could work here: Fire at a lower temperature, perhaps cone 04. Or, greatly slow rate-of-rise for the last 100F, perhaps over several hours.
Thursday 22nd April 2021