If your functional ware crazes you absolutely must fix it
This glaze and body and not compatible, the body thermal expansion is too low and the glaze it too high. It not only creates a germ zoo but severely weakens the piece. I tapped this lightly with a spoon about 1-2 cm below the rim, doing so repeatedly. Right from the start the piece had no ring, no strength. With each tap the pitch dropped. The piece is breaking apart the same way auto-glass does in a collision. This type of functional ware is entirely unacceptable. If you use commercial glazes test their compatibility with the bodies you use. If you make your glazes, adjust their recipes to reduce the thermal expansion until no crazing occurs using a thermal stress test (e.g. the IWCT test, which anyone can do).
Can you help with some math? Dry shrinkage is obvious. Fired fired shrinkage is not the total but the shrinkage from dry to fired. And you cannot just add the dry and fired numbers together to get the total because the fired shrinkage value is based on the dry length, not the original. In this example, 6.25 dry shrinkage plus 6.66 fired equals 12.9 whereas the actual total shrinkage is 12.5. But what if you need to calculate the actual total shrinkage if you only have drying and fired shrinkage? I am having trouble deriving the formula. Anyone can help?
Measuring clay test bars done by Luke Lindoe 40 years ago
Luke Lindoe prospected Montana and Idaho for clays during the 1970s. He found an amazing variety of fireclays, earthenwares and stonewares. Every color, texture, plasticity. For each he made test bars to fire at different temperatures. Our M2 and Troy clays originated from this work. We just found these bars, but do not have Luke's shrinkage and porosity data, so are measuring them now. He code-numbered each and stamped them with four-inch marks. So we can derive the total fired shrinkages and measure the porosities. We can tell a lot about the plasticity of each by the nature of the cut lines. The texture also is obvious. Now we just need to start searching Luke's map archives to find out where all of these are.
Wednesday 3rd October 2018
We have to fight with the fibreglass industry to get kaolin!
These are bags from three recent truckloads of 880 bags each. Order-delivery delays are getting longer and longer as the fibreglass industry is making more and more demands on kaolin suppliers. This means we have to store this material in larger quantities and for longer periods than in the past. And we must be more diligent in testing for consistency because manufacturers are catering to fibreglass instead of ceramics. When this is coupled with the decline of ceramic manufacturing in North America it means maintaining and documenting the properties important to ceramics are becoming less important to kaolin manufacturers.
Here is what it takes to make sure P700 has minimal fired specks
We had to sample every pallet of a 1500 bag bentonite shipment. On testing each one we found dark-coloured particulates. Then we determined which pallets where the worst and did a second round of testing. Then we mixed up 5000 gram P700 test batches from three pallets, made ware and tiles, clear glazed and fired it all to cone 10R (with heavy reduction). We also prepared samples and returned them to the manufacturer for further testing in their lab. As it turned out, the dark particles were not iron-containing and we found only a few tiny specks.
Three visual glaze mechanisms make this piece unique
"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954J black engobe on the inside and part way down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).
This is G2934 matte (cone 6) and G2931K glossy base recipe having the same Cerdec yellow encapsulated stain been added (about 10%). You really need to see these side-by-side to appreciate how much richer the silky matte version looks (and feels). AMACO has popularized the bright glossy transparent look in its 'Celadon' product line (while the effect looks good on smaller areas, for cover glazing it appears 'washed out'). An additional factor to consider is that encapsulated stains begin to decompose around cone 6, this can produce orange-peel and blisters on glossy glazes, but with this matte base we have not seen it happen with any color. Google 'G2934 recipe' to learn more, it is being talked about everywhere, a truly amazing base.
Friday 21st September 2018
A mug cracks before your eyes because of glaze compression
A highly vitreous, thin walled mug is glazed inside-only. The glaze has a thermal expansion that is too low and it is under compression, pressing outward. A tap with a spoon is enough to trigger a sudden crack. It opens under the pressure. Had it been glazed on the outside also it would likely survive, but this test still indicates that it would be better to raise the expansion a little.
Program your firings manually, calibrate the final temperature
Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). We need it to actually go to cone 10, the only way to do that is verify with a cone (self supporting cones are the only accurate way). Then make a note in the record for that schedule in your account at insight-live.com.
The Polar Ice data sheet has been changed. In the past we have been hesitant about firing it over cone 6. This is because it is just so vitreous and translucent that firing it higher seemed to be asking for trouble (like warping, blistering). But at cone 10R it is still resistant to warping. And fires this beautiful blue-white. The translucency, amazingly, is about the same: Incredible! This is among the most incredible pieces we have ever made in the studio here, it is hard to believe it is possible produce this kind of quality in a pottery studio.
This contains 6% Mason 6666 gunmetal black stain. The base recipe, G2934, is an excellent balanced-chemistry host for a wide range of stains to produce equally stunning reds, yellows, oranges, etc. The fritted version of the recipe, G2934Y, provides an even better host. This glaze is affected by the clay it is on. The body on the right is highly vitreous, this has produced a finer texture that glistens in the light. The body on the left is a whiteware having 1% porosity (Plainsman M370). Firing schedule is also a factor, slower cooling will dull the color more. We use the PLC6DS firing schedule.
This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glaze compression (the inside glaze is pushing outward) is important to avoid crazing. But, when suddenly heated it pushes harder. Outside surface imperfections, even microscopic in size, provide crack-initiation-points to relieve the stress (the solution is an inside glaze with carefully tuned thermal expansion or an outside glaze to counter the forces and fill in all the imperfections).
Mother Nature's Porcelain - From a Cretaceous Dust Storm!
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 terrasig)! The feldspar and silica are built-in, producing the glassiest surface I have ever seen (despite this, pieces are not warping in the firings at cone 6). I have not glazed the outside of this mug for demo purposes (a practice sure to fail in a crack when hot coffee is poured in).
Clive Tucker and me (Tony Hansen) had a throw-off today. I got beat. I have never had to face a thrower as capable as this! He made the higher mug (on the right, we had 520 grams of clay each). It is a privilege to have, not just talented and artistic customers like this, but also technically capable ones like him. The real winner is the M370 clay we used. Even though it was too soft, neither of us had issues with twisting, it wanted to compete as much as we did.
Clay and dinosaur country in southern Saskatchewan
This is the Frenchman river valley and is the home of the "Whitemud Formation" and two of the mines of Plainsman Clays. The Whitemud layers are clearly visible on the badlands side of the valley (half way down on the left). However the Plainsman quarries are on the rolling hills side, where many places can be found having much less over-burden above the white layers. These materials were laid down as marine sediments during the Cretaceous period. Below the Whitemuds are formations from the Jurassic period. A complete T-Rex, dubbed "Scotty", was found nearby and can be seen in the T.rex Discovery Centre in Eastend, Saskatchewan.
Mining the Battle Formation in our quarry at Ravenscrag, Saskatchewan - June 2018
This is the top layer. Battle clay is highly bentonitic, it is the "super hero of plasticity" in the quarry, it is unbelievably sticky. We have considered it "over-burden" in the past, but now will be looking for ways to employ Battle clay in our products and seeking special-purpose markets for it. Only 10% of this can turn a silt into a plastic throwing body! It is also high in fluxes (melts by cone 6). That means we can use it to improve the fired maturity of bodies, reducing the need for talc. Removal of this layer has exposed the top of the White-Mud Formation, the "A1" layer. A1 is employed in high fire bodies to impart brown color and fired speckle.
Ever wondered why your dealer can quickly get the clay you need?
This is our warehouse. It is really big! There are 20,000+ boxes in stock of almost every kind of clay we make (about fifty). Plus a hundred different ceramic material powders, many of which we buy in truckload quantities. We keep all kinds of equipment and supplies in stock also (in other storage areas), having a total value exceeding that of the clay. This means that when your dealer orders a truckload of clay, materials, supplies, tools and equipment from us, they get it fast.
Wednesday 20th June 2018
Why is there a health warning on the top of each box of clay?
Like any dirt, clays contain quartz. Quartz particles, if inhaled in just the right size, can block the tiny air passages in your lungs. Quartz is all around us, it makes up about 12 percent of the land surface and about 20 percent of the Earth's crust. This label is a reminder to reduce dust levels in your studio and working area. You can see specifics about hazards of any of our products by clicking links (on any manfuacturers website) to view the SDS (Safety Data Sheet). It contains references to where you can learn more about working safely.
Mel Noble at Plainsman Clay's Ravenscrag, Saskatchewan quarry
Plainsman 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).
G2926B has proven to be my most durable, crystal clear, non-crazing, easy-to-use general purpose cone 6 base glaze (from dozens I developed). However, some porcelains (e.g. Plainsman P300) need an even lower thermal expansion. G2926S adjusts "B" (by adding low-expansion MgO at the expense of high-expansion KNaO). Yet it has the same gloss. The insides of these P300 mugs use it (with 10% added Zircopax to make white). "S" is not an all-purpose recipe, it could shiver on high silica bodies, use it if G2926B fails an IWCT test for crazing. These mugs were fired using the PLC6DS firing schedule, the outside glazes are G2934Y silky matte with added stains.