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Ravenscrag Slip produces a much better Floating Blue
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Technical Tips Blog
Incredible Titanium Dioxide in a calcium matte
The glaze is G1214Z1 cone 6 base calcium matte on Plainsman M390 fired at cone 6 using the PLC6DS schedule. 5% titanium dioxide has been added. Titanium can create reactive glazes, like rutile, with no other colorants added. This effect also works well on matte surfaces, but the glaze needs good melt fluidity (that is good because functional mattes melt well). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible compared to rutile (iron is still needed by it is coming from the body here). Like rutile, the effects are dependent on the cooling rate of the firing, slower cools produce more reactivity. Even application without drips is important (mixing as a thixotropic dipping glaze is best). This appearance also depends on using dark burning body or engobe.
Sunday 2nd October 2022
Ravencrag Slip vs Alberta Slip floating blues at cone 6 oxidation
Usable, reliable, non-crazing floating blue glazes are difficult to achieve at cone 6. Not these, they pass all the tests yet fire like the original classic G2826R floating blue from David Shaner. Both have been applied at moderate thickness on Plainsman M325 (using a slurry of about 1.43-1.45 specific gravity, higher values end up getting them on too thick). The Ravenscrag version highlights contours better (the edges are black because of the black engobe underneath). It also produces the blue color whether or not the kiln is slow-cooled (although drop-and-hold PLC6DS schedule usually fires more blue). The Alberta Slip version has zero cobalt so it is less expensive to make (but it does require the C6DHSC slow-cool firing schedule). It produces a deeper color over the L3954F black engobe on these pieces. Both of these produce a wide range of effects with different thicknesses, bodies and firing schedules.
Wednesday 28th September 2022
Ravenscrag floating blue color affected by cooling speed
Ravenscrag Slip really shines in its ability to produce a good floating blue glaze at cone 6, this is the GR6-M recipe. The speed of cooling in the kiln affects the appearance. The mug on the left was cooled faster, using our drop-and-soak PLC6DS firing schedule. The other one was slow-cooled using the C6DHSC schedule. The latter schedule is preferable for these because the G3914A black has a much smoother surface. The blue could be recovered by adding more cobalt.
Wednesday 28th September 2022
A gummed engobe made this possible
This is L3954F cone 6 black engobe (these mugs will also be glazed completely black). Three factors make this workable. First, a non-gummed dipping engobe will not work for this (it will not apply by brush evenly or thickly enough). To make the brushing version we mixed 500 grams of L3954F (with black stain) into 280g water and 75g of Laguna Gum Solution. Second, the recipe is tuned to have the same degree of vitrification as the body so pieces won’t stick to the kiln shelf during firing. Third, this procedure introduces a possible issue: Applying the engobe rewetted the bases of these leather hard mugs, extending drying time. Since the handles were in danger of shrinking too much as the bases caught up I painted gum solution on the thinner sections to slow down their drying.
Saturday 24th September 2022
Mother Nature's Porcelain: The lumps in the quarry
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.
Friday 23rd September 2022
Pure nepheline syenite mug glazed and fired to cone 02
It is near stoneware strength. How was it possible to make this? Actually, it is 90% nepheline syenite and 10% bentonite. The latter imparts enough plasticity that it can be thrown easily on a potter's wheel. By about cone 1 it begins to warp. This is fired to cone 02 with standard Spectrum low fire glazes. No crazing is evident and the coverage is normal. The next stage will be to use Veegum to get a 95:5 mix, then we will bisque to cone 01 for translucency and then glaze fire at cone 04. This really demonstrates the amazing ceramic properties of this material. We aged this for several weeks before throwing and it was stable and unchanged in softness or plasticity.
Friday 23rd September 2022
Testing a found clay for its pottery suitability: First steps
Would you like to be able to use your own found-clays, ones native to your area or even your property, in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do exactly this. I will use ordinary tools that any potter either already has or can buy at low cost. We will describe this clay in terms of plastic clay bodies and common ceramic materials that most potters already use. The potter who submitted it has worked enough with the material to suspect it has potential and he wants to know how to best utilize it (e.g. at what temperature, with what glazes, mixed with what, processed in what way). In technical terms what we are doing is called "characterization".
Sunday 4th September 2022
Here’s how to remove agglomerates in a gummed engobe
This brushing engobe is thick and gooey (because it contains CMC gum), so it is very difficult to sieve. It contains tiny lumps of New Zealand kaolin that our propeller mixer is not able to break up. But 30 seconds in this kitchen blender and a litre of it is as smooth as silk.
Sunday 4th September 2022
Iron red on porcelain and a red burning stoneware
This is the G3948A recipe fired to cone 6 using our standard C6DHSC schedule. The color "breaks" to black where thinner around contours so it seemed like a natural that the inside glaze should be G3914A Alberta Slip black. The contour of the foot ring is important or the glaze will run onto the kiln shelf. My standard fluted ring foot is working well. An option would be to glaze the bottom inch or so with the black and the iron red down to that.
Tuesday 16th August 2022
New iron-red glaze on porcelain at cone 6 oxidation
This is the G3948A recipe. Iron red glazes are easy to do in high-temperature reduction but not so in medium-temperature oxidation. Most people just try a bunch of recipes they find online hoping that one of them actually fires the way it is shown in the picture! A better approach for us was to study a range of ones claiming to be iron reds looking for things in common with the chemistries and recipes. G3948A, on these two M370 mugs, is a product of that. Unlike many, the original recipe we found, G3948, did have a suggested firing schedule. It seemed strange so we just used the standard C6DHSC slow-cool schedule. That one is also ideal for the liner glazes, giving them a better gloss finish. It was not tempting to even try the original recipe (because it measured up poorly against common sense recipe limits), but it did make sense to fix obvious issues and then try it. Unlike every other recipe we have seen, this one suffers no issues with gelling of the slurry because it contains no Gerstley Borate and uses black iron oxide. It has very good application properties and requires only 80 water for each 100 powder to mix as a creamy dipping glaze. And it does not need any lithium carbonate.
Sunday 14th August 2022
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