|Monthly Tech-Tip |
Modified: 2023-04-15 23:22:42
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
|Alberta Slip 1000F Roasted||40.00|
|Ferro Frit 3134||20.00|
This glaze creates a rich blue yet contains none of the world's most expensive common ceramic material, cobalt oxide. On dark bodies the depth of color is incredible. It has a great glossy surface, and, because it contains no raw metal oxides or stains, toxicity issues are less of a concern. On lighter bodies it variegates from medium steel blue, where it is very thick, to amber clear (or a brown if the body is dark) where thin. The blue color does not develop well on porcelains or white-burning stonewares (unless you underlay it with a dark-colored engobe). This glaze requires a slow-cool firing schedule to work (see the paragraph below).
Experiment to get it the right thickness in your circumstances. Try it on different clays and different thicknesses to find the best combination. If it is melting too much or too little, you can increase or decrease the frit to compensate. But don't change the frit without testing first (the blue likely won't develop). Be sure your kiln is actually firing to cone 6 (using self-supporting cones).
One possible caution: This glaze relies on the "rutile variegation effect". Rutile can vary in chemistry over time and from place to place, so test this first before using and test it again when you get new supplies of rutile.
THE DEEP BLUE EFFECT REQUIRES SLOW-COOLING (otherwise the glaze will just fire an ugly brown). This can happen naturally if you fire packed loads or have a well-insulated kiln, otherwise, you must program the cool (use the Slow Cool C6DHSC schedule, it drops the temperature, then holds, then slows the cooling to about 1400F). You can also add 0.25% cobalt oxide to restore the color if you want to do a faster cool (to prevent transparent glazes clouding, for example)! If the blue is working, but less than you want, then add a little less cobalt.
If the glaze shrinks and cracks too much on drying, then increase the calcine Alberta Slip and reduce the raw Alberta Slip. If it is too powdery on drying, increase the raw against the calcine.
There is also a Ravenscrag Slip version of this glaze, it employs iron, cobalt and rutile (like the original David Shaner recipe).
Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about 1 kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying (if used raw the GA6-B and similar recipes will crack as they dry and then crawl during firing). Roasting eliminates that. Calcining to 1850F sinters some particles together (creating a gritty material) while roasting to 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.
Left: Plainsman 3D, a raw quarry material that fires as a buff stoneware at cone 6-8. Right: Plainsman Coffee clay (stained black using raw umber). Both were fired using the C6DHSC firing schedule. The inside glaze is the GA6-B Alberta Slip base. As of Dec/2021 cobalt retails at $100/500g! Yet the deep blue color on the mug on the right contains zero cobalt, the color is from titanium and iron. This GA6-C blue happens with any dark burning body, or with light burning ones having a dark engobe (e.g. L3954B with a dark colored stain) under the glaze.
The glaze recipe is GA6-C. The firing schedule is C6DHSC. The black engobe (applied inside and halfway down the outside) is L3954B. The clay body is Plainsman M370. This demonstrates how different this glaze fires on a white porcelain (bottom half outside) and a black porcelain (the engobed top half).
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 (left) 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.
Cone 6 mugs made from Plainsman M350 (left) and M390 dark burning cone 6 bodies. The outside glaze is Alberta-Slip-based GA6-C rutile blue and the inside is GA6-A base (20% frit 3134 and 80% Alberta Slip). That inside glaze is normally glossy transparent amber, but crystallizes to a stunning silky matte when fired using the C6DHSC schedule.
These have to be seen to be believed, it is the deepest, richest blue we have ever produced. This is Plainsman M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt (not raw) Umber instead of the normal 10% Zircopax) was applied inside and partway down the outsides (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue. Firing schedule: Cone 6 drop-and-soak.
Originally popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. It is loved and hated. Why? The high Gerstley Borate content makes it finicky. But the magic ingredient is not the GB, it is the rutile, Rutile makes the cobalt and iron dance. This recipe actually produces a number of different mechanisms of variegation. Color and opacity vary with thickness. Small rivulets of more fluid glass flow around more viscous phases producing micro-areas of differing colors and opacities. Titanium crystals sparkle and calcium-borate creates opalescence. Bubbles of escaping gases (from GB) have created pooling. Small black speckles from unground or agglomerated particles of iron are also present. Surprise! This is actually Ravenscrag Floating blue. All the visuals, none of the headaches.
GA6-C (left) and GA6-E (right) at cone 6 oxidation. The E version adds 4% spodumene onto the 4% rutile in the C (the base is 80% Alberta Slip and 20% frit 3134). The spodumene eliminate the overly whitish areas that can appear. This glaze requires the "Slow Cool (Reactive Glazes)" firing schedule. It looks the best on dark bodies.
2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If more rutile is added the surface turns an ugly yellow in a mass of titanium crystals.
This mug has thin walls and was bisque fired to cone 04 (so it had a fairly porosity). As a result the glaze went on thinner when it was dipped. This was not evident at the time of glazing but at firing the thinner sections produced the brown areas.
This is Alberta Slip (GA6C) on the left. Added frit is melting the Alberta Slip clay to it flows well at cone 6 and added rutile is creating the blue variegated effect (in the absence of expensive cobalt). However GA6D (right) is the same glaze with added Tin Oxide. The tin completely immobilizes the rutile blue effect, it brings out the color of the iron (from the rutile and the body).
The first is on GA6-A, the rest are on GA6-C (Alberta slip glazes). The last has been applied too thickly, the brown band is dry and blistered.
Plainsman Cone 6 Slow Cool
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, fastdrop to 2100, hold 30 minutes, 150/hr to 1400
GR6-M - Ravenscrag Cone 6 Floating Blue
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe.
GA6-H - Alberta Slip Cone 6 Black
Pure Alberta Slip can be made into a black adding only 20% frit and 3% black stain
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Alberta Slip Glaze Recipes
Alberta Slip is a substitute for Albany Slip that has gained a life of its own so that there are now many glazes based specifically on it.
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|By Tony Hansen|
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