|Monthly Tech-Tip |
Alberta Slip makes a great base for glazes because not only is it almost a complete glaze by itself but it has low thermal expansion, it works well with frits and slurry properties can be adjusted.
Plainsman Clays has manufactured Alberta Slip since 1988 as a substitute for Albany Slip. The project to duplicate Albany was done with an early version of Digitalfire INSIGHT software and is an excellent example of the power of glaze chemistry calculations in creating material substitutes.
However Alberta Slip is much more plastic (and therefore shrinks more during drying) than the original Albany and this has been a concern for some. However on rethinking, the extra plasticity is actually an advantage because it creates the possibility of a shrinkage-adjustable material (by blending raw with calcine). Calcining is not a problem for most companies or people, only red-heat and a short soak are required to destroy the plasticity of a bisqued container full of the powder.
When you begin to experiment with it you realize why the original Albany Clay became so popular and widely used. It worked so well all by itself as a glaze that many recipes employing it specified more than 75% Albany.
The simplest way to utilize Alberta Slip is just mix it with water and fire on a stoneware clay at cone 10. In oxidation it melts to a transparent brown glossy. In reduction it is a tenmoku, a very deep brown glossy maroon that breaks to a rust on edges. This tenmoku works on stonewares and porcelains and fires with a low thermal expansion that is unlikely to craze. It is not quite as glossy as classic tenmokus.
Alberta Slip is a great base glaze for browns and darker colors that employ iron as part of the staining mechanism. Adding more iron to the material produces a progression that goes to metallic and then crystal dominated deep maroon rust colors.
One place where Alberta Slip really shines at high temperature is for black. There is just no better what to make a black glaze. All it takes it 1% Mason 6666 or 6600 to get a jet black glossy in reduction. Adding more up to 5% of either moves toward metallic effects with an increasing silky and then matte surface.
All you need to do is add 20% Ferro Frit 3134 and the material melts in a manner similar to what it does by itself at cone 10. Amazingly, you can produce a surprising range of colors from this brown burning material. Adding rutile turns it into a beautiful blue by 5%. Adding Tin lightens the brown color.
The classic Albany recipe was 85 Albany, 11 Lithium Carbonate and 4% tin. This produced a beautifully variegated brown that exhibited all manner of dark and light tones depending on thickness. Alberta Slip I did not work well with this recipe because of its high drying shrinkage, but Alberta Slip II is perfect. However, like Albany, the glazes tends to shiver on many clays. I thus altered it to 5 lithium, 21 Ferro Frit 3195, 4 tin and 75 Alberta Slip. It fires to a very similar effect and is obviously going to be less likely to leach lithium into food or drink. Like the original, it melts to a very smooth and defect free surface even where it is applied very thinly.
Like cone 10, the material excels as a base for black glazes. All you need to do is add a black stain to the 80:20 base. You can add a small amount of cobalt (1%) for a glossy black or large amount of cobalt or copper (10%+) to get a metallic black (larger amounts of stain also increase the degree of black). You can also transplant the color mechanism of one of the many existing black glaze recipes in the public domain. One of the popular ones uses a mix of cobalt, iron and manganese.
G2451B Alberta Slip cone 6 glaze with 5% alumina calcined added.
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 is 85% Alberta Slip, 11% lithium and 4% tin fired at cone 6 in oxidation. Like the original Albany version, it has a very low thermal expansion (because of the high lithium content) and likes to shiver on many bodies.
This is 100% Alberta Slip (outside) on a buff stoneware (left) and iron stoneware (right) fired to cone 10R. The glaze is made using a blend of roast and raw (as instructed at the PlainsmanClays.com product page). Alberta Slip was originally formulated during the 1980s (using Insight software) as a chemical duplicate of Albany Slip. The inside: G2947U transparent. The intensity of the color depends on firing, add a little iron oxide (e.g. 1%) if needed.
Duplicating Albany Slip
How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip
Alberta Slip data sheet at PlainsmanClays.com
About Plainsman Clays