Modification Date: 2017-06-07 13:17:31
Member of Group: AS10
Alberta Slip at 60:40 calcine:raw makes a great tenmoku-like glaze at cone 10R
Alberta Slip, like the original Albany Slip, melts to a beautiful glossy deep brown at cone 10R. Use as a pure glaze, it stops just short of being Tenmoku at cone 10R (a 1% iron addition is needed). Unlike many Tenmokus, it is not too fluid.
The color of this glaze varies considerably with thickness. Its thermal expansion is low enough that it does not easily craze on stonewares or porcelains.
Like Albany, Alberta Slip is a clay. It shrinks during drying, more than Albany did. Using pure Albany as a glaze required calcining part of the mix (to prevent cracking during drying). Alberta Slip is the same, however it requires a higher calcine-to-raw proportion. While calcining is an extra step, the capacity to change the calcine:raw proportion gives you control over the properties of the slurry. Ideally it needs to be plastic enough to suspend well and harden on the ware, but not so plastic that it shrinks too much during drying. For calcining instructions please visit http://albertalslip.com.
In our lab we can make 1 Canadian gallon using a mix of 2800 water and 3000 dry (1500 Alberta Slip, 1500 Calcined Alberta Slip). This produces a specific gravity of 1.43 at about the right viscosity for dipping. We add a 1-2 grams of Epsom Salts to this to gel the slurry a little for better application properties. A 1-2 second dip in 1850F bisque ware produces the right thickness.
Roasted Alberta Slip (right) and raw powder (left). These are thin=walled 5 inch cast bowls, fired to 1000F and held it for 30 minutes. Why calcine? Why 1000F? Because Alberta Slip is a clay, it shrinks on drying. Roasting eliminates that, a 50:50 raw:roast mix works well for most recipes having high percentages. And 1000F? Calcining to 1900F sinters particles together (creating a gritty material) while 1000F just does not. But note the difference: The raw powder has an LOI of 9% (looses 9% weight on firing). 1900F-calcine has zero%. 1000F-roast has only 3%. So we need 6% less (9%-3%) of it than of the raw to supply the same chemistry. For example, if a recipe calls for 1000 grams of Alberta Slip: Use 500g raw and 456g 1000F roast (500*.94).
The far left has 1% iron oxide, the far right 7%. Crystallization of the iron begins around 3%.
A 13 inch vase glazed with 100% Alberta Slip fired at cone 10R. The glaze was sprayed on. It is about 60% calcine and 40% raw powder. When it is very thin, as on the shoulder, it is quite metallic and varies from deep red to brown (depending on thickness). Where thick it is a tenmoku high gloss. The spots on the shoulder are thicker areas that have glossed.
This is 100% Alberta Slip (outside) on a white stoneware clay fired to cone 10R. The glaze is made using a blend of 60% calcine and 40% raw (as instructed at the albertaslip.com support website). Alberta Slip was originally formulated during the 1980s (using Insight software) as a chemical duplicate of Albany Slip. The inside: A Ravenscrag Slip based silky matte.
These mugs are the same clay and glazed with a 50:50 raw:calcine Alberta Slip mix (GA10-A) and fired to cone 10R. Both looked like the one on the left. The one on the right has a decal on the inside, it was fired to 1500F. This firing has made the glaze significantly glossier, darker, deeper and more vibrant. Why? I have no idea. I have 20 more using this glaze and made from this and other clays, they all did the same thing.
GR10-K1 Ravenscrag tenmoku (left) compared to Alberta Slip tenmoku GA10-B (center) and pure Alberta Slip (right).
B-Mix is a popular high-ball clay very plastic grey cone 10R stoneware in North America. The two mugs on the left have pure Ravenscrag Slip on the inside (the middle on the outside also), it fires almost transparent with a slightly silky surface. Pure Alberta Slip is employed on the outside of the left one and the inside of the right one. The outside of the right one is RavenTalc silky matte. In all cases the Ravenscrag and Alberta Slip are mixed half-and-half calcined and raw. B-Mix fires dark enough and with enough specks that a normal transparent glaze is not very interesting. But these Ravenscrag ones look much better (for liner glazes).
Fired cone 10R. The one on the right contains 10% of Plainsman A1:St Rose Red mix to add speckle.
Out Bound Links
New York Slip, Albany Clay
Archie Bray Slip
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By Tony Hansen