|Monthly Tech-Tip |
Alternate Names: Archie Bray Slip
This material was formulated as a physical and chemical substitute for the late and very popular Albany Slip from New York state. Like Albany, it is a low melting iron stained clay. Alberta Slip is more consistent than Albany was since it is made from a blend of raw materials.
Alberta slip has a slightly lower iron content than Albany had so some glazes may not fire as dark (this can be counteracted by adding additional iron oxide). Alberta slip melts as well and works in most glazes that call for Albany. Alberta slip is more plastic (less silty) so recipes containing larger proportions may shrink and crack during drying (requiring the use of a calcine:raw mix).
Plainsman Clays has made this material for many years and it is established in the market place across North America. It has its own website at albertaslip.com. You can use Alberta Slip at 100% (raw:calcine mix) to create a chocolate brown glossy glaze at cone 10. Many Albany glazes were based on the addition of an active flux that increased melt fluidity so that greenish and yellowish iron crystals grew on cooling. Many black glazes were based on Albany Slip, since it already contained lots of iron only a little more and some extra cobalt or manganese were needed.
The plasticity of Alberta Slip is very helpful in suspending glaze slurries. However if there is more than about 40% (with no other clays), then you must use a raw:calcine mix. The albertaslip.com website has information on how to do the calcining and how to adjust for the change in LOI.
The analysis of this material was changed here in Sept 2013, not because the material changed, but because they switched to an actual assay instead of a calculated analysis.
Fired to cone 6 using the C6DHSC schedule. Top: GA6-B. This recipe is 80% Alberta slip and 20% Ferro Frit 3195 (we used to use frit 3134 but have found frit 3195 works much better). Bottom: We added 1, 2, 3 and 4% iron oxide. At about 2%, the color matches the rich reddish effect you would get if you used an 80:20 Albany:3195 recipe (without an iron addition). An added benefit is that the iron acts as a fining agent to remove micro-bubbles to achieve better transparency.
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.
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.
Compared to a slow-cooled, high-melt-fluidity iron crystal glaze fired at cone 6 oxidation (right).
This is an example of how a glaze that contains too much plastic clay has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. The Alberta Slip website has a page about how to do the calcining.
A nice thing about this is that the percentage of metallic oxide is comparatively low compared to other metallic glazes. And, it is iron oxide, which is not toxic at all.
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.
Albany Slip was a pure mined material, Alberta Slip is a recipe of mined materials and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance.
This is 100% of the pure material. Notice how the iron is fluxing it more on the left, it is beginning to run. And how the reduction atmosphere amplifies the color of the iron (by changing it to the metallic state).
The glaze on the left is 85% of a calcine:raw Alberta Slip mix (40:60). It was on too thick so it cracked on drying (even if not too thick, if others are layered over everything will flake off). The solution? The centre piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. How is this possible? 1% added CMC Gum (via a gum solution)! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied. No cracks. CMC is the secret of dipping-glazes for multi-layering. The down side: More patience during dipping, they drip a lot and take much longer to dry.
These three melt flows and mugs were fired at cone 6 (using the C6DHSC firing schedule). The benchmark recipe is 80% clay and 20% Ferro Frit 3195. The center melt flow and mug (made from a Plainsman 3D-based stoneware) employs original Albany Slip as the clay portion. The one on the far left uses an Albany Slip substitute that was developed by calculating a mix of RedArt and other materials to have the same chemistry as Albany Slip. The one on the right employs Alberta Slip. Notice that, although the Alberta Slip version has a very similar melt flow, on the mug it is apparent that it needs a little iron oxide for a better match (e.g. 1-2%). And the glaze on the left: The chemistry of RedArt is different enough from Albany that some compromises in chemistry-matching were needed to avoid over-supplying the iron even more (and firing even darker than this). Although this Redart version runs in a very similar pattern on the melt flow, the character of the glaze is somewhat different on the mug (a better match can be achieved by increasing the frit percentage slightly, or firing to cone 7).
The inside glaze is pure Ravenscrag Slip and the outside glaze is a 50:50 mix of Ravenscrag and Alberta Slips. Each of the glazes employs an appropriate mix of calcined and raw clay to achieve a balance of good slurry properties, hardening and minimal drying shrinkage. Ravenscrag needs less calcined since it is less plastic than Alberta Slip.
This high-Alberta-Slip glaze is shrinking too much on drying. Thus it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe (common with slip glazes). Clay is needed to suspend the other particles, but too much causes the excessive shrinkage. The easiest way to fix this is to use a mix of raw and calcined Alberta Slip (explained at albertaslip.com). The calcined Alberta Slip has no plasticity and thus much less shrinkage (but it still has the same chemistry). Many matte glazes have high kaolin contents and recipes will often contain both raw and calcined kaolin for the same reason.
At cone 5R pure Alberta Slip (left) is beginning to melt and flow down the runway of this tester. It is producing a matte gunmetal surface. Pure Ravenscrag Slip (right) is just starting (it needs frit to develop melt fluidity at this temperature). The iron in the Alberta Slip is melting it because of the reduction atmosphere in the kiln (it does not move like this in oxidation).
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
|Materials||Alberta Slip 1900F Calcined|
|Materials||Alberta Slip 1000F Roasted|
GA10-D - Alberta Slip Black Cone 10R
You can make a black glaze at cone 10R using only 1% black stain in a 100% calcine:raw mix of Alberta Slip
GA10-A - Alberta Slip Base Cone 10R
Alberta Slip at 60:40 calcine:raw makes a great tenmoku-like glaze at cone 10R
GA10x-A - Alberta Slip Base for cone 10 oxidation
Alberta Slip creates a glossy transparent brown at cone 10 with the simple addition of 10% frit.
GA6-A - Alberta Slip Cone 6 Amber Base Glaze
An amber-colored glaze that produces a clean, micro bubble free transparent glass on brown and red burning stonewares.
GA6-C - Alberta Slip Rutile Blue Cone 6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
GA6-D - Alberta Slip Glossy Brown Cone 6
Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com.
Alberta Slip data sheet at PlainsmanClays.com
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.
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Alberta Slip, 20 Years of Substitution for Albany Slip
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.
Duplicating Albany Slip
How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip