Alberta Slip: An excellent base for cone 6 glazes

These are "not guaranteed-to-work glazes". This page focusses on the value of this material to make base glazes and provides some examples of variations. However circumstances differ, do testing and alter for your situation (for information on variegating, opacifying and coloring glazes click here).

Care should be taken not to fire glazes too fast or at too low of a temperature. Witness cones are recommended to assure you have reached cone 6. These glazes have worked in our studio at Plainsman Clays but we recommend testing them in your circumstances and on your clay bodies for fit.

Pure Alberta Slip can be made into a black adding only 20% frit and 3% black stain

A glossy black. The small amount of frit needed is due to the fact that Alberta slip is a dark burning material already. If it is not black enough, increase the percentage of stain. If you need a glossier surface, increase the frit. If it crazes switch to Ferro Frit 3195. Should be ball milled.

For mixing instructions please see the master recipe, GA6-A.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

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.

Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com.

Alberta slip is well suited to oatmeal glazes because it already has the iron content needed. Vary the titanium for more or less variegation and oatmeal appearance.

One issue you might encounter is pinholing or blistering if it is too thick (a common problem with this type of glaze). Try using it on different bodies and thicknesses to find the best combination. Adjust the frit if you would like it to melt lower or higher. Do not hesitate to reduce the rutile and titanium to experiment.

An advantage of this recipe is that the oatmeal effect is achieved without the use of manganese.

This recipe was referred to as GA6-B in past.

For mixing instructions please see the master recipe, GA6-A.

How much rutile can a glaze take before it becomes unstable?

The 80:20 base GA6-A Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left: 6% rutile, 3% titanium; right: 4% rutile, 2% titanium). That oatmeal effect is actually the excess titanium crystallizing out of solution into the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is unpleasant to touch and susceptible to cutlery marking and leaching (not as stable or durable as in glazes which are pure amorphous glass). For functional ware, rutile glazes are among the most troublesome to keep consistent, one way of avoiding problems is keeping the percentage as low as possible while still getting the desired variegation (of course that will vary depending on the melt fluidity of the glaze, more highly fluid ones can handle more rutile or titanium).

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

An amber-colored glaze that produces a clean, micro bubble free transparent glass on brown and red burning stonewares.

This is the base cone 6 Alberta Slip recipe. The 20% frit makes it melt well to form a transparent amber glossy.

Frit 3134 has traditionally been used and it is the best for some additions (e.g. rutile for floating blue). However we recommend Frit 3195 when possible, it imparts a lower thermal expansion for better fit (less crazing). For use on P300 we recommend Ferro 3249 (or Fusion F-69) for the lowest possible thermal expansion.

This base can be used as-is (with no colorant or variegator additions). It is most useful to replace regular transparent glazes on dark-burning clays (to avoid issues with microbubble clouding). If not cooled slowly, the effect is a very clean, speck-free amber transparent glossy that showcases the dark body color below. On light-burning clays and even porcelains, celadon green effects can be produced.

However, if this glaze is cooled slowly (which often happens in heavily loaded kilns) it will form surface crystals. These can produce beautiful effects (depending on cooling speed). For consistency in appearance, program the descent of firings to match the slowest natural normal descent. Add 1% tin oxide to prevent crystallization (see photo below).

Reactive rutile and titanium effects work well with this base. Its tendency to crystallize on slow cooling, when used as a honey transparent, provides insight into what kind of base glazes are best for floating blues.

As noted above, assess the fit with your clay body to be sure there is no tendency to craze (by stressing ware using a 300F-to-icewater IWCT test ice). If crazing occurs switch to frit 3195 (GA6-B). If it still occurs (e.g. with a porcelain), switch to frit 3249. Note again: While these frits still produce the glossy amber transparent effect they may not react the same with colorants, especially the rutile blue.

Glazes having a high percentage of Alberta Slip are most often prepared using the traditional method of simply adding water until the preferred viscosity is achieved (the material has inherent properties that produce functional slurries for dipping). Control of drying shrinkage and slurry character is achieved by varying the proportion roast and raw powder in the recipe. For us, a weight ratio of 88 water to 100 powder (2200 water for 2.5kg of powder) produces a 1.45-specific-gravity slurry that, although fairly runny, gives the right thickness on 1-2 second dip on 1850F bisque-ware (there is some dripping, but coverage is even and it is quick drying). This recipe actually does not respond to flocculant additions that gel traditional mineral-blend glazes to a thixotropic state.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

Alberta Slip GA6-B base darkened with iron oxide

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.

A cone 6 clear glaze plus iron vs. Alberta Slip amber base

These two mugs are made from a dark red burning stoneware and fired in a cool-and-soak firing schedule. A white engobe (L3954A) has been applied on the inside and half way down the outside. Both are glazed inside with G2926B whiteware transparent glaze. The outside glaze on the left is the same transparent with 4% added iron oxide. It has been sieved to 80 mesh. Notice the iron agglomerates and still produces specking (an effect that may be desired, but difficult to keep consistent). Interestingly, that iron is producing a clear amber-colored glass about equal in color to the Alberta Slip GA6A base glaze (80% Alberta Slip, 20% Frit 3195) on the mug on the right. Neither has the micro bubbles that mar a typical clear glaze on bodies like this.

A transparent that looks good on cone 6 red burning bodies

The GA6-A Alberta Slip:Frit 3134 (80%:20%) glaze is excellent as a liner for dark burning bodies, it looks much better than a regular transparent recipe (which often form clouds of bubbles on red bodies). The iron in this glaze makes it fire an amber color on buff burning bodies (not very attractive), but on red bodies it brings out the natural color of the clay.

Plainsman dark bodies with Alberta Slip floating blue

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.

A typical transparent glaze vs. Alberta Slip amber base on a red burning cone 6 body

The body is Plainsman M332, a coarse particled brown to red burning cone 6 body. With the G2926B transparent cone 6 glaze (left) and the GA6-A Alberta Slip base (right). The latter brings out the color of the body much better, the former is milky, bubbly and yucky!

GA6A Alberta Slip base using Frit 3249 and 3195 on buff body

The body is buff burning Plainsman M340 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe on the inside of mug 1 is L3954A (also glazed inside using transparent G2926B). These frits are producing an amber gloss glaze of high quality. On the outside of mug 1 we see the 3195 version on the white slip until midway down, then on the bare buff clay (the other has the 3249 version). These mugs are fired using a drop-and-soak firing schedule. A couple of caveats: Frit 3249 has a very low thermal expansion, use it on bodies that craze other glazes (like Plainsman P300), it could shiver on stonewares like this. Both of these frits prevent the formation of bloating blues (with additions of rutile or titanium).

A white engobe on dark and buff burning cone 6 stonewares

Left is Plainsman M340. Right is M390. Each mug has been white-engobed inside (by pouring), using L3954B, and halfway down the outside (by dipping). The insides have been glazed using G2926B clear. The inside surface has more depth and a richer appearance than could be achieved using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6-B. The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and the visual contrasts and character on the outsides of the stoneware mugs.

Crystallization of this glaze gives insight into why it can host the floating blue effect

These two mugs have the Alberta Slip base cone 6 GA6-A glaze on the inside and GA6-C on the outside (it just adds rutile to GA6-A). The left one was cooled normally (kiln off at cone 6 after soak). For the mug on the right, the kiln was soaked for half an hour at 1800F on the way down to develop the rutile blue glaze on the outside. But during this period crystallization occurred on the inside also. This provides an insight into my this GA6-A base hosts floating blue effects but GA6-B does not: The amount of Al2O3 is much lower, that improves melt fluidity and acts as a catalyst for crystallization.

Same glaze, same kiln, same clay: The right one crystallized. Why?

Well, actually they are not exactly the same. This is 80% Alberta Slip and 20% frit. But the frit on the left is Ferro 3195 and on the right is 3134. By comparing the calculated chemistry for these two we can say that the likely reason for the difference is the Al2O3 content. Frit 3134 has almost none whereas 3195 has 12%. Al2O3 stiffens the glaze melt, that impedes crystal growth. And it stabilizes the melt against running during firing. Frit 3195 is thus much more "like a glaze" than is 3134, it is what Alberta Slip needs to melt as a transparent glass under normally cooling in the kiln.

GA6A Alberta Slip base using Frit 3124, 3249 and 3195 on dark body

The body is dark brown burning Plainsman M390 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit. The white engobe, L3954B, on the inside of two of the mugs is L3954A (those mugs are glazed inside using transparent G2926B). The Alberta Slip amber gloss glaze produces an ultra-gloss surface of high quality on mugs 2 and 3 (Frit 3249 and 3195). On the outside we see it this glaze on the white slip until midway down, then on the bare red clay. The amber glaze on the first mug (with Frit 3124) has a pebbly surface. These are fired using a drop-and-soak firing schedule. Some caution is required with the 3249 version, it has low thermal expansion (that is good on bodies that normally craze glazes, but risks shivering on ones that do not).

Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.

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 with fully packed kiln-loads or with well-insulated kilns, 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 roasted Alberta Slip and reduce the raw Alberta Slip. If it is too powdery on drying, increase the raw against the roast.

There is also a Ravenscrag Slip version of this glaze, it employs iron, cobalt and rutile (like the original David Shaner recipe).

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

GA6-C on a light and dark burning clay body at cone 6

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.

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 (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.

Alberta Slip rutile blue on a porcelain at cone 6

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).

Plainsman dark bodies with Alberta Slip floating blue

Deep, deep blue without any cobalt. How?

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.

Close-up of Floating Blue on cone 6 dark/buff burning bodies

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.

Adding spodumene to this floating blue tones down the white patches

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.

Alberta Slip Rutile blue glaze too thin on a dark body

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.

GA6-C Alberta Slip rutile blue at cone 5R

On Plainsman P300 (left) and M350 (right). The blue effect is darker and richer than oxidation. The richer effect is also partly because the reduction kiln cools slower.

The rutile mechanism in glazes

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.

Tin oxide can stop the rutile variegation effect dead in its tracks!

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).

Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com.

Works well on all types of bodies, very reliable.

For mixing instructions please see the master recipe, GA6-A.

Tin oxide can stop the rutile variegation effect dead in its tracks!

Variegating effect of sprayed-on layer of titanium dioxide

The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However, on the outside the dried-on glaze was over-sprayed with a very thin layer of titanium and water (VeeGum can be used to help gel the sprayable titanium slurry and suspend it). The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile. Another advantage is that the variegation can be selectively applied in specific areas or as a design. This effect should work on most glossy glazes having adequate melt fluidity.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

One of the most popular Albany Slip glazes was 11% lithium, 4% tin oxide and 85% Albany Slip. This recipe reduces the lithium by more 50% eliminating the shivering problems that were common. This also employs a frit to help melt the glaze (in the absence of the 6% lithium carbonate). The added alumina darkens the color and slightly dulls the gloss (leave it out if you want).

Visually, this glaze works very well on porcelain showing variegated effects even on smooth surfaces, especially with variations in thickness. However, it may craze on porcelains, add more lithium to fix that.

Our G3933EF is similar, it employs 6% lithium carbonate.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

Alberta Slip used in the common lithium-tin cone 6 glaze

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 clay bodies.

A variation of Albany lithium brown glaze

Alberta Slip 75, lithium carbonate 10, tin oxide 4, nepheline syenite 11, calcined alumina 5.

A classic Albany glaze that often shivers

These mugs have experienced very serious shivering. This is an Albany Slip glaze with 10% lithium carbonate, it is known to have a very low thermal expansion. This problem can be solved by reducing the amount of lithium or adding high-expansion sodium or potassium. However these fixes will likely affect the appearance.

Adding 6% lithium carbonate to an Alberta Slip glaze does this

Left is G3933A, it is an 80:20 mix of our matte and glossy cone 6 base recipes (plus a mix of iron oxide, tin oxide and rutile). The body is Plainsman Coffee Clay. Because of repeated issues with crawling a project was started to create the same effect using Alberta Slip to supply as much of the chemistry as possible. Along that road, the opportunity arose to add lithium (to duplicate Amaco PC-32, a classic Albany/Lithium recipe). That is the glaze on the mug on the right, G3933G1, it has 6% lithium carbonate. Lithium is a super powerful melter, turning this into a very reactive glaze! In 2023 a 500ml jar of this glaze required about $7 worth of lithium carbonate. That makes this an ideal candidate to prepare as a brushing glaze - a benefit of that is better control of thickness, a key to the visual effect this gives.

Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com.

The regular Alberta Slip lithium brown recipe crazes on porcelain. This one was formulated to maintain the appearance but reduce the thermal expansion. It does this by reducing the KNaO and increasing the MgO. This was effected by employing MgO sourcing frit 3249. This frit is more expensive and difficult to get but it is the only way we have found to effectively reduce the thermal expansion and maintain the aesthetic.

Like the original Albany glaze, this recipe contains lithium carbonate (which is partially soluble), thus the slurry can gel over time. This necessitates the addition of water and increases the drying shrinkage and there cracking (which results in crawling). We are working on substituting a lithium frit to eliminate this issue.

For mixing instructions please see the master recipe, GA6-A.

Fine tuning glaze shrinkage vs. hardness

These mugs are fired at cone 6 with GA6-G1 Alberta Slip lithium brown. The difference: the ratio of raw to calcine Alberta Slip. In this glaze, a 50:50 ratio was not working well (left). The glaze was shrinking too much on drying, then crawling on firing (it needs to be thickly applied to get the visual effect I want). I mixed the recipe using pure calcine Alberta Slip, then repeated a cycle of pouring a little of this into the 50:50 mix and trying it. I kept doing that and glazing another mug until I had a minimum of drying cracks (while still having good gelling, application properties and dry hardness). The mug on the right was the last cycle, it has fired perfect. Using this technique I can perfect the ratio of raw:calcine for each Alberta Slip glaze I use.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

Alberta Slip is a product of Plainsman Clays and is mined in North America's premier "Clay Country" (near Ravenscrag, Saskatchewan, Canada and also in Montana, USA)

Cone 6 Alberta Slip Glazes