Modification Date: 2016-09-11 11:28:55
Member of Group: AS6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
|Alberta Slip Calcined||40.0||38.5%|
|Ferro Frit 3134||20.0||19.2%|
|Rate (F)||Temp (F)||Hold (Min)||Step|
This glaze creates a bright blue yet contains none of the world's most expensive common ceramic material, cobalt oxide. It has a great glossy surface and variegates a from medium steel blue where it is very thick to amber clear (or a brown if the body is dark) where it is too thin.
You will need to experiment to get it the right thickness in your circumstances. Try it on different clays and different thicknesses to find the best combination. It works best on stonewares. If it is melting too much or too little, increase or decrease the frit to compensate.
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 FLOW COLORATION REQUIRES SLOWER COOLING. This can happen naturally if you fire packed loads or have a well insulated kiln, but it is generally best to program the cool (use the Slow Cool schedule link to learn more, 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).
Calcined Alberta Slip (right) and raw powder (left). These are just 5 inch cast bowls, I fire them to cone 020 and hold it for 30 minutes. Why calcine? Because for glazes having 50% or more Alberta Slip, cracking on drying can occur, especially if it is applied thick (Alberta Slip is a clay, it shrinks). I mix 50:50 raw:calcine for use in recipes. However, Alberta Slip has an LOI of 9%, so I need to use 9% less of the calcine powder (just multiply the amount by 0.91). Suppose, I needed 1000 grams: I would use 500 raw and 500*.91=455.
2,3,4,5% rutile added to a 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.
Left: What GA6-C Alberta Slip rutile blue used to look like. Middle: When it started firing wrong, the color was almost completely lost. Right: The rutile effect is back with a vengeance! What was the problem? We were adjusting firing schedules over time to find ways to reduce pinholing in other glazes and bodies. Our focus was slowing the final stages of firing and soaking there. In those efforts the key firing phase that creates the effect was lost: it happens on the way down from cone 6. This glaze needs a drop-and-soak firing (e.g. cooling 270F from cone 6, soaking, then 150F/hr drop to 1400F).
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.
The rutile blue variegation effect is fragile. It needs the right melt fluidity, the right chemistry and the right cooling (during firing). This is Alberta Slip GA6C recipe on the right (normal), the glaze melt flows well due to a 20% addition of Ferro Frit 3134 (a very low melting glass). On the left Boraq has been used as the flux (it is a calcium borate and also melts low, but not as low as the frit). It also contains significant MgO. These two factors have destroyed the rutile blue effect!
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.
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 recipe is GA6-C. These are from the same firing (slower cooling is needed to develop the rutile effect).
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% Ravenscrag Slip and 20% frit 3134). This glaze requires slower cooling. It looks the best on dark bodies.
And it contains no cobalt! Fairly close in appearance to the classic cone 6 floating blue recipe used across North America, this is a variation of the Alberta Slip Rutile Blue glaze (except this adds 1% tin oxide, 1% black copper oxide and 2% ceramic rutile, it is GA6-C1). Because of the melt fluidity, it thins on the edges of contours and breaks to the color of the underlying body. It looks best on dark bodies, but if thick it is OK on light ones also.
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, but crystallizes to a stunning silky matte when fired using the schedule needed for the rutile blue (cool 100F and soak, slow cool to 1400F).
Cone 6 oxidation. GR6M Ravenscrag version is on the left. The Alberta Slip version (GA6C) is more fluid, but that also means it will run more during firing and blister more if too thick or on re-firing. Generally, the Alberta Slip version appears better on dark bodies and the Ravenscrag one on lighter burning clays. The Alberta Slip version gets its color only from Rutile (and thus requires a special drop-and-hold firing scheduel), the Ravenscrag one produces blue in any firing schedule (although the color will be better in the drop-and-hold schedule).
Left: 4% rutile in the Alberta Slip:frit 80:20 base. This glaze has been reliable for years. But suddenly it began firing like the center mug! Three 5 gallon buckets of glaze (of differing ages) all changed at once. We tried every combination of thickness, firing schedule, clay body, ventilation, glazing method on dozens of separate pieces with no success to get the blue back. Even mixed a new batch, still no color. Finally the 'crow bar' method worked, 0.25% added cobalt oxide (right mug). It is identical ... amazing. It is not the same mechanism to get the color and it is not exactly the same, but worked while we figured out the real issue: the firing schedule (the secret turned out to be cooling, soaking, then slow cooling to 1400F).
These two cone 6 mugs have the same glaze recipe: GA6A Alberta Slip base. 4% rutile has been added to each. They were fired in the same kiln using a slow cool schedule. The recipes and chemistry are shown below (the latter gives a clue as to why there is no blue on the right). The mug on the left is the traditional recipe, 80:20 Alberta Slip:Ferro Frit 3134. Frit 3134 melts at a very low temperature and a key reason for that is its near-zero Al2O3 content. Al2O3 in glazes stiffens the melt and imparts durability to the fired glass (normally we want adequate levels in functional glazes). When Al2O3 levels are low and cooling is slower molecules in the stiffening glass have much more freedom to move and orient themselves in the preferred way: crystalline (fast cooling produces a glass). Thus the rutile in the glaze on the left has had its way, dancing as the kiln cooled, producing all sorts of interesting variegated visual effects. The glaze on the right employs Ferro Frit 3195. It has lots of Al2O3 and has contributed enough to stop the rutile dead.
Out Bound Links
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.
2013-09-04 - David Shaner's cone 6 floating blue has been used ...
Archie Bray Slip
In Bound Links
An amber-colored transparent glaze that works better on brown and red burning stonewares than typical transparent glazes.
2003-12-12 - This is the base cone 6 Alberta Slip recipe, it fi...
Pure Alberta Slip can be made into a black adding only 20% frit and 3% black stain
2003-12-12 - A glossy black. The small amount of frit needed is...
Variegated, or mottled, glazes are those that do n...
By Tony Hansen+