The outside glaze is GA6-B, the liner glaze is GR6-A. On M340 at cone 6 oxidation. These glazes are just Alberta Slip and Ravenscrag Slip with 20% added frit. Both of these apply well and fire to a brilliant surface with minimal defects if firings are slow cooled.
This is 100% Alberta Slip (outside) and 100% Ravenscrag Slip liner glaze. White stoneware H450 clay fired to cone 10R. Both glazes have been made using a blend of calcine and roast (60:40 and 50:50). By Tony Hansen.
Historically, Albany Slip was a 'standard' within the North American pottery community and ceramic industry. It was a silty glacial clay that melted easily at high temperatures to form a 'natural glaze'. With additions of a little frit, feldspar, lithium carbonate, or gerstley borate, it produced a wide range of earth tone glazes in middle fire oxidation. Albany Slip responded well to dark coloring oxides and stains to produce deep blacks, browns and even blues. It produced glazes that had a low thermal expansion (to avoid crazing). At cone 10R, with only a small iron addition, excellent tenmoku surfaces were possible. In the late 1980s Hamill and Gillespie Co. discontinued Albany Slip leaving thousands of users with less than ideal alternatives.
In 1988 we formulated Alberta Slip as the first widely available substitute material. We published the process we used to do it. Alberta Slip had the same chemistry and very similar firing properties (the original material was somewhat variable, we patterned our work after an average sample). Alberta Slip has been proven over many years and is used by people across North America to make many stunning glazes. Other substitutes have appeared from time to time but no others have had the success of this material.
November 2022: We began producing this material at 60 mesh, this greatly streamlines our ability to keep up with demand. Sieve your glazes through 80 mesh (to prevent roughness on contour edges of fired glazes). Only one pass is needed. Sieving through 100 mesh will likely yield about 3g/1000 of oversize material.
Left: Raw Alberta Slip powder. Right: Roasted at 1080F. This is a plastic clay, thus it has a significant drying shrinkage. Where a glaze is applied thickly or the percentage of Alberta Slip is high, shrinkage cracks (which produce crawling during firing) will occur. We recommend a mix of roast and raw material in recipes. Roasting the Alberta Slip powder at 1000F greatly reduces the shrinkage. Use a firing rate and hold-time-at-1000F appropriate for the wall thickness and size of your bisque vessels (e.g. 500F/hr and 30 minutes for thin walled small vessels, slower and longer hold for large ones). If any of the powder within is black, increase hold time. Adjust proportions as needed (more roast if the glaze cracks on drying or more raw if it is drying too powdery or not bonding well).
The roasted material has a weight-loss of about 3% on firing (vs. 9% for the raw powder). This difference can be ignored in most cases. But, to be more precise, use 3% less of the roasted powder (multiply the amount by 0.97). For example, suppose you need 1000 grams of a 50:50 raw:cacline Alberta Slip mix for a glaze recipe. Use 500 raw and 500*.97=485 roast.
Alberta slip is much more plastic than Albany was. This means that it dries harder and less dusty but also has a higher drying shrinkage. Thus, part of the material needs to be roasted (to cut shrinkage). Roasting is absolutely necessary to prevent cracking as the glaze dries (and thus crawling during firing). Using a mix of raost:raw also enables control over slurry properties (e.g. with a 50:50 blend in a glaze having 80-100% Alberta Slip, the slurry suspends and applies well, dries hard and does not crack for normal thickness).
We roast it to 1000F, high enough to destroy the plasticity (firing higher than that could can calcine it, sintering particles together and creating grit). Use a firing rate and hold-time-at-1000F appropriate for the wall thickness and size of your bisque vessels (e.g. 500F/hr and 30 minute hold for thin-walled small vessels, slower and longer hold for large ones). Watch for black powder remaining in the center as an indication of insufficient roasting.
Adjusting percentages: This roasted material has an LOI of about 3% (vs. 9% for the raw powder). This difference can be ignored in most cases. But if you want to be more precise, use 3% less of the roasted powder (multiply the amount by 0.97). For example, suppose you need 1000 grams of a 50:50 raw:cacline Alberta Slip mix for a glaze recipe. Use 500 raw and 500*.97=485 roast. Then watch how the slurry performs and adjust the proportions if needed (more roast if the glaze cracks on drying or more raw if it is drying too powdery or not bonding well).
Alberta Slip is a plastic clay, if used in recipes in higher percentages, it can cause them to crack during drying. A mix of roast:raw must be used.
In our studio we use pure Alberta Slip as a glaze at cone 10R. We mix it using a 60:40 roasted:raw mix of Alberta Slip powder. Glazes are most often prepared using the traditional method of simply adding water until the desired consistency is achieved (rather than targeting specific gravities and conditioning with flocculants). This works well in recipes having a high percentage of Alberta Slip, the clay:roast mix (which you control) coupled with its predominance in the recipe creates a slurry that applies well across a range of viscosities without the help of any rheological chemicals. Care is needed for more viscous slurries, these tend to go on too thick. When frit is added to make it melt at lower temperatures, lower specific gravities (higher water content) will likely be needed. In recipes that have low percentages of Alberta Slip, calcining is likely not needed and they can be treated normally (gelled to a thixotropic state).
Like classic Albany Slip, Alberta Slip begins melting at middle temperatures and by cone 10 it produces a glossy brown in oxidation and a tenmoku in reduction. One of the major advantages of this material is the ease with which it can be adapted to different temperatures. 20% frit produces very good melting at cone 6 and 50% melts at cone 06.
Since it is basically a clay material, it generates a significant amount of gas as it decomposes during melting. However, despite this, it can produce stunningly smooth and defect-free surfaces (we do not have a good explanation for this). For example, GA6-B produces a far better transparent for dark burning bodies like M390 or Coffee Clay.
As a raw material, the plasticity of Alberta Slip makes it an ideal base for 'slip glazes' for use on leather-hard ware. These do not need to be made using a mix of roast and raw, the pure raw powder is better.
Like Albany Slip, Alberta Slip has a low thermal expansion. Thus glazes will tend to be craze free. Glazes having significant lithium carbonate can lower the expansion enough to produce shivering. Using this with materials containing high K2O/Na2O can produce glazes that craze (especially on porcelains).
Our melt-flow tests show that Alberta Slip displays the same characteristic blistering as Albany in fast firings. However, it does not melt quite as vigorously (although it does flow as well). In addition, Alberta slip is not as inherently fine and silty as Albany. Alberta slip will tend to gel glaze suspensions a little more than Albany did and it does not deflocculate easily.
Since Alberta Slip fires by itself to a dark gloss at cone 10, it is an ideal base for dark shiny colors (requiring the help of a flux at lower temperatures). Alberta Slip provides one of the best ways to create difficult-to-make black glazes. As little as 2-5% combined cobalt oxide, copper oxide, black stain, etc. can be employed to make range of excellent glossy blacks. The more fluid the glaze (i.e. more frit, added iron) the more the likelihood of crystalline effects. Saturating the color can produce gunmetal blacks.
For cone 6 we recommend the GA6-B base (40:40:20 roasted Alberta Slip:raw Alberta Slip:Frit 3195). Do not use Gerstley Borate instead of frit, it is plastic and will increase shrinkage (causing cracking during drying) and turn the slurry into jelly. If you insist on using GB, it may be necessary to roast the entire 80% of the Alberta Slip complement in the recipe.
Drying Shrinkage: 5.0-6.0%
Cone 6: 3.0-4.0 cm Cone 8: 5.0-6.0 cm Cone 10: 7.0-8.0 cm
If your analysis data for this material is different it is not because the material changed, but because we have switched to an actual assay instead of a calculated analysis.
|Plainsman Clays Ltd.|
702 Wood Street, Medicine Hat, Alberta T1A 1E9
Phone: 403-527-8535 FAX:403-527-7508