|Monthly Tech-Tip |
How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip
Albany slip has long been a "standard" within the North American pottery community. It has proven invaluable for the production of craze-free earth tone glazes, and many potteries use some in most of their glaze formulas. In the late 1980's after increasing difficulties in mining the product, the Hamill and Gillespie Co. discontinued it. Many companies have been working on a substitute, with limited success.
As luck would have it, I had access to a material called "Redearth", which burns somewhat similar to Albany slip. The task seemed to be to calculate the recipe of ceramic minerals that when combined with Redearth would chemically duplicate Albany slip. The result turned out to be "Alberta Slip".
I came to recognize early that there are some limiting factors to consider to maintain a realistic view of calculation and make the best use of the information it provides:
Here is Redearth and Albany calculated alone after adding the former to INSIGHT's materials database.
I have set unity with the fluxes, since the material will be used in glazes, and will be compared to Albany, which is a flux-dominant material. The LOI figure shown was derived as described above.
Since Albany slip has a reputation for being somewhat inconsistent, obtaining a representative analysis of it proved elusive. I finally decided on an arbitrary textbook analysis. This source did not include a loss on ignition, so it was necessary to fire my own sample in the kiln (yielding an 8.4% figure). I added the Albany formula, with unity set to the fluxes, to the materials definition table in INSIGHT and a standard formula report is shown above beside the Redearth. Notice that this formula does not appear very similar to that of Redearth, but you may be surprised to find out how little material had to be added to convert Redearth to a close chemical duplicate of Albany.
It is important to note that I did not know how well the sample of Albany slip clay I had matched this formula, nor did I know whether this formula could be considered representative of "normal" Albany clay. I was attempting to duplicate Albany clay on two levels, chemically and physically. The first step of the plan was to duplicate it chemically "on paper".
Firing and physical properties tests would indicate further changes necessary to mimic the physical properties (like apparent plasticity, smoothness, dry strength, shrinkage, raw color), firing properties (like shrinkage, absorption, strength, fusibility, melt viscosity), and glazing properties (like its effect on physical viscosity, drying shrinkage, application characteristics, etc.). I could not, however, duplicate the material's mineralogically, that is, produce a substitute that would have the same proportions of the same clay minerals. Mineralogical analyses were not available but this was okay since Albany clay is used mostly in glazes where the kiln fires break down all minerals into their basic oxides.
The next step was to set up the Albany analysis as a reference in the formula window of INSIGHT. I then entered a recipe with 100 Redearth and calculated to compare. It was just a matter of introducing materials to source oxides that are lacking in Redearth, one at a time, until the Albany formula was duplicated using the highest amount of Redearth possible. The process is an oxide juggling act, but one develops an aptitude for it quite quickly. Even by beginner's trial and error, it doesn't take more than a few minutes.
Following is a detail report of the first calculated mix of materials to produce a substitute. Take special note of the "UNITY FORMULA" totals line near the bottom.
Notice that I duplicated the Albany formula goal almost exactly (disregard the second redundant decimal on any numbers).
DETAIL PRINT - ALBERTA SLIP #1 MATERIAL PARTS WEIGHT CaO MgO K2O Na2O Fe2O3 TiO2 Al2O3 SiO2 WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 160.0 79.7 102.0 60.1 -- ------------------------------------------------------------------------------------------ MATERIAL Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.13 0.14 0.06 0.04 Cost/kg -------------------------------------------------------------------------------------------- ------- Redearth........... 68.00 936.47 0.01 0.02 0.02 0.01 0.02 0.01 0.10 0.79 0.00 WHITING............ 5.00 100.00 0.05 0.12 DOLOMITE........... 8.75 184.00 0.05 0.05 0.00 NEPHELINE SYENITE.. 13.50 446.40 0.00 0.00 0.01 0.02 0.03 0.14 0.33 KAOLIN............. 3.00 258.14 0.01 0.02 0.24 IRON OXIDE RED..... 1.75 160.00 0.01 2.90 -------------------------------------------------------------------------------------------- ------- TOTAL 100.00 0.11 0.07 0.03 0.03 0.03 0.01 0.14 0.95 0.11 UNITY FORMULA 0.11 0.07 0.03 0.03 0.03 0.01 0.14 0.95 PER CENT BY WEIGHT 6.05 2.77 2.91 1.88 4.44 0.42 14.78 58.36
Cost/kg 0.11 L.O.I. 8.40 Si:Al 6.70 SiB:Al 6.70 Expan 6.82
The next step was to mix a batch of this to compare with pure Albany. I did the comparison by treating each as a glaze and applying it to test tiles at a variety of thicknesses and fired at a range of temperatures. Disappointingly, the substitute had a very coarse and relatively rough surface compared to the smooth and silky nature of the Albany. It seemed obvious then, that the substitute mix needed to be ground to a finer particle size. I milled for an hour, redid the test tiles, and made melt flow tester samples. The substitute fired very close to Albany slip in character, but about 1-2 cones less mature and it was a little lighter in fired color.
The next step was to change the recipe of the substitute in such a way that I did not diverge too much from the target formula, but still moved closer to the physical firing characteristics of Albany. Here is what I decided to do:
ALBERTA SLIP #2 =============== Redearth............ 67.50 67.50% WHITING............. 9.50 9.50% TALC................ 7.00 7.00% NEPHELINE SYENITE... 14.00 14.00% IRON OXIDE RED...... 2.00 2.00% ======== 100.00
CaO 0.10 5.92% MgO 0.07 3.10% K2O 0.03 2.92% Na2O 0.03 1.94% Fe2O3 0.03 4.69% TiO2 0.01 0.42% Al2O3 0.13 13.66% SiO2 1.00 61.65%
Cost/kg 0.13 L.O.I. 5.70 Si:Al 7.66 SiB:Al 7.66 Expan 6.72
I ended up diverging slightly from the Albany formula goal; however, the mix fired out, in most cases, identical to my Albany slip specimen. On the melt flow tests, the two were identical at cone 9 but at cone 7, the Albany fused slightly better, probably because of a finer particle size.
The final test of compatibility was to put the two materials into a flow tester and a glaze and compare.
I chose a recipe with a high percentage of Albany as follows
GLAZE A GLAZE B BATCH RECIPE BATCH RECIPE ------------ ------------ ALBANY........... 85.0 ALBERTA SLIP..... 85.0 LITHIUM CARBONATE 11.0 LITHIUM CARBONATE 11.0 *TIN OXIDE........ 4.0 *TIN OXIDE........ 4.0 FORMULA & ANALYSIS FORMULA & ANALYSIS ------------------ ------------------- *CaO .24 6.05% *CaO .23 5.86% *Li2O .39 5.31% *Li2O .40 5.38% *MgO .15 2.74% *MgO .17 3.11% *K2O .08 3.41% *K2O .07 3.18% *Na2O .07 1.89% *Na2O .07 1.87% *Fe2O3 .07 4.82% *Fe2O3 .07 4.74% Al2O3 .33 15.33% Al2O3 .30 13.82% SiO2 2.23 60.45% SiO2 2.25 61.60%
RATIO 6.71 RATIO 7.58 EXPAN 6.22 EXPAN 6.47
Following are some encouraging things I found after using these glazes:
Alberta slip tends to flocculate a little when added to a glaze in large amounts, so it is necessary to add some deflocculant to thin it out. Even after the deflocculant was added (I used Allied Colloids #311 sodium polyacrylate dispersant), the glaze tended to gel after sitting for a while but as soon as it was stirred it loosened up and flowed well. This might be an advantage, since it prevents the glaze from settling out and most people would sooner stir a glaze to "loosen it up" rather than try to re-mix one with settled solids.
In general, True Albany seems to fuse a little better at lower temperatures than the current version of Alberta slip, so its glazes flow a little more at cone 5. I attribute this to the fact that Albany is finer. At cone 9 the Alberta slip glaze is actually more fluid, and at cone 7 the difference is very small. Certainly, it is possible to flux Alberta slip to fuse more at lower temperatures (possibly boron for magnesia). The True Albany glaze has a tendency to de-vitrify more than the Alberta slip material. The added CaO was supplied by whiting, but one could easily source it from another material that tends to seed crystals better (e.g. wollastonite). However, this might increase the cost or reduce the amount of Redearth that could be used. One real advantage of Alberta slip is that Redearth deposits of raw clay are extremely large and very consistent. From a purely "visual appeal" point of view, Alberta slip produces a more appealing glaze in my tests.
Alberta Slip is available from plainsmanclays.com.
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% 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.Tap picture for full size and resolution
This is fired in cone 10R. The effect becomes more intense by 5%. To achieve this same effect using Ravenscrag, which has much less natural iron content, 10% added iron is needed (which is, of course, much messier to work with).
Albany Slip successor - a plastic clay that melts to dark brown glossy at cone 10R, with a frit addition it can also host a wide range of glazes at cone 6.
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|By Tony Hansen|
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