Duplicating Albany Slip — Digitalfire

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:

It is important to have a "representative sample" of any native materials used in development, so that testing work can later be reproduced in production. Obtaining a proper specimen is usually a matter of sampling a stockpile in many places.

It is important to have a "reliable analysis" of the "representative sample". Make sure that a lab with applicable experience does the tests, or have them do it twice to verify. If possible, calculate your own percent weight loss on ignition (LOI) by weighing a bone dry sample, firing to cone 02, and weighing again. If the lab's percentage plus your LOI figure do not total somewhere between 98 and 102, then find out why. They may tell you to adjust the LOI or SiO₂ , retotal, or they may repeat it.

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).

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:

Drop dolomite from the recipe and supply magnesia with talc instead. This would allow the use of more magnesia to achieve better fluxing and retain Albany's low expansion. Had I increased dolomite to source more magnesia, it would have resulted in unwanted extra calcia.
Drop the Al₂O₃ slightly to improve maturity and flow.
Increase the black iron oxide to give more color to the raw material.

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

Following are some encouraging things I found after using these glazes:

Both shivered on my clay body as evidenced by the fired glaze flaking off at the rims of pottery pieces. Albany is well known as a non-crazing material and this shivering confirms it. Alberta slip shivers at least as much, possibly more. It is very encouraging to see this important property of Albany duplicated.
Both glazes blistered in some fast firings, so the evolution of gases, glaze melt viscosity, bubble passage characteristics, etc. of the two seem similar.
The Alberta slip seems to show a little better color when highlighting thin edges on the pieces (e.g. on incised designs).
Alberta slip is more plastic than Albany and this tends to cause the glaze to crack when it is drying on the piece.
Alberta slip is not as inherently fine as Albany, and therefore, might require ball milling in many glazes. This does not seem to be the case with highly fluxed glazes, but could be true in others.

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.