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This glaze is 85% Albany Slip and 20% Ferro Frit 3195. These bisque tiles were dipped in a brushing glaze version of it (just water and powder). The glaze is applied quite thin on the front tile and thicker on the back one. The material gelled slurries and required a lot of water to make them thin enough to use. For assured success, this or any glaze that had a high percentage required mixing the raw Albany Slip with a calcined Albany Slip (which people had to make on their own).

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These three melt flows and mugs were fired at cone 6 (using the C6DHSC firing schedule). The benchmark recipe is 80% clay and 20% Ferro Frit 3195 (the standard GA6-B recipe).
-The center melt flow (and matching buff stoneware mug below) employ the original Albany Slip.
-The one on the right employs Alberta Slip. Notice that, although having a very similar melt flow, it needs an iron oxide addition to darken the color (e.g. 2%).
-The one on the far left uses an Albany Slip substitute made from 80% Redart, 6.5% calcium carbonate, 6.5% dolomite and 6.5% nepheline syenite (code L3613D). The chemistry of RedArt is different enough from Albany that some compromises were needed to avoid over-supplying the iron even more (and firing darker yet). Although this Redart version runs in a very similar pattern on the melt flow, the character of the glaze on the mug reveals it needs a little more melting (increasing the frit percentage would take care of that).

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Layered dipping glazes usually peel like this because they contain clay and shrink as they dry (the fact that all of them don't do this is actually amazing). Success, even with single layering, is a matter of the shrinkage being low enough, the drying being fast enough, the layer being thin enough, the bisque being absorbent enough, the water content being low enough and the bond with the bisque being good enough. Glazes with high clay content (including Gerstley or Gillespie Borate), thick applications or multi-layering are the main offenders. Thixotropic slurries apply most evenly and are least likely to go on too thick. Dipping glazes having 15-20% kaolin or ball clay are easiest to slurry up and have the best application and drying properties. Mixing base layers as first-coat dipping glazes is also important.
The problem with this piece: The addition of 7.5% bentonite to make up for the otherwise low raw clay content in the recipe produced a recipe that does not pass a sanity check. When that was replaced with kaolin it worked. There is a crowbar approach to fix these without any other changes: Add CMC gum (e.g. 1%) to make them brushing glazes (if you don't mind long drying times).

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Six layers of any normal dipping glaze would be impossible (flaking usually starts with the second). Yet this slurry is 85% plastic clay, it shrinks so much that it would be like a "dried-up lake bed" on the first layer. By the second it would all just fall off! How was it possible to dip six layers of this with no cracking? A 1% CMC gum addition added enough adhesive power to not only stick it on better but also counteract the drying shrinkage! Of course, there is a downside: A drying period is needed between each layer, the time depends on the porosity and wall thickness of the ware and the amount of gum. This also demonstrates the difference between the function of Veegum (and similar materials) with CMC. The former, if added to this recipe, would gel the slurry, require more water and increase the shrinkage, making the cracking much worse. We typically use a mix of calcine:raw Alberta Slip to control drying shrinkage enough to enable crack-free application for one coat (without CMC). But, as a first coat dipping glaze, a CMC addition is required.
| Materials |
Albany Slip
A pure low plasticity clay that, by itself, melted to a glossy dark brown glaze at cone 10R. It was a popular glaze ingredient for many decades. |
| Troubles |
Crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms. |
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