Paint another layer onto a fired glaze? Yes. With CMC gum.

The cone 6 mug on the left has the G3933A glaze, applied as a dipping glaze. It turned out poorly - crawling from corners and looking thin and washed out. I made a brushing glaze version of this (which adds 1.5% CMC gum), I keep it around for this very purpose. It has a high specific gravity (unlike commercial ones that have high water contents - they will run and go on too thin if you try this). Because of the gum it dries hard, there is no shrinkage or cracking. On a second firing, using the C6DHSC schedule again, (mug on the right) the surface is transformed - thicker, more vibrant color (being picked up from the underlying body).

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Six layers, 85% Alberta Slip in the glaze, yet no cracking. How? CMC gum.

A small cup with multiple layers of glaze applied, and no cracks

Six layers of any normal dipping glaze would be impossible, flaking usually starts with the second layer. 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 layer it would all just fall off! How was it possible to dip six layers here? A 1% CMC gum addition (via a gum solution). Gums are often added to low-clay-content glazes to dry-harden them. But with all the clay in this one, no help is needed for hardening. This is an incredible demonstration of the power of a gum as an adhesive and hardener: It has sufficient power to actually counteract drying shrinkage! Of course, there is a downside: A drying period is needed between each layer, the length 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 drastically increase the shrinkage, making the cracking even worse. Of course, one could simply use a mix of calcine:raw Alberta Slip to control drying shrinkage and gum would not be needed.

Control gel using Veegum, brushing properties with CMC gum

This is G1214Z1 brushing glaze (with 5% titanium added). For a 340g powder batch (to get a pint) my initial target is 5g CMC gum and 5g Veegum. CMC controls drying speed and Veegum the amount of gelling. I first mix the CMC with the powder and shake the whole batch in a plastic bag. Then I add it all to 440g of water in the blender jar and mix it well (making sure no agglomerates remain (stage 1). Stage 2 is adding the VeeGum slowly, while high-speed blender-mixing, this is important because it enables tuning the degree of gel (which cannot be predicted). Because this recipe has little clay, it took all 5g of Veegum without overgelling (the entire mass moved freely in the mixer jar). But it did gel overnight (so 4g would be better next time). By contrast, it is the brushing behavior that demonstrates whether the amount of CMC is right. Not enough and coats dry too fast and go on too thick. Too much and it dries too slowly and too many coats are needed.

Convert a pint of dipping glaze for brushing using CMC gum

We normally make a low specific gravity pint of slow-drying brushing glaze using 340:440:5:5 powder:water:Veegum:CMC proportions (this is ~1.5% of each of the gums). But where thick coverage of large areas is needed a potter can quickly convert a dipping glaze for painting. How? By blender mixing some CMC gum into it (with no Veegum the specific gravity is high enabling thicker layers). Since we mix most of our dipping glazes to about 50:50 water:powder then 340g powder is yielded by ~680g slurry. I can thus blender mix 5g CMC gum into 680g of slurry to quickly make a pint. Why no Veegum? Faster drying, and as noted, thicker coverage (still much slower drying than dipping consistency). This piece is difficult to cover using dipping or pouring (without having a large bucket of slurry). In addition, I need a thin glaze layer inside the flange and quite thick elsewhere. These issues were easy to accommodate by brushing on three coats. In fact, I can quickly glaze large 15 and 35 lb bowls, while rotating on the wheel, inside and out, using this same technique.

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Materials	CMC Gum CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness.

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