In order to add 0.5% of CMC gum to this glaze I need to calculate the water content to know how much powder is present. Other scenarios for this are adding Frit 3135 to cone6ify a cone 10 glaze, adding stain to color it, adding zircon to opacify it.
I put a teaspoon of the glaze (well stirred) on the tared tray. It weighed 5.04g (this 0.01g scale is accurate to 1/100th of a gram). WhenI dried the tray under a heat lamp, it weighed 2.46g. The water content of the slurry is thus (5.04 - 2.46) / 5.04 * 100 = 51.2% (the solids content is thus 48.8%). To blender mix 0.5% CMC gum into this I weighed the jar with glaze at 680g and the jar itself at 35g. Thus 680 - 35 = 645. 48.8% of 645 is 315g of powder. 0.5% of 315 is 315 * .005 = 1.6g of CMC gum.

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Right: My attempts to apply a layer of watery white underglaze to leather-hard clay. Not surprising that I measured the water content of this at 68%. Yikes!
Left: After adding about a gram of Epsom Salts (and blender-mixing it in to dissolve), the slurry became thixotropic and gelled enough to enable applying a thicker layer with no problem. Note how it stays on the spoon. However, there is still an issue: Multple layers are needed, with lots of drying time between them. The low solids loading of this product also makes it tricky to use on bisque ware (more time is needed between the thinner coats it creates). Allowing it to settle and pouring off water, or evaporating some of the water to raise the specific gravity, would make it even better.

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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.
| Glossary | Water Content |
| Tests |
LOI/Density/Water Content
LDW LOI, density and water content test procedure for plastic clay bodies and porcelains |
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