Forming a glaze into balls for melt fluidity testing

The vast majority of glazes are somewhat plastic (but less than clay bodies). They can thus be dewatered on a plaster surface and formed. Why do this? To make 9-10 gram balls and fire them on flat tiles (or inclined flow testers) to see their melting characteristics. We call this the GBMF test, it is surprising how much it can tell you about a glaze or melting material. To make the ball, mix the slurry well and pour a little on the plaster. It should dewater in less than 30 seconds (although there are exceptions e.g. glaze with Gerstley Borate). As soon as the water sheen is gone, scrape it up with a rubber rib, hand-knead it and flatten it back down to dry a little more if needed (leave it only for five or ten seconds and rework it. Repeat until it is stiff enough to form balls of about 12 grams. Stamp them with ID numbers and dry them.

Related Pictures

VeeGum and CMC gum can plastify non-plastic powders for making melt-flow test balls

Frit and veegum powder ready to mix with water

Fluxes used in ceramics are almost always non-plastic, they cannot be formed like clay - frits and feldspars are good examples. And they don't dry hard. To make balls for use in the GBMF test for melt flow a binder needs to be added. Traditionally we have used Veegum, however, it interacts with materials enough to affect melting - CMC gum does not. That being said, Veegum dries better (these balls can dry very slowly). When simply comparing the melt of two materials either is fine.

Each ceramic powder responds differently to being water-mixed with a gum or plasticizer. Some material-gum mixes uptake water so well that it can be worked in drops at a time until a plastic material is produced. Others require vigorous mixing into a slurry and then dewatering on a plaster surface. We target 9g balls, they fit into the reservoir of the melt flow tester. To make one ball we start with 11 or 12g of powder (to allow for waste) and then form them into 12g (wet) balls - these dry them down to the 9g weight. We are almost always comparing the flow of two materials, in these cases it is only important that the two balls be the same weight - so we trim the heavier one down to the weight of the lighter one. Does cornstarch work? No, the mix is not plastic. Psyllium? Yes, but it has a flakey texture and demands more water.

If you are testing a plastic material then a binder is not needed.

Links

Articles	A Low Cost Tester of Glaze Melt Fluidity This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Tests	Glaze Melt Fluidity - Ball Test A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension.

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