Preparing balls for a melt fluidity test using CMC gum

We use 11g of the material being tested, e.g. a frit, (the right amount for one ball), 0.11g CMC (1%). Of course, you need a 0.01g scale to be able to accurately weigh 0.1g (that is a really small amount). Put them in a small ziplock bag, zip it to entrap air and roll the zipper down to inflate it. Shake well to mix. Stir the powder into water (~5-8g) in a small bowl. Pour it onto a plaster slab - it dewaters very quickly (e.g. as little as 10 seconds) - just as the water sheen is gone, peel it up with a rubber rib. Smear it back down and peel it up every few seconds until it is plastic and formable (but not sticky). For better formability use 1.5% CMC gum - however, the ball will dry slower - drying time is an issue with this method, you will need a heat lamp or other drier of some sort (e.g. a dehydrator). You will also need a plaster surface to absorb the excess water (which invariably happens).

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Preparing balls for a melt fluidity test using Veegum

This method uses 3% Veegum. A 0.01g scale is required (the amount of Veegum needs to be precise since it affects melting). Weigh 11g of the frit and 0.33 of Veegum. Put them in a small ziplock bag, zip it to entrap air and roll the zipper down to inflate it. Shake well to mix. Stir the powder into water (~5-8g) in a small bowl. Pour it onto a plaster slab - it dewaters very quickly (e.g. as little as 10 seconds) - just as the water sheen is gone, peel it up with a rubber rib. Smear it back down and peel it up every few seconds until it is plastic and formable (but not sticky). Roll it into a ball, label it and dry it under a heat lamp (this could take an hour, the Veegum really holds on to the water). When dry we use a sharp knife to cut off enough to bring it down to 9 grams. One caveat: The Veegum is a melting catalyst, balls will flow noticeably more, this is not an issue if you are comparing the melts of two materials (but it is for tests meant to be absolute).

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

Glossary	Ceramic Binder Binders are glues that harden ceramic powders as they dry. They enable improved surface adherence. And slower drying.
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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