These tiles were fired at cone 6. The melt-flow comparison behind them shows how much more freely the G3808A clear base flows than G2926B. G2926B produces a crystal-clear glass by itself and works well with some colorants. But with 2% added copper carbonate, its melt does not clear the bubbles and heal the surface completely. G3808A has enough melt mobility to produce a smooth surface.
That extra fluidity comes at a price: Notice the glaze running down the right-hand tile. To get this clear, glossy copper color while controlling movement, strategies include thinner application, keeping the glaze farther above the foot, restricting it to more horizontal surfaces or using a tested catcher glaze below it.

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The first glaze, G2926B, is a standard functional transparent glaze with added copper. The other three are part of a project to find a copper blue (G3806B has the best color and the best compromise of flow and bubble-clearing ability).
The GLFL testers for melt flow are at the back, and the GBMF test melt-down-balls contain 1% copper carbonate. The glazed samples in front have 2% copper carbonate. But why do the recipes containing half the amount of copper have far more bubbles? Because they are thinner? Not really; in use on ware, they also have fewer bubbles. Why? A small CuO addition can change where and how bubbles nucleate and how viscous the melt is. At some point between 1% and 2%, a threshold is crossed that affects nucleation and coalescence. For example, a little copper could encourage lots of tiny bubbles to form and stay trapped, while more changes the melt chemistry enough that they coalesce and escape (or simply aren’t nucleated the same way). Phase separation could produce Cu-rich droplets that enable copper to be its own fining agent.
| Recipes |
G3806C - Cone 6 Clear Fluid-Melt transparent glaze
A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others. |
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