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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 (L3806B has the best color and the best compromise of flow and bubble-clearing ability).
The GLFL testers for melt flow 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.

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By itself, without copper, the G2926B recipe produces a better and more durable glass (on the cup at the back). But a 2% copper addition, front, turns its surface to a mass of unhealed bubble-escapes. The G3808A recipe, on the left, develops much more melt fluidity, the extra mobility enables the bubbles, created by the decomposing copper, to coalesce, grow, break at the surface and heal before the melt stiffens too much.
| 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. |
| 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. |
| Materials |
Copper Carbonate Basic
This form of copper carbonate is the article of commerce, a mixture of theoretical copper carbonate and copper hydroxide. |
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