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Metallic or Bronze Glazes

Glassy iridescent metallic glazes can most easily be produced in oxidation using very high a percentage of manganese dioxide (the metal fumes of which can be very dangerous) in a borax or lead based frit or glaze. Manganese is an active melter, so 50% or it and a borax frit will produce a very fluid glaze at cone 6. Other metal oxides like copper and cobalt are also active fluxes and melt even better than manganese, but they want to form crystals during cooling (the micro-crystals of copper completely matte the surface). To utilize copper and cobalt a frit base of high alumina is required to make the melt stiff enough to resist crystal formation.

Up to 80% metal oxide is sometimes used. If crystals are desired, their development can be encouraged by adding a catalyst (e.g. barium carbonate). As noted, these glazes can be very toxic to fire because of the danger of the metallic fumes. They are completely unsuitable for use on functional surfaces.

In reduction firing it is obviously easier to produce metallic surfaces, thus much lower amounts are needed. A key reason for this is that iron, while refractory in oxidation, is an active flux in reduction. In addition, iron oxide is inexpensive whereas the other metal oxides useful for this purpose are very, very expensive. Bronze-like surfaces can also be made by the addition of rutile.

How do metal oxides compare in their degrees of melting?

Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.

A metallic, silky crystal black glaze based on Alberta Slip

This is a 50:50 mix of calcine and raw Alberta Slip plus 5 parts Mason 6600 black stain, 5 Mason 6666 black and 7 iron.

Metallic deep purple by firing pure alberta slip at cone 10R, then refiring at cone 6 oxidation.

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By Tony Hansen

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