This demonstrates the amazing melt behaviour of lead-as-a-flux for ceramic glazes. Not only does it melt early, but it softens slowly over a 300F range of temperatures before it goes off the end of the runway on this GLFL test. Then, when fired 200F hotter than that, it remains a stable, clear and uncrazed glass. Beginning around 1750F, this becomes a transparent glaze, by itself.

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These 10g balls GBMF test balls were fired far lower than typical bisque; notice how many of them are already melting well! Frit 3602 is lead bisilicate. Maybe you always thought lead was the best melter. But it got "smoked" by the Fusion FZ-16 high-zinc, high-boron, zero-alumina frit! However, notice the FZ-16 is crazing badly; that is a problem for many applications using this high KNaO frit. Want to get some of this frit for pottery? You can't; Fusion Ceramics doesn't want to handle retail sales of smaller quantities.
Notice that, at this low temperature, each frit has a distinctive melt fingerprint that makes it recognizable.
Notice that the common North American pottery frits on the bottom row: 3195, 3249, 3124, 3110 (and 3134 in the second row) fire distinctly differently. Notice Fusion F69. It looks like 3249 because it is a substitute for it! F-75 is a substitute for 3110 (sharing the same melting pattern but having different micro-bubble clouding).

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This piece was bought in Sinaloa in 2020 (made in Puebla). By breaking it and refiring shards I estimate the firing temperature around 1800F. This lead test procedure involves leaving white vinegar in the piece overnight, pouring some of that into a test tube, dipping a cotton swab into a reagent solution and then stirring the vinegar with it. Darkening of the color indicates the concentration of lead in the leachate. It has turned black! Yet a typical fritted lead bisilicate PbO:2SiO2 glaze (having 10-15% clay to suspend it) does not leach lead (when melted well). The very thin glaze application suggests potters were trying to save money (although thin or thick does not make it more reachable). Frits are expensive, so it seems likely they are using white lead powder. But, they are not mixing enough silica to produce a stable lead silicate chemistry (likely because adding silica would require firing at a higher temperature to get a good melt).
Yet this pottery is a tradition in Mexican culture (and elsewhere) and is used for food and liquid surfaces everywhere. Some manufacturers are trying to make stoneware that retains the traditional terra cotta appearance, but many prefer this.
| Materials |
Ferro Frit 3602
A lead bisilicate frit with 2.5% alumina oxide. |
| Materials |
Lead Bisilicate Frit
A standard frit of 1 molar part of PbO and 2 of SiO2. It is considered stable and non-leachable. |
| Glossary |
Lead in Ceramic Glazes
Lead is a melter in ceramic glazes and performs exceptionally well and must be misused to be toxic. It is also now environmentally pervasive. It is toxic and cumulative at any level of exposure. |
| Projects | Comparing the Melt Fluidity of 16 Frits |
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