Lead bisilicate frit data sheet claims high resistance to leaching

Notice it is specifically "formulated for use in pottery glazes". And that "Lead bisilicate is extremely resistant to leaching by dilute acids, including gastric juices, which reduces its toxicity and offers the maximum safety of any of the lead products." In many countries, the use of lead glazes is still considered normal and safe. There is zero use of lead in pottery glazes in North America. Is it possible that we have tarred all lead products with the same brush? Could we at least be using it on non-functional decorated surfaces of low-fire stoneware? This would drastically reduce the energy consumption of kilns.

Related Pictures

Is Mexican Terra-cotta pottery lead-glazed? Yes. Does it leach? Yes.

A lead-glazed handled cup sold in markets in Mexico

This piece was bought in Sinaloa in 2020 (made in Puebla). By breaking it and refining 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. The 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. Frits are expensive so it seems likely they are using raw white or red lead powders. But they are not mixing enough silica to produce a stable lead silicate chemistry.

Yet this pottery is a tradition in Mexican culture (and elsewhere) and is used for food and liquid surfaces everywhere. There are manufacturers trying to make stoneware that retains the traditional terra cotta appearance, but people prefer this.

A lead bisilicate frit fails a leach test. Yet as-a-glaze it passes. Why?

I have soaked a lead bisilicate frit in vinegar overnight. To test whether it is leaching I pour the vinegar leachate into a test tube, soak a Q-Tip in the sensor solution and dip it into the vinegar. It turns black immediately - so we have lead in the leachate! But this is not as it seems.

Remember a key point here: The frit glass had no opportunity to be annealed - it was crash-cooled by being quenched in water. Annealing and associated toughening of the surface is a natural consequence of a glaze cooling slowly in a periodic kiln - which is why pieces made using an 85:15 mix of this same frit and kaolin, pass this test. The same 85:15 mix also still passes a lead check test if melted into an ingot and crushed into a granular powder (this is amazing given the exponential increase in surface area).

The secret of the higher gloss glaze on the right? A lead frit addition.

These cone 04 glazes have the same recipe (a version of Worthington Clear sourcing B₂O₃ from Ulexite instead of Gerstley borate). But the one on the right is more glassy, more transparent. Why? It has 10% added lead bisilicate frit. Lead bisilicate produces dazzling transparent glazes. no other method matches it. While potters gasp at the thought of using lead consider this: They thrive on unstable flux-deprived, glass-deprived and alumina-deprived base stoneware glazes with additions of large percentages of toxic colorants like chrome and manganese!

Links

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.
Glossary	Slipware Slipware, in the UK, is terra cotta pieces decorated at leather hard with thixotropic high ball clay slips, then bisque fired and clear glazed with lead bilisicate.

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