• Materials

Lead Bisilicate Frit

Notes

This term refers to frits having approximately one molar part of lead and two of silica. This ratio of silica and lead produces a stable low solubility powdered glass material that can be used in production with relative safety to workers. Of course it can be put into a recipe of unbalanced chemistry to create a glaze that is leachable.

Many lead bisilicate frits contain from 1-3% Al₂O₃ and are referred to as "lead alumina bilsilicates". This addition further stabilizes the frit glass powder itself (and prevents phase separation is the glass during firing).

Many manufacturers used to make this frit formula. But today, especially in North America, it is difficult to find a source. While available in some places, they only sell to manufacturers (not to potters, schools or hobbyists). Ceraflux from Hammond Lead Products might be the best choice.

For certain glazes care must be taken not to ball-mill these frits too fine (eg. tin glazed earthenware). Some products are dry milled by the manufacturer, others are well milled (much less common).

Related Information

Ferro Frit 3602 melt flow over many temperatures

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.

What is the secret of the higher gloss glaze on the right? Yikes, it is lead!

These cone 04 glazes have the same recipe (a version of Worthington Clear sourcing B₂O₃ from Ulexite instead of Gerstley borate). While the one on the left is OK, the one on the right is great! Why? It has 10% added lead bisilicate frit. Of course, I would not recommend this, I am just demonstrating how well it melts. Still, we gasp at the thought of using lead while we thrive on unstable flux-deprived, glass-deprived and alumina-deprived base stoneware glazes with additions of toxic colorants like chrome and manganese!

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By Tony Hansen