Lead Bisilicate Frit

Oxide	Analysis	Formula
PbO	65.00%	1.00
SiO2	35.00%	2.00
Oxide Weight		343.38
Formula Weight		343.38

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

A 9 gram ball of frit melts and flows downward more and more as temperature increases.

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!

Links

Materials	Ceradel Frit C 1249
Materials	Ceradel Frit C 1250
Materials	Frit 3647
Materials	Ferro Frit CE VTR 29
Materials	Lead Carbonate
Materials	Red Lead
Materials	Ferro Frit 3403
Materials	Ceradel Frit C 1251
Materials	Frit Welte FR 2015
Materials	Ferro Frit 4064
Materials	Solargil Frit FR2
Materials	Ferro Frit 3602
Materials	Potclays Frit 2261
Materials	Pemco Frit Pb-700
Materials	Ferro Frit 3498
Materials	Frit
Materials	Lead Sesquisilicate Frit
Materials	Ferro Frit 4364
Materials	Frit B300
Materials	Lead Monosilicate Frit
Materials	BPS Lead Bisilicate
Materials	Hommel Frit 437
Materials	Lead Bisilcate B-15
Materials	Ceraflux
Materials	P29 Frit
Materials	Pemco Frit Pb-545
Materials	PotteryCrafts Frit P2950
Glossary	Metallic Glazes Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
Typecodes	Frit
Typecodes	Leaded Frit Frits can contain 1% or 80% PbO so this category can be misleading, check the chemistry to find out.
Hazards	Lead in Ceramic Glazes: What Did We Learn? The long slow death of lead use in ceramics taught us some lessons. Manufacturers who still use acid tests to determine glaze toxicity risk liability.
Hazards	Lead in Frits: The Hazards
Hazards	Lead Toxicology

Data

Co-efficient of Linear Expansion	7.1 x 10-6
Frit Softening Point	880-1050C M
Frit Softening Point	1390F

By Tony Hansen