|Monthly Tech-Tip |
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).
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.
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!
|Materials||Ceradel Frit C 1249|
|Materials||Ceradel Frit C 1250|
|Materials||Ferro Frit CE VTR 29|
|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||Lead Sesquisilicate Frit|
|Materials||Ferro Frit 4364|
|Materials||Lead Monosilicate Frit|
|Materials||BPS Lead Bisilicate|
|Materials||Hommel Frit 437|
|Materials||Lead Bisilcate B-15|
|Materials||Pemco Frit Pb-545|
|Materials||PotteryCrafts Frit P2950|
Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
Frits can contain 1% or 80% PbO so this category can be misleading, check the chemistry to find out.
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.
Lead in Frits: The Hazards
|Co-efficient of Linear Expansion||7.1 x 10-6|
|Frit Softening Point||880-1050C M|
|Frit Softening Point||1390F|