• Materials for Ceramics

Lead Bisilicate Frit

Description: Lead bisilicate frit

Notes

This term refers to frits having one molar part of lead and two of silica. This ratio of silica and lead that is said to produce a stable low solubility powdered glass material that can be used in production with relative safety to workers.

In pottery circles, lead glazes carry a lot of “legal baggage” in North America, irresponsible use in the past contributed to this. Pottery supply companies do not carry lead bisilicate frits. Yet manufacturers have been using them all along and claim they can prove their recipes are resistant to leaching. Potters, especially on the hobby level, would not be good candidates for the use of lead bisilicate.

Ceraflux from Hammond Lead Products is the most common North American lead bisilicate. Many lead bisilicate frits contain from 1-3% Al₂O₃ and are referred to as "lead alumina bilsilicates". This addition further stabilizes the glass powder itself and helps prevent phase separation (crystallization) in the glass during firing.

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 wet milled (much less common).

While the frit powder itself is stable (compared to raw lead carbonate or oxide), that does not mean that glazes created using it automatically resist leaching. Theoretically, the most stable non-leachable glass would be created using the pure frit without additives (however our lead testing indicates lead leaching). To make a practical glaze slurry 10-15% clay is typically added, the add SiO₂ and Al₂O₃ should add stability.

Lead borosilicate is different than lead bisilicate, the former has almost as much lead but the rest is a mix of B₂O₃, SiO₂ and Na₂O.

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.

A way to get crystal clear glaze on terra cotta: Lead bisilicate

The mug on the left is a brush-on version of a boron-based clear glaze for cone 05. Three coats were applied and the often-encountered clouding occurred. The one on the right is an 85:15 lead bisilicate:kaolin mix. Three coats were also applied. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully. And my lead testing kit passes it with no detectable lead release. Underglaze brushwork here we come! I have sought this effect for decades, this is it! Recent realizations about the slipware tradition in the UK (and their standard use of this same glaze) motivated me to get some of the frit. All I could get was a sample of the frit shards, but these milled down to a powder easily in our ball mill.

Of course, to be safe, I would still glaze the insides of pieces with a boron clear, likely as thin a layer as possible of G1916Q (with 2% added iron as a fining agent for the micro-bubbles). And, I will obviously fire these lead glazed pieces with the kiln exhaust system turned on.

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!

Lead testing a lead bisilicate frit

I have soaked an aluminum lead bisilicate frit in vinegar overnight. To test whether it is leaching I will pour the vinegar leachate into a test tube, soak a Q-Tip in the sensor solution and dip it into the vinegar. If it turns black we have lead in the leachate!

Keep in mind something important here: While it is true that the vastly increased surface area of the frit contributes to its failing the leaching test another important factor is that 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 by-product of a glaze cooling slowly in a periodic kiln.

Links

Data

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk