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Lead Bisilicate Frit

Description: 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 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


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.

Lead bisilicate with his ugly borosilicate cousins at a cone 05 party


Boron and lead transparent glazes on terra cotta

The middle front mug is glazed with an 85:15 lead bisilicate:kaolin mix, the G3971 recipe. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully regardless of whether the clay is smooth or coarse or the glaze thick or thin (this one was applied as a brushing glaze in three coats on L215). My lead testing kit passes it with no detectable lead release. The other pieces are done using brush-on versions of boron-based clear glazes (commercial and made from a recipe). At almost any thickness and whether on L215 or L4170B clouding occurs. The worst one is a commercial three-coater on the right, the best is G1916W (it has 2% added iron as a fining agent for the micro-bubbles). My terra cotta plan: Glaze the inside functional surfaces with that and the outsides with the leaded one (and using a kiln exhaust system).

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


Lead frit smooths a glaze

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!

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).

Links

Materials Lead Sesquisilicate Frit
A standard frit of 1 molar part of PbO and 1.5 of SiO2. It melts lower than a lead bisilicate.
Materials Frit
Frits are made by melting mixes of raw materials, quenching the melt in water, grinding the pebbles into a powder. Frits have chemistries raw materials cannot.
Materials Ferro Frit 3498
Materials Pemco Frit Pb-700
Materials Potclays Frit 2261
Materials Ferro Frit 3602
Materials Solargil Frit FR2
Materials Ferro Frit 4064
Materials Ferro Frit 4364
Materials Ceraflux
An alumina lead bisilicate from Hammond Lead Products in Indiana. The data sheet claims it is safe and insoluble in stomach acid.
Materials PotteryCrafts Frit P2950
Materials Pemco Frit Pb-545
Materials P29 Frit
Materials Ceraflux
Materials Lead Bisilcate B-15
Materials Hommel Frit 437
Materials BPS Lead Bisilicate
Materials Lead Monosilicate Frit
A standard frit of 1 molar part of PbO and 1 of SiO2. It melts lower than a lead bisilicate.
Materials Frit Welte FR 2015
Materials Ceradel Frit C 1249
Materials Ceradel Frit C 1250
Materials Ceradel Frit C 1251
Materials Ferro Frit 3403
Materials Red Lead
Materials Lead Carbonate
Materials Ferro Frit CE VTR 29
Materials Frit 3647
Hazards Lead Toxicology
Hazards Lead in Ceramic Glazes
Lead glazes may or may not be hazardous. This topic is not as clear as you might think.
Typecodes Leaded Frit
Frits can contain 1% or 80% PbO so this category can be misleading, check the chemistry to find out.
Typecodes Frit
A frit is the powdered form a man-made glass. Frits are premelted, then ground to a glass. They have tightly controlled chemistries, they are available for glazes of all types.
Glossary Metallic Glazes
Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
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.

Data

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