Lead Bisilicate Frit: Why is it used in glazes at high temperature (≥2000 °F)?

[Ukar]

Hello everyone,

How many times have I already read in the literature: "The maximum firing temperatures for glazes made with Lead Bisilicate frit is 1950 °F (≈ cone 04)”.

Or: "There is never lead in a high temperature vitrified ceramic glaze (stoneware, porcelain): it is quite simply chemically impossible because the melting point of lead is much too low to be used as a high temperature flux (stoneware, porcelain)".

However, I note that several renowned international ceramists use the lead bisilicate frit in glazes recipes (mainly based on feldspars) for T° of  2000°F (≈ cone 5, 6...) and above!!

MY QUESTION : Could someone explain to me when and for what reason(s) it would be wise to use lead bisilicate at T° 2000°F and above?

Thank you.

[liambesaw]

I'm not sure there is a good reason, other than it's a cheap and easily available frit in Europe.  I think the lead bisilicate actually becomes unstable above cone 04, which is why it's not recommended.  In other words, the lead begins evaporating out of the frit, and upsets the stochiometric balance, which can lead to leeching of the oxide, and contamination of the kiln furniture and bricks, and air around the kiln with lead fumes.

[Ukar]

Thanks for your answer!

[Min]

2 hours ago, Ukar said:

Could someone explain to me when and for what reason(s) it would be wise to use lead bisilicate at T° 2000°F and above?

 

 

I would argue that it's not wise to use lead bisilicate at any temperature.

Frank Hamer states lead bisilicate as partially volatilizing above 1200C / 2192F. Before it gets to that point it's going to cause issues with the glaze as it passes out of it, I would imagine bubbling/craters would be an issue. 

[liambesaw]

33 minutes ago, Min said:

I would argue that it's not wise to use lead bisilicate at any temperature.

Frank Hamer states lead bisilicate as partially volatilizing above 1200C / 2192F. Before it gets to that point it's going to cause issues with the glaze as it passes out of it, I would imagine bubbling/craters would be an issue. 

It's one of the more common fluxes internationally, and completely safe to use under a certain temperature.  This is what a lot of commercial dinnerware factories use as their main flux.  It tests well under the allowable limits for import.  I just think (as evidenced above) it is really easy to abuse.  Best left to factories with chemists on staff.

[Min]

I have issues with the mining of lead and the making of the lead bisilicate, frits etc. It’s inexpensive and makes lovely looking glazes but I don’t think the cost to those involved with the processing of it is justified in this day and age. It’s also available to hobby potters in parts of the world, including here in North America. (US Pigments sells it)

[liambesaw]

Oh yeah it's available here, but it's expensive ($20/lb) and we have been conditioned (rightfully so) to be wary of lead in glazes.  There seems to be more of a lessaiz fair attitude about lead glazes in Europe (especially eastern europe) so the traditions persist.  At least a lot of people have moved to lead bisilicate as a safer alternative to using lead oxide itself.