Robert Marshall Simpson on fri 21 apr 00
------------------
I hope someone on the list will set me straight on the use of lead =
bisilicate in
glazes. I have been under the impression that they were not food safe---to =
be
used only on the nonfunctional. In =22The Complete Practical Potter=22 by =
Josie
Warshaw there is a glaze used on a cup, pitcher, plate, etc. that contains =
72=25
lead bisilicate and fired to cone 03/02. The glaze does not look like the =
dry
matt I associate with lead bisilicate but the recipe is printed at least 3 =
times
so I doubt it is a typo.
Kayte-in Oklahoma and losing sleep over my misunderstanding of lead frits
Smart on sat 22 apr 00
Dear Robert,
In certain conditions lead bisilicate frits can be used and food safe for
tableware articles. The most important is to follow J.W. Mellor rules and
verify them with leaching tests (FDA testing method).
J.W. Mellor rules (for lead frits and glazes) :
It's a calculation with molecular ratios based on sum of basic oxides
components (Na2O, K2O, Li2O, PbO, BaO, ZnO, CaO, MgO, SrO...), sum of acid
oxides (SiO2, B2O3, TiO2, ZrO2...) and Aluminium oxide (Al2O3)
Calculation :
Mellor coefficient = (% Basic oxide sum + % Aluminium oxide ) / (% acid
oxide sum)
Solubility limits will be at very low level if the Mellor cofficient is
lower than 0.5 This rule is valid with glaze recipe until 42.5-65% lead
(mol). In all the cases you have to minimize the Mellor coefficient (by
increasing of SiO2) to be sure of your ceramic food safe and verify it by
leaching tests.
I hope you understand my english,
Cordialement,
Smart
----- Original Message -----
From: Robert Marshall Simpson
To:
Sent: Friday, April 21, 2000 9:37 PM
Subject: lead bisilicate frit
----------------------------Original message----------------------------
------------------
I hope someone on the list will set me straight on the use of lead
bisilicate in
glazes. I have been under the impression that they were not food safe---to
be
used only on the nonfunctional. In "The Complete Practical Potter" by Josie
Warshaw there is a glaze used on a cup, pitcher, plate, etc. that contains
72%
lead bisilicate and fired to cone 03/02. The glaze does not look like the
dry
matt I associate with lead bisilicate but the recipe is printed at least 3
times
so I doubt it is a typo.
Kayte-in Oklahoma and losing sleep over my misunderstanding of lead frits
| |
|
