A new version of this law says to leave the glazed ware for 24
hours in 4% vinegar, after this period no more than 3 milligrams
Pb must be dissolved from 100 square centimeter glaze surface,

Often the impression is given that the higher the proportion of
silica to lead is, the less soluble the lead will be. This com-
prehension may result out of the names of lead frits, mainly
known as "Monosilicate” and "Bisilicate", Tests which have
been performed by several laboratories came to the following
results:

Pho. 1.5 SiO», = 10.4 % dissolved lead
Poo. 1.75 SIO9= 6.7% ‘ »
Pho . 2.00 5109= 7.0% 7 .
Poo, 2.25: 31095 2.3% u id
Poo . 2.50 5102= 3.0% ? ’
Pho . 2.75 810,= 3.3% , .
Pho . 3.00 S109= 3.3% : .
Pho . 3.25 S105= 5.1% .
Pho . 3.50 S109 = 13.7% " :
Pho . 3.75 S109 = 25.8% 7

We see from this row that the proportion of Pbo . 2.50 SiO is the
ideal. A pure lead glass or frit can be traced easily by its
colour, which is yellowish on a white background, like white
clay for instance, as long as free (and herewith easy soluble

in weak acids) lead is present. This appears in the glass
"Monosilicate", where we find a distinctive yellowish tint.

The “Bisilicate" shows a white glass, especially as a small
amount of Alumina oxide is added for an even safer compounding
of this frit.

To make the safe use of lead compounds absolutely sure, only
the bisilicate should find entrance in the products of a novice in
glaze making. Besides this step of using bisilicate, the
finished glaze should contain at least the permitable amount of
Calcium carbonate, and if possible, an addition of Alcalies will
do further good towards insolubility of lead sources in glazes.

Where a professional is using raw lead compounds, he always
will apply great precautions in handling the stuff, and make
sure his endproduct is either safe for food use, or there is no

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