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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