Lead bisilicate frit refers to frits having approximately one molar part of lead and two of silica. This ratio of silica and lead produces a stable low solubility powdered glass material that can be used in production with relative safety to workers. Of course, it can be put into a recipe of unbalanced chemistry to create a glaze that is leachable.
Many lead bisilicate frits contain from 1-3% Al2O3 and are referred to as "lead alumina bilsilicates". This addition further stabilizes the frit glass powder itself (and prevents phase separation is the glass during firing).
Glazes based on lead frits produce a shiny, durable finish and give brightness and clarity of colour when used in conjunction with oxides, stains, slips etc. While they can be used on all earthenware clays, they are particularly suitable for red clay.
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 well milled (much less common).
A frit is a type of ceramic glass. It is a combination of materials that, when melted together, are rendered insoluble and resistant to acid attack. They are, therefore, a means of introducing certain materials into a glaze which would otherwise be toxic. Frits can be used alone as low temperature glazes, e.g. raku and majolica, but generally they form the basis of a glaze recipe.