Since this conversation started about French faiences, maybe
Brongniart's terminology can be mentioned.
In his 19th-century ceramic encyclopedia, Brongniart noted that
several names for glazes were used interchangeably. In order to be
more accurate, he decided to restrict the use of  "vernis" to simple
lead-glazes, "émail" for tin-based glazes (or tin-lead glazes), and
"couverte" to porcelain and stoneware.
His reasons were that a vernis or lead-glaze was a low-fired glaze
that would melt at a temperature lower than the temperature needed for
the body. So vernis applied mostly to coarse earthenware and refined
earthenware, and that's why coarse earthenware in France are sometimes
called "terres vernissées."

An émail for Brongniart was a similar glaze, but more opaque and most
often made with tin.

A couverte was a high-fired glaze that would melt at the same
temperature as the body, hence its use for porcelain and stoneware
glazes.

Meanwhile, Brongniart also called all of the above "glaçures" or even
"enduits vitreux".

Today the name "émail" in French is used for a variety of objects, but
it is also the preferred word of most faience "experts" (included Jean
Rosen).

Now, I hope that I haven't bored everyone to death with this!


-- 
Myriam Arcangeli, MA, RPA
Ph. D. Candidate
Department of Archaeology
Boston University

people.bu.edu/arcange