thanks Christine
Raymond Slack, quoting from the Pottery Gazette for 1st May 1880, shows beyond any doubt that Blanc-de-Lait was the end result of Sowerby's experiment with opalescent glass......"this new glass was a beautiful opalescent material" (Slack's words)............."three years have been spent in experiments on the opalescent product by Mr. Sowerby and his art workmen" (the Pottery Gazette).
In addition to the more common form of plain opalescent Blanc-de-Lait', Hajdamach shows a green stained example in his 'British Glass 1800 - 1914'.........'and within a very short time the added novelty of decorating it with opaque-staining was to become a popular feature of production.' (Slack).
In addition to Slack and Hajdamach, Lattimore also identifies Blanc-de-Lait' correctly, as the opalescent production, but unfortunately then makes the mistake of adding the word 'opaline' to the description.
There appears to be no difficulty (for all authors) in distinguishing correctly 'Queens Patent Ivory Ware' - for a variety of reasons, not the least of which is that for us (with u.v. torches) it glows green.
This leaves the Vitro-Porcelain only - and whilst Hajdamach and Slack have no problem with describing this correctly, Lattimore seems to have problems with terminology, and his descriptions are inadequate and insufficient to be relied on, although his accompanying photographs are what we know to be Vitro-Porcelain.
Quoting from Slack..........................."If on the other hand four parts of cryolite were used, an opaque mass closely resembling china or glazed earthenware in appearance was obtained. This opaque substance...........etc. etc. and marketed under the name of Vitro-Porcelain by Sowerby's, and later under this and other names by most other manufacturers of pressed glass in the north east". (page 35).
I believe that white was the first of Sowerby's Vitro-Porcelain colours, to which they gave the commercial name of 'Opal', although subsequently other colours were produced, using the same original recipe.
The products discussed above are for uniformly coloured wares and, description wise, are not to be confused with other pressed products that Sowerby produced, for example 'Sorbini'/Malachite/slag or marble glass.
You could, with hindsight argue that Sowerby were perhaps lacking in foresight in calling an opaque Vitro-Porcelain ware 'Opal', since it gives rise to confusion with the word opalescent, and brings to mind a quite different looking fiery glowing appearance (as we know), but it's possible that with the newness and novelty of brand new products and zero history, plus a need for words that were commercially appealing, they went a little OTT. Their choice of words has been our undoing.
Literally, of course, the colour opal can quite legitimately be described in Cottle's words........"Opal is a milky semi-transparent glass".............since that is what I guess an opal looks like, although Cottle appears to be at odds with original descriptions, historically, and has further confused the issue with his description.
According to Lustrousstone's quote of Cottle's description of Blanc-de-Lait (not 'du'), it would seem that his description does not match that of other authors, or the Pottery Gazette.
Must admit that I was surprised to see that Bernard was unsure of the correct description of Blanc-de-Lait, bearing in mind the length of time of his association with British pressed glass
I hope that this might have helped, but if not, then apologies.