thanks - I'm not surprised at this multitude of recipes for 'opal' - an association word that carries with it many of the qualities and attributes of the real thing - it seems to have been a fashionable description for so many glass inventions over the past couple of centuries, to include the word opal. As to the C19 recipes for 'opalescent' glass, I'm unsure as to the extent of the arsenic content as an opacifier, or as a means of contributing to the 'opal' effect i.e. the sunset sparkle - much of the literature is unclear on this - possibly some of each. Generally, with these C19 products, and for obvious reasons, glass houses were no doubt reluctant to publish details of what went into their pots.
In the U.K., John George Sowerby was responsible for the invention Vitro-Porcelain, and the first colour they produced was a plain white which they called 'opal' (it's a truly opaque material with the appearance of a milk-white opal, so no chance of transmitted light). I don't see that the ingredients included arsenic, but can't be sure - the main content appears to have been cryolite plus some oxide of zinc and the sand - no mention of ashes of calcined bones.
A little later with his Patent Queen's Ivory Ware, he speaks of "adding to the usual ingredients of common flint glass, arsenic to make the glass opaque, and as we know, he also added uranium to the Ivory batch "to give it the yellow tint". However, Sowerby appears not to have been happy with this recipe, and later he speaks of improving the appearance of 'Ivory' by dispensing with arsenic and substituting cryolite spar, plus some other ingredients - the u. was of course retained.
So, as with 'Vitro-Porcelain, the main opacifier for Ivory, appears to have been cryolite, since there isn't any mention of arsenic or ashes of calcined bones in the final recipe - again, as we know, Ivory is a fully opaque product, and no chance of any sunset glow with opque products.
Sowerby continued with his inventiveness and produced Blanc-de-Lait, about which the Pottery Gazette said ..................... "Three years have been spent in experiments on the opalescent product by Mr. Sowerby etc. etc. ......." it's a mostly translucent glass and according to one observer ............. "Held up to the light (transmitted light), it shone like opal in a hundred delicate hues, chief amongst which was a glowing yet tawny gold, that suggested the sun shining through a purple mist in evening. "
Unfortunately, I can't see the recipe for Blanc-de-Lait - the opacifier may have been arsenic and there may have been ashes of calcined bones added too.
Someone else said of Blanc-de-Lait .......... "it was of a rich, lustrous, milky colour, verging in the thinner portions into that sky-blue which milk assumes when it's vendor has had dealings with 'the cow with the iron tail'" .......................... so might it be lead arsenate or cobalt that gives this bluish hue - with Jobling, much later, it was the inclusion of cobalt.
Other than Sowerby, Ray Slack mentions Edward Moore & Co (U.K.) who used calcined oats; or, other calcined cereal; or, vine stalks as ingredients in the opaque batch - Edward Moore (in Slack) gives an account of his recipes for 'Eau de Nil (soft opaque green (probably nothing like the Nile), and a type of 'caramel brown'. Moore appears to be saying that he adds these calcined cereals as colouring substances.
Coming forward some decades - and without getting too bogged down in chemistry - but just to show perhaps a more modern take on this issue of opal/opalescence etc. Jobling produced a very well known product called 'Opalique', the idea for which appears to have had its origins in their lighting bowls - similar to the Chance panels and of course to the famous opalescent products of Lalique in France. Jobling's translucent light bowls were opacified using phosphate, fluorine and alumina - so no ashes of calcined bones - but amongst a very varied bunch of ingredients they experimented with was white arsenic - plus they included a trace of cobalt which "was successful in producing an attractive blue glass with an inner golden fire (sunset glow), similar to that found in some Lalique products.
"The density of the opalescence depended on the rate of cooling of the outside relative to the centre: the opal within cooled more slowly, allowing crystals to form which refracted the light."
Much of the above is taken from the original research work of Ray Slack and of course Baker & Crowe re the Jobling material published in the Tyne & Wear County Council Museums Collectors Guide - and much credit to these authors that their work is publicly available for all of us to read - I just do the typing plus the italic bits. If you've not seen the Baker & Crowe publication it really is worth getting.
Topic: Richardson's Vitrified Opaline (Read 2269 times)
