hello m............I've just gone through all my books looking again at entries for opaline and opalescent - and am confused. However, I suspect you are better informed than me on this subject. I also think that some authors have muddied the waters along the way - and I get the impression that the original Baccarat product which they called 'opalin' didn't show a sunset glow.
my opinion is that if the piece is without the suset glow, then it isn't opalescent, and I don't have a problem with that, and had got into the habit of using the word 'opalescent' when referring to non-coloured products only i.e. Jobling's 'Opalique' and the bulk of Sabino and Lalique etc. (these all have a sunset glow) - so cannot be anything other than opalescent).
I also possess a number of coloured pieces which show the sunset glow, but seem to remember that when the piece is coloured I was told to call it 'opaline' and not 'opalescent'.
In the famous French 'bulle de savon' and 'gorge-de-pigeon (both opaline products), how are the colours produced - calcined animal ashes??
As I understand it, the opalescent effect seen in some of the famous pressed pieces from Manchester in the C19 would, as you say, have been produced using calcined animal bones and arsenic, although later similar products like Jobling's 'opalique' did away with the calcined bones and substituted a chemical composition of phosphates (but still included the arsenic).
Too many manufacturers have used words, or variations, on opalescent/opal/opaline - each with their own interpretation.
Harold Newman (dictionary) says ..........................Opaline - opacified with ashes of calcined bones and coloured with metallic oxides
Mark West says .............................when an opaline piece is held up to the light it is possible to see a red and orange tint, known as 'fire'.
Raymond Knotley says (speaking of opalescent)..................If glass of a uniform thickness was used, an even opalescent effect was produced and was called 'opaline'.
Did you mean to say ................opaline can be made with or without the sunset glow? In order to simplify the matter, and to get back to the definition of Baccarat's orginal product, perhaps we should adopt the word opaline to cover only those pieces without a fiery glow, and to reserve the word opalescent for those which do.
P.S. Re a set of four egg cups................. what happens if Mum and Dad have three children - who is forced to go without their boiled egg and toast soldiers