I have quoted below from a Treatise from 1832. (link to Treatise given at the bottom)
It seems to me that what is being described is glass replicating the opal stone, but not using reheating and not producing what we call today as opalescent glass. It seems to me the mix of glass in the batch once cooled and hardened as a vase, gives off the play on colours as a result of the constituents of the batch and is not reheated to produce an opalescent (as we know the word) effect. I'm open to correction here.
Also, the description is given under the heading 'Opaque white glass'. I think opaque is being used as a word to describe any glass that is not transparent. i.e. it is not opaque v translucent v transparent, but merely describes the difference between glass which is transparent and glass which is not. There is no in between (i.e. translucent or semi-opaque) just it is transparent, or it's not. If it's not then call they call it opaque.
This is a 'treatise' from 1832
Source: The treatise on the progressive improvement and present state of the manufacture of porcelain and glass - 1832 (I think it was written by George Richardson Porter , produced for Longman, Rees, Orme, Brown & Green )
Page 281
It comes under the header Chap XIII White Opaque Glass. Prior to writing the description I give below, the author gives two formulas for producing white opaque glass from two makers and then goes on to say:
'Fontanieu has given directions for imitating the peculiar lustre of the semi-transparent opal, by mixing 576parts of hi sglass no 8 (Chap VII) with 10 parts of muriate silver, 2 parts of magnetic iron ore, and 26 parts bone ash. The beautiful play of colours exhibited by the "precious opal" is deservedly an object of much admiration, and it has always been a subject of interest to imitate successfully so pleasing an effect.
Ornamental pieces of opal glass have usually been obtained from France, but in their production, of a quality fully equal to these importations, may now be witnessed in the London Glass-works.
The peculiar delicacy and beauty of this glass does not appear while it remains in a state of fusion or at a red heat; and are not fully developed until it is sufficiently cooled to acquire its quality of brittleness.
Other coloured glasses which are opaque, are made by the same processes as are followed by the transparent glasses of the like colours, substituting for the common vitreous base, one of the above described, opaque-white glasses.'
http://books.google.co.uk/books?id=yr8UAAAAQAAJ&pg=PA282&dq=richardson+opal+glass&hl=en&sa=X&ei=XEwCUYO9NKuk0AXq_oGwDw&ved=0CC4Q6AEwAA#v=onepage&q=opalescent&f=falsem