I have seen the "hebrewhistory" site before and I found the sections on glassmaking very interesting. But I was not convinced by several of the "conclusions" drawn, primarily because I could not see actual, referenced evidence for some of the statements made.
I agree with Marcus's query about Dr Plot and the inferences about Ravenscroft. If there was a reference via which the comments could be validated, I would be happier to maintain an open mind about what has been suggested.
But from what I have read elsewhere Ravenscroft very likely learned about the inclusion of oxide of lead in glass batch from a translation by a Dr Merret of an Italian work by Antonio Neri. The "hebrewhistory" articles do not seem to mention Dr Merret, or the work by Neri. Neverthless it is agreed in various literature that Ravenscroft was not the first person to use lead in glass.
But what Ravenscroft did was to use a high proportion of lead in order to produce a glass that met the requirements of the time as determined by The Glass Sellers Company (which has, at least, been mentioned in the website articles). It was this high-lead batch that was referred to as "glass of lead" or "lead glass". And it seems that at about the same time, the use of higher lead content was also developed in Bohemia, but it was the new English glass that made the most impact.
If the earlier glass made by Jews was a lead glass, in the sense of it having a high lead content, then why did this not become the standard batch used by such as the Venetians? Why make items of soda glass with its great fragility, if the stronger "lead glass" formula had been known?
Going back to the quote about, "Not only lead glass but all glass were then attested by two roman emperors as being made by the jews", my interpretation of that was that Jews were definitely known to have been working as glassmakers at the time. I did not read it as meaning that all glassmakers of the time were Jews. And that point has not been proven to my satisfaction in any of the "hebrewhistory" articles - and particularly for the millenia preceding the Roman empire.
But I really ought to read it all again.