Nice Hora set, Michael.
I've looked closely at my opalescent piece. It does have slightly petrol-like sheen when the light hits at a certain angle, so, yes, hints of of iridescence. The opalescence varies throughout the piece, most likely because of the wide range of thicknesses in the glass. Some places where the glass is thick, the glass looks almost white. That would argue against the coating idea as the effect is present throughout the glass. This probably came out around the same time as Fratelli Toso's "Starry Nights" pattern, which was fantastically successful, so that would give a reason for the experimentation.
I checked a few definitions of opalescent and that didn't help much. The term was used to describe different processes in different factories. One was where iridescent and opalescent were synonymous. Another method entailed cooling a piece quickly with compressed air, then re-heating. And finally the effect could be achieved by adding bone ash and arsenic. I would guess Skrdlovice used the third method, but wouldn't want to for very long. Interesting couple of pieces.
Yes, my Vizner piece is very heavy.
PS Michael, Vancouver fell from top city about three years ago, much to the anguish of the citizens. I could never understand why it was so highly ranked, especially when one considers that the ratio of average income to average house price makes it one of the most expensive cities in the world in which to live.