Only Michael Harris made them at Mdina, they were very difficult - especially when you consider how short a time he had had actually working with hot glass (by the time he left, only about 3 years, given the time he took out to get set up there) and that he had no "teacher" as such.
The wide shape was achieved using centrifugal force - spinning the hot glass on the end of the rod.
The crizzle effect is produced by shocking the surface of that gather of glass into cracking in cold water, then introducing sliver chloride salts into the cracks before further casing and blowing.
Iridescence appears on the surface if the gather "escapes" from the rod at some point, and the silver vapours escape from the inside of the glass and sublimate on the surface. Sometimes an electric blue-y sheen can arise inside a layer of casing from these vapours too.
The vapours are very toxic, and it's tragic to think that this beautiful effect may well have contributed to his early death from emphysema.