There is a lot of rather well executed silver deposit on the outside - along with delicate trails that look a bit like comets.
This was something that originally happened by accident. When the silver salts are heated, they separate from the chloride or nitrate ion and some silver gas gets created in the hot metal. If the hot glass comes away from the iron, some of that gas escapes and gets deposited on the outside of the piece.
Because it was so attractive, they started doing it on purpose - but not everybody there could manage it. Michael Harris could do it really, really nicely and eventually so could Boffo and both Said brothers, (Paul and Joseph) but theirs was often a bit splodgier than Michael's.
I have not yet found out exactly who could do it or who couldn't.
They eventually gave up trying to do it.

Incidentally, when you find an electric-blue haze inside a casing on Mdina, that is from silver ions/gas that got distributed throughout the metal. (rather than deposited on the outside.)