I've been pondering for a couple of days whether to mention anything about the sfumato techniques. Sfumato, of course, means smoke. It is used in two different ways when it comes to the art forms. To painters the term means the gradual shading from one color to another, so that there are no distinct lines. It is what makes paintings looks so different to coloring books. In art glass it may also be used to describe the glass that is exposed to smoke to give it a smoky gray appearance. Barbini is thought to have developed this technique when he was at VAMSA. It (or a descendent of the technique) is still in use today to make much of the "grey effect" that we see. I have to say that "grey effect" is so common now that it may be made using a simpler technique. I don't know.
In her book on Archimede Seguso, Pina describes Arhimedes' polveri pieces as sfumato. There is nothing wrong with that, because the technique does fit with the painter's definition of the term. I prefer to use the word polveri, because it is more descriptive of the technique used. I reserve the word "sfumato" to pieces that were made using smoke. Because I'm not sure if all "grey effect" is made using smoke, I simply call it "grey effect" so I don't have to figure it out.
Gold foil is not polveri or sfumato. It is just a whisper thin sheet of gold rolled onto the gather before the glass is expanded. Polveri is actually ground (pulverized) colored glass. Gold foil is often used with the colored glass to help richly blend the shades, but itself is not "polveri."