Fumato

It consists in a glass that, under a transparent colourless layer, contains inside it coloured corpuscles that, due to the diffraction of light, give the visual impression of smoke.

It is obtained as follows: during production, the surface of the item is exposed to the smoke of a wood fire; a certain quantity of greyish particles (unburnt carbon and ash) adheres to the surface of the glass that is in turn vigorously marked with metal tools so as to form some grooves. 
The entire surface is later coated again with another layer of molten glass. 
This technique was presumably invented by Alfredo Barbini at the V.A.M.S.A. glass factories at the end of the 30s and was used to execute both vases and figures of birds, among them the famous ducks that met with such great success. Some minor furnaces took this technique up again in the 80s.