The sommerso technique was originally developed in the 1930's by Antonio Da Ross. Sommerso (Italian for "submerged") is the Murano glassmaking technique of creating two or more layers of contrasting glass without the colours mixing together. It is formed by dipping coloured glass into molten glass of a different colour, before blowing the glass into the required form. The outermist layer is often clear. During the 1950's the sommerso style became world famous due to the work of Flavio Poli, artistic director of Seguso Vetri d'Arte. There have since been many other factories in the Murano region which have made pieces in this style, and as such it is difficult to be certain of the manufacturer without labels or signatures present. Other well known manufacturers that have produced glassware in the sommerso style include Mandruzzato, Galliano Ferro, Formia, Oball, Arte Nuova and Luigi Onesto.
Important note on Flavio Poli: As mentioned, Flavio Poli helped to make the sommerso style famous. This has had the unfortunate side effect that nowadays, whenever someone sees a sommerso design, they often assume it was designed by Flavio Poli, despite the fact that there is strong evidence, via catalogues and labelled examples, that almost every factory on the island of Murano has produced sommerso glassware. There are also actually very few designs that can be positively attributed to Flavio Poli, who aside from an early stint at I.V.A.M. from 1929 - 1934, only worked for the Seguso Vetri d'Arte factory, from 1934 - 1963. Anything that wasn't made by SVA and designed before 1963, cannot possibly be a Flavio Poli design. In over 15 years of dealing in vintage art glass, and selling many thousands of sommerso glass items, I can honestly say that I don't think I have ever had a piece that was designed by Flavio Poli. I have had a few pieces by Seguso Vertri d'Arte, but even those were either designer unknown, or designed by Mario Pinzoni, who took over as chief designer after Flavio Poli left. Sadly, this myth just keeps on growing, as people simply assume "if everyone says it, it must be true". The truth is that 99.9% of Flavio Poli attributions found online, or at antique fairs, or even famous auction houses and a few Murano glass books, are wrong. I'll say it one last time loud and clear in a futile attempt to dispel the myth: