This is glass that is coated with a thick layer of colourless or differently coloured glass.
Carlo Scarpa produced the first objects for Venini & C. in the second half of the 30s. He used layers of colourless glass, with inclusions of air bubbles or gold leaf and coated them with a colourless, transparent layer two centimetres thick.
Many other manufacturers took up this technique, and they achieved a variety of results.
The best results however were achieved by Flavio Poli at Seguso Vetri d'Arte in the 50s.