There are three types of classic filigrana: the “retortoli” type, also called “zanfirico”, characterized by subtle twisted white or colored threads inside the thin wall of the blown piece; the “reticello” filigrana, characterized by a fine netting of threads; the “mezza-filigrana” with diagonal threads, an intermediate phase in making “reticello” filigrana.
The oldest filigrana is the “retortoli”, patented in 1527 by Fililppo Catani, whose furnace bore the sign of the Mermaid (Sirena), the founder of the Muranese Serena dynasty. Ahead of time, the glassworker prepares clear rods with white or colored threads inside wrapped in a spiral. On a slab he positions equal segments of this rod, parallel and adjacent. He then uses his blowpipe to pick up the rectangle obtained by fusing these segments together in the heat, so as to obtain a cylinder which is then smoothed by rolling it over the marver and closed at the end. The piece is subsequently blown and shaped.The “mezza filigrana” is obtained by preparing a rod with a straight thread inside. The glassmaster lays equal and parallel segments of this rod on a slab, fuses it together in the fire and picks up the vitreous rectangle with his blow-pipe. By marvering and closing the resulting cylinder at the end, he twists it to make the threads run diagonally.
If two identical glass pieces in mezza-filigrana are prepared with opposite thread patterns and one is blown inside the other, by overlaying the two vitreous walls a grid effect is obtained, the “reticello”. Since the surface of the two blown pieces at the moment of their adhesion is still wavy, an air bubble remains imprisoned in each square of the grid, conferring yet another decorative effect.