Incalmo

Incalmo is a glassblowing technique in which two or more separately blown glass sections are fused into one object, usually a vessel. The edges to be joined must have the same diameter. The method can create distinct zones of colour, but the join forms part of the vessel wall itself.

Incalmo should be distinguished from applied glass decoration, paint and surface layers. A sharp colour boundary alone is not proof. The construction of the transition, wall thickness, pontil mark and other manufacturing evidence should be assessed together.

Source: Corning Museum of Glass. See also glass and crystal.