|Incalmo vessels by Tapio Wirkkala for Venini.|
|16th century incalmo plate,|
The above description is the classical way of achieving incalmo, but modern materials and equipment allow artists to achieve a similar effect with considerably less skill. For instance, precise diameter glass tubing is now available in a wide variety of colors. This can be cut into rings with a saw, then stacked in a kiln and fused together. From there, this “prefabricated incalmo tube can be worked by traditional methods. Whether or not this meets the definition of true incalmo depends entirely on whether one focuses on the method or on the end result.
|9-10th century incalmo vase,|
Syria or Iraq.
All glass expands a little when it is heated and shrinks when it cools. Different formulations of glass generally expand by differing amounts. When a single piece incorporates more than one type of glass, and the thermal expansions differ significantly, the result is disaster. After the piece is finished it is placed in a kiln where it slowly cools back to room temperature. Because of the mismatch, one area wants to shrink more than the adjacent area and the glass cracks along the join. The expansion and contraction is microscopic, but it is enough to ruin hours and hours of work, leading to much gnashing of teeth the morning after, when the finished work is inspected.
The Venini glass masters had the benefit of this knowledge, but for earlier artisans, trial and error must have played a big role in determining which formulas were compatible. Different colors mean different metallic additives and to match expansion other ingredients would need to be adjusted. Today, manufacturers produce glass in a series based on expansion; artists can be relatively sure that two different colors from the same series can be “grafted” and not self-destruct when cooled.
 I have not absolutely confirmed this, but authoritative secondary references credit Venini, and I can find no mention to "incalmo" as a glass technique prior to the twentieth century.