Technique of decorating beneath the surface of an object which consists of interposing a painted decor between two layers of glass. This technique, employed in three steps which take the glass from hot to cold and back to hot, requires a delicate approach. In the first step, the piece is blown like any ordinary piece and is then decorated cold with enamels or high temperature paints before being reheated and softened while retaining the painted decor. The piece is then usually covered with a layer of clear glass that traps the painting, enhancing the pictorial surface and giving it a finished quality without altering it.

Among the pioneers of this process, already studied by Burgun, Schverer and Co. as well as Emile Gallé, are the brothers Daum who registered a patent titled “decoration intercalaire a grand feu” on June 23, 1899 in Paris; a patent which moreover does not limit itself to painting but includes other forms of decorating beneath the surface, like etching, thereby paving the way for the technique the Scandinavian glass artists would later call “Graal”. Here is an excerpt: “We designate under this name a decoration, executed ahead of time on a cold glass bowl of any shape, open or not at the ends to prevent shattering when heated, which we reheat until soft in order to then overlay on the inside and/or outside with one or more layers of molten glass and ultimately give the vase or object its definitive shape. The decoration is thus interposed between vitreous surfaces. In the first stage the bowl is prepared and shaped hot like any other piece of rough glass, and can be of several layers of colored glass (doubled, tripled, etc.) and roughly decorated with any necessary additions ( bits, stoppers, etc. of any shape, color, or thickness. The work of decorating the bowl then continues cold with enamels, paints, etching, or any other known process.” The “decoration intercalaire”, which the Daums presented with great splendor at the World’s Fair of 1900, remained a largely experimental technique and was soon abandoned for being too risky and costly. As for interpolated painting, technical progress made in recent years on the stability of the medium has resulted in greater mastery of this complex process, as revealed in Ulrika Hydmmann-Vallien’s Kabale series.

Let us take the example of the powerful work of American glass artist John Dewit which superimposes abstract and gestural friezes on tall, massive incalmo vases as well as the work of the young Anne-Lise Sibony-Riond whose striking vase portraits are imbued with quasi-expressionist sensitivity.

SALMON Béatrice, BARDIN Christophe, “Daum-Collection du Musée des Beaux-Arts de Nancy”, Réunion des Musées Nationaux, Paris, 2000