Small Val Saint Lambert vase in blue and clear double cut cristal.

The technique to make bicolor crystal is called doubling or bubbling. The vase is mouth blown into clear crystal, then rested for a while, and then doubled with colored crystal. Afterwards the colored crystal is partially cut away, so that the clear crystal is visible again. Then facets are cut and finally the crystal is polished with lava stone and a linen cloth.

Val Saint Lambert introduced this technique in 1908, an innovation with which the Belgian crystal gained world fame.