The Boromini vases are made of so-called studio glass, which is glass that clearly shows its handcrafting character with small bubbles and irregularities, each piece mouth-blown in deep-black opaque glass. Then the workpiece is polished by hand and given a fine satin finish. The result is an almost ceramic look with an intensive colour depth. Small irregularities and bubbles are typical characteristics of this special glass. Such unique pieces should only be cleaned with warm water and fine dishwashing liquid. Tough limescale rings and plant remnants can be removed with vinegar cleaner.