聚氯乙烯
Burri's artistic research became personal in short time, between 1948 and 1950 he began experimenting with using unusual, 'unorthodox' materials such as tar, sand, zinc, pumice, and Aluminium dust as well as Polyvinyl chloride glue, this last material being elevated to the same importance as oil colors. During this artistic transition, the painter showed his sensitivity to the mixed-media type of abstraction of Enrico Prampolini, a central figure in Italian Abstract art. Nonetheless Burri went one step further in his "Catrami" (Tars), presenting tar not as a simple collage material, but as an actual color which – by way of different lucid and opaque shades in monochrome black–, blended itself with the totality of the painting.