The invention concerns the use as additive for improving properties of resistance to arc tracking and arc erosion: of a mixture A, B or C formed by: concerning mixture A: A1: platinum; and A2: at least an inorganic compound A2.1 of a metal with several degrees of oxidation selected from the group formed by V, Cr, Mn, Fe, Co, Ni, and Cu, in which the metal atom is at a degree of oxidation higher than zero, the least possible (abbreviation: DOX mini); or A3: a combination of at least an inorganic compound A2.1 with at least an inorganic compound A2.2 in which the atom of V, Cr, Mn, Fe, Co, Ni or Cu is at a degree of oxidation higher than DOX mini; concerning mixture B: B1 platinum; and B2: at least an inorganic compound B2.1 of cerium-IV or of lanthanum-III; or B3: a combination of at least an inorganic compound B2.1 with at least an inorganic compound B2.2 of titanium-IV; concerning mixture C3: C1 platinum; and C2 a combination of constituent A2 or A3 with the constituent B2 or B3; in a polyorganosiloxane composition D for obtaining a silicon elastomer, either cross-linkable at room temperature or by heat by reaction of polyaddition in the presence of a metal catalyst, or cross-linkable at high temperature by the action of organic peroxide(s).