coatings on for example paper or molds especially those used in the pouring of concrete these catalysts are especially effective in the preparation of materials used for making human or animal dental impressions when the platinum catalysts are used in accordance with this invention for cross linking organopolysiloxane compositions the organopolysiloxane composition contains a an organopolysiloxane containing preferably at least two alkenyl groups and b organohydrogenpolysiloxanes having silicon bonded hydrogen atoms organopolysiloxanes a containing alkenyl groups are known in the art and are described for example in u s pat nos 3 723 497 and 3 950 300 suitable examples of organopolysiloxanes are preferably diorganopolysiloxanes having triorganosiloxy terminal groups having at least one vinyl group in each terminal unit these organopolysiloxanes have a viscosity of from about 500 to 300 000 cs and more preferably from about 2000 up to about 100 000 cs at 25 u00b0 c the organohydrogenpolysiloxanes b containing silicon bonded hydrogen atoms are described for example in u s pat nos 3 723 497 and 3 950 300 preferably these organohydrogenpolysiloxanes have at least three silicon bonded hydrogen atoms per molecule generally these organohydrogenpolysiloxanes contain from about 0 01 to about 1 7 percent by weight of silicon bonded hydrogen atoms and the silicon valences.