Ti has a strong affinity to elements like O, N, and C. During melting a non-reacting crucible and vacuum and/or Ar-gas protection therefore have to be used. If the impurity content becomes too high, the material turns hard and brittle. In the present work vacuum/Ar-pressure casting of crowns has been carried out at different air pressure levels. Recording of hardness on cross-sections of the castings and the degree of mold filling show that an air pressure level < 1 torr in the melting chamber and < 10 torr in the mold chamber is sufficient to obtain sound castings of a small size like a crown.

Effect of casting atmosphere on the quality of Ti-crowns.
Sunnerkrantz PA, Syverud M, Herø H.
European Journal of Oral Sciences 1990 June ; 268-272 DOI: 10.1111/j.1600-0722.1990.tb00972.x