Journal of Dental Research, Vol 72, 566-571, Copyright © 1993 by International & American Associations for Dental Research Online Journals
Effects of initial temperature and tempering medium on thermal tempering of dental porcelains
B. Hojjatie and K. J. Anusavice
Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville 32610-0446.
The objective of this study was to test the hypothesis that quenching of
porcelain in silicone oil rather than in compressed air will significantly
increase the flexure strength by reducing the potential for crack formation
during transient cooling. A secondary hypothesis to be tested is that the
initial tempering temperature can be reduced significantly below the
porcelain maturing temperature of 982 degrees C but well above Tg without a
decrease in strength. Opaque-body porcelain disks, 16 mm in diameter and 2
mm in thickness, with a thermal contraction mismatch (delta alpha) of -1.5,
0, and +3.2 ppm/degrees C were tempered from initial temperatures of 650,
750, 850, and 982 degrees C in silicone oil with kinematic viscosities of
50, 1000, and 5000 centistokes. Porcelain disks were also subjected to
three cooling procedures in air: slow cooling in a furnace (SC), free
convective cooling in a laboratory bench (FC), and tempering (T) by
blasting the surface of body porcelain with air. The crack size induced by
a Vickers microhardness indenter was measured within one minute after crack
development. For determination of the influence of initial cooling
temperature on biaxial flexure strength, six body porcelain disks (delta
alpha = 0) were tempered in air from initial temperatures of 650, 750, 850,
and 982 degrees C. The mean crack size of specimens tempered in oil was
significantly smaller (p < or = 0.001) than that of specimens that were
slowly-cooled or fast-cooled in air for all thermal contraction mismatch
cases.(ABSTRACT TRUNCATED AT 250 WORDS)
