An assessment of the influence of mixing induced variability on the bi-axial flexure strength of dentine porcelain discs and the implications for laboratory testing of porcelain specimens

Objectives: The proportions of dentine powder and modelling fluid employed to product a "creamy" porcelain slurry consistency used in veneer construction are assessed by the technical experience of the operator so that variability is inevitable. The present study investigated the implications of mixing variability on the slurry consistency used in the manufacture of dentine porcelain disc specimens for laboratory testing. Methods: Vitadur-Alpha dentine porcelain was fabricated into disc specimens and the mean fracture strength, standard deviation and associated Weibull Moduli (m) were determined by fracturing sets of 30 specimens prepared to different powder contents for a constant 0.33 ml of liquid using bi-axial flexure (ball-on-ring). Results: The strength data showed variation in magnitude and consistency ranging from 85.1 +/- 12.9 MPa (m = 7.2 +/- 1.3) at 0.82 g to 87.3 +/- 11.9 MPa (m = 8.0 +/- 1.5) at 0.91 g and 81.9 +/- 13.2 MPa (m = 6.7 +/- 1.2) at 1.0 g of powder, increasing or decreasing the powder content of the slurry (compared with specimens condensed using 0.91 g of powder) resulted in an increase in porosity and a decrease in the apparent solid density. Significance: No significant decrease in the mean bi-axial flexure strength or reliability of the fracture strength data was identified for the slurry consistencies investigated, however, the results suggested that an optimum consistency existed wherein consistent reproducibility of test results was achieved. A comparison between materials can only be achieved if specimen preparation occurs consistently between test centres and therefore these results appear to have implications for the laboratory testing of materials. (C) 2000 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.