Objective: The high-temperature strength behaviour of dental phosphate-bonded investments is not clearly understood. The present purpose was to determine their variation in strength with temperature and to seek explanations for that variation.
Methods: The clamped-edge disc-rupture test was used in a sequential testing protocol, with a variable casting metal mass, to determine the effect of burnout temperature (ranging from 400 to 1100 degrees C) on the strength of six phosphate-bonded investments.
Results: The strength of phosphate-bonded investment is temperature sensitive, generally showing a distinct minimum in the tested temperature range. Low temperature strength gives little information about high temperature behaviour. The variation of strength with temperature seems amenable to explanation in terms of investment composition.
Conclusion: Since strength varies greatly with temperature and brand, the inconsistency of previous results is explained. On the other hand, low-temperature strength is no guide for the high-temperature strength, so that information on both the room and the service temperature strength is required. Inversion of silica at high temperature may decrease the strength of phosphate-bonded investments.