Just participated in a digital forum at Whip Mix where Dr. Russell Giordano discussed the material science and dynamics of Zirconia and Zirconia layering ceramics.... lots of "i's" and "t's" to dot and cross if you really want to ensure a scientifically sound interface between your Zr substrate and the Zr core... one thing he advocated in particular, and I know we have heard different variations on this from a variety of Zr and Zr layering mfg.s, but he stated quite emphatically, that anytime one introduces stress (grinding and/or finishing of Zr frames and full-contour Zr restorations) that a 'conditioning firing' is imperative! This conditioning firing consists of running the Zr frame up to 1000 C and holding it for a minimum of 15-20 mins., then a slow cool coming down- not 1-2 mins., but a REAL slow cool down to a min. temp of 600 C... then bench cool down to room temp. If one doesn't observe this conditioning cycle, the Zr will remain in a Monoclinic 'stressed' state and not return to a tetragonal/metastable state! That is to say, if a crack starts to propagate, it will not necessarily force the crack together, thus stopping it or encapsulating it. What this could spell for us is short-term, false-sensed "success..." OR LONGTERM Failure, depending upon one's mindset... Noritake & Vita were proponents of this necessary cycle long before others joined the band wagon... — tagged Whip Mix Corp.