[Tony Atkins]

I have found that all ceramics can suffer with steam tearing, This can be caused by various factors if I list them it may help diagnose some of the possible causes,
1: The temperature with which you place your ceramic build up into the furnace, be careful as the glass point of many ceramics is actually quite low some are as low 575degrees c, As mentioned before if a build up is placed into a furnace to close to this temperature the surface crystalisis and prevents the moisture from escapeing, Hence we get steam tearing, this effect is exagerated with larger bulid ups or where there is greater mass of porcelain. Allowing the temperature to drop to it's preset temperature after the previous firing will help this, generally I place all my ceramics in at 450degrees c and since have not experienced steam tearing.
2: When you have a combination of large masses of alloy and thin areas of ceramic this tends to occurr most in the interstitial areas of bridge work, Others would generaly say that shrinkage occurs in the areas of least resistance ie thin section of ceramic large mass of alloy. Another instance where tearing of the ceramic can occur is when there is equally a large mass of alloy and a large mass of ceramic especially where implant supported crowns are concerned there are generally two ways of eradicating exsesive shrinkgae in these two cases.
1: in the instance of bridge work I make sure I completley seperate through to the opaque before fireing, therfore allowing the porcelain to shrink evenely and ultimatley avoid tearing trying to fill in tears without opening it up first is nearly always impossible, then on my second addition I mix low fusing correction 50:50 with a half shade darker dentine to maintaine colour this will allow the ceramic to melt more easily into the proximal areas.
In the case of implant supported crowns it doesnt appear to be so straight forward, Basically what we have to do during the fireing process is to allow the alloy and ceramic to expand and contract at the same rate this is due to the fact that metal is a very good conductor of heat and ceramic isnt, therfore the alloy expands before the ceramic does so we must lower the rate of climb 55 degrees c per minute being the optimum for a ceramic. My starting point would be say 45 but when lowering the rate of climb it may be an idea to lower the final temp by the same number of degrees with which you lowered the heat rate, then allow the restoration to slow cool to 7oo degrees before removing from the furnace keep an eye out for devitrification of the porcelain as lucite will grow within the ceramic more readily when slow cooling which can result in a very glassy looking ceramic I hope this has helped
best wishes.
Tony