"I just opened the Muffle table; white powder and a square piece of glass just fell out. I looked up inside and I see white powder and cracking on my muffle windings. What is going on?"
Here is what's going on:
The quartz tube that the heater coils or muffle windings run through is a dynamic piece of the heat dispersion in your muffle. Remember that it is heating and cooling, expanding and contracting with each cycle you run in the furnace. The quartz can convert to Crystobalite, which is the white powder you see, on the outside of the tubing.
“At 870C quartz ceases to be stable, but in the absence of fluxes, does not alter until a much higher temperature is reached, when, depending on the temperature and nature of fluxes present, it is converted into the polymorphs of Cristobalite and/or Tridymite” (1) . This is a conversion that happens at a much higher temperature than exists in the muffle; however in the presence of contaminants, which act as fluxes on the quartz, this temperature is dramatically lowered.
If left untreated, this will lead to weakening of the tubing and eventually the cracking (crazing) and breaking away of pieces of the quartz. Once this happens the heater coil can begin to stretch during heating and even extend through the opening in the tube. Eventually the muffle will fail.
How can I prevent it?
In order to help prevent this from occurring, you should be removing the contaminants or “PURGING” the muffle on a regular basis. There are products on the market, which advertise use in this process, but the most important part is reaching a high temperature and holding the temperature to drive contaminants away from the surfaces and into the colder areas of the muffle. These purge programs can be obtained through the Technical Support arm of the furnace manufacturer. If you are using a Whip Mix Pro100 Series Furnace click here for the purging instructions, for the Pro200 Series Furnce, click here.
(1) ^ Wikipedia… Dictionary of Ceramics. Arthur Dodd & David Murfin. 3rd Edition. The Institute of Materials. 1994.
About the Author
Craig A. Pickett, RG, CDT, TE is NBC certified in Crown & Bridge with Technologist designation. A 30 year CDT, Craig is currently Technical Support Manager for Whip Mix Corporation. In addition, Craig is the NADL 2014 recipient of the Excellence in Education award.