On April 4 I started to blow glass, but the furnace was acting up. Ramping from 1900 to 2100, it went to 2000 and then really didn’t get any hotter. The temp readings were acting up and not settling down, so I set it back to 1900. April 5 I tried again. This time I was watching and saw the temperature reading go past 2000 no problem, but at about 2050 it started to “go unstable”. Eventually it read UUUU which means “no reading, upper limit”.
The only reasonable causes were; broken thermocouple, faulty wiring or connections, or controller failure. The sane response was to shut the furnace off and do a complete check of all components.
After turning the furnace off, I noted the crucible was welded to the maintenance lid by spilled glass. It would not budge, even hot. In an effort to free the crucible from the lid, I blocked the maintenance lid up a bit (about 1/2inch) and left it.
Sure enough, when I returned 12 hours later the crucible was free.
Once the furnace was completely cold I removed the crucible, lids and alumina board to have a look. The crucible is in excellent shape, though full of glass. The kanthal heater wires also look in perfect shape, which is amazing for the age of the furnace. The lids were all good, but there was glass on the top lip of the crucible and on the bottom of the rammable gathering ring. This was what welded the crucible to the ring and thus the lid.
I’ll have to remove the glass carefully so the crucible doesn’t weld next time I run hot.
The next steps are to inspect all the wiring and connections, and then to make sure all connections are tight. The most likely cause of the temperature readout issue is a loose thermocouple connection as these are usually pretty robust if not touched. Only if the connections are all tight will I start further tear down.
My action plan for the late spring (May-June) is to first check the wiring connections. Second is to remove the grog in the base of the hot box and sieve it so it’s clean, then reinstall the two lids and take the furnace up to 2100F to check the wiring and controller. With the crucible out of the furnace, this can be done much quicker as the crucible is the limiting factor on temperature rise.
While this is going on, I’ll also clean up the gathering port ring and crucible lip. If the heating test is good, I’ll install the ring and crucible to the again cold furnace and start it up.