....pure copper melts at 1984 F. Copper wire is about as pure as you'll get. I wonder if melting some wire is a good test for stainless temps?]
Copper wire is not pure copper. It is normally an alloy of copper and oxygen called ETF copper that melts at 1981F/ 1083C. Close, but not the exact same as pure copper. IIRC, it is around 99.5% copper and .5% oxygen.
Melting metals to determine temperature has the problem of what I call "slump", which means it doesn't suddenly change from solid to liquid at a certain point like ice does to water. The "slump" phase can be over a good size temperature range in the oven for some metals unless in an inert atmosphere. Also, we have the issue of surface oxidation at high kiln temperature that created a skin of harder material that holds the shape far beyond the melting point. Finally, the metal temperature will rise to its fusion point (melting point) and won't rise any more until all the metal has reached the fusion point and becomes liquid. The HT oven temperature is not necessarily the same as the metal temperature during this point. All this is part of the enthalpy of fusion. A simple example is placing a block of ice in a 400° oven. If you froze a TC in the ice, it will read 32°F when the ice starts reaches the fusion point. It will not rise above that temperature until the entire block of ice is at 33°F. The oven is 400°F, but the ice is in the enthalpy stage.
If using a HT oven to calibrate the TC, you will have to fully soak the oven with a ceramic black in the center under the TC, once soaked place the test metal on the ceramic surface and then hold the metal at that exact temperature for at least 15 minutes. Check if still solid and raise te temperature 5°. Repeat for each step until the metal or chemical melts to know when you reach the melting point. Using 5° steps might get you close to an accurate reading. Each step will require the oven rising to the next point and soaking for a good 15 minutes before peeking to see if it melted.
The best idea is to get a good quality thermocouple and reader from a known source (Omega, etc.) and use it in your oven. Run some test coupons to determine the best temperature reading for a certain result. Use that data to adjust the temperatures from Larrin's charts to reflect what your oven should be set at.
Example - If using salt to check the TC and your oven readout says 1490 when it is actually 1475 (salt melts), use a setting 15 degrees higher than the chart to get an adjusted internal temperature. So, if your HT target is 1485°, set the oven to 1500°.
BTW, thermocouples are like most things and have a useful life. If you do weekly HT runs and are a stickler for getting a perfect reading, change the TC every year.
