Just a thought. You are trying to apply the heat externally and measuring the
air temperature surrounding the solder sample.

Since the solder is a thick wire, how about just attaching a thermocouple to
the solder piece with an insulating pad under it and then running current
through the solder section to heat it up. A low voltage transformer with a
variac on the primary could control the temperature.

Ideally, you would measure the temperature with a non-contact optical
thermometer.

Earle Rich
Mont Vernon, NH