The varnish I used is intended for oven-drying at 140 to 150 degrees Celsius. I don't have an oven large enough to fit this coil, so I heated it by applying roughly 20A of direct current to the high voltage winding. I monitored the temperature by measuring the resistance change of the wire.
If you know the temperature coefficient of the wire, this isn't particularly hard to do! You can do it easily by applying a constant voltage roughly of the wanted amperage and then calculating the resistance via voltage and resulting amperage, then feedback to adjust voltage.
It's fairly popular in a wide range of applications, mostly either machines with too much current or too large areas to reasonably measure temperature or parts that are too small (vape coils for example can be used in a temperature coefficient mode).
It's interesting, as a hobbyist in electronics I tend to see the components I order (inductors, capacitors, etc) as atomic 'units' so watching someone build their own is fascinating. In some ways, it connects what we do abstractly to the concrete reality.
I built a transformer once, for some high-school project, and while it wasn't anywhere near the quality of the commercial ones, it worked. It's actually not that hard if you have the right tools and a basic design that you can copy - it just takes some precision and patience. I definitely encourage you to try it sometimes if you're interested in electronics...
