In early June 2008 I bought an old Vigor Burnout Furnace (model CA 1065) for $95 from a seller on Craigslist. The furnace appears to have been built in 1979, but does work.
The furnace came with a so-called "infinite" control, which is an electromechanical contraption with ten intensity settings (from mostly off to always on). What varies is the average duty cycle of power on versus power off. There is no feedback control. This is far too crude and clumsy to heat-treat metals, so I decided to add an electronic furnace controller.
So I bought a 1/32 DIN PID controller and solid-state relay (SSR) from Omega Engineering. I'm still using the thermocouple that came with the furnace, but not for long, as the original thermocouple is old and oxidized and a bit off. But it works well enough for now, and will hold to within a degree or so of a temperature, far better than is required in heat treating of say O1 oil hardening steel or A2 air hardening steel.
The big problem was that the original design didn't have a big enough heatsink on the SSR, and there wasn't enough thermal isolation between the control box and the furnace box, so the controller and SSR both got too hot when the furnace went to max temperature (about 990 degrees C). The controller would get to 50 degrees C, which is right at its upper temperature limit, which is asking for reliability problems. The SSR would get almost to 60 degrees C, which is also in its range, but asking for trouble.
So, for the SSR I got a large heatsink from Omega, which dropped the temperature to 36 degrees C.
The controller was a bit more difficult. I ended up making a set of spacers from 0.5" diameter gummy aluminum rod on the lathe, and cutting two aluminum plates with clearance holes, all to space the control box away from the body of the furnace, with two parallel plates between, thermally isolating control box from furnace body. Now, the PID controller temperature is 35 degrees C max.
At this point, I have declared victory, as 35 or 36 degrees C is low enough that reliability won't be much reduced.