Actually come to think of it, it could technically be possible to do the same thing with a flame heat source. In that scenario, you would heat a piece of metal that is channeled and evenly interfaced with the etched disc via geometry. At some point along the channel, a thermochromatic indicator could tell you when to apply/not apply the flame. It would be prone to more error, but still way more precise then lining up a lighter on a pipe and hoping for the right temp airflow (especially when you're aiming for a stiff dose)