So here we are, at the heart of the operation. Although you can fail miserably before this step, this piece of equipment is critical to calculate both time and temperature. I'll post on safety in a different entry, let's just say that you do not use this unless it's in a well-designed, vented, tested, fume hood.
After the plate is polished, fumed and exposed, it comes to this unit. Here. the plate is fumed again but this time with heated mercury - Hg. There are many methods, times and temperatures, but the general rule is 100 C (212 F) for 2 minutes. The mercury is heated with an alcohol lamp - this is quicker and the "cool down" is quicker, too. I have nothing against electrical heaters for mercury - if that's what you like, go for it.
With René's design, I can inspect the plate through the "safe windows". Again, you can do this by time and temperature only, and a lot of people do, I prefer the visual, if I can get it.
All material © 2010 Studio Q - Quinn Jacobson Photography and René Smets.