Back before modern mechanization, the chemical silver nitrate was used to make mirrors. When it hits a glass panel, it instantly becomes reflective.
The worker is wearing a gas mask because the solution they’re working contains ammonia, most likely mixed with silver nitrate and sodium hydroxide. Time is of the essence in this reflective creation. If the three are left together for too long, the mixture can become explosive.
Instructables has good instructions on how to build what you see here, but there are many ways to make DIY mirrors. One of the more fun ways resulting in burning things.