What 640 degrees actually does

People assume firing a painted panel is like baking. It is not. At 640°C the glass has not melted and is nowhere near melting — soda-lime glass will not run until well past 1000°C. What is happening at 640 is that the flux in the paint has softened enough to grip the surface of the glass and pull the pigment into it.

Ten degrees under and the paint sits on top and can be scratched off with a fingernail in two years. Ten degrees over and it sinks, spreads, and loses its edge — a trace line that was crisp on the bench comes out of the kiln soft, and a soft trace line in a figure’s eye is the difference between a face that looks at you and a face that does not.

Our kiln has three thermocouples and we believe the middle one. The other two are there to tell us how much the middle one is lying.