You can tell the temperature of molten glass by its color, and a good blower reads it the way a baker reads a loaf — by eye, every few seconds.

The scale

White-hot is the furnace. Amber is workable. A deep red is the moment it starts to stiffen, and by cherry-red you're either moving fast or starting over. Get the color wrong and the bubble fights you.

Why it matters

There's no thermometer at the bench. The color is the thermometer, and it never lies — it just moves faster than you'd like.