How do you know when the clay is done firing?

You know the clay is done firing when it has reached the appropriate temperature for its specific type, indicated by the kiln's temperature gauge or pyrometric cones. The clay will also have a change in color, becoming more vibrant or muted depending on the type. Finally, when the kiln cools down, you can safely open it and check for a solid, durable finish, indicating that the firing process is complete.