Definition: Hard firing (verb and adjective) refers to firing a kiln slightly past the target cone's rating. The term comes from a description of the state of the cones in a cone pack. When the target cone has melted beyond the target 90 degree angle and has partially or fully flattened against the clay of the cone pack, it is deemed to be hard fired, or hard.
Examples:
Hard firing can make glazes more glossy, but also more fluid. (verb)
The hard fired cone 2 glaze has lost some of its color due to volatilization. (adjective)
I'll be taking the kiln to a hard cone 9. (adjective without "firing" included)