Not yet metallurgy but the same basic concept... 75,000 years ago

New archaeological research at Pinnacle Point, South Africa, evidences that people living there some 75,000 years ago had elaborate techniques of fire usage in order to alter the properties of rock (silcrete specifically) to best fit their manufacturing needs.

Lead researcher, Kyle Brown, from Arizona University, experimented with their hypothesis, based on field evidence: he left a piece of silcrete beneath their fire pit and the next morning he found the color had changed exactly as with their material evidence and the rock had become much more easy to flake and work into replicas of the prehistorical tools they had been studying.

For him:

Here are the beginnings of fire and engineering, the origins of pyrotechnology, and the bridge to more recent ceramic and metal technology.

Read more at Science Daily.
.