We document the evolution of productivity in a steel mini mill with fixed capital, producing an unchanged product with Leontief technology working 24/7. Despite—almost—unchanged production conditions, output doubled within the sample period (12 years). We decompose the gains into downtime reductions, more rounds of production per time, and more output per run. After attributing productivity gains to investment and an incentive plan, we are left with a large unexplained component. Learning by experimentation, or tweaking, seems to be behind the continual and gradual process of productivity growth. The findings suggest that capacity is not well defined, even in batch-oriented manufacturing.