user: Question: What is the casting temperature for the alloy with hardness 21?
Columns: ['Usage', 'Sn/Sb (%)', 'Liquid at (°C)', 'Casting at (°C)', 'Remelting at (°C)', 'Hardness']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Typograph', '3/12', '250', '280…290', '310…330', '19']
SQL:

assistant: SELECT "Casting at (°C)" FROM "Table" WHERE "Hardness" = '21';