Related Clues:

• Temper, as metal
• Temper
• Strengthen
• Harden
• Toughen
• Harden by heat
• Strengthen by tempering
• Heat to remove stress
• Temper, as steel
• Toughen, as glass
• Heat to remove stress from, as glass
• Temper with heat
• Fire in a kiln
• Heat and cool, to strengthen
• Heat and then cool