Tempered glass or toughened glass, is made by heating annealed glass at a temperature of approximately 650 °C(1200°F) and then rapidly cooled by air. This process locks the outer surface of the glass in compression which compression which dramatically increases the glass strength. When fully tempered this glass is approximately four times stronger than a piece of annealed glass.
More importantly, when tempered breaks it dissociates into small pieces which is considered a break-safe pattern.