Broken Laboratory Glass Can Sometimes Be Repaired by Using Heat

Short Answer

Broken laboratory glass can sometimes be repaired by using heat to melt the glass pieces back together at the break or to melt shut a crack. Often the glass will crack easily at these repair joints, unless it undergoes a process called annealing. Annealing raises the temperature of the glass to just below the melting point and slowly lets it cool down to room temperature. After annealing, the glassware is often just like new. Using your knowledge of amorphous solids, why does annealing strengthen the glassware?