Thinner walls can survive higher average-power loads due to lower mechanical strain across the thickness of the material, which is caused by a temperature gradient between the hot plasma and cooling water, (e.g. 1 mm thick doped quartz has limit of 160 W/cm2, 0.5 mm thick one has limit of 320 W/cm2). For this reason, thinner glass is often used for continuous-wave arc-lamps. Thicker materials can generally handle more impact energy from the shock wave that a short-pulsed arc can generate, so quartz as much as 1 mm thick is often used in the construction of flashtubes. The material of the envelope provides another limit for the output power; 1 mm thick fused quartz has a limit of 200 W/cm2, synthetic quartz of same thickness can run up to 240 W/cm2. Other glasses such as borosilicate generally have less than half the power loading capacity of quartz. Aging lamps require some derating, due to increased energy absorption in the glass due to solarization and sputtered deposits.