A note about sand - it's critical not to let it overheat as sand will retain that high heat for a long long time.
I was using sand and got distracted with my food prep and forgot to check the temps and catch them on the way up. By the time I got out there, the temp was in the mid-300º and climbing.
Not fully realizing what was going on, I attempted to lower the temp through vent adjustment - no effect. Indeed, even closing them had no effect. So I attempted to reduce the amount of lit coals - no effect. After futzing with the coals for a bit with little change, I decided to just start all over. It was then that I noticed the temps were still high even though the top+mid section had been sitting to the side for awhile. Lifting the lid a nice warm updraft greeted me and the light bulb went off - it's the sand!
The sand had absorbed all that heat and, unlike water which gives off excess energy in the production of steam, it was now releasing all the stored up energy.
If you let that sand overheat, you're going to have a really tough time getting temps back down.