It contains minerals such as pyroxene, amphibole, feldspar and 63-68 percent silica. The eruption temperature for dacitic lava is 1,500 to 1,800 degrees Fahrenheit. Dacitic to rhyolitic lava is so dangerous because it contains a high amount of crystals.Mt. Vesuvius erupts according to the Plinian model of eruption.
Foil has low emissivity, so will greatly reduce radiative heat transfer (act like insulation). It should face the side that is supposed to be cooler. But foil will not reduce conductive heat transfer, so it must be used judiciously with something that reduces conduction.
Still air is just about the best insulator there is. At the molecular level, heat transfer is easier when atoms are packed closely together. So the actual point of most insulations is to get rid of as much solid material and trap still air inside little pockets. Most closed cell foams trap different gases than the air we breathe, but they still work on the same principle, replace as much solid material with gas as possible. But air is also a fluid, and as such large volumes of air are great at convecting heat. If you have a large air current, it will transfer heat from hot to cold pretty effectively.