sling vs bow

[Matthew]

Could the heat of a sling bullet be due to the lead bullet being used straight from the mold?

The one that puzzles me is Caesar's report of heated clay bullets.

At BGall 5.43, he says that the Nervii, besieging Q. Cicero in his camp, "began to throw red-hot bullets of molten clay from slings and heated (i.e. burning) javelins onto the huts which were roofed in Gallic fashion with straw."

The question is: how do you "melt" clay so that it is hot enough to set fire to a thatched roof?! And how do you sling a red-hot bullet, anyway?
(Rice Holmes suggested a metal-lined sling pouch. I suppose that might work.)

Maybe what is being described are clay projectiles [i.e. pots] filled with some sort of burning substance.

Matthew James Stanham