From Concrete Planet:
A small aperture at the kiln's base, just large enough for someone to crawl inside when it was fully open, controlled the amount of air reaching the flames and enabled the adding of more fuel. Once the structure had been completed, men would crawl through the opening to tightly stacked firewood in the pit and then place pieces of limestone- usually the size of a small fist or smaller-over the wood. More wood was then stacked on top of the layer of limestone. The
reason the limestone was no more than fist-size was to allow the heat to completely permeate the stone and fully calcify it. The wood stacking and limestone placement alone usually required a day to complete. The kiln workers would then carefully crawl out of the oven, set the wood alight, and
then close the opening with a flat rock, leaving it ajar enough to permit a
steady flow of air to feed the fire. Once the fire was started, it needed to be
worked regularly; men would poke the embers, fan the flames, and contin-
ually add more wood every hour or so.
Reading this made me uncomfortable. I imagine myself doing the placement, lighting the fire, closing the kiln, and noticing there was no heat an hour later. Then, I picture myself opening the kiln, going in, and looking for what’s wrong but being really uncomfortable and unable to tell what’s going on.