"At the back of the horno, two temperature release holes are plugged with rocks wrapped in wet cloths. After one hour of baking, the stones and cloths are removed and a light is shined into the horno. If the bread seems to be baking, the stones are replaced and the bread is left to bake. If the bread is too brown, the temperature release holes are left open so that steam can escape."
Interesting, but one could always open the door a bit to achieve the same effect. There is however a tradition of having "smoke holes" in the hornos. I find the aesthetics of this oven most pleasing, although the tall conical shape and large door opening make for a very inefficient oven. Also, everything is being baking on trays rather than directly on the floor of the earth oven. Ok for cookies and muffins, but not for breads. Lots of photos on the website. www.periodistacostilla.org/