When the tiles are cured in caves (or kilns), the tiles are stacked on end, overlapping slightly, so that they will not fall over. Before firing, the kiln is sealed with clay to prevent heat from escaping. Then fire is started at the bottom of the cave and fuel is added to increase the heat until the proper temperature is reached. Obviously, fire will not burn without oxygen, and at some point during the firing process the available oxygen in the atmosphere in the cave is used up. The flame then seeks the next available source of oxygen which it finds in the form of iron oxide in the clay tile, and burns it, flashing the surfaces of the tile. The light buff color in the tile is the area that was flashed; the peach color in the tile is the area, which is shielded from the flame by other tiles. The color of the tile is also effected by:
- How the tile is stacked in the kiln.
- How the flame/heat is controlled inside the kiln.
- Whether or not a color additive is added to the clay during the manufacturing process (see the section on Moreno Saltillo tile.)
- Sometimes a green hue is apparent from the firing process.