The gloss on this low fire red clay is not the product of a glaze or any kind of glass development. It is from a layer of incredibly fine clay on the surface (a process called Terra Sigillata). It is the product of a lengthy research project by Monika. She investigated many materials, techniques, clay bodies and firing schedules.
This is a road-side stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a flame under them to keep the food inside warm. The pedestal is unglazed. The ware is thick and heavy. The casseroles are hand decorated with under glaze slip colors and a very thin layer of lead glaze is painted over (producing a terra sigilatta type appearance, but with brush stoke texture). These have been made and used here for hundreds years. How can they not crack over an open flame? The flame is small. The clay is fired as low or lower than potters in Canada or the US would even fired their bisque. It is porous, open and able to absorb the stresses. They know these pieces are not strong, so they treat them with care.
Tera Sigilatta at Wikipedia
The use of some traditional firing techniques is still popular among modern potters and sculptors (who are accustomed electric and gas kilns, often with computer controllers).
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.