Left: Plainsman L215 terra cotta bisque and glaze fired at cone 04 using the G1916Q recipe. Right: L215 bisque fired to cone 04 and then glaze fired to cone 2 using the G1916V recipe. While glazing ware only on the inside can create glaze compression issues, that has not appeared to be an issue so far. I preheated both of the glazed mugs to 300F and quickly dipped each in milk (both steam-dried rapidly). I was careful not to touch the surface of either while they dried (since touch marks show up after fired). However, the one on the right did not work. While the milk did apply thickly enough to fire on normally, it has only created a film on the surface. After a couple of weeks in service, with repeated cold-to-hot and wash cycles the milk is shedding off! It thus appears the milk needs an absorbant body to form a bond, low fired terra cotta, like the mug on the left, appears to supply that best. Another issue is achieving a drip and bubble-free application, application using a milk-soaked sponge creates a variegated pattern, under more control.
Using milk as a glaze
Don't just try this, go into it as a project with your eyes open to the issues you will meet.