Earthenware discolouration

I have a tureen that has these big discoloured beige areas, under the glaze I assume (picture below).
What are they caused by and is there a way to remove or fade them? About 50% of the lid is discoloured, only the center of it is not. What could cause such an effect?

 

Hi,
The glaze on the item has tiny little weak spots. The heat from the steam would let tiny fat globules enter under the glaze and discolor them. The recipe using high strength H2O2 (hydrogen peroxide) soak should remove them. Some times it is quick sometimes takes a week or so of soaking. DO NOT use bleach since it will do crazy things like fall apart or just keep oozing for months.
greg