This is an example of the importance of allowing a bisque piece to dry after glaze the inside surface before glazing the outside face. This hand-built caserole lid is thin and was glazed on the inside first. That wetted the bisque enough that when the outside was poured there was not enough absorbency remaining to build a sufficient thickness on the darker-colored areas of thinner cross section. The problem is exacerbated by the fact that the underlying red body is darkening the color of the thinner glazed sections.
Uneven Glaze Coverage
The secret to getting event glaze coverage lies in understanding how to make thixotropy, specific gravity and viscosity work for you
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.