Scarva Earthstone ES130 White Earthenware Clay From  £19.32

I love working with this clay but I’m worried about the porosity. The glaze makes the piece water tight, but after washing up there is a damp ring underneath the piece from where the exposed clay absorbed the water.
I bisque fire to 1000’C and glaze fire to 1180’C
At what temperature does this clay vitrify?
Is it normal I have this damp ring underneath a fired piece? Is the piece functional this way?