Definition of a clay vessel

As is known, only earthenware vessels receive ritual impurity. I wanted to know if all clay that has been fired in a kiln is called earthenware, or only a certain type of clay, and what is the law regarding modern materials such as mortar, gypsum, bitumen, and why does glass only receive ritual impurity by a Rabbinic decree, since it is made of molten sand? Or is it only one type of clay that is called earthenware?

Dear …!
A clay vessel is a vessel of any earth that has undergone a firing and final drying in a kiln. In the days of the Sages there were both clay vessels and vessels of natron, and also clay vessels made of various types of earth [white and red, etc.].
A vessel of earth is any vessel that, after it’s been given its shape, did not undergo firing, and this includes earthenware and mortar and others.
A glass vessel does indeed undergo melting in a fire, but the firing is not intended to preserve the final shape of the vessel, but rather to shape and change the vessel, and it is actually its cooling that gives the vessel its stability and final shape.

Tractate Keilim, ch. 2