High heat transforms clay vessels from a fragile state ("greenware") to a form that is stone-like. The short end of it is that the clay melts! However, this "melting" takes place at the molecular level and should not alter the form of the vessel. The tool used to deliver this high heat is the pottery kiln.
The technology of kilns and kiln-building has been known in the Mediterranean world for thousands of years. Kilns usually take one of two forms. The first is horizontal in orientation; the second is vertical. Vertical kilns were widespread in the Roman empire and continue to be built and used today in Palestine. In the image above, portions of three vertical kilns are visible. The first and most obvious rises between the two men. We peer through the door to the firebox. Fuel goes in here. The vessels to be fired go in through a (barely visible) "manhole" at the top.
Two other kilns are visible on the edges of the above image. On the left, you can see the manhole topping a concrete "hill." On the right, a third kiln rises and is cut off by the picture. Wood piles and oil in a drum suggest the means by which these contemporary kilns of Hebron (al-Khalil) are fired.