why earthenware vessels keep water cool?

Answer 1

Earthenware like pitcher etc are very porous.
Through these pores some amount of water seeps out.
The water which has seeped out has access to a large surface area.
That means that a little quantity of water is in contact with the atmospheric air.
The tendency of EVAPORATION is largely enhanced by these factors.
When water evaporates it TAKES HEAT FROM THE ATTACHED SOLID SUPPORT WHICH IS THE EARTHENWARE
As a result of which the surface temperature of the earthenware drops.
When the water inside the vessel comes in thermal equilibrium with the vessel overall lowering of water temperature occurs.

Thus earthenware vessels keep water cool.

Answer 2

The idea was used by the Egyptians thousands of years ago. They realised that water could be kept cool by placing it in porous earthenware vessels. These containers allow sufficient liquid to escape and evaporate, and in so doing cool the water remaining. The same idea has been used with wine coolers, and also with pieces of muslin draped over jars.

Answer 3

vaporization thru pores of earthen ware vessels takes away latent heat of vaporization, thus cooling the bulk of the water