The phenomenon you're referring to is known as "sweating" or "condensation" on the surface of a clay pot when it is kept in a refrigerator. This process is a result of the interaction between the temperature difference and the humidity inside the fridge.
When you place a clay pot in the fridge, the temperature inside the refrigerator is typically lower than the ambient temperature outside. The clay pot absorbs the cold temperature, causing its surface to cool down. As a result, the air in contact with the pot's surface also cools down.
The second factor involved is the humidity in the refrigerator. Fridges are designed to maintain a relatively high humidity level to preserve the freshness of food items and prevent them from drying out. This means that there is moisture present in the air inside the fridge.
When the cold clay pot comes in contact with the humid air, the air around the pot's surface is cooled, causing the moisture in the air to condense into tiny water droplets. This is similar to how water droplets form on the outside of a cold glass on a hot day. The temperature difference between the cold surface and the surrounding air causes the water vapor to transition from a gaseous state to a liquid state, forming water droplets.
The clay pot, being a porous material, can absorb some of this condensed water, and over time, the droplets may accumulate and become visible as moisture or water droplets on the pot's surface. This phenomenon is more pronounced with clay pots because their porous nature allows for greater absorption and retention of moisture.
It's worth noting that this sweating or condensation process is purely a physical phenomenon and does not indicate any specific chemical reaction or unique property of clay pots. The same condensation can occur on any surface that is colder than the surrounding air and exposed to sufficient humidity.