Why do earthen pots lose their efficiency in keeping the water cool after being used for a few years?
K. SAI SUMANTH
Fresh earthen pots have multitudinous and crisscross narrow channels running from inside to outside of their walls like in a sponge. Or, we say that the walls of fresh earthen pot are highly porous.
When water is held in such fresh earthen pot, part of the water bleeds out through these pores by capillarity and interfacial affinity and makes the outer surface of the pot rather wet.
However, the water from inside cannot gush out though these pores like a shower because of the crisscross nature and narrowness of the channels and the fineness of the pores.
Thus, the effective exposure area for water in a fresh earthen pot, is not only the top meniscus in the pot but also the wetness on the outer surface of the pot. When water evaporates from these surfaces, the molecules leaving the pot take along heat from the pot’s walls and the water in the pot becomes cooler.
Potable water is not pure water; it contains some minerals such as potassium, sodium, magnesium and calcium salts, suspended particles, dissolved oxygen and carbon dioxide, etc among others.
During the course of usage, some of these calcium and magnesium ions in water tend to precipitate as insoluble carbonates and sulphates which gradually plug the pores and channels of the pot walls.
The suspended particles, dust collected inside and outside the pot during usage and some algal and moss growths also block these pores and discourage the development of wetness of the outer walls of the pot.
In the event of decreased net area of exposure, the rate of evaporation and the scope for loss of internal heat is drastically reduced.
That is why earthen pots lose their efficiency in keeping the water cool after being used for a few years.
PROF. A. RAMACHANDRAIAH
Department of Chemistry
National Institute of Technology Warangal
Warangal, Andhra Pradesh