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as stated in

https://books.google.com.br/books?id=Kz4FKczHm2gC&pg=PA643&lpg=PA643&dq=ice+india+Sir+Robert+Barker.+F.R.S.&source=bl&ots=ngGVcv3jQr&sig=tsbt494sRQ2NJ5pwn_3fIppgK2Y&hl=pt-BR&sa=X&ved=0ahUKEwjli8S2oIPSAhUFHpAKHcc9BN4Q6AEINDAE

“The process of making ice in the East Indies at Allahabad, Mootegil, and Calcutta”.

On a large open plain, three or four excavations were made, the bottoms of which were sugar-cane, or the stems of the large Indian corn dried. Upon this bed were placed in rows, a number of small, shallow, earthen pans, for containing the water intended to be frozen. These are unglazed, and made of a prous Earth. Towards the dusk of the evening, they were filled with soft water, which had been boiled, and then left in the afore-related situation.

The ice-makers attended the pits usually before the sun was above the horizon, and collected in baskets what was frozen.

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  • $\begingroup$ If you read on it tells you that it is evaporation, the same process by which swearing keeps you cooler. Evaporation of the water occurs at the surfaces of the porous pots. Evaporation requires energy which it takes from the remaining water left in the pots and if enough energy is taken from the water it starts to freeze. $\endgroup$ – Farcher Feb 9 '17 at 16:58
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The night sky is very cold. A pan of water, insulated from the warm ground, but open to the sky, will lose heat by radiation. If that heat loss is not balanced by warm air (and a pit in the ground doesn't get much ventilation), radiative heat loss of water in the pan might freeze it even if ambient air temperature doesn't allow frost formation.

Thermal radiation to the sky will cool the apparatus well on clear nights, but not if there is cloud cover. It is consistent with language in the cited text that indicates 'the more clear and serene the weather' favors ice production.

I'm dubious that evaporation dominates the process. Evaporation will certainly cool recently-boiled water rapidly, but evaporation is a slow process when water is chilled or frozen. Freezing 1 kg of water releases about 333 kJoules of (heat) energy, after freezing point is reached. If that energy came from liquid evaporation, after forming an ice skin only sublimation, not evaporation, would happen thereafter.

Evaporation removes 2257 kJ per kilogram, so evaporating one kg of cold water might produce 7kg of ice, after reaching freezing.

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