The Sealing of the Pot

The CC received a clay pot as a gift recently.

It was a classic meen chatti (मीन चट्टी - literally: fish clay pot). The first word is from Sanskrit (= fish) and the second most likely from the local vernacular.

Instructions were provided to "season it" but the CC needs to understand what that means so he proceeded down the rabbit hole (as he is wont to do) about what that really entails.

Clay is quite porous as a material. However, it can be fired at high temperatures after which it becomes hard. That's how bricks are made which has been known since at least Sumerian times.

Clay is also an extraordinarily poor conductor of heat.

These combined facts act as an advantage in fish cookery.

The critical point about a clay pot is that no matter how hard you try to heat it in a reasonable time (= less than five hours, for example), it's very difficult to raise the temperature of the interior of the pot to above 70°C^† — which is the outer limit of the temperature that fish proteins denature.

Of course, if you fired the crap out of the pot, eventually it would rise to the temperature of the flame but we're talking cooking here not cremation!

The other critical point is that if you don't raise the temperature beyond the denaturation point of fish proteins, they will stay soft independent of the cooking time. If you held the temperature precisely at a certain point, your fish would stay perfectly tender whether you cooked it for ten minutes or ten hours!

This is how airlines control food, for the record. The temperature is never allowed to get above the denaturation point of the proteins in question — chicken, beef, fish, vegetables, whatever!

The principle of clay-pot cooking is the same as that of sous-vide cooking except that it is being done in an intuitive way rather than formally. It also has a massive advantage over sous-vide in that you can fry or sear the fish and have it cook in the complex juices generated by it. You don't need to worry about the problem of precise timing either which was a useful fact in the frenetic frazzled world of yore unlike modern day times.

There's a price to be paid for this freedom. Nothing comes for free.

The pot is a complex apparatus. If you drop one of your metal pots, it'll get dented. If you drop a clay pot, it shatters and you get to start all over again. (Luckily, they are cheap but that's not the point.)

Poor conduction also goes both ways. It's really hard to raise the temperature and it's equally hard to lower the temperature. A certain boring-ness is required in the usage of a clay pot. You can't just dial the temperature up and down like a metal pot.

Much more importantly, you can't add hot water to a cold pot, or cold water to a hot pot. It will crack. The poor conduction means you cannot have the interior and the exterior at significantly different temperatures.

The seasoning of the pot is really quite simple once you grasp the concepts. You alternate boiling cold water in the pot and toss it out followed by coating it with cooking oil overnight. The water percolates through the pot and mends micro-holes in the clay and the oil conducted via the water generates enough carbon residues to plug the holes. It also generates a non-stick surface at the bottom of the pot over time (exactly like a great cast-iron frying pan.)

There are two fallacies about clay pots and since chez CC, we tend to be the analytic sort, we're going to dismiss them.

The first is that the seasoned clay pot is not porous — it absolutely is! The water inside the seasoned pot has to be going somewhere. It's not evaporating significantly at 70°C^‡ so it must be disappearing somehow. It's evaporating through the porosity of the pot. It's being wicked^§ away.

This porosity created a bit of a problem for the CC in the second round of water seasoning when the pot had been coated with oil overnight. Enough oil/water was being wicked away through the porous clay that the CC's gas flame went a little crazy. It's basically a baby version of a grease fire. It was such a minuscule amount of oil that it posed no risk whatsoever but it was quite unnerving until the principles had been grasped.

The second which is really the flip of the first is the myth that you can't cook in a dry pot without water. You absolutely can as long if you want to raise the temperature above 70°C. You could quite plausibly cook the clay pot to up to 1000° C but the CC really doubts that your puny stove generates enough BTU's for that. (Your oven only goes up to 275°C!)

Remember, if you do amp up the temperature of the pot, you will not be able to add cold water to it. You must either cool the pot down and start over or heat the water to the same temperature and add it. Most cooks do the former not the later since it's too risky.

In fact, many recipes ask you to first boil stuff till the water disappears, take the fish out, and then use the pot just like a frying pan adding the fish back at the last step. (The reverse is also true. Some recipes ask you to use it like a frying pan and then add the fish and warm water a little at a time thus lowering the temperature back down.)

Let's abstract out the principles of the perfectly seasoned clay pot.

[1] With water inside, the temperature will not go above 70°C until the water disappears.

[2] The clay pot is a very poor conductor of heat.

[3] The dry clay pot will allow you to fire it up to 1000°C.

Now that we've grasped the principles, we're ready to roll.

One of the great glories of Mughal cooking is the perfectly cooked daal. This involves the denaturing of lentil proteins and some of these lentils tend to get slimy if the skins split. The clay pot is the ideal apparatus since it preserves the skin perfectly while letting the water and spices to enrich the dish via osmosis but not allowing the lentils to burst.

The truly great dish of Gujarati cooking is undhiyu (ઊંધિયું — literally: upside-down.) The Mughals totally envied this dish! It's a classic mélange of spring vegetables, complex vegetarian meatball-esque preparations, and spices cooked in a sealed upside-down (sic) pot in the dying embers of a fire overnight at low heat. Same logic goes for long-term low-temperature cooking and the denaturing of the proteins.

If you go to North India during winter, you'll find yogurt served in clay pots. The clay pots are being used for two purposes — heat preservation and water-evaporation. Even in the cold temperatures of the North-Indian winter, the clay pots retain the heat of the warm mixture of milk and bacteria^¶ and wick away enough water to make the yogurt rich and delicious.

So this device which is at least 20,000 years old (based on Chinese excavations that date back to 18,000 BC) is truly a magic apparatus. It just requires a little understanding.

We're just the latest in a truly ancient lineage.

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† You can verify this quickly and empirically by sticking a finger for less than a second into the water that has been boiling in the clay pot for an hour. Nothing much will happen. If you stuck your finger in real boiling water for even less than a second, you would get second-degree burns!

‡ The CC will not bore you with the details but roughly speaking in an environment with no air-flow (e.g. your exhaust fan) the evaporation rate is linear in the temperature differential between water and air for temperatures around 100°C. Estimating your house to be about 25°C (80°F), you are only getting roughly 60% of the evaporation of boiling water. The rest of the water has to be going somewhere!

§ You can test this by covering a pot and heating water in it. It will never boil but soon much of the water will be gone.

¶ The clay pots have the advantage of porosity. The yogurt bacteria are sitting in there and they help develop the culture implicitly particularly if you make yogurt every day or week in the same pot. (The clay pots that they use are small. Individual use, one might say, in modern parlance.)