Can someone explain the science behind the directions to cure & season my terracotta tagine?

Submitted by JohnQuincyAlias t3_1041hqa in askscience

I received a new, unglazed terracotta tagine (Moroccan clay cooking vessel) for Xmas, which came with directions for curing & seasoning before its first use. I sort of understand the science behind seasoning cast iron, but not this.

  1. Submerge tagine in water for 12 hours
  2. Let dry in open air for 48 hours
  3. Rub interior with olive oil
  4. Put in cold oven, set at 350F and leave for 2-2.5 hrs

My questions:

  1. What's the point of submerging in water if we're just going to let it dry? What's happening there?
  2. When seasoning cast iron, I understand to use oil with higher smoke point and use a much higher temperature in oven, like 450-500 degrees F. Is there any logic behind using olive oil besides "authenticity"? Is the lower temp because of the olive oil, or the clay? Is the olive oil creating a polymer layer, like in cast iron?
  3. Is there any reason why the manufacturer would ship this tagine uncured and unseasoned? Is there a benefit for the purchaser/user doing it themselves - like, maybe my tagine is more fragile now?
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Corvis_74 t1_j36bkg0 wrote

The soaking is probably to get out water soluble iron or calcium compounds that, while not unwholesome, might impart an unwelcome rust or chalky flavor to the first dishes cooked in it. The olive oil is to put a food safe coating into the pores and surfaces. I’d imagine being particular about the oil is sticking with the known procedure to avoid experiments with adverse results

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djublonskopf t1_j37dihb wrote

Replying to Corvis_74 (#1,294,404)

In addition to getting out mineral compounds, the soaking helps loosen any clay "dust" that might be left over from firing/shipping. The soak/dry is basically a way of gently washing the pot (and all of its tiny pores) without scrubbing it.

As to the temperature, vegetable oils can even polymerize at room temperature; heat merely speeds up the process. So choosing a low heat like 350° F, and a long time like 2.5 hours, is basically just a "low and slow" way of achieving the same polymerization as a higher heat for a shorter time. However, I suspect that by staying below the smoke point of the oil, you're going to not get that deep black carbonized look associated with cast-iron seasoning, but a much lighter-colored, low-carbonization seasoning that is nevertheless fully polymerized.

As to the choice of oil, olive oil specifically has one of the highest ratios of monounsaturated fats to polyunsaturated fats, so there's fewer carbon bonds available to cross-link for polymerization (meaning polymerization will be slower than with an oil higher in polyunsaturated fats). However, I suspect that u/Corvis_74 is right that the choice of oil is probably not the result of rigorous terracotta-seasoning research.

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Indemnity4 t1_j3jiw8t wrote

> What's the point of submerging in water if we're just going to let it dry? What's happening there?

You really want to avoid sudden temperature shocks that can cause cracking, which is why you first soak the earthenware in water. The water acts like a buffer for the temperature changes as the water in the clay will never exceed boiling point of 100°C. The thin layer of oil on the surface will get hotter, then polymerise and make the pot more hydrophobic, less likely to stain, prevent the unpleasant taste of raw clay, etc.

> When seasoning cast iron,

The metal expands when it's hot. Oil can diffuse into the gaps. When the pan cools the metal will shrink and grab onto the polymerized layer. Overall the heating/cooling is like scrunching up / unfolding paper.

You want the metal pan to be as hot as possible for the maximum amount of expansion, which is going to result in at least smoky oil, if not burnt carbon. Domestic oven / burner can't really get hot enough (it's fine, but people still have problems with the seasoning cracking or failing to adhere), however, a gas BBQ outside is much hotter. You can get a much better seasoning by really getting the pan hot before adding the seasoning oil.

Earthenware has a very different expansion/contraction issue compared to metal. Earthenware is already porous, so lots of little handhold grips for the seasoning to adhere into, no need for expansion/heating. The temperature only needs to be hot enough for the oil to polymerise, but not too hot or too long otherwise the earthenware will dry out and crack.

> Is there any logic behind using olive oil besides "authenticity"?

No logic. Other people use animal fats or other vegetable oils.

You can play around with using a drying oil such as linseed to form a hard, tough polymer film. Or you can use a non-drying oil such as olive and it's just fine.

Extra virgin olive oil isn't the best. It contains lots of non-oil stuff like proteins that make it tasty, but will also prevent it forming a nice tough barrier. You want to use "refined olive oil" or any other refined food oil.

> why the manufacturer would ship this tagine uncured and unseasoned?

Cost and time. It's tradition to season it yourself so you get that local family flavour, so manufacturer will allow for that and save money on an additional step.

Mold and rancid oils. The manufacturer has no idea how long the product will sit unsold on the shelf. Any residual water + oil = microbe food.

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