In all the videos of Portuguese egg tart molds I see on Youtube, they all appear to be fully black. Carbon steel doesn't seem to be as dark as that (although it could simply be lighting), and I'm not too sure what sort of reaction would cause metal molds to darken like that.

I first thought that it was aluminum oxide forming on the molds, but from what I can find online that would seem to taint any tart with black residue, so that doesn't appear to be it.

I grabbed a picture from this video around 3:08, but I found another example here too:

enter image description here

  • 4
    Please link pictures (screenshots of the videos, or give us links to the videos at the moment in which the mold is seen). It is difficult for us to keep guessing what you are seeing when we are not seeing it. I even tried finding videos myself, but out of three, one used a shiny grey metal mold (probably aluminium) and two used teflon-coated muffin pans.
    – rumtscho
    Oct 8, 2021 at 8:46
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    It is doubtful to be aluminium oxide. If the videos are of production facilities where the egg tarts are mass made then it is likely that the blackening is a patina developed from carbonized fats that have built up through countless uses. If the videos are from food bloggers and the like, then it is more likely to be non stick moulds.
    – Mr Shane
    Dec 19, 2021 at 20:06
  • Is suspect that you were thinking of anodized aluminum, which is a typically dark grey or relatively clear. Aluminum oxide is when bare aluminum rusts (reacts with oxygen), and is white. There are some anodizing solutions that impart other colors, such as what you can get for iPhones, it’s not typically used for cookware. (I don’t know if it’s because of cost or durability issues)
    – Joe
    Jan 3 at 14:16

1 Answer 1


That looks to me to be seasoned carbon steel for both the individual cups and the trays.

When first seasoning cast iron or carbon steel, it can look orange or brown, but as you cook it repeatedly (or to a higher temperature), it will turn black, as in the videos that you linked to.

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