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.

  • 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 '21 at 8:46
  • 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 '21 at 20:06

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