They are most likely carbon deposits. This happens due to overheating of fats and oils.
The black residue or black specks coming off your cast iron cookware is carbon deposits formed during the cooking process. It is generally created from overheating of cooking oil or fats used while cooking, or bits of burnt food that accumulate in the pores of the cookware.
Seasoning Cast Iron
The interesting thing about cast iron is that the more you use it, the more seasoned it will become. However, as you use and clean it, it's important to habitually reseason a pan to give it a fresh, clean layer of seasoning to aid with cooking and to protect from rust.
Over time, dirt, dust, spray starch and fabric fibers build-up on the bottom soleplate of your iron. In addition, old water inside your iron's water reservoir can begin to cause rust spots. While you may be inclined to toss your iron for a newer, cleaner model, regular cleaning is relatively easy.
White vinegar – white vinegar can help you here, but check the manufacturer's instructions first as putting vinegar in the reservoir may not be recommended for your model. Mix equal amounts of vinegar and water, put the mixture in the reservoir, turn the iron to medium heat and give it about ten minutes to steam.
Since olive oil is such a pantry staple, many people naturally wonder if they can use olive oil to season cast iron. You can use olive oil to season cast iron, but you need to choose a high quality extra virgin olive oil, which should have a smoke point close to 400 degrees Fahrenheit.
Generally, black oxide or black iron oxide refers to the oxide of iron known as magnetite (Fe3O4) that is formed when a ferrous metal is exposed to special oxidising chemicals. It is a type of conversion coating, thus the surface layer of a metal is formed into its oxide through conversion.
Seasoning is a natural process and requires oil to be heated in the pan. The oil reacts with the iron, creating a visible black layer. You can build a seasoning yourself from scratch, but it's much easier to buy a preseasoned skillet set, like mine from Uno Casa.
You should only need to fully re-season your cast-iron cookware one to two times a year, but you may also want to give it some extra seasoning love anytime you cook something that requires a heavy-duty cleaning.
If food or oils are burned onto the pan's surface, they can create blackened areas. "This can be very problematic with nonstick aluminum pans, where burnt-on residues can be challenging, especially if you want to remove them without damaging the nonstick coating."
Put the oiled pan in a preheated 450°F (230°C) oven, and leave it there for 30 minutes. It may get a little smoky, so keep your kitchen well ventilated.
Oil, especially canola oil (bad, unhealthy oil), should not be used. If a quick oiling (not seasoning) in between is desired, it's best to use a high temperature oil like avocado. A quick rub can do the trick, if your pan looks a bit dry. I have one pan that is about 100 years old!
When using carbon-steel and cast-iron pans, some people like to heat the dry pan first, then add the oil. This is because high heat causes the oil to oxidize and polymerize, allowing it to bond to the metal, filling in any small pits and divots—a process that seasons the pan, essentially giving it a nonstick surface.
We get lots of questions about this, so we thought we'd share our guide on how to properly season a cast iron pan: Wash the pan well and rinse, then dry completely. Coat the interior of the pan with a thin layer of vegetable oil, lard or shortening. Avoid using butter or a low smoke point oil such as olive oil.
Your new skillet may be have been pre-seasoned to keep it from rusting in the store. This seasoning is not meant to last forever and can start to flake after the first few meals you cook. The oil or wax coating will start to disintegrate with high heat and will not hold up for long.
Occasionally, the seasoning on your pan may break down and leave black specks, especially if the cookware is not well-seasoned, but it is not harmful in any way.
Common cooking oils like olive oil will gradually produce seasoning, but won't be as effective as grapeseed oil. Canola, other vegetable oils, and shortening are a little better.
However, iron when exposed to a highly alkaline salt bath at about 285 degrees F forms the other oxide of iron, namely ferrous oxide, Fe3O4, which is an aesthetically appealing black oxide or what could be called black rust. In this sense, it's good rust!
Any acid will remove Black Oxide finishes, even vinegar if it is in it long enough. Whire wheel will also remove Black Oxide.