To remove black buildup from cast iron, use a combination of heat, an abrasive like coarse salt or baking soda with water, and vigorous scrubbing with a stiff brush or chainmail scrubber, then dry thoroughly and re-season; for extreme buildup, you might need sandpaper or a drill with a wire brush attachment.
Sprinkle baking soda generously into your pan along with warm water. Bring to a gentle boil for about five minutes. Pour the water out, then scrub. The residue should be all gone and if it's not, just repeat once more.
I use vinegar and water (in equal parts) to remove stubborn carbon build up and also surface rust when I'm restoring cast iron it works well for that but I would recommend removing it and scrubbing it after 20 minutes then repeat as necessary.
On stubborn deposits, use a putty knife, wire brush or steel wool, taking care not to bear down on the metal surfaces. Clean away the remaining carbon with solvent, using fine steel wool to smooth rough spots. You can also soak metal parts for up to 15 minutes to remove stubborn deposits.
How to clean an iron with vinegar
Cast iron rusts when exposed to moisture for long periods. A water soak can undo months of seasoning and require a full restoration. Wash quickly with warm water and a soft brush while the pan is still warm. Dry it immediately on low heat until all moisture evaporates.
Rescuing Cast Iron
Here are a few signs it's time to replace your cast iron cookware.
It may help to heat the pan slightly so the dirt and grease soften. Then, we use sandpaper to sand the cast iron completely bare. It takes some effort, but it's all worth it. If the pan is completely devoid of rust and you only see a grey metal layer, rinse the pan thoroughly under running water.
Always dilute the vinegar with 50% water at least, and only soak for about 40 minutes to an hour at a time. Long term soaking (like you have done) leads to permanent chemical deformation of the iron.
But eight places you should never use vinegar include:
Yes, you can cook acidic foods in cast iron.
5 Mistakes That Could Ruin Your Cast Iron Skillet
You shouldn't cook eggs in a cast iron skillet because they tend to stick, especially in new or poorly seasoned pans, leading to frustrating messes and potential damage to the pan's seasoning, though proper preheating, fat, and low-heat techniques can make it possible. The porous surface of cast iron, especially newer ones, grabs eggs, and its excellent heat retention can easily brown or overcook them, making delicate foods like eggs challenging compared to Teflon or stainless steel.
Traditionally, lard was used to season cast iron, and while that is still okay, we do not recommend it unless you frequently use your cookware. If the cookware is stored for too long, lard and other animal-based fats can go rancid. Using flaxseed oil to season cast iron pans is a growing trend.