Cast iron pan has black residue on it I have a small cast iron pan which I am working with, trying to make sure I understand seasoning/cleaning before moving on to a larger one. I seasoned with flaxseed oil (4 light coats, hour in the oven), and have cooked a few things - I made some Ghee, fried some onions, stuff like that. After each I washed with hot water (no soap), heated until dry on the stove and coated with a thin layer of canola oil.

Now, when I take a dry paper towel and rub it on the pan, a black residue shows up on the paper towel. I can't feel anything with my fingers and I haven't noticed anything cooking with it. I'm fairly confident this occurred after the initial seasoning - although I'm not sure I did this exact test with the paper towel.

Is this a) normal, or b) what is wrong and c) how can I fix it?

There are quite a few questions (and good answers!) regarding cast iron cookware on this site but I haven't been able to find one that deals with this specific issue.
You may notice the black residue if you fry eggs in the pan as well.  

Most likely, the black residue is charred (greasy) food sticking to the seasoned oil.  Since Flaxseed oil has low heat tolerance, it could be that disintegrating, too.  Otherwise it could be related to the iron in the cast iron which isn't bad for you (some say even good).

a) Is it normal: Yes if you keep with the same regiment (recommend you don't).

b) What is Wrong:  Water won't perfectly wash non-polar chemicals such as burnt solids in grease (milk solids while making ghee), so the hot water no-soap routine leaves the stuff in pan.

c) How can I fix it: Try the following going forward:


After you finished cooking and while the pan is somewhat hot, put a heap of salt in the middle and with a paper-towel spread and lightly scrub the pan.  The salt will pick up the would-be black stuff, largely disinfect the pan, and the abrasive properties of the salt helps with the cleaning.
Wipe off the now brown salt from the pan and rub-in the coat of oil as you normally do. The new coat will stick better to through the salt's abrasive effect.


This routine won't eliminate residue altogether.  But it seems to work great long term.

Note on oils:
Flaxseed oil is best consumed cold and within three weeks.  I haven't heard of a credible source promoting seasoning the pan with flaxseed oil as it has one of the lowest smoke points in the oils.  Use a more heat tolerant oil like light (not extra-virgin) olive oil or canola.