Does a steel pestle corrode while pressing soot?

Question

While experimenting with a few different powders, I noticed the resulting small flakes were ferromagnetic. Then I decided to find out what was the cause: after repeating the experiment with only soot, the flakes were ferromagnetic again.

The experiments were simple:

1. On a piece of pure copper, I placed a small amount of sulfur and/or carbon (soot from burning hydrocarbons in air) powder.
2. I used a handheld steel pestle to pressurize the powder and obtain a few barely visible flakes.
3. I observed the flakes' behavior near a permanent magnet.

The working surface of the pestle was just large enough to obtain a visible quantity of flakes after about 5 minutes of work. It is made of steel, which is strongly ferromagnetic. I could not see any damage on it by naked eye after the experiments.
The estimated pressure was slightly over $\pu{1 GPa}$.

Using only sulfur yields non-magnetic flakes, but their color is changed by the reaction with the copper surface.

Using only carbon yields slightly ferromagnetic flakes (totally non-magnetic before applying enough pressure).

Using sulfur and carbon simultaneously yields ferromagnetic flakes already with somewhat lower pressure.

What is the cause of ferromagnetism here? Iron from the pestle or some allotrope of carbon?

Is there an easy way of increasing the amount of product without reducing the applied pressure, as a home-experiment? Right now, it is impossible to see, if the pestle surface is corroded.

Answer 1

Yes.

Further experimentation with non-magnetic $\ce{CaCO3}$ also resulted in slightly ferromagnetic flakes. Harder materials (which are not reactive nor magnetic) produce the effect with less effort, suggesting the ferromagnetism originates from iron contamination. The reason for sulfur being an exception is probably a reaction which happens easily with pressurization, consuming all iron contamination.