I don’t remember the first time I noticed that my Daniel Smith Hematite Genuine watercolour was ferromagnetic.
If you produce a nice flocculating wash of hematite you can have fun playing with a magnet and you can get quite long strands of pigment to form.
During Covid lockdown #3 in 2020 I played around with this. Recalling a school experiment with iron filings showing magnetic field lines I decided to try the same with watercolour.
I’ve made subsequent experiments using a variety of magnets. Typically I set up a static field with 2 or three magnets and then disturb the field with other magnets and steel items and generally doodle a bit.
I have found that Daniel Smith Hematite Genuine and Lunar Black work very well..
Some other pigments show a slight reaction to a magnetic field. I find an easy test is to make a very wet wash of the colour and then wave a magnet about behind the paper. Even though the magnet is hidden you can see its form as it moves past the wash if the pigment is influenced by it. Another test is, assuming the paint tube itself is not ferromagnetic, to see if a magnet is attracted to the tube of paint.
If you modify a wash with a magnet it is best to try to avoid getting much pigment on the magnet, as it is difficult to remove. Placing a magnet very close above a wash can cause pigment particles to be pulled out of the wash and attach to the magnet.