Years ago, Becky Stern did a post about the wonders of “super paramagnetic silicone putty.” It was a commercial product and I assumed it was beyond the ability of the average nerd to make at home. Wrong. It couldn’t be easier. This Instructable shows how magnetic putty is nothing more than regular Silly Putty/Thinking Putty mixed with black iron oxide powder.
To create the magnetic putty, you mix the iron oxide with the putty at about one teaspoon per 0.8 oz size “egg” of putty. Black iron oxide is sold as a black pigment in art supply stores and you can find it easily online.
Black iron oxide, like all kinds of fine metal powders, is nasty business, so you’ll want to wear gloves and a dust mask when handling it. This is definitely a project to show children, not one for children to do themselves.
The black iron will also stain, and the resulting putty might stain, too, so be mindful of what surface you use it on. And best to keep it in an airtight bag or tin when you’re not recreating that 50s movie classic, The Blob, on your kitchen table.
You may think that, while it’s nice to make your own magnetic play-blob, the results are less than exciting. But look at the video here, the ones on Becky’s old post, and others on YouTube, and you’ll see that this stuff is a lot cooler and creepier than you might imagine. I can’t get enough of watching that stack of neodymium magnets getting digested by a blob of writhing black goo.