A long time ago, probably 200 years, the scientists experimented with electromagnets by winding copper wire onto a bar or rod of steel. Most anyone that is taking a physics class has been introduced to this electromagnet. This works ok for a battery or direct current but when the current is alternating the solid steel piece has eddy currents that waste power and cause the coil to be lossy. In the late 1800’s they figured out how to reduce these losses by making the coil core out of many short lengths of wire bundled together. This works ok for audio frequencies but as the frequency goes up even these wires have more loss. So someone came up with the idea of making the coil core out of iron filings. This works even better at higher frequencies . As the frequency got higher the iron particles had to be made smaller to reduce losses. One way of doing this was to make iron precipitate out of a solution and you have very fine particles.
I decided to try make my own toroid core out of iron powder, but not with chemicals. One way to get iron powder is to go to the river or stream and run a magnet through the sand or soil. So I got a powerful neodymium magnet from a crashed hard disk and a pill bottle and walked down to the creek.
I spent about 45 minutes running the magnet through the dirt and pulling off the particles into the bottle. I ran the magnet through about a quarter square meter or two square feet each of soil down to about 2 centimeters or less than an inch deep. I collected about half a pill bottle or a few ounces of iron powder. When I got home I sifted the iron powder through a lemon-lime strainer and that left about half the powder and the rest were larger grains or pebbles. The powder sticks to the magnet so it is mostly iron.
When I finished sifting, I had 3 choices of what to do with the coarse grains. I could use them along with the fine powder in the mixture. Or I could leave them out altogether. Or I could crush them with a hammer and use the magnet to get the iron out of the crushed powder.
I chose this last option, and I’m glad I did. I went outside and found a smooth spot on the concrete walkway, sat down, and poured a small amount of the coarse grains into a pile and pounded it with a hammer until the coarse grains were broken up. I ran the magnet through the pile and put the iron powder into the container with the fine powder. When I finished, the leftover pile of nonmagnetic powder was three times as much as the magnetic powder. In other words, I got rid of 3/4 of the useless nonmagnetic stuff, and further concentrated the iron.
Okay, now I have to make a mold to hold the liquid powder until it solidifies. I chose a salad dressing bottle cap with a plug in the center that can hold the form for the hole. I put a short length of green soda straw on the plug, then I built up several layers of heat shrink tubing, a total of 5 layers.
I haven’t figured out what kind of binder to use. I was thinking that white glue might work, or maybe hot glue. Whatever I use it has to stay liquid long enough to mix the powder in before it solidifies. The hot glue may cool too quickly. The white glue might shrink as it dries, which may not hurt. I could pour more than one layer and then let it dry between layers. I need to check my glue supply to see if I have any two part epoxy. But I have to make sure that it will catalyze and get hard. If I have some, it has been sitting around for years and might be too old to get hard (I’ve had that happen before).
When you are done, you will have a distributed air gap, like with iron powder toroids. I did what you are doing, but with ferrite from working toroids, and my results were poor because I couldn’t get rid of enough air gap, though I pulverized with a hammer for quite some time. Hope your experience is better.