Freezing Magnetic Fields

For centuries, people have used iron filings to observe magnetic field lines. But there's a certain magic in seeing finer details, especially in three dimensions - which I saw when using a block filled with clear oil and iron filings. These same patterns can be frozen forever by using casting resin instead of oil.

I used the following:

• Casting resin. I used EasyCast® Clear Casting Epoxy
• Measuring cup, stirrers, gloves, etc. to mix the resin
• A container (mold) to cast the resin in. For some, I used plastic packaging; for others, I used these silicone hemispheres from American Science and Surplus.
• A magnet. (See Step 1 for details.)
• Iron Filings. I used these (also from sciplus.com)
• A disposable plastic pair of tweezers. I used these (also from sciplus.com)

I'm not affiliated with American Science and Surplus, but they sell some neat stuff.

Bar magnets, horseshoe magnets, disk magnets, etc. all produce interesting and different magnetic field patterns. Decide what you want to see.

Similarly, looking at the magnetic field in one flat plane is beautiful - as is seeing it in three dimensions. Decide which you want to emphasize.

Finally, curved surfaces magnify and distort what you see. That is a very nice effect that people often use for casting projects; but if you want to avoid that, your mold should have mostly flat faces.

All of these will influence your choice of magnet and mold.

When considering magnets:

• A strong magnet will draw the iron filings to itself into a fairly interesting "filing ball" that's not really good for seeing larger patterns. So, ceramic magnets are usually good - no large neodymium magnets. The smaller your cast piece, the weaker your magnet should be.
• Pairs of magnets that attract each other produce different patterns from those that repel each other. It can be nice to compare and contrast the two. But without support, the two magnets won't stay where you want them. I glued pairs of magnets to a piece clear stiff plastic (from packaging); the plastic was barely visible in the final piece. If you do it, make sure the glue is fully cured before submerging it in resin.

When considering molds:

• If you don't have much practice, it's easy for the resin not to cure properly, leaving a gummy surface. If your mold is clear and disposable, you can leave your cast in the mold (and never remove it). For my first attempts, I used clear plastic packaging that had a reasonable shape. For rectangular blocks, the plastic from Hot Wheels cars works OK; but I prefer larger circles.
• The magnet doesn't have to stay embedded in the mold if you choose a broad and flat container. Rest the mold on top of the magnet. This works best for bar magnets, and allows you to use a relatively strong magnet: you can take the magnet away before the resin has solidified. By taking the magnet away, the iron filings settle straight down from where they are, preserving parts of pattern that would have gotten drawn to the magnet.
• If your mold is too close to the size of your magnet, most of the filings may clump against the magnet, making it hard to see the full pattern.
• If your mold is shallow, some iron filings and resin may stick out of the top of the mold. This looks similar to ferrofluid and can be nice to look at, but you need to be careful to avoid that if you're planning for a smooth surface. If you like that effect, you may need to add clear acrylic spray or another very thin coating of resin - otherwise, some iron filings may be exposed (which is not nice to handle).

Several of these are shown in the examples in Step 6.

As mentioned above, I used EasyCast® Clear Casting Epoxy. It has a yellowish tint for thicker clear castings like these. There are many brands and varieties (including perfectly clear which you may prefer), but I've stuck with it so far since experimenting with different options costs time and money. If you want to try others, make sure it can be used the way you want. It must be able to cast in the depth you want (some kinds only work with small or shallow casting). And it helps if it has a slightly thick consistency: if it's too runny/watery, the iron filings may fall to the bottom of your mixing cup or stick to the magnet relatively quickly - which can make it harder to get an interesting pattern.

Some varieties give off bad fumes, others less so. Consider your work space, and be considerate of others who need to share it. (The low odor is the main reason I chose to start with EasyCast®.) No matter what, make sure there's enough ventilation.

During the mixing process, you will introduce bubbles; for some resins, bubbles naturally rise and pop. Others require more work to eliminate bubbles. Decide if this will bother you; and if so, how you plan to reduce them (heat, degassing, pressure pot, etc.). There are plenty of good tutorials online; the best method depends on the resin you use. Of course, mixing carefully reduces the number of bubbles you have at the start.

As with anything you're new to: for your first tests, start with things that are as small and simple as possible, so you're not disappointed if they don't work out well.

Make sure your work area is clear and free of things that can spill onto your project, or that can get damaged by your work tools. Some people like covering their work area with a silicone mat, since resin can peel from that easily. I use wax paper: it's a clean surface that resin doesn't stick to, that I don't mind throwing away. (Some of the photos show aluminum foil, which is also a clean surface that can be thrown away - but resin sticks to it, so I've stopped using it.)

Most types of casting resin need to be used within a certain range of temperatures, and you may need to watch the humidity.

Make sure your surface is level - otherwise, the surface of your piece will take on the same slope as your table.

Arrange everything you're going to use so that you know where it is, can get to it when you need to, and can get it out of the way when you're done with that step.

Be sure to follow the manufacturer's instructions carefully. As mentioned above: if you don't, the resin may fail to cure properly. For most types, the resin and hardener will have streaks while you're mixing, and it won't be fully mixed until after it's clear.

Two to four separate pours usually work best. Here's a reasonable routine:

1. Fill a small portion of the mold with clear resin, and wait for it to mostly cure. This makes a clear layer beneath the magnet, and will ensure there's not a level of unpatterned iron filings on the bottom. This level must cure more completely than for most inclusions: magnets are much more dense than resin, so they will sink much more than paper, flowers, or other common types of inclusions, so expect a long wait.
2. (Optional.) Once that layer is sufficiently cured, mix another batch of clear resin. Coat the bottom of the magnet with this resin by dipping it in the fresh resin (using the tweezers), then place it on the almost-entirely-cured resin. (Again, use tweezers to place it exactly where you want.) This coating has two purposes: 1. It reduces the chances of bubbles staying trapped against the magnet. 2. It helps the magnet stay where you want it. Then, pour that resin to cover less than half of the magnet - but do not let it cure. Instead, wait for it to get to more of a jelly-like texture.
3. Once that layer is only partially cured (jelly-like), mix another batch of resin, large enough to fill most of the depth you plan to use. Add between a quarter and half of a teaspoon of iron filings to this third batch of resin, and stir very thoroughly. (The exact amount depends on the size of your mold, and how much you want stuck to the magnet. Plan to experiment before settling on a routine you like.) The filings settle quickly, so you must keep stirring. Slowly pour the filing-containing resin into the mold, pouring closer to the edges of the mold than the magnet. If you pour directly onto the magnet, the iron filings will stick to it, and resin that fills most of the mold will have very few filings. Pause occasionally, looking for areas where you would like to have additional iron filings, and pour additional resin there.
4. The iron filings poured near the magnet will stick to it making an attractive three-dimensional pattern. Those farther from it will sink to the boundary with the first layer - still aligned with the magnetic field of the magnet. These should stick to the gelled layer rather than sliding toward the magnet (which they may do if you use a fully-cured layer). So, most of this layer (except very near the magnet) will end up clear.
5. (Optional.) If the top has iron filings poking out of the top surface and you want it to be smooth, then wait for the second layer to be partially cured. (The second layer does not need to be cured as fully as the first layer, since you're not adding any heavy inclusions.) Prepare another batch of clear resin and pour it on top of the previous layer. Pour until the surface is smooth. (It probably won't take much.)

A hint from Chemistry labs: you can direct the resin where you want it by holding the side of your stirring-stick against the mixing cup, and letting resin run along the stick to the point you want it.

To increase the three-dimensional effect farther from the magnet, you can use more layers, each containing iron filings.

Any time the resin is curing, use a box to protect it from dust or other things that can fall in.

Follow the manufacturer's instructions for cure time and temperature; the iron filings should have little effect on those.

Since most resin shrinks as it cures, you may need to sand the edges. Be sure to read tutorials about that - it's easy to mess up your finished piece.

If you want the pattern to visually stand out, you can paint one face with a glossy white paint. If iron filings end up exposed on a face and you want to cover it, or if the cast comes out gummy and you removed it from the mold, you can help by coating it with clear acrylic spray. Only resort to that if you've waited several days for it to fully cure.

Share and Enjoy your frozen fields!

Here are some of my experiments; I hope you learn from them, to be happy with what you make.

This attempt used a scrap piece of clear plastic as a mold (and I never removed the cast from the plastic). The bar magnet (the same type as used above in the Instructable) was below the mold, so it's not embedded in the acrylic. Because the container is shallow, some of the iron filings stick out of the acrylic in a way that reminds me of ferrofluid.

This attempt used a slightly stronger ceramic block. Here, I poured the filing-containing resin directly onto the magnet, rather than letting it flow in from the sides. This led to a clump of filings on the magnet, and relatively few elsewhere. It has an interesting look, but isn't good for handing to others since some sharp points of iron filings are exposed.

Here, I didn't let the bottom layer cure sufficiently before inserting the magnet. The filings far from the magnet stayed on the boundary with the bottom layer as expected, but the magnet and the filings attached to it sank to the bottom. That wasn't a problem since I used a clear plastic mold; but if I'd removed it from a silicone mold, it would leave the magnet and some filings exposed for people to touch.

These three were cast in silicone hemisphere molds and turned out rather well, but used fewer filings than I prefer. The lower layer was also fully cured, so the filings that were there tended to slide toward the magnets. The first uses a metal bar magnet (as shown in most of the steps), the second uses two ceramic block magnets attracting each other, and the third uses two repelling magnets.

Notice the difference between the fields for the attracting vs. repelling magnets. For the attracting magnets, the field is perpendicular to the center line between the magnets (similar to a single bar magnet); for the repelling magnets, it's parallel. This can be explained nicely by adding the magnetic field vectors created by the two magnets.

For future projects, I plan to try:

• Magnetic fields created by wires (straight wires, flat coils, solenoids, toroidal solenoids)
• Embed a magnet in a little plastic globe, and cast the iron-filing-embedded resin around it (to mimic the earth's magnetic field)
• Using bits of hair instead of iron filings, and showing electric fields for assorted shapes (using electric charge rather than magnets)

If you try any of these, let me know - I'd love to know how it worked for you!

Iron filings suspended in oil is a fairly common physics lecture demonstration that you can find sold at science supply houses, and instructions for making similar viewers for both electric and magnetic fields have been posted online for many years.

After a few years of teaching, I thought it should be possible to do the same in resin, which would make it easy to pass around to a classroom full of students. I looked around to buy one and... I couldn't find any. With the busyness of life, it was a few years more before trying to make it myself.

Do you know someone interested in this type of thing? Let me know.