On the coins thread, I posted a test photo of iron filings...
I took the iron filings, immersed them into Karo syrup on a well slide, then applied a magnet to it. Karo syrup is thick enough to keep the filing in place once I line them up with a magnet, but (mostly) optically neutral.
The only problem is that I wasn't patient enough to wait for the bubbles to disperse.