One of my favorite river sites produces a fair number of fossil sea urchins, some of which are covered in a crusty harder-than-clay-but-not-quite-hard-as-rock matrix that I can't seem to remove without breaking (or at least cracking) the fossil. Any suggestions?

I looked around the site and couldn't find an answer, but if it's covered somewhere else please let me know.

They're all from the same Virginia (Surry County) river site. It's really more like crusty crumbly cement/clay than what I think of as rock. I've got some older pics up in flickr, but they'll need to be resized before I sling them up here. Actually, if I can get home while the light is still decent, I should be able to take some more.

It looks like your urchins are from the Yorktown formation. The matrix is a slightly calcified sandstone / sandy limestone. The problem isn't going to be the matrix, it's the thin tests of the urchins that will be hard to work with. There are several ways to get the matrix to soften up a bit. Test any method with broken urchins first.     Soak the specimen in white vinegar, right off the grocers shelf. You will need enough vinegar to completely cover the urchin  but not too much to start with. There should be some fizzing as the vinegar attacks the calcium in the matrix. when the fizzing stops or slows to almost nothing remove the specimen  from the vinegar and try to remove the matrix by scrubbing with a stiff bristle toothbrush in water. If this is working dry the specimen completely and soak in fresh vinegar. Keep an eye on the urchin to make sure it's not dissolving along with the matrix.     I have used concentrated acetic acid from a photography shop in the same way. This acid is basically concentrated vinegar, but be carefull using it or any acid. Use safety glasses and rubber gloves to work with the acid in a well ventilated area.     If using vinegar or acetic acid you must soak the specimen in several changes of clean water when it's completely prepped, to make sure no acetate crystals form in the fossil and cause it to explode / fall apart. I usually soak specimens overnight between water changes.     Muriatic acid is dilute hydrochloric acid and must be used with caution. When I use muriatic acid on a fossil I use a disposable eye-dropper to place one drop at a time on the spot I want to dissolve. As soon as the spot is clean I rinse the specimen in lots of water. Make sure you use safety glasses and rubber gloves and have good ventilation.     I have heard that you can boil some lightly calcified matrix pieces in distilled water to destabilize the calcite but I have not tried this.     For mechanical matrix removal you can use a rotary tool like a Dremmel to slowly get rid of unwanted matrix. You will find that carbide or diamond tips work best. You may want to coat any areas of the urchin that are clean of matrix with Butvar, dilute white glue, or some other type of stabilizer to strengthen the test before working on it.     I'm sure there are other ways to remove the type of matrix you have, but these are a few I'm familiar with. Good Luck.