An alternative damping material I would like to mention is Silica sand. The variety I am using is for sand blasting and is distributed by Clemtex of Houston, TX. The damping it provides is greater than playground sand or (clay) kitty litter, and has mass in between the two.
Each Silica crystal is sharp, extra dry and screened for contaminates. These crystals must pass through a sand blast nozzle, and do the job of sandpaper. Their natural shape, plus screening ( for particle size), makes them a natural to pack upon themselves much tighter than the other products mentioned.
The health hazard is low compared to lead, and should lead be desired, silica may be mixed with #7 shot in a ratio that provides the mass to damping ratio you prefer. I like a 75% Silica, 25% lead mix for the hollow upright steel towers of my Soundlab U-1's. In this mix, the lead is more to the bottom, to offset the pendulum effect of too much weight toward the top of this seven foot tall speaker.
For my turntable and equipment stands, I use Silica only, as the mix with lead was not as good sounding for this application. I bought a huge 25 pound bag for less than $12.00 from a local supplier, found in the Yellow Pages under Sandblasting.