I just purchased a significant clock with a mercury pendulum that came with maybe 15lbs of 125-year-old mercury. As I have smallish children and the clock is in a carpeted area as well as the fact that spilling more than 1lb of mercury requires a mandatory notification to the EPA and probably lots of $$$$$ to clean up, I think it's prudent that I take the mercury out of service. I have heard of people replacing the mercury with a steel slug. The mass of mercury is 13.5 g/cm^3 and the mass of iron/steel is just over half that - around 8 g/cm^3. Will this have a significant effect on the rate of the clock - that is will it require the pendulum being lowered farther than it can likely go or is the mercury centered over the effective pendulum length such that weight is less of a factor? Tungsten carbide is 15 g/cm^3 so I can go that route if I need to, but it's more costly and less available. Thanks! P.S. - a related question - has anyone attempted to permanently seal the mercury in something more robust than open glass jars - acrylic maybe? It's a shame to have to disable such a cool aspect of the clock. If there was a way to significantly reduce the likelihood of exposure in the event of a suspension spring break or other issue, it would be great to be able to still use the mercury. I don't think I'm comfortable with just sealing the existing glass jars with something like silicone - the jars themselves could crack.