Lead
Lead is an element, number 82 on the periodic table. 

Old 23rd century type impulse packs (part of the impulse engines) were insulated with an element that could decay to lead. When a ship had been through trauma it was best to check the points of the packs, because if they had decayed, when activated they could take out the entire impulse deck.

After encountering the galactic barrier, Lee Kelso checked the impulse engines and did not notice that the damaged points on the impulse pack had "about decayed to lead." ()

The surface temperature of the innermost planet of system L-374, of the two planets that survived the planet killer's attack, was that of molten lead. ()

In 2268, Spock reasoned that to hide from a sensor scan, the dikironium cloud creature would have to have been able to change its molecular structure, likening the process to gold changing itself to lead or wood. ()

Later that year, after multiple phaser strikes placed a strain upon the 's warp engines, Montgomery Scott complained that Spock wouldn’t be satisfied until the engineering panels were a puddle of lead. ()

In 2269, the tunnel walls on Phylos were found to be six hundred times denser than lead. ()

In 2368, a fatigued Geordi La Forge claimed that his eyelids felt like they had lead weights attached. ()

The soil of a planet in the Gamma Quadrant surveyed by Nog and Jake Sisko in 2370 contained lead, traces of which were found in nearby water. ()

In 2371, Crewman Henley remarked that her legs felt "like lead pipes" even before Tuvok made her and other Maquis members of the  crew run ten kilometers. ()

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Elements