Pencil "plumbago" (Latin for "lead ore"). Because the pencil core is still referred to as "lead", or a "lead", many people have the misconception that the graphite in the pencil is lead, and the black core of pencils is still referred to as "lead", even though it never contained the element lead. The words for pencil in German ("bleistift"), Irish ("peann luaidhe"), Arabic (قلم رصاص "qalam raṣāṣ"), and some other languages literally mean "lead pen". The value of graphite would soon be realised to be enormous, mainly because it could be used to line the moulds for cannonballs; the mines were
Lead of uranium-235, thorium-232, and uranium-238, respectively, so traces of all three of these lead isotopes are found naturally. Minute traces of lead-209 arise from the very rare cluster decay of radium-223, one of the daughter products of natural uranium-235, and the decay chain of neptunium-237, traces of which are produced by neutron capture in uranium ores. Lead-210 is particularly useful for helping to identify the ages of samples by measuring its ratio to lead-206 (both isotopes are present in a single decay chain). In total, 43 lead isotopes have been synthesized, with mass numbers 178–220. Lead-205 is the most stable