“The radioactive lead that we’re interested in has a half-life of only 22 years. This means that if you start with a small quantity of this radioactive isotope of lead, which is called lead-210,[1] then in only a few hundred years it would have disappeared. However, in rock, where there is uranium, the uranium keeps feeding the elements following it in the series, so that as fast as they decay they are reproduced by the element before them.” [Illustration: _The Uranium Series. In this simplified diagram, the double vertical arrows represent alpha radioactivity and the single slanted arrows represent beta radioactivity. The times shown on the arrows are the half-lives for each step._] Uranium-238 ⇓^α 4½ billion years Thorium-234 ↓^β 24 days Protoactimum-234 ↓^β 1⅕ minutes Uranium-234 ⇓^α ¼ million years Thorium-230 ⇓^α 80 thousand years Radium-226 ⇓^α 1600 years Radon-222 ⇓^α 3⅘ days Polonium-218 ⇓^α 3 minutes Lead-214 ↓^β 27 minutes Bismuth-214 ↓^β 20 minutes Polonium-214 ⇓^α less than one second Lead-210 ↓^β 22 years Bismuth-210 ↓^β 5 days Polonium-210 ⇓^α 138 days Lead-206 (Not Radioactive) “I don’t quite understand how that works,” said Harley. “What do you mean ‘it keeps feeding it’?” “Well, think of a series of lakes connected by waterfalls. At the top, the highest lake has an enormous supply of water. Following the waterfall coming out of the lake you find a smaller lake and then maybe a medium-sized lake, and after another waterfall, a smaller lake, then a tiny lake, and so on. “As long as that big lake on top is full or nearly full, all the other lakes, whether they are small or medium-sized, will still be getting water as fast as it pours out. But if you cut off the supply of water from the upper lake to the next lake, then the smaller lakes will in time run dry. The same thing works with the radioactivity. In this series headed by uranium, as long as uranium is present all the other elements below it are kept supplied so that they don’t run out.” “I understand that,” said Bill, “but how do we use that to date a painting?” “One of the pigments used by artists for over 2000 years is known as lead white and it is made from lead metal. The lead metal in turn is extracted from a rock called lead ore, in a process called smelting. The radioactive lead, this lead-210 that I mentioned, behaves like ordinary lead metal and goes along with it. “The radium, which has a fairly long half-life, doesn’t follow the lead metal, but is removed with other waste products in a material called slag. Since the longer-lived ancestor of the lead-210 is removed, the supply of lead-210 is cut off. (Or we can say that one of the waterfalls is shut off.) The lead-210 will then decay with its 22-year half-life.”