“How’s that?” asked Harley.
“There are 3 or 4 different compounds with about the same chemical composition as lead white. Chemically, they are almost impossible to distinguish. But with X-ray diffraction, a chemist can easily tell them apart. The hope is that the type of lead white will indicate how it was manufactured. Until the middle of the 19th century, lead white was produced mainly by packing strips of lead in clay pots with a little vinegar in the bottom. The clay pots were stacked in a large building with layers of decaying organic matter on the floor. The building was sealed for several weeks during which time the lead corroded in the fumes and became covered with a white substance. The white substance, lead white, was scraped off, ground, and washed to make the pigment.
“But, in the 19th century, when people began to learn more about chemistry, they looked for faster ways of making lead white and some of these methods produced a lead white of somewhat different composition. By using X-ray diffraction, chemists now hope that they can tell how the lead white was manufactured. This may provide another means of dating the lead white in a painting.”
“Are there any other methods?” asked Harley.
The stack process for making lead white. Rows of clay pots containing lead and vinegar are packed to the ceiling of the building, and fermenting tanbark on the floor produces carbon dioxide and heat. The fumes of vinegar and the carbon dioxide corrode the lead in 2 to 4 months, and the corrosion is lead white.
“Yes, isotope mass spectrometry is one. All lead consists of 4 different isotopes or atoms of different weights. Three of these 4 are the end products of a radioactive decay chain. Depending upon the history of the rock formation in which the lead ore occurred, the relative amounts of the lead isotopes vary in a special way. In other words, if we know the different amounts of lead isotopes in the world’s lead ore deposits, and we have a sample of lead white from a painting, we can tell from which deposit the lead, which formed the lead white, came. If, for example, we find that the isotope pattern in a sample from a painting is the same as in lead ore from Australia, then the painting can’t be very old because lead white wasn’t produced from lead mined in Australia until about 100 years ago.”
X-ray diffraction patterns from three different lead compounds that might occur in lead white. The middle one is the ideal lead white produced for over 2000 years. While some of the bottom compound may be found mixed with it, the compound shown at the top is only a 20th-century invention.
4PbCO₃ · 2PB(OH)₂ · PbO 2PbCO₃ · PB(OH)₂ PbCO₃