Go figure

It was oil and pine resin. There you go!
From the Australian:

It has long been suspected that the beauty of a Stradivarius violin was skin-deep, with its gleaming lacquer responsible for the instrument's supreme, bright sound.
But after years picking apart scraps of varnish taken from Stradivarius instruments, scientists have ruled out any secret ingredient as the key to the fiddles' superb timbre. They reveal that Antonio Stradivari (1644-1737) applied two simple products in his workshop in northern Italy: oil and pine resin.

"There is no indication that allows us to say that the lacquer has an influence on the sound," says Jean-Philippe Echard, a chemical engineer at Paris's Museum of Music, which took part in the four-year study.

"There's been talk of fossil amber or propolis, which is produced by bees. But we haven't found any of these ingredients, which may have given the impression of something mysterious or rare."

Far from being a magic ingredient to make the instruments sing, the resin was more likely used to give them a rich colour.

"We also found red pigments in the varnish," says Echard.

The study was carried out on lacquer from five instruments -- four violins and a viola d'amore -- made over a period of 30 years. The chemists scrutinised the samples under infra-red light, identifying the two layers of varnish and their chemical composition.

"We have found that Stradivari employed and easily available components," the researchers wrote in their report, published by the journal Angewandte Chemie (Applied Chemistry). "He might not have possessed an unusual or `secret' ingredient, but he was an outstandingly skilled craftsman who had mastered the art of violin making."