Passionate men who love their careers are rare but important. Those that I'm speaking of become so consumed by their work that they forever change how the world views their craft. For luthiers, the Stradivari family exemplified these qualities. The term Stradivarius has become a way of saying something is excellent because of this.
This excellence was more attributed to the work of Antonio Stradivari, born 1644 in Italy, than any other family members. While his sons inherited some of his skills and techniques, his violins were the pieces everyone valued most.
It is not known exactly what processes were used or what made these instruments so great. Many theories exist about this and a few experiments have been done. For example, during this period of time in Europe the temperatures were significantly colder, causing trees to grow with higher density wood. This wood was assumed to have been used in his violins. An alternate theory states that wood from maple trees in northern Croatia were used. This also relates to higher density wood.
When it comes to the processes of making these violins, certain chemicals were found in the composition that gave it a higher quality sound. These were suspected to be oxidizing agents. These chemicals may have been added for the purpose of enhancing the sound quality or merely to protect the wood from worms. It is not yet known what exact chemicals were used. This is not due to the lack of science, but due to the lack of samples to analyze.
Antonio Stradivari pumped out around 1,200 violins through his long life (he died at around 93 years of age). About half that number remains. Each violin may be worth up to $5 million, but estimates vary. The most one is recorded to sell for was in October of 2010; the price was $3.6 million. This was the Molitor, created in 1697 and supposedly owned by Napoleon Bonaparte. Between the years 1700 and 1720 were known as Stradivari's "golden period" and may sell for more in good condition.
Is the sound quality really that good? As time went on their quality became controversial. Blind tests have been done in which listeners were requested to guess which violin was the Stradivarius, by sound, out of a lineup. One test pitted a Stradivarius violin against three others from various time periods and makers. None could correctly guess more than two correctly. Two of the listeners even thought the 20th century violin was the Stradivarius.
Four Ronheimer violins were compared to a Stradivarius violin in a 2009 test. Most listeners misidentified the Stradivarius.
So does this mean that the quality of the widely-loved Stradivarius violins is a fluke or that Antonio Stradivari didn't create excellent violins? Personally, I wouldn't press it to that degree, though claims may have been exaggerated. The rumors of incredible quality may have been started by Stradivari's personal connections combined with his location. While most instruments endure time very well, it is possible that some of the original quality was lost as a whole due to handling and other factors. Until more studies are done, it is up to the best guess of the listener.