I've always thought that the secrets behind a Stradivarius were a combination of both the Varnish as well as how it was applied to the instrument as well as the wood that was used.
The European Spruce that was used most likely came from the Italian, Swiss, or possibly even Austrian alps, and was much differant than that available today because of the fact that the wood utilised was in the construction thereof was taken from trees that were growing during the 'Little Ice Age' global cooling climatic event which ran from about the 14th Century to the early 19th Century.
Wood from these trees was denser and harder due to a shorter growing season resulting in a much finer annular ring growth pattern. --Even though the article mentions that fine quality wood is readily available to build copies, (Of which BTW, many Violins are copies of Stradivarii) European spruce that grew through the Little Ice Age has long since been logged, and is non-existant because of this.
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