Did Stradivari have a secret?
Probably not one that can be recovered, and — more uncomfortably for the legend — the evidence that his instruments are audibly superior is much weaker than the reputation suggests.
The instruments themselves. Antonio Stradivari worked in Cremona from the late seventeenth into the early eighteenth century, producing perhaps a thousand instruments of which several hundred survive. They are exceptionally well made, they command enormous prices, and their status has been essentially unchallenged for two centuries.
The secret theories, and what has been found:
The varnish. Analysed repeatedly, and found to be ordinary materials of the period — oils, resins and pigments consistent with what other craftsmen used. No exotic recipe has emerged.
The wood. The suggestion that timber grown during a colder climatic period was denser and finer-grained is genuinely plausible and remains unproven as an explanation of sound.
Chemical treatment. Evidence of mineral treatment, probably as a preservative against woodworm, which some argue affected the wood's acoustic properties.
Geometry and craftsmanship, which is the least romantic and best-supported explanation: he was an outstanding maker refining a design over a long career.
The blind listening tests, which is where the story turns. Studies in which experienced soloists played old Italian instruments and high-quality modern ones while unable to see or identify them found that players could not reliably tell which was which, and in several trials preferred modern instruments. Listener panels have produced similar results.
The objections to those studies — small samples, unfamiliar instruments, hall acoustics, limited playing time — are real and have been argued vigorously. The results have nonetheless been replicated, and no comparable body of evidence supports the opposite conclusion.
What the value actually rests on: scarcity, provenance, history, craftsmanship, and the self-reinforcing belief of an entire market — which is not nothing, and is a different claim from acoustic superiority.
Age also changes instruments, and centuries of repair mean few are as originally made.