"My violin? It is a Stradivarius—the same which once belonged to the celebrated Baillot. I think it is good for a violin to rest, so during the three months when I am not playing in concert, I send my Stradivarius away to the instrument maker's, and only take it out about a month before I begin to play again in public. What do I use in the meantime? Caressa, the best violin maker in Paris, made me an exact copy of my own Strad, exact in every little detail. It is so good that sometimes, when circumstances compelled me to, I have used it in concert, though it lacks the tone-quality of the original. This under-study violin I can use for practice, and when I go back to the original, as far as the handling of the instrument is concerned, I never know the difference.
"But I do not think that every one plays to the best advantage on a Strad. I'm a believer in the theory that there are natural Guarnerius players and natural Stradivarius players; that certain artists do their best with the one, and certain others with the other. And I also believe that any one who is 'equally' good in both, is great on neither. The reason I believe in Guarnerius players and Stradivarius players as distinct is this. Some years ago I had a sudden call to play in Ostende. It was a concert engagement which I had overlooked, and when it was recalled to me I was playing golf in Brittany. I at once hurried to Paris to get my violin from Caressa, with whom I had left it, but—his safe, in which it had been put, and to which he only had the combination, was locked. Caressa himself was in Milan. I telegraphed him but found that he could not get back in time before the concert to release my violin. So I telegraphed Ysaye at Namur, to ask if he could loan me a violin for the concert. 'Certainly' he wired back. So I hurried to his home and, with his usual generosity, he insisted on my taking both his treasured Guarnerius and his 'Hercules' Strad (afterwards stolen from him in Russia), in order that I might have my choice. His brother-in-law and some friends accompanied me from Namur to Ostende—no great distance—to hear the concert. Well, I played the Guarnerius at rehearsal, and when it was over, every one said to me, 'Why, what is the matter with your fiddle? (It was the one Ysaye always used.) It has no tone at all.' At the concert I played the Strad and secured a big tone that filled the hall, as every one assured me. When I brought back the violins to Ysaye I mentioned the circumstance to him, and he was so surprised and interested that he took them from the cases and played a bit, first on one, then on the other, a number of times. And invariably when he played the Strad (which, by the way, he had not used for years) he, Ysaye—imagine it!—could develop only a small tone; and when he played the Guarnerius, he never failed to develop that great, sonorous tone we all know and love so well. Take Sarasate, when he lived, Elman, myself—we all have the habit of the Stradivarius: on the other hand Ysaye and Kreisler are Guarnerius players par excellence!
The complete book "Violin Mastery, Talks with Master Violinists and Teachers" by Frederick H. Martens
can be found at http://www.gutenberg.org/etext/15535