Very interesting BBC article online
http://www.bbc.co.uk/news/entertainment-arts-13856203
We often get asked if the violin someone has found in the attic is a Strad. Often the fact that the instrument is old and has a label marked
"fecit anno domini 17XX" with mentions of Cremona, has the excited owner convinced it's the real and genuine article.
Sadly this is (to date) not the case. We may yet find that one of these instruments is indeed the product of the great master but so far, no; they are almost uniformly Victorian handmade or factory hand crafted copies. The rest may be slightly older or more modern. Some are fiendishly clever - right down to the fake "reduction" of the neck as if to imply it was originally a strad period length. Others are shiny and glossy and not even of stradivarian dimensions.
Many of the instruments labelled Stradivarius are not bad violins. They are decent, middle range instruments sometimes worth 2-3,000e depending on the standard of making involved. Many are quite standard instruments but again comparable to a well made handmade modern one around 7-1000e and some are dreadful. The label is not always a trick to defraud; it was perfectly understood that it meant "in the manner of" or "a copy of" a strad. Concepts of copyright and ownership were different and it is really a product of the twentieth century to assume that a label means authenticity.
In the end there are few strads and although one or two may well be discovered it is a little unlikely that great aunt Mary's old fiddle is one, no matter what the label says. However if you like the instrument and the sound is worth the asking price (the only true estimation of an instrument's real worth) then enjoy it. And maybe dream a little that the great man's hands did indeed fashion it, many moons ago.
