I've heard that the top of late 19th century stringed instruments can turn almost black because they used a varnish with a high iron content. Is there truth to this? And if so, can it be restored to it's lighter color while keeping the integrity of the original varnish? How much would it affect the value if one where to have the top completly refinished? I know on vintage slab basses, it's borderline blasphomy. Does the same hold true for DBs?