I have 2 old (?100 years) marquetry pictures where the wood is drying out. Many years ago I applied linseed oil but this darkened the pictures. Is there something I can use to prevent this drying out and improve the surface?
If these have sentimental or monetary value they could probably do with professional attention as they're likely to need a multi-level intervention, including but not limited to removing the linseed oil, re-gluing certain pieces and some level of refinishing, possibly a complete refinish. While we could actually help with all of the above steps I think it would be better to have a single person with knowledge and experience tackle this in an integrated way.
Obviously this won't be cheap (and be suspicious of a cheap quote!) so it's only really viable if the pieces have enough real or perceived value.
If you did want to try to tackle these yourself be aware of some things in advance:
removal of the linseed oil will require stripping, and many consumer-level strippers are no longer suitable for pieces of this kind*;
the re-glueing work will be best accomplished using hide glue;
if you want to refinish it as it may originally have been finished it'll involve using shellac, and applying this to a high standard is not easy for the first-timer.
But as I mention above in my Comment, the main issue facing old marquetry and veneered items is their environment — homes typically being much drier than they once were, something that is difficult or impossible for the homeowner to do anything about.
1 Because they contain water, rather than being exclusively based on organic solvents such as the highly effective but hazardous methylene chloride. In addition, these newer safer/'greener' strippers tend to act much more slowly, so the stripper needs to be in contact with the workpiece for extended periods; this could undermine all of the glue joints, with disastrous consequences.....