Some years ago the British rail Pension system invested in blue chip art with expert advice. I think that over time they more or less achieved the same return as they would have in the stock market.
Right now, collectors/dealers seem to be trying to out do each other in a game of who can get the most for a work of art. Steve Wynn would have held the title if he hadn’t put his elbow through a Picasso, but now David Geffen has the title, having sold his Pollock for a reported $145 million.
I call that a bubble.
Regarding that possible Sargent, I would take it to a reputable auction house on their “free evaluation” day. They will do a good job for you, and unlike dealers they have nothing to gain by misleading you.
I think Bonhams and Butterfields has at least one evaluation day per month.