system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5. Why are old paintings painstakingly restored, but other antiques lose their value when the patina is removed?
gpt
As an antiques restorer,

most paintings need cleaning and often removal of the 'varnish' which was the last coat applied by the artist.

Because old varnishes had linseed oil as part of the formula, and linseed oil darkens when exposed to sunlight.

The point of the exercise is to return the painting to its original hues and appearance without removing the artist's brush work, and then to conserve or stabilize the painting in that condition.

When it comes to furniture and other objects, some of the 'patina' has been described as validating the item's passage through time and use.

So a restoration would include repairing damage, cleaning to a reasonable degree without disturbing the long term accumulation of wax, darkening oils, dirt and wear spots.

A restoration to the original as manufactured condition would remove the accumulation of all that evidence of the passage through time and present essentially what would be a new appearing thing much like a modern copy visually, which lowers the value of it as an antique.

And then there is the fake antiques biz, a whole level of artistry above mere copying an old design. And fun too. Depending on the ethics of sale.