Restoring Edo-period Japanese woodblock prints: removing aging artifacts without losing historical character

Two decades ago I worked on pretty much the exactly same assignment at university: scanning in originals and copies of Sakoku period prints related to Deshima.

At the time I used the same method as I would do now: Balance out the Levels to remove the overall browning and apply careful curve adjustments to regain contrast. 

Perhaps nowadays I would apply a subtle frequency filter; but only to enhance local details of textures (such as the textile textures in your example) where it assists in understanding the detailed art.

We were told not to fix damaged areas at the time. We'd build up local enhancements using multiple layer masks and adjustments.

But I would not see any way to do all this automatically. because each case had its own specific adjustments.

I agree with ​@D Fosse  here as well.

That’s a very good question. I work as photographer at an art museum, so I need to consider this constantly. I don’t have much time now, so just a quick rundown.

Obviously, for archival purposes you don’t alter anything, but for presentation purposes it can be absolutely legitimate to do a digital restoration. But again, I always do as little as possible.

There are two simple things that usually get you 95% there:

One, make sure the lighting is absolutely even. If it wasn’t during shooting, you can fix that. A very effective way is to put a steep curves layer on top, temporarily, to reveal any lighting falloff. Then correct with Curves or Levels under that temporary layer, using masks and soft brushes at low opacity.

Two, correcting the white point will have a huge impact. Look at the histogram for each channel and use a basic tool like Levels to drag each channel up to where they should meet at a creamy warm white paper color. It’s a very simple operation, not complicated at all, and it will drastically freshen up the impression.

Note - major tonal adjustments should be performed at the raw stage if at all possible. The raw file contains a lot more headroom data at the high end.

Next, consider whether you really need to retouch defects and flaws. It may not be as important as you think. But if you do, the healing and spot healing brushes usually do whatever’s needed.

Don’t, don’t, don’t, fall for the temptation to use AI-based tools. You’ll quickly regret that. I have all those tools permanently disabled where possible, and don’t touch the rest.