Need Advice on How to Flatten Curling Classic Stamps

Curling stamps are not an easy fix.
The main issues are humidity, residual gum and the type of paper which often changes in a given issue
You might try soaking in warm water (some older stamps you have to be careful esp. reds & indigo)
The warm water may help remove the gum.
Watermark fluid may also be a solution.

Although I have not tried them
Showgard Desert Magic Stamp Drying Books appear to be a good drying alternative & not expensive (come in 2 sizes)
Some say they prevent curling??

With older stamps the glues were often made on the spot using gum Arabic, starches, powdered bone ...
They were applied after the printing and before perfing as the paper needed to be wet.
Given that gumming and paper was not an exact science, don't be surprised that some in the same series will curl & others do not.
Gum breakers were used on many stamps from about 1920 up to about 1970 to prevent natural curling (especially on coils)

Many thanks to JKJ Blue (I like to call him "Big Blue"!) for an illuminating response about the issue with curling stamps. He posted that info on my thread about Austria 19th Century Classics, so I wanted to be sure that it appeared here, too.

Blue mentioned that he had some old Persian stamps that would curl when he took them from his home at 400-ft elevation to his cabin in Oregon at 4000-ft elevation. I responded:

"Thanks for your comments about the differences in elevation having an effect on my stamps. I think you have hit the nail on the head! Where I live in northern Arizona is at 5200 ft of elevation. I have noticed many more problems with curling since I moved here than when I lived in Cleveland, Ohio at 650 ft of elevation.

"That probably explains why earlier this year, after having purchased some stamps while on travel in Singapore (hot, humid, and elevation near sea-level), many of them curled like crazy after I got them home!"

Unfortunately, this doesn't really solve my problem, but it explains a lot. I will apply some of Falschung's suggestions and see where it gets me. I promise to report my results!

Many thanks to Falschung, Big Blue, and Steve for the helpful comments about the curling stamps. I have had some success using your suggestions, and will post some new photos of the 19th-Century Austria with the now-flat stamps mounted in the album at long last!

Thanks for your additional comments, Chris W!

Yes, I have come to the conclusion after all these years that I should finally break down and get a drying book. I have never had one, but it sounds like I will get better results by doing that.

I have also used stock books to try to flatten curling stamps, but with mixed results. Sometimes, I take a stamp out of the stock book after a couple of years (!) and they still curl up again within a few minutes. Other times, it seems to help.

Anyway, a drying book will be one of my acquisitions early in the New Year!

True, but I decided some time ago to use only hinges in my primary collection albums to get a consistent look. Also, in these very old albums, which were printed in the 1930s, the pages are not made of a heavy enough stock for mounts.

The current Scott Specialized album pages are much heavier than what is in my albums and they are also only printed on one side, which makes them much better for using with today's mounts.

It is clearly one of the advantages of using mounts, but I have not gone in that direction.