I have my method to soak stamps from old 100++ years documents. (for my Thailand revenue collection)
If I use room temperature clean water, it will take days to soak off and then when I remove stamps, most will be damaged or in pieces because some area of the stamps still stick to the paper.
So, I cut excess paper away. Put the stamps in a glass bowl inside the microwave, use the same program I use to heat my coffee water for 2 minutes. After that I take stamp by stamp out of the bowl then slowly peel the backing paper away.
I never put my stamps to dry on paper, they either curl too much or some perforations or even complete stamps stick to the paper due to some glue remnants. I put the stamps upside down on a bath towel and just let dry in the air for maybe 2 hours. After that some stamps are slightly curled. I take a magazine with glossy paper and put stamps close to the spine, then every few pages I put maybe some 10 stamps. Let it flatten for a week by putting some weight on the magazine
After that my stamps look flat, are not discolored and look in general cleaner than before I started to soak.
I guess that old stamps use ink more stable against high temperatures than modern stamps. Also in the old days many stamps were stuck on the documents using so called bone glue, which will never soften or dissolve in room temperature water.
I live in Bangkok Thailand, using hot water you can call me a stamp barbarian, but its the only way to get stamps in 1 piece off the documents.