There are several methods for separating mint postage stamps that are stuck together. The stamps that I am referring to are the ones with the gummed backs, not the self adhesive ones. The results vary and invariably a few will be damaged beyond any collector value. Value should be considered before trying to separate stamps. The most valuable stamps are from before 1940, but I have never come across mint stamps before that time that were stuck together. So, determining value and how to handle them is up to the individual. The following are methods you can try:
Steam – I have tried steaming by holding the stuck together stamps with stamp tongs over the spout of pot for boiling water for tea. I have also put them in a vegetable steamer. The stamps will come apart, but I never had good results. The stamp paper will take a beating and the colors readily fade. So, I don’t recommend that anyone use this method.
Soak – Soaking stamps in warm water works well, but there will not be any gum left on the back side when finished. If many stamps are stuck together (I have had 15 or more) then the soaking process should be done in steps. That is, soak them for a while and then see if you can get a few apart. Put the rest back in the water and repeat. It may take a few soakings to get them all apart. Place the separated stamps on some sheets of newspaper face side down. When dry, put the stamps between pages of a thick book for a few days to flatten them. The stamps are then classified as unused, no gum. You can use them as space fillers in your album, sell them as described or use them as postage.
Sponge – This method gives the best results, but takes the longest to achieve. You are going to need two large kitchen sponges for this method. First, soak the sponges and then squeeze out all the water. Place the stuck together stamps on top of one sponge and then place the other sponge on top of the stamps. The humidity created by the sponges will soften the gum to the point where the stamps can be separated. This may take a few hours to overnight. This method works well when just a few stamps are stuck together and not very well if there are several layers of stamps. When separating the stamps try to slide them apart. Doing so does the best to preserve the gum. When apart, dry the stamps on some sheets of newspaper face side down. When dry, put the stamps between pages of a thick book for a few days to flatten them. You may have to go back with a damp sponge and wipe off any gum residue from the face side of some of the stamps. If you have sheets of stamps that are stuck together, place the sponges side by side inside a large zip-lock bag. Place the sheets on top of the sponges and close the bag. Again, this may take a few hours to overnight to work. The results are going to vary. At best, the stamps will look to be almost mint with very little gum disturbance. Other times, there is going to be a lot of gum disturbance. It all depends on the stamps and how careful you are when separating them. Again, the stamps can be used as space fillers, as postage or sold as mint with disturbed gum.
There probably are other methods for separating stamps and I would like to hear from anyone that has had success with other methods.