I generally will put them into stock sheets (the ones with the plastic holders, making sure the entire stamp is covered), and then rest a bunch of heavy books on them. Helps, but doesn't always cure the issue completely. However, once placed into mounts and then album pages, the remaining curl tends to straighten out.
Don, There are a few things that may be at play too. If they are pre-1900 and they are curling, it is a sign (though not definitive) that the stamp has been regummed. If it is curling, you should check it closely for regumming in those issues (particular culprits are Colombian and Trans-Mississippi). Some modern stamps, especially coils have a natural tendency to have a bit of a curve to them. So that's another occasion where you don't really want to try to take the curve out.
As an "advanced" technique (and I suggest doing this first with a few low-value common stuff), take a pair of stamp tongs (not spade tipped), and lightly run the tong the opposite direction of the curve. This can gently restore the natural shape, but you don't want to crease it, or break the gum. It's subtle.
Placing them in stock books as George mentions might work for a while, but usually, they will just curl again if put into a glassine. While "pricey" this is where dealer cards are worth their weight in cold, especially if you're just going to keep stuff stored, and they are of high value. Put them in a 1 or 2 row dealer card, and place THAT in a glassine.
Comments
There are a few things that may be at play too.
If they are pre-1900 and they are curling, it is a sign (though not definitive) that the stamp has been regummed. If it is curling, you should check it closely for regumming in those issues (particular culprits are Colombian and Trans-Mississippi).
Some modern stamps, especially coils have a natural tendency to have a bit of a curve to them. So that's another occasion where you don't really want to try to take the curve out.
As an "advanced" technique (and I suggest doing this first with a few low-value common stuff), take a pair of stamp tongs (not spade tipped), and lightly run the tong the opposite direction of the curve. This can gently restore the natural shape, but you don't want to crease it, or break the gum. It's subtle.
Placing them in stock books as George mentions might work for a while, but usually, they will just curl again if put into a glassine. While "pricey" this is where dealer cards are worth their weight in cold, especially if you're just going to keep stuff stored, and they are of high value. Put them in a 1 or 2 row dealer card, and place THAT in a glassine.