Yesterday's entry covered reasons why you should not soak a stamp from a cover due to reducing the rarity of the stamp. Today, I will discuss why you should also exercise care in determining stamps to soak off, when the act of soaking will physically harm the stamp.
The dyes used in colored envelopes may bleed onto the stamp if it is soaked. Since many envelope dyes are not color-fast, it is very common to have some of the color transfered onto the stamp during the soaking process. This can be a particularly pesky problem during holidays when colored envelopes are used, such as for the Christmas and St. Valentine's holidays. The red envelopes prevalent during these holidays are notorious for the damage it causes to stamps. If you look at a bulk mixture of Christmas stamps, for example, some of the stamps will have tinges of pink on the stamp where the red dye has bled out and tinted the stamp.
Stamps on colored envelopes may need to be collected on-cover (i.e, collected along with the entire envelope) or at the very least, be carefully trimmed within 1/8th of an inch around the stamp.
Paper composition of the stamp is another caution when soaking stamps. Stamps printed on chalky paper, a paper type that is coated with a chalk-like substance, should not be soaked; the stamps can only be lifted from the cover by wetting the underside of the envelope until the glue dissolves and the stamp lifts free. If you soak stamps printed on chalky paper, they will be damaged. Because of the care that must be exercised in removing these stamps, you are better off to collect them on-paper.
Not only is it a crime, but postal administrations lose money when someone alters a stamp to make it appear to be unused so that it can be re-used. To combat this problem, some early stamps were printed in water-soluble ink. The second that it is soaked, the ink will begin to deteriorate, and thus alert the postal authorities that the stamp had seen previous duty. These types of stamps must never come in contact with water; a collector must either collect the complete envelope or trim the envelope paper down in size so that it can be collected on-paper.
Early stamps are not the only stamps that can be damaged by a soaking; modern-day self-adhesive stamps are getting a reputation for being very difficult to soak off of envelopes. Attempts at soaking these stamps sometimes require such intense effort that the stamp is sometimes reduced to a soggy pulp. Some recent French Marianne stamps, such as the one shown nearby, are notoriously difficult to soak.
The soakability of self-adhesive stamps has become such a matter of concern in the United States that the Board of Directors of the American Philatelic Society recently lobbied the U.S. Postal Service:
On behalf of its 42,000 members, the American Philatelic Society calls upon the United States Postal Service to produce stamps that can be immersed in water, reliably removed from paper intact, and added to collectors’ albums, as U.S. stamps of of the past 160 years traditionally have been collected.
Think twice before you automatically try to soak the stamps off of envelopes. While most stamps of recent years are made with stable ink, on paper that holds up well, and with water-soluble gum, there are stamps that can be damaged by soaking. If in doubt, save it until you know for sure that the stamp will hold up.
Update: Sharp-eyed reader Dominique reminds me that the French stamp that I show in this post, the 3.40 denomination, was not issued as self-adhesive, but others, such as the 2.30 denomination was. Thanks for spot-checking me!