The first polyester fibers were produced by reacting ethylene glycol and either terephthalic acid or one of its esters to give poly(ethylene terephthalate). This polymer is still used to make thin films (Mylar) and textile fibers (Dacron and Fortrel).
This stamp also may be used for mailing items weighing up to 2-ounces. These Forever stamps will always be equal in value to the applicable price for the price category printed on them, at the time of use. The initial price and value for this stamp is 71 cents, effective May 31.
7. Determine how rare the stamp is. The rarity of a stamp depends on how many copies of the stamp were produced to begin with. Stamps produced within the last 60 years are generally worth only their face value to mail postage because there are so many of them in existence. In some cases, a stamp's approximate age can be determined from the design on it, if the stamp was issued to honor a historical event at the time of the event. 2  Older stamps were also made with different grades of paper than more modern stamps.