In 1931, Japan invaded Manchuria following the Mukden Incident ,
In 1931, Japan invaded Manchuria following the Mukden Incident ,
In 1933, issued a set of Airmail Semi-postals picturing early modes of flight (Scott #CB21-4,CB21a-24c). The set was issued in both perforated and imperforate form, in quantities of 7,420 and 6,210 respectively.
Chile began issuing souvenir sheets featuring designs in conjunction with the issuance of some of its more popular stamps in the 1950s, and many of the these souvenir sheets are quite scarce. Printed on thick card stock, these souvenir sheets were not intended for postal use, and should technically be classified as souvenir cards or presentation cards. Nevertheless, Scott, the other catalogs, and collectors of Chile treat the items as souvenir sheets. Scott notes these issues after the listings for the related stamps.Chile is considered the most "European" Latin American nation, and the proportion of stamp collectors within the population is moving towards European levels. Given a rapidly developing economy spurred by a government which implements sound economic policies, the future looks bright for this nation of 15 million. I expect to see continued increases in the values of better Chilean stamps and souvenir sheets.

In 1958, Panama issued a set of stamps (Scott #C199-202) and a souvenir sheet (Scott #C202a), commemorating the 10th Anniversary of the United Nations, which had occurred in 1955. Better late than never, I suppose. Both the set and the souvenir sheet are cheap (Scott Value for the Unused set is $ 4.05 ; for the Unused souvenir sheet: $ 5.25 ) and both had very low printings. Only 10,000 of the sets and 3,000 of the souvenir sheets were issued.

Excessive exposure to moisture can ruin a stamp's gum, or transform it into a meal for mold or insects. The best ways to avoid this threat are not to store stamps in damp environments or rooms in which flooding is a risk, and endeavor to keep stamps out of the rain.
Acidity poses both an internal and external threat. Some stamps are composed of paper which has a high acid content, and will turn yellow and brittle over the years if exposed to excessive moisture or light. In the vast majority of cases, this process can be avoided if the stamp is not over-exposed. Also, in the cases of used stamps and stamps issued without gum, much of the acidity may be removed by soaking the stamp in warm distilled water for several hours.
The external threats of acidity damage involve handling and storage. Always use tongs when handling stamps, because oils secreted from the skin have a high acid content. Stamps should be mounted using acid-free mounts in modern acid-free albums or stockbooks. They should be removed from old albums in which the pages are printed on paper which has a high acid content, so that the acid doesn't migrate to the stamps.
Excessive exposure to light should also be avoided, though fading probably will not occur unless there is extreme exposure to light over a prolonged period of time.
Extreme variation in temperature is another stressor, and should also be avoided. Storing a stamp collection in an attic which freezes in the winter and becomes an oven in the summer is a very bad idea.
All of these suggestions for preserving stamps are basically just common sense, and others that are equally valid may occur to you. It's best not to learn the hard way how a stamp may become more rare.