Christmas Seals & other Charity Seals
This is an impression on the gum intended to prevent curling of the paper. In some sheets, many seals may show only a faint impression or none at all (sometimes called flowed gum). Reflecting light off the gum at a shallow angle may make the breakers more visible. Three types of broken gum have been used:
HBg (Horizontally Broken Gum) shows lines crossing at oblique angles forming diamonds wider than they are tall (used by Eureka between 1919 and 1931)
SqBg (Square Broken Gum) shows a web of lines crossing at right angles forming squares (used by Leonhardt in 1908 and Eureka in 1922)
VBg (Vertically Broken Gum) shows lines crossing at oblique angles forming diamonds with the long dimension of the diamonds vertical to the seal design (used by Eureka between 1922 and 1932)
This is also sometimes called "tropical" or "dry" gum and was used beginning in 1963 on some seals intended for Puerto Rico and other humid climates. Seals with this gum appear to be ungummed, though the tiny gum crystals (rather than paper fibers) can usually be seen under magnification. Blue coloring is sometimes added to assure users that gum is actually present.
This has a frosty or satin appearance. It is shinier than dull gum but less reflective than traditional shiny gum.
These seals may be used without moistening. They are issued on backing paper, sometimes with slogans or other information printed on the backing paper. They were first tested in 1972. Self-adhesive test designs, Spring seals and Christmas seals have been issued since 1989.
Shiny and smooth gum should be assumed if it is not specifically mentioned. See also the above note on Broken Gum.
Green’s Catalog of the Tuberculosis Seals of the World, Part I, U.S. National Christmas Seals, 2010 ed. p.4