Along with never hinged gum, perfect, micrometer exact centering is another factor that collectors in recent years have been willing to pay a great deal of money to have. Like gum, it has sprouted a practice of re-perforating stamps with large margins down in order to obtain a perfectly centered stamp. Also, many stamps from the late 1800's and early 1900's were printed in large sheets of 4 or more panes of stamps that were guillotined apart, rather than being fully perforated, resulting in the stamps from the outer margins of these sheets having straight edges. The 1893 Columbian Exposition Issues and 1898 Trans-Mississippi Issues of the US, as well as most other US issues before 1930 are good examples. Collectors have traditionally shunned these stamps, due I think, to their uneven appearance. I believe that this is changing, as booklet stamps become more popular, since a sheet stamp with a straight edge does not look much different from a booklet stamp, but I digress.
Because of this preference for stamps with perforations on all sides, re-perforating is extremely prevalent on early US stamps, and Canadian stamps issued between 1922 and 1935 when Canada followed much the same practice as the US with regard to its stamp sheets. Another reason why many stamps have been re-perforated is to transform a stamp with a common perforation into a scarcer variety. An example is the 5c Large Queen from 1876, which is commonly perf. 11.75 x 12 and rarely found perf. 12.1. Many examples have been re-perforated to create stamps apparently perforated 12.1.
The marketplace is not kind to these stamps, valuing them typically as if they had the perforations clipped off, or are otherwise damaged. So, falling victim to this type of alteration is also very costly in the long run, and often comes as a nasty surprise when the collector goes to sell their stamps. Fortunately, there are ways to tell if a stamp is re-perforated.
Usually when a sheet of stamps is perforated, the spacing between the pins in the perforator is consistent, and the sheet being perforated is held relatively still while the perforating is being done. Consequently, the perforation teeth on opposing sides of the stamp will generally be perfectly aligned, so that:
- If you place a straight edge horizontally through two perforations on opposing vertical sides of a stamp, the angle of that straight edge will either be 0 degrees or very close to 0 degrees.
- If you place a straight edge vertically through two perforations on two opposing horizontal sides of a stamp, the angle at which that straight edge sits will be either 90 degrees or very, very close to 90 degrees.
When the stamp you are examining has been comb perforated, these angles will be perfect, as the perforating pins are laid out in a pre-set pattern. If the stamp is line perforated, the angles may be perfect, or they may be very, very close to perfect, but not perfect, as each column and row has to be perforated separately, and the paper can move slightly, or the perforator starts out in a slightly different position on each column or row. You can read about the difference between line and comb perforations in either the Philatelic Glossary section, or the section titled "Correctly Measuring Perforations".
When a stamp has been re-perforated, these perforations will be badly out of alignment, resulting in angles that are wildly different from 0, or 90 degrees. Also, the perforations themselves will lack the microfibers that perforations that have been separated from their neighbour through tearing will have. Of course there are some instances in which scissors have been used to separate perforated stamps, so this is not always a foolproof test. But it is a good starting point. A third test is to look at the depth and shape of the perforation holes, which are the valleys between the perforation teeth. Although the spacing of the pins on the perforating wheel or comb can vary, the size of these pins does not usually vary significantly, so that the holes on most perforated stamps should be approximately the same diameter, or in the case of single stamps, depth, as well as the same general shape. Re-perforated stamps will usually have holes that are the wrong diameter or shape.
Another way to check for re-perforating is to carefully measure the gauge. On some issues there are known variations in the spacing of perforation pins such as the early issues of stamps printed by Waterlow and Sons, from North Borneo, Labuan, Niger Coast Protectorate and Costa Rica, to name a few countries. However, on most issues, the gauge will be consistent on all sides of the stamp, or at least on two opposing sides. If you measure the perforations on a stamp and find significant differences in the measurements of two opposing sides, chances are very good that you are dealing with a re-perforated stamp, unless the issue is known to come in several different perforations, due to significant differences in the spacing of the perforation pins, as discussed.
Let's take a look at an example. The stamp below is re-perforated: