Using a Scanner to Extract Postmarks

Peter Aiken, Compulatelist

Have you ever been faced with a used stamp where you cannot quite make out the details of the postmark? Postal history collectors, who are interested in place names and dates, sometimes encounter this problem. If the postmark is smudged there’s not much you can do, but if the problem is that the postmark “blends in” with the stamp design the following two techniques may enable you to make out more details. It can be used with stamps off-paper as well as stamps still on cover.

The first procedure involves scanning the postmarked stamp and an unpostmarked copy of the same stamp, then subtracting the two images. In theory the stamp design will cancel out leaving only the image of the postmark. In actuality the technique is difficult to use and requires a steady hand, some experimentation, and lots of practice.

Here’s what to do:

  1. Mount the postmarked stamp and an unpostmarked copy of the same stamp in some sort of holder.
  2. It is essential that the edges of the stamp design be exactly parallel.
  3. Scan the stamps using a fairly high resolution.
  4. In your graphics program select an area that contains only the postmarked stamp.
  5. Copy and paste this selection to a new image.

Repeat step 3, this time for the unpostmarked stamp. Be very careful to select exactly the same area, with respect to the stamp, that you did for the first stamp.

Use your graphics program’s Arithmetic command to subtract one image from the other. In Paint Shop Pro the command is Image, Arithmetic. Use either the Subtract or Difference command.

The resulting subtracted image should show very little trace of the stamp design. If the design is clearly visible it probably means either that the two stamps were not parallel during scanning or the areas selected in steps 3 and 4 did not match exactly. Go back and redo these steps if required.

If the postmark is not immediately readable in the subtracted image, you may be able to improve it by adjusting the contrast and/or brightness.

This technique is based on the two stamps being the exactly the same, except for the postmark. This is often not the case, unfortunately. For example, a used stamp may have changed size slightly with respect to a mint stamp as a result of being glued to an envelope and then soaked off.

Also, colors may fade which prevents the subtraction from completely canceling the two stamp images. This technique does not always succeed, and you should not expect perfect results the first time you try it.

The second technique was told to me by Mr. Robert W. Hisey. It works best used when the postmark is black and the stamp design has little or no black in it. It involves using your graphics program to separate the image into its 4 color components: cyan (blue), magenta (red), yellow, and black. It is described in detail in the following section.

Lifting Postmarks and Overprints

Sometimes there is not a problem reading a postmark, but rather you want to “lift” the postmark to make a clean copy of it – in other words, an image of the postmark without the stamp. This is sometimes needed for overprints, surcharges, and the like as well as for postmarks. The technique described here can also be used in some situations for reading a difficult postmark. This technique works best if the postmark or overprint is black and the rest of the stamp is colored with no black elements in the design.

  1. Scan the stamp in true color mode. Here’s an example of a Canada Scott #51 with a great CDS.
  2. Canada Scott #51.

  3. Use your graphics program’s channel splitting command (or equivalent) to create a CMYK split. This creates four new grayscale images, each containing the color information for one of the four colors of ink (Cyan, Magenta, Yellow, and blacK) that would be used to print your original image. You need only the black channel, which contains a negative image of the black parts of the original scan.
  4. Canada Scott #51 negative image.

  5. Invert the image colors, using the Negative Image or Invert Colors command to replace black with white and vice versa:
  6. Canada Scott #51 with inverted colors.

  7. Crop and rotate the image as desired for the final product: In this example I have also done some retouching to “fill in” missing parts of the outer ring.
  8. Cancel from Canada Scott #51.

Detecting Forgeries and Printing Varieties

Scanning techniques can be useful in some situations to detect forgeries and printing varieties. The principal behind this technique is that the printed design on the forgery or variety will differ from the design on a normal stamp. By scanning both stamps and subtracting one image from the other, any differences should stand out. The technique is essentially the same as described for reading difficult postmarks. End of article marker.

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