The Wet and Dry Printing Methods on U.S. Stamps

By Bob Allen

The Bureau of Engraving and Printing constantly experimented in search of a better and more cost effective product. Dry-printing came about as the result of such innovation. The traditional, wet-printing process used paper containing a moisture content of from 15% to as much as 35%, while the newer dry-printing method used paper with only 5% to 10% moisture content. This dry-print process required much higher pressures and fast-drying inks and a thicker and stiffer paper to accommodate this.

The first U.S. dry-prints were the multi-color Overrun Country commemoratives of 1943-1944, printed by the American Bank Note Company. The dry-print method held promise for printing bi-color stamps cost effectively and it is perhaps not a coincidence that the first dry-print stamp issued by the Bureau of Engraving and Printing was the 8¢ bi-color of the 1954 Liberty Issue. This eight-cent Statue of Liberty stamp was issued on April 9th, followed shortly thereafter by the Lewis and Clark commemorative, the $2 Duck stamp of 1954-1955, and the red violet $1 Presidential bi-color stamp.

The flat plate stamps used the same presses for both the wet- and dry-printings, while the rotary press printings used a Stickney press for the wet- printings and new experimental presses for the dry-printings. In all cases however, a much greater pressure was needed to get the ink to print properly on the drier paper, resulting in the surface of the dry-print being harder, smoother, and shinier than the duller, rougher surface of the wet stamp. This has been the traditional method for separating the wet and dry stamps, but has been shown by Bob Rufe to break down when separating the flat plate wet and dry stamps, in particular the Special Handling stamps (see the United States Specialist, Oct.-Dec. 2007 journals). It turns out that although many of the Special Handling stamps printed in the 1940's have a very crisp look and a color not usually associated with wet-printings, they fail the size and flick tests described below. Further, this was a decade or more before the dry-printing method was implemented. Thus, many of the wet-printing flat plate Special Handling stamps have been erroneously classified as dry-printings. These stamps, the 10¢, 15¢, and 20¢ Special Handling stamps, are now listed in Number as QE1-QE3 for the wet-prints and QE1a-QE3a for the dry-prints. For many years, the listing order had been reversed, that is the wet-prints had been listed as the variety, QE1a-QE3a. The scarlet $5 Postage Due stamp is still listed anachronistically in the 2010 Number Specialized, with J78b for the earlier wet-printing and J78a for the later dry-printing. It is a certainty that many of the flat plate stamps now labeled as dry-prints are in fact wet-prints, meaning it is very probable that the dry-printings are scarcer than their catalogue pricing may suggest.

The gum on the flat plate wet-printings was applied by the Bureau after printing the sheet of stamps, while the sheets used for the flat plate dry-printings were pre-gummed. The wet-printings, with the gum applied after the printing, have a thicker layer of smooth gum that may appear slightly yellowish and show grooves, whereas the pre-gummed sheets used for the dry-printings, with gum applied by the paper manufacturer, is nearly clear with a rough texture and never shows the cracks and grooves of the BEP applied gum. The rolls of paper provided for the printing of rotary press stamps were not pre-gummed, so any difference in gum will not distinguish wet- and dry- rotary press printings. The fact that the gum was not applied until after the printing on the wet-printings on flat plate stamps had the inadvertent side-effect of adding offsets to the back of sheets placed on the stack above the newly printed sheet. The sheets of thicker, stiffer paper used with the flat plate dry-printing stamps were pre-gummed by the manufacturer, limiting any offset to the gum on the sheets above and not to the stamps themselves. Thus, used flat plate stamps with offset marks have a high chance of being wet-printings.

The definitive test for any of these stamps is to measure the paper thickness with a sufficiently accurate micrometer, since the high pressures involved in the dry-print method required a thicker, stiffer paper. Many collectors can distinguish the thicker, stiffer paper by applying what is known as the flick test. Holding the stamp gently along the long edges, gently flick the stamp. The sound and feel of the resulting snap is sometimes enough to tell whether the stamp is on thick (stiff) or normal paper. However, this can be dangerous, an over exuberant flick can cause irreparable damage to the stamp. Since most collectors do not own a micrometer, perhaps the safest test for flat plate stamps is to compare the design size. This can be done using a template stamp or in your photo-editor by overlapping scans of a known wet- or dry-printing with the subject stamp, a method familiar to those who distinguish rotary and flat plate printings.

There are other methods that will verify whether the printing method used was "wet" or "dry" and they are listed here.

Wet Printings Dry Printings
1. Thinner paper - from .0030 to.0034 inches 1. Thicker, stiffer paper - from .0037 to .0042 inches
2. On flat plate stamps, the design of the wet-prints measures smaller than dry-prints by as much as  .25mm in the direction of the grain of the paper 2. On flat plate stamps, the design of the dry-print measures larger than wet-prints by as much as  .25mm in the direction of the grain of the paper
3. The gum is slightly yellow, thicker, and is smooth, but often shows grooves 3. On flat plate stamps, the gum is nearly colorless, thinner, rougher and does not show grooves 
4. Unused stamps can be examined from the back in watermark fluid; since the gum is thicker, the wet-print stamps will appear more opaque than the dry-print stamps 4. Unused stamps can be examined from the back in watermark fluid. Since the gum is thinner on dry-printing flat plate stamps it will appear more transparent than the wet-print stamps
5. Offsets on the gum side are common and may be significant (flat plate) 5. Offsets on the gum side are unusual and insignificant if present 
6. There is an overall tone in the color of the stamp (both flat and rotary) 6. On rotary press printings there is very little overall tone. This is not a factor for the flat plate stamps.
7. Dull, rough printed surface 7. Hard smooth sheen on rotary printings
8. Printing is usually softer, but may be as sharp as the dry-printings 8. Printing is nearly always sharp and crisp (particularly on rotary press)

Wet Printing                                            Dry Printing

The above illustrations clearly show the difference in color and toning between the wet- and dry-printings on the rotary press stamps. Because the dry-printings used a quicker drying ink of different composition, there will sometimes be a noticeable color difference between the wet- and dry-printings of the same stamp.

Suggested further reading:

U. S. Special Handling Issues - Robert G. Rufe in a series of articles from Oct.-Dec. 2007 in the United States Specialist (USSS)

Size Differences Between Wet and Dry Printings from Flat Plates - Wallace Cleland in the Nov. 2003 United States Specialist (USSS)

New Stamp Varieties Due to Manufacturing Changes, et al - A contemporary series of running articles on the dry-printing method - George W. Brett , Sol Glass and Norman W. Kempf - The Bureau Specialist - Volumes: 25-30. Printed by the Bureau Issues Association (USSS)

Q and A Corner - David G. Lee - p. 498 in the Nov. 1987 United States Specialist (USSS)