Flat Plate Vs Rotary Vs Offset - Printing Methods for the Classic U.S. Stamps

The three methods employed in the printing of the Washington Franklin Heads were: the flat plate press, the rotary press and the offset press. All designs and denominations of the Washington Franklin stamps were printed using the flat plate method. Only the 1¢, 2¢ and 3¢ stamps were printed using the offset method and only the 1¢, 2¢, 3¢, 4¢, 5¢ and 10¢ stamps were issued using the rotary press method. The Washington Franklin rotary press printings were primarily for coil stamps, although some interesting experimental and "coil waste" stamps, stamps from leftover coil stock, were issued fully perforated. The following information applies to the Washington Franklin stamps, as well as the Series of 1922 stamps and certain commemorative stamps of the era, e.g. the Harding and Edison stamps.

In order to better understand the differences in these three types of printing methods, it would be helpful to have a little background knowledge regarding how each of the methods worked. In all three methods plates typically had between 150 and 400 stamp images, depending on the size of the stamp and the quantity needed. The vast majority of the stamps were printed on 400 subject plates.

An engraving method was used in both the flat plate and rotary press printings. This is a somewhat complicated process that takes advantage of the fact that steel can exist in both a "hardened" and a "soft" state. First, a small block of steel, the die, is softened. This soft metal die is then etched by the engraver or engravers, with the design of the stamp in reverse. This die is then hardened. In the meantime, a small cylinder of steel is softened, this is often termed a "transfer roller". In its softened state the design can be applied by rocking the softened roller under great pressure over the hardened die. The design on the transfer roller is in normal orientation and in relief, that is raised above the surface, since on the die the design was recessed and reversed. This transfer roll is then hardened. In the meantime a sheet of steel that will become the "plate" is softened. The hardened transfer roll with the design of the stamp in relief is then rocked into the softened plate, again under great pressure, to "lay down" the image of the stamp, below the surface of the plate and in reverse. This is done as many times as is necessary to fill the plate with as many images of the stamp as is desired. Finally, the plate is hardened, inspected and prepared to print stamps. Note that the engraving method was not used to manufacture the offset stamps.

The flat plate method employs a hardened plate with the design of the stamps "engraved" below its surface. Ink was applied to the plate in an economical fashion by rolling a saturated ink roller across the surface of the plate. Ink naturally settled into the grooves of the design, but also adhered to the surface of the plate and so the plate was then wiped to remove any ink that was not in the grooves. The ink within the grooves, bearing the design of the stamps, was then transferred to a sheet of slightly dampened paper, in a sense laying the ink on top of the paper. This is an important feature of the engraving process since the raised ink can be felt, a security feature still used on currency today.

The rotary press method: All of the features of engraved printing mentioned above apply to the rotary press method; the excess ink needed to be wiped off the plates, the paper needed to be slightly dampened, and the ink lay above the surface of the stamp it was printed on. The rotary presses in general employed two curved plates, very similar to the plates used in the flat plate method, only a little more flexible. The two curved plates were placed on a cylinder with only a tiny gap (or joint) between the plates. The process involved inking, wiping and printing as the cylinder revolved and a large roll of paper was pulled through the printing apparatus. Since typically two plates were on the cylinder, each revolution of the cylinder produced two plates of stamps, for example in the production of sidewise coils (with 170-subject plates) each revolution would produce 340 stamps, with a joint line every seventeenth stamp.

Why are rotary press stamps wider or longer than their flat plate counterparts?
Rotary plates are curved around a cylinder, while flat plates are of course "flat". It is this curving that increases the length or width of the stamp design. The stamp designs were wider if the direction of the curve was in the sidewise direction, for example on the 170-subject plates used to make sidewise, i.e. horizontal, coil stamps (coil stamps connected to their left and right neighbors). The stamp designs were longer if the direction of the curve was in the lengthwise (endwise) direction, for example on the 150-subject plates used to make endwise, i.e. vertical, coil stamps (coil stamps connected to their top and bottom neighbors) .

This knowledge is of paramount importance to philatelists who wish to separate varieties of stamps. For U.S. collectors, many of the rarest 20th century stamps are examples of these wider or longer rotary press stamps given the perf 11 perforations of the flat plate stamps. Knowledge of this "stretching" is also important in separating coil varieties and is quite useful in ferreting out fake coil stamps manufactured from stamps of the incorrect dimensions.

Offset stamps, on the other hand, were not engraved. The Washington Franklin offset plates were prepared by taking a photograph of a proof made from the die used to make the flat plate and rotary stamps, enlarging this photograph and retouching it until all the lines were crisp and sharp, taking a negative of this touched-up photograph, reducing the negative to normal size, and finally transferring the negative to a sensitized plate one stamp at a time until the desired number of stamps per plate was obtained. A negative celluloid mask was made from this sensitized, positive plate, which in turn was used to manufacture the offset plates. It may sound complicated, but it was actually much easier than making the engraved plates. The key to the offset method is that on the printed surface the ink lies flat. Thus, all offset stamps will have a smoother feel; the ink will never be raised as with an engraved stamp. The process also resulted in a lack of fine printed lines, giving the offset stamps a somewhat blurry appearance. Many offset stamps can be identified merely by the poor quality of the printing.

Distinguishing the Flat Plate Vs Rotary Press Stamps - A Matter of Size

There are two conventional ways to determine whether a stamp was printed using the flat plate or rotary press method: measure the design with a ruler or calipers or compare the design size with a known stamp, sometimes called the template method.

Using a Ruler or Caliper - Measuring the height and width of the frame with a ruler would seem to be the obvious way to determine the size of the design. A set of calipers is often used for more exacting measurements. The design on flat plate printed stamps measures, on average, 18.5-19 mm wide by 21.5-22 mm high, but can sometimes measure close to 19.5mm wide or 22.5mm tall and this can cause confusion. Rotary press printed stamps are either slightly wider or slightly longer, sometimes both wider and longer, either 19.5 mm or more wide or 22.5 mm or more high.

However, this method may prove difficult for several reasons. The distances we are dealing with are very small, a tenth of a millimeter is a very short distance. In fact, of the three rulers in my desk drawer, none has accuracy greater than millimeter subdivisions and all are woefully inadequate for the exacting measurements required here. Inspection of my perforation gauges shows more hope, these have subdivisions of .5mm, but still not as accurate as needed. To further complicate the issue it appears that none of my measuring devices exactly matches the subdivisions of any of the others. Even my trusty and worn cardboard perforation gauge measures 50mm as being a full .75 mm longer than my metal ruler! We invite the reader to inspect their measuring devices and feel fairly confident that similar results will be found. This does illustrate one of the problems in taking the measurements.

To further complicate matters, no two people measure exactly in the same manner. For example, does one start with the beginning of the hash mark, the middle of the hash mark, or the end? In our case, the width of a hash mark, nominally .2mm to .4mm, can have a profound effect on the resultant measurement.

We therefore strongly recommend forgoing the ruler measurement in favor of the template comparison described below.

The Template Method - An easy method to sort flat plate and rotary press stamps in quantity is to cut a damaged common flat plate stamp in half and use it as a template; perf 12 Washington Franklins make good examples since they will always be flat plate. You will need two templates, one cut horizontally and one cut vertically, since the rotary press stamps can be longer in either direction, or you can make a single template with four cut corners, as in the picture below. Note that you can use any flat plate stamp of the same size as a template, it does not have to be of the same denomination or color, only the size matters. You might also want to make a rotary press template if you have a few beat up rotary press coils to spare. If you feel uncomfortable cutting up your own stamps, with minimal diligence you should be able to pick up a small packet of Washington Franklin stamps for a dollar or two on eBay. You can rest fairly assured that none of the stamps in such a mixture are those rare rotary coil waste stamps, but it will in all likelihood contain a few rotary coil stamps that can be used as templates.

Examples of Templates

Important caution: Insure that the stamp being considered as a template has the proper measurements; we would recommend close to 19 mm wide and close to 22 mm tall for a flat plate template and more than 19.5 mm wide or more than 22.5 mm tall for a rotary press template. Due to variances in shrinkage in the printing process, it is possible to find flat plate stamps somewhat wider than 19.0mm or somewhat taller than 22.0mm. It is suggested that these larger flat plate examples not be used for templates, unless as proof positive that the stamp in question is rotary - if the stamp is somewhat larger than this "large" example, it must be rotary. On the other hand, don't use a particularly small flat plate example either or you will get too many false positives for rotary. Try to find a nice 18.8-19.2 x 21.8-22.2mm stamp to use as your flat plate template.

Template Placed Over Top of Stamp
Stamp in question is Wider Template Placed Over Right of Stamp
Stamp in question is Taller

How to use the template - simply place the template on top of the stamp in question. If the design is similar in size it most likely is that printing method. In the examples shown the white line of the frame of the larger stamp is clearly outside the white line of the frame of the template stamp. Note that there will be slight differences in size, most often due to paper shrinkage during the printing process. Remember too that this method is a quick and dirty method of sorting flat plate and rotary press stamps, you should still verify any stamp by actually measuring it.

The Grid Method - With the seeming ubiquity of scanners we would propose an alternate method of measuring the design. Simply scan the stamp at actual size, turn on the grid feature in your photo editor, set it to millimeter increments, and count the number of grid lines. Once you have the general number of divisions, zoom in on the two ends of the design to get even more accurate readings, with a little addition and subtraction you can get amazingly accurate readings assuming your scanner is calibrated to give actual image size. If you count the divisions in the stamp above you will note that the design size measures approximately 19mm by 22mm. We have the advantage of measuring the image using the zoom method in our photo editor and have determined the "exact" measurement to be 18.75mm by 22.0mm, certainly accurate enough to make the determination that this stamp was printed using the flat plate method.

You can verify the calibration of your scanner by scanning a metric ruler as a bitmap, ".bmp", image at actual size and measuring the image of the ruler with your photo editor's grid. There should be a one-to-one correspondence; if not, determine the "scaling factor" by dividing the size read from the grid by the actual size of the ruler itself. You will need to use this scaling factor on all subsequent "grid-method" measurements.

On the surface, this may seem nearly as difficult, possibly even more so, than simply measuring the stamp with a ruler. However, this technique can prove invaluable if you are considering purchasing a stamp via the internet, particularly if the stamp was scanned at actual size. Even if the stamp was not scanned at actual size a comparison can be made with another stamp from the same seller; the grid method should provide some insight.

Distinguishing the Offset Stamps

The offset method of printing resulted in some very poor quality stamps. It is rare to find an offset stamp with anywhere near the color depth or crispness of image found on the flat plate and rotary press stamps. In most cases a stamp can be identified as offset just by looking at it, the color and printing quality are that poor.

Sometimes visual inspection is not enough and another method of determination must be made. The offset method does not employ engraving. This means that the stamp feels flat as opposed to the flat plate and rotary stamps. Don't let the "flat" in "flat plate" fool you, engraved stamps have raised ink on the paper surface and flat plate stamps are engraved.

Some collectors can feel this engraving or lack thereof by merely running their fingers across the stamp, but many of us can't. A semi-soft nib may be ran gently over the surface, particularly in the area of several parallel lines. This must be done very carefully in order to not scratch the ink from the stamps surface or damage the surface of the stamp in any way. Often a pair of stamp tongs is used for this determination. The tongs will glide across an offset stamp, but bump across engraved stamps.

The "Thin Foil" Method

Another favorite method is to use a piece of aluminum foil, or other thin foil, the thinner the better. Place the foil over the stamp in question and rub your finger very gently over the foil. The frame of the oval portrait and other details will show on the foil on flat plate and rotary stamps, but will not show on the offset stamps. Again, care must be taken not to damage the stamp by rubbing too hard on the foil. Gentle pressure should bring out the features on the engraved stamp. Practice these techniques on some of the more common varieties and you will soon be an "expert" at separating engraved and offset stamps.

more... both the offset 2¢ and offset 3¢ stamps have distinguishable design types.

More on Engraving (Used in the Flat Plate and Rotary Press Methods)

The flat plate and rotary press stamps employed an engraving technique in which a pool of ink settled into the engraved grooves on the printing plate, resulting in an actual 3-dimensional transfer of ink to the paper when the ink was, in a sense, laid on top of the paper. This is the opposite of typography, a more common method of printing, in which a raised design or lettering is inked and then impressed into the fibers of the paper - think of a typewriter. Although engraving is a more expensive means of printing, it is most often done to thwart counterfeiters. Securities of all types are printed by engraving, and most paper currencies to this day are engraved. For this very reason, forgeries of the Washington Franklin flat plate and rotary press stamps made by drawing in the missing design feature are actually quite easy to detect. When the design is drawn in by the forger, the ink lays flat against the paper and will not create an impression on thin foil gently pressed against it. If you should run across a stamp that is more valuable than its unaltered counterpart, it is a good idea to check the design area in which alterations may have been made. This is particularly true for Types Ia and II of the 2¢ stamp. A familiarity with the design features of all engraved 2¢ Washington Franklins would be of great aid in this matter.

Good luck on sorting your Washington Franklin stamps. We hope this helps.

Bibliography and suggested further reading:

Printing Postage Stamps by Line Engraving by James H. Baxter. Printed by the American Philatelic Society

The Development of the Rotary Press Printing by Max G. Johl. An article in The Stamp Specialist Volume 1: Part 1, Published 1939

United States Stamps 1922-1926 by Gary Griffith. Published by Linn's Stamp News.

Fundamentals of Philately by L. N. and M. Williams. Published by the American Philatelic Society