Grill

under construction
Beginning in 1867, US stamps had grills applied to absorb and hold cancel ink and prevent re-use. They are an embossed pattern in various sizes and orientation. This was an idea adopted by the N ational Bank Note Co. who would later use it on stamps of Peru from 1874 to 1884.

A problem with the idea was that grills could not be consistently impressed into the paper – too hard and the stamp would be seriously damaged, too light and the grill would not work as intended. So, the different grill types represent a continuous string of experiments until re-use became less of a concern by about 1876.

The grill pattern is a series of squares enclosing a hip roof type of pattern. the tops of those "roofs" are lines that usually show a direction, vertical or horizontal. If that pattern and detail is not visible, the likelihood is high that a forgery is present. There is also a size and orientation for the various types, the latter being points (rooftops) pointing towards you ("up") when viewing stamps from the back, the points pointing away from you ("down") otherwise.

With time and production flaws, the pattern is not necessarily uniform or clear. Also, weak impressions do exist, appearing to be errors without grill, or fraudsters have ironed out grills, weak or not, over the years. Just because you have, say, an 1869 issue without grill does not necessarily mean you have an 1875 reproduction – other tests have to be met like paper and color.

The types are as follow:
 * A grill. The grill covers the whole stamp. This makes the stamp very fragile with intact examples being rare.

 File:Grill AAA.jpg|front view 

Measurement
The two measurements that are given above must agree to properly identify a grill type. An old method was to take graphite from a soft pencil and lightly rub it on the back of a grilled stamp to make the grill stand out for measurement – traces may be found on stamps today. This is a rather stupid thing to do today. It is easier to immerse the stamp face down in watermark fluid, make an image of it. Embiggen the image to make it easy to count the points.

There is also the Sonic Imagery Labs Precision U.S. Specialty Multi-Gauge that can be overlaid on stamps to measure grill sizes.

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