4c Abraham Lincoln plate proof
During production of the Liberty Series, the Bureau of Engraving and Printing experimented with "dry" printing. Stamp paper was normally wetted to a moisture content of between 15% and 35%. This method of "wet" printing helped the paper to pick up ink from the printing plates. However, it also caused the ink to spread out on the paper, resulting in a blurred appearance. To remedy this, the moisture content was reduced to between 5% and 10% and the amount of pressure used to transfer the ink to the paper was increased. This made the ink fan out less, and the resulting image was sharper and clearer. The "dry" printing technique was used to create 4¢ Lincolns beginning sometime in 1958.