In early 1855 Perkins, Bacon & co. found that some of the plates used in printing the 1d. and 2d. stamps were suffering from corrosion, the cause unexplained.
Plate 4 of the Two Pence, at press at the time, was one of the plates so affected by this corrosion. The effects were represented on the stamps by colored dots and smears. These printed as the corrosion cause small areas of roughening on the polished surface, rendering this disturbance retentive of the ink.
Another development which some attribute to this corrosion was observed in the later printings. These took the form of short parallel markings mainly found in the marginal spaces between rows. In Plate 4 they are always of vertical orientation or N/S striations. One conclusion is that they may be caused by the original grinding operation showing through as the polished surface deteriorated.
Whatever the cause, the progress of the corrosion is a good indicator of when the particular impression(s) came from the press.
A strip of three, lettered "JD", "JE" and "JF" are shown with progressing states of corrosion.
As the imperforates were pressed before corrosion began, they are the 'base' form of the stamps.
Stamps with perforation 16 were issued in March 1854 (operations began 31st January for this plate) and perforation 14 followed nearly a year later, being issued in late February 1855.
As Plate 4 was withdrawn in mid 1855, stamps showing corrosion, particularily those in perforation gauge 14 are from the final few months of printing.
All printings of "JF" show a mark in the upper white line, right of the "E" of "POSTAGE".
Imperforate - Printed 1849-1854
Perf. 16 - Printed 1854-1855
Perf. 14 - Printed in 1855
Mis-Aligned - Perf. 16
Although the corrosion was found across the plate, the heaviest effects were on the right side (left side of printed sheet).
This is an early printing for perforation 16 with some corrosion beginning in the letter squares, best seen on the "IK" portion of this markedly misperforated copy.