There are several very interesting reasons why the back of the stamp is worthy of further investigation.
INK MEETS PAPER
The early stamps of both Britain and the Cape of Good Hope are printed on blue-tinted paper, which gradually becomes whiter over time (Figure 1).
The blued colour is caused by Prussiate of potash (potassium ferrocyanide). This was added to printers’ ink to make it singly fugitive, so the cancellations could not be cleaned off by petroleum-based chemicals. The effect disappeared thanks to the addition of inhibitors and a reduction in the amount of Prussiate, but not before it had created one of Britain’s most spectacular varieties. A detailed Ivory Head can be worth 50 to 100% more than a normal stamp (Figure 2).
Later attempts to increase security resulted in the use of aniline inks, which used a coal tar compound and were water-soluble. This ink soaks right into the fibres of the paper and can be seen from the reverse.
Stamps printed on the gummed side make highly collectable errors. Another effect occurs when a printed sheet of stamps has not quite dried when another lands on top of it. This is known as ‘set off’.
FIBRE CRUNCHING
From 1867 until 1870, the US Post Office took a sledgehammer approach to the problem of reused stamps: they treated the paper with a series of grills to break up the fibres and make the ink of the cancellation soak in. The first three grills were ‘points up’.
