Prior to the issue of any surface-printed postage stamps—of which this value was the first to be created—the Inland Revenue authorities were supplying adhesive stamps, for various fiscal purposes, nearly all of which were intended to be obliterated with pen-and-ink : these were consequently printed on a so-called “ safety ” paper, into which, while in a state of pulp, some chemical ingredient had been introduced, so that any attempts to remove the cancelling mark might be immediately detected.

To effect this, a small amount of prussiate of potash (Potassium ferrocyanide) was mixed with the pulp whilst still in the vat ; and the paper, though it possessed to a certain extent the desired properties, was discoloured by a blue tinge, varying in its intensity from a deep shade to almost imperceptible traces of colour.

As can be seen from the image above, this was not the first use of prussiate of potash, it was experimented with during the rainbow trials.