Research for Writing on Cape of Good Hope Triangles

For many years collectors supposed that their blueing was due to the action of the gum on the printing inks or on the paper, but it is clear now that the cause lay in the introduction of potassium prussiate, the old name for potassium cyanide, or ferrocyanide, into the inks or the paper. The purpose of using this chemical was to make it less possible to clean off a cancellation or postmark so that the stamps could be used again through the post. Loss to the revenue from such a practice and from forgeries was an obsession with the British Post Office of those days…

The addition of the cyanide made the print fugitive, so that if the cancellation were removed by other chemicals the print would be so affected as to show that the stamps had been tampered with. The blueing resulting from the use of cyanide was not intended. The apparent colour of a stamp is affected by the extent of blueing.

Bacon considered that damping the paper caused the prussiate to spread over it and that, as the sheets were not always of the same degree of dampness, the blueing varied—the wetter sheets becoming more deeply blued than the drier ones. Subsequent gumming of the sheets moistened them again and may have increased the spread of the blueing.

This explains the formation of the ivory or cameo heads and blue frames seen on the backs of some stamps—the lines of engraving around the Queen's head of the British stamps and the figure of Hope of the Cape stamps being finer than the rest of the design and holding less ink and prussiate—the result was that the centre and gutters were white areas amid the blueing.

The "size" of the paper was particularly variable in its chemical composition and was considered to be the cause of the transformation of the blueing into a greenish shade. Yellow gum might have the same effect. The paper was sometimes blued in spots- an effect that was put down to imperfect washing out of chemicals after bleaching the rags from which it was made.