The Penny Black was a revolutionary creation. The world's first postage stamp, it set a precedent that has forever changed the world's postal systems.
Famously, it came as a result of Sir Rowland Hill's 1837 proposals to reform the British postal system, which at the time employed a post-pay system in which the recipient would pay for their mail on delivery.
However, just a month into its use in 1840, the stamp's creators were made aware of a problem. Several of the red "Maltese Cross" cancellations, showing that the stamp had been used, had been removed from the stamps by cunning members of the public, who immediately spotted an opportunity to outwit Hill's system.
Shocked by the idea that his beloved creation could be so easily misused, Hill initiated an exhaustive series of tests that would determine how to overcome the problem, experimenting with a range of inks and printing techniques. These became known as the Rainbow Trials.
It was decided that the black ink used for the 1d stamp was too permanent, and that the cancellation ink should be more lasting, so that the stamp's design would be damaged far sooner than the cancellation could be removed.
The Rainbow Trials tested many colours and different types of stamp inks, all of which were intended to be less permanent than the original black. The concurrent obliterating trials tested different inks for cancellation, analysing each one to see if it could be removed without harm to the stamp.
Plates and states
The first trial plate contained just three impressions, which were printed without corner letter and with the top right corner of the plate plugged with wax to void the resulting proofs. Impressions from this first trial show the "O" flaw, a transfer roller flaw that can be seen on the later stamps from plate 7,8,9 and 10.
The second trial plate displays 12 impressions, which exist in three states. It was used for experimenting with printing inks, cancellations inks and a variety of papers, and it was the tests using this plate that resulted in the stamp colour eventually changing from black to red in February 1841.
Identifying proofs printed from these plates can be tricky. Second plate "state one" proofs are printed in black, orange-red, reddish-brown or deep blue on a variety of papers.
The second state of this plate is almost identical, aside from a small mark in the bottom right hand corner square of the second stamp of the third row.
The third state proofs are more easily identifiable from the rough edges of the voided corners.
Fugitive ink trials
As well as experimenting with inks, papers and cancellations, the Rainbow Trials also saw tests with different chemicals that could improve security and help prevent forging, beginning in September 1840.
Many of the proof sheets used ink mixed with prussiate of potash, or potassium ferrocyanide, which would react with any chemical used to remove cancellations and immediately turn the paper blue, allowing postal workers to easily identify a reused stamp.
It is this chemical which gives us the blued paper found on a number of early stamp varieties. It is also used as a pigment, creating the Prussian Blue colour employed for the rare King George 2½d stamp of 1935.
The trials also experimented with tartaric acid, potash, prussic acid, creosote and turpentine.
Collecting Rainbow Trials proofs
With many different varieties, proofs from the Rainbow Trials provide a fascinating collecting area, charting a crucial point in both Great Britain and the world's postal history.That's not to mention the fact that most are annotated in the hand of some of the most important figures in the development of adhesive stamps, such as Joshua Bacon and Rowland Hill.
The proof sheets are very rare, and many examples are housed in the Royal Philatelic Collection. However, opportunities remain on the market for the collector to acquire and build a collection.
Paul Fraser Collectibles has one of the finest selections of Rainbow Trials proof sheets for sale, including some of the scarcest and best condition examples available on the market.