British stamps explained: the introduction of phosphor and automatic letter facing equipment
Today we accept phosphor as a normal feature of a British stamp.
When subjected to ultra-violet light, the phosphor gives a short after-glow that can be detected. This enables equipment to turn envelopes so that the stamp is positioned in the top right hand corner, ready both for cancellation and for the address to be read.
The initial trials of automatic letter facing (ALF) equipment were conducted in the Southampton area using stamps with black lines printed on the reverse. However, work on developing a machine that could detect the location of stamps on envelopes had begun in 1937. It entailed optical scanning to identify in which corner of the envelope was the stamp. Understandably this work ceased during the War.
Soon after, the Post Office established a Mechanical Aid Committee, which first met on 24 May, 1946, although its progress was slow as, following the War, the priority of the Post Office was the telephone service.
The development work was undertaken at the Post Office Research Laboratories at Dollis Hill, and by 1947 the general concept was put forward that ‘letter collections would be fed, via hoppers, into a mechanical segregator and facing machine, whence the letters would pass automatically through a stamp cancelling machine then be fed automatically to a machine sorter who would read the address and either code for subsequent mechanical sorting, or code and sort in a combined process for delivery to the appropriate destination box’.
By the following year photo-electric scanners were being tested that could ‘detect the stamp on each letter or postcard, and, by turning the items where necessary, place them all in the same relative position ready for the stamp cancelling process’.
By September 1949 a machine having four scanners to check each corner of an envelope was being live tested at Mount Pleasant, using reflectivity to distinguish between the colour of the stamp and the colour of the envelope. It could separately detect whether items bore the red 1d printed paper rate stamp. It worked at 320 items per minute, but could not turn envelopes over, nor did it simultaneously cancel the items. Difficulties arose when stamp and envelope were of similar colour, or labels other than stamps were affixed to the envelope.
Other ideas considered for detecting the stamp included adding a metallic foil to the face, treating the stamp paper with zinc, aluminium or ferric oxide, or even adding a small quantity of table salt so that the stamp would become a good conductor of electricity.
In the end the option chosen was to use graphite inks printed on stamps: initial thoughts were to print as a frame around the stamp design, or as two lines either side, but the chosen option was to print on the back of the stamps under the gum. The machinery, known as ALF, was thus able to detect the stamps, turning the envelopes if necessary, before cancellation and stacking: unstamped mail was segregated.
Live testing was planned for late 1956 at Liverpool, but the location did not have sufficient space. So Southampton was chosen, at the end of 1957 to include the Christmas period. The ‘graphite-lined’ stamps were issued on 19 November and ALF was put into use on 7 December, although the ‘official’ start date was 19 December in the presence of the Postmaster General.
The trials were not a complete success, so work continued to find an alternative to the graphite lines. This came with phosphor, less visually conspicuous and less costly to produce the stamps.
Information for this article has come from files within The Postal Museum.