Cambridge University Machin Colour Trials of 1969 - Part 1

[Note: This is the first of three posts summarizing articles written by Tony Walker, FRPSL, that appeared in The GB Journal, published by the Great Britain Philatelic Society. My thanks to Tony for permission to do these posts and to Mike Jackson, editor of The GB Journal, for the illustrations. Visit the GBPS web site to find out more about the society and how to purchase the journal back issues that contain these articles.]

Preparation for a new series of Machin definitives with decimal currency began in 1968, the year after the introduction of the pre-decimal series. The work was informed by two mishaps that needed to be corrected soon after the Machins were issued.

The first mishap was the loss of some of the desired three-dimensional effect in the portait used in stamps with a dark background. This was the portrait we now call Head A. A new negative was used to create a new version, now known as Head B. Head B eventually completely replaced Head A. See here and here.

The other mishap was the confusion between two very dark colours, the 4d olive sepia and the 5d royal blue. To resolve this problem, in 1968 the 4d was reissued in vermillion and the 8d, issued only six months earlier in vermillion, was changed to duck egg blue.

In 1968 Don Beaumont of the Marketing and Printing Section of the Post Office hired the Applied Psychology Unit of the Medical Research Council at Cambridge University to help him establish a palette of colours for the decimal currency definitives.

Sadly, much documentation from this effort has been lost, but with the aid of material held by the British Postal Museum & Archive (BPMA) and colour trials that have appeared on the market, it is possible to reconstruct what happened.

Trials were printed in 61 different colours, most of them denominated 8d, 1s or 1s 6d. The group of 61 included 11 colours that were used for pre-decimal stamps. Eight of the 61 colours are shown at the top of this post, and the complete set of 61 can be seen here.

The first step reduced the 61 colours to 40 based on these criteria:

- Some colours result in a weak signal from phosphor bands applied to the stamps
- Some colours vary unacceptably during the printing process
- Some colours are aesthetically unpleasing

The second step was to subdivide the 40 colours into 25 groups. The members within each group were similar to one another, but each of the 25 groups was distinctive from the other groups.

One stamp from 22 of these groups was used for further testing, along with three additional colours that were not part of the original 61, making a total of 25.

(To be continued)

--Larry