All sorts of things turn up in boxes of old photographic equipment. Over the past year I have been buying daylight-loading developing tanks for my other blog/website which is intended to be a resource centre for present-day users of these tanks as well as a source of information on their history.
In an Essex 35 tank that arrived I found a cardboard calculator for Unitol developer. Unitol was made by the Johnsons of Hendon Ltd. It was introduced in late 1950. The BJ Alamanac included Unitol in its 1951 edition:
The essential new feature of "Unitol" is that, unlike most fine-grain developers, it may be diluted within wide limits so that it becomes possible to use it with the utmost economy – and simplicity – by following the slogan "One ounce; one film; use once; then discard". It is reckoned that one ounce of the concentrated solution is necessary for the proper development of one film, and that it is by then exhausted for practical purposes…Quite apart from this convenience, however, "Unitol" is a fine-grain developer of the highest grade…A most convenient calculator and film group chart has been prepared for use with "Unitol". Sold at the nominal price of 4d, it enables the time of development for any dilution, at any temperature, of any film to be immediately calculated by the rotation of a disc.
"Unitol" was extremely popular, at least in UK. I am pretty certain that the main active developing agent was Meritol. All the clues (sold in liquid form, not compatible with Johnsons 142 retarder, half-a-stop loss) suggest Meritol, and, as far as I am aware, the formula of the commercial mix has never been published. Meritol was Johnsons own patented agent. Another clue is that the Johnson advertisement in the same edition of the BJ Alamanac states, After long research Johnsons are now able to present a concentrated liquid fine grain developer. Meritol itself was patented in 1937 (applications made in 1936). Two patents (GB466625 and GB466626) cover the chemical synthesis and the formulation of a developer (essentially Johnson's Superfine Grain). Meritol was invented by Frank Clement Starnes, who worked for Johnsons. Starnes had died in 1945, aged 62, and I wonder whether his death, together with getting the company running properly after the war, was responsible for that statement, After long research.
Meritol was formed from paraphenylenediamine and pyrocatchin (pyrocatechol, catechol), each a developing agent. The additive product is a new compound with different properties from each of its constituents. Claims that the two components can be used instead of Meritol are not, therefore, accurate. Research on Meritol continued. I even found a paper in a chemical journal from 1989 that shows the distinct structure of Meritol.
Finding the Unitol Calculator (which would now of course be launched as an app) was a reminder of that 1950s mantra: Ilford Film; Kodak Paper; Johnsons Developer. It has to be said though that D76/ID11 caused wavering with the last part.
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| Johnsons of Hendon advertisement in BJPA 1951 |
The Unitol Calculator:
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| Centre was pushed out to form the central disc of the calculator |
Finally, the headings of the Starnes patents that covered Meritol:
