15 Apr 2021

Plastics Conservation: Comptometers and Patents

It’s probably a sign that you have an emerging passion for plastics conservation when you get excited by the words: “there’s a deteriorated plastic stuck to the top of an object that needs treatment”

by Anna Beaumont

This was the case with a model J Comptometer found in the storage of the Museum of London. As you can see in the photograph, a cracked, discoloured mat which smelled of rubber had slumped over the finger keys and at some point had gotten stuck to them. A search of the collections database yielded no information on the provenance of either object, leaving Jannicke Langfeldt (my supervisor) and me to speculate on both items’ material composition and use. Determining the various plastic components was going to be made easy through the use of Durham University’s Fourier-transform infrared (FTIR) spectroscopy and resources at an upcoming Plastics Fair. However, COVID-19 had other plans for us and a change of strategy was necessary.

Figure 1: Comptometer when brought in vs re-constructed image of it using photographic sources (A.Beaumont, 2020)




 First of all, I want to clarify what a Comptometer is and its importance when discussing women in the 20th-century workplace. The descendant of the Pascal calculator and the ancestor of electronic calculators and computers, the Comptometer is a calculating machine that was used prolifically in accounting firms, banks (seen in the 1938 film You Can’t Take it with You), and even NASA. First patented in 1887, it remained popular until the 1970s and was largely used by women.

Becoming a Comptometer operator was considered a feminine job and was a relatively popular career for women. The practical skills of operating the machines and arithmetic were taught in specialised schools that permitted women from age 16. Like with typing pools for secretary work, companies needed a great number of individuals to do calculations in their finance and accounting offices, with Comptometers used as their tool of choice. An example of this is the mainly female teams of human calculators from NASA (as seen in the 2016 film Hidden Figures), who worked on different parts of equations for the male engineers, or physicists.

The reason for mass employing women in these jobs, at the time, was that they could be paid less than men, which made it more profitable for the company or institution. Professions in this field started to decline in the 1960s with the arrival of modern computers, and while women initially were the coders and programmers for these, they were gradually replaced by men. Comptometers are, therefore, important objects in museums to tell stories about women at work.

Figure 2: Advertisement for J model in Forbes magazine, 1937 (Alan Jewell, flickr.com, 2018)


Getting back to the Museum of London’s Comptometer, the first task was to separate the mat from the Comptometer. Indeed, it is hard to appreciate the object as a calculating machine when all the keys are covered by a deteriorating black plastic mat. The first task, therefore, was to carefully try to release the mat from the keys. It was firmly adhered to them, as if someone had glued it on. The glueing effect was probably a combination of two deteriorating plastics - one from the material that made up the keys and the other from the rubber mat. This was most likely amplified by the limited air exchange between the two surfaces.

I began to excavate the finger keys from under the mat with small dental tools and slipped pieces of Melinex on top of the keys to provide a barrier between the two surfaces, so they would not re-stick themselves. I found that I could move some keys up and down by gripping the lower part of the key with a specially adapted pair of tweezers. Only nine keys remained adhered when the first Covid lockdown started. Separated from the object and with stretches of time before me, I had time to ponder what plastics could have been used for the finger keys, causing the reaction with the rubber mat. Not only was the Plastics Fair cancelled, but the lockdown also meant that I was not able to travel anywhere to analyse the plastics.

After using several plastic databases suggested by online seminars, I narrowed down possible options but felt I could go further. The idea of using patents as a way of finding more precise answers came from a memory of a middle school Modern World History class: patents became very common in the 19th century as people kept copying each other’s inventions and taking credit for them. The accuracy of this statement may be questionable, and my middle school teachers might not have said exactly those words, but it got me looking for documents patenting the design of the finger keys, hoping that they would say what plastic they used.

As I had previously researched the engraved US patent numbers on the base of the Comptometer I wasn’t too out of my depth. The trick to patents is to know the format for the country you are dealing with (US+ number) and to be aware of the exact technical term for the object you are looking for. I personally used the Comptometer patents to find the term ‘finger keys’ as they came up when the engineer explained how the mechanism worked (I also ran a search with ‘typewriter’ as a key word, for good measure). Dates are key, of course, so I kept mine in the range of a few years before the first Comptometer patent and after the last engraved number on the object. I found what I wanted: a finger key patent dated to 1910 (a further confirmed no other had been made till the 1950s).


Figure 3: Diagram of the finger key on patent US956485 (Conklin, 1910)



Google patents was my primary resource, and it is great for US patents, especially because the documents often have illustrations, which make reading and understanding the technical vocabulary easier. The patent stated that the keys were made of a “non-metallic plastic material” that can be molded under pressure. It mentions rubber, fibre or “a less expensive composition material” (Conklin, 1910, p.2). It continues by stating the addition of tar, pitch, and asbestos as part of the make-up of the finger key body. With this information, the best guess today is that the keys are made of cellulose acetate; a material that can be easily softened and molded, with the addition of plasticizers. It has also been mentioned as being commonly used for typewriter keys.

When Jannicke lifted off the mat she found the Felt and Tarrant plaque with new patent numbers, this time from the UK. I am still researching these to gain a better understanding of the materials. Google patents does not contain as many inventions from the UK as from the US, but through research and conversations with peers two resources have come up: Rechnerlexikon, a German site that has a large database of worldwide patents the 19th century onwards, and the European Patent Office website. Another option for the UK is to contact the British Library, as it holds all national patent documents.

While the search for patents can seem quite time consuming, it was an amazing way for me to research and identify materials without the use of analytical equipment. It is a great tool that, when combined with the plastic identification databases available, I believe will be especially useful when trying to identify plastics linked with machines from the 19th century onwards. Using patents also gave me an appreciation of the technological developments at the time, and an insight into the production methods of plastics.



Conklin, E., (1910). Finger Key For Typewriting And Similar Machines. 956485.


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