Swapping-out cellulose acetate keys on a Comptometer

To start, five columns of white keys for a Comptometer.

These are enuine, old keys. Not new reproductions, but relatively good originals. They do have some yellowing and have started to deform, but still very serviceable. These keys were harvested from a ~1930 Comptometer Model J with severe rust and -oddly- loss of many of the green keys.

 

Usually it is the white keys that degrade instead of the green. That's because the white keys did not have enough colorant in them to act as antacid to halt the decay of the cellulose acetate.

This very common 'key-rot' on Comptometer Model J machines was introduced by changing the keys material from cellulose-nitrate to cellulose-acetate. It would have seemed a sensible idea to change to this new material - just as film moved from cellulose-nitrate to cellulose-acetate (i.e. safety-film; not as flammable as the nitrate!). 

This change could have been fine. However, the unfortunate (in hindsight) choice was to also change from the completely opaque white nitrate keys to a slightly translucent white. This likely looked great and 'gem-like' when new and perhaps made the keys a bit cheaper, but with too little zinc-oxide the cellulose-acetate mass remained slightly acid. Then the acetate starts to degrade, adding more acetic acid (vinegar) to stimulate further decay. The acetic acid can even 'sweat' out of the key and then mingles with the (wax?) lettering, making it look like the lettering just oozes off the key. It also makes them sticky (yuck).

The green keys on Model J machines are fully opaque; i.e. seem to have much more pigment and these are generally fine. It is quite unusual for green keys to also have crumbled. The white keys all being fine on this machine suggests that these were already a replacement set, probably fitted sometime in the 1940s.

The replacing of keys requires removal of the entire key-stem. Removing keys-with-stem from a Comptometer in the usual manner. Then using a slotted wood-block and hammer to tap the key from the stem, as it is held in a vise. 

Replaced the keys one column at a time - not removing all stems from a column at once, to make re-fitting a bit easier. (Otherwise it can be tricky to ensure all the levers and rods go at their correct side of the stem.)

The right-most column with the new keys, the contrast with the raisin-like old keys is noticeable. After replacing the keytops on all the white (well, cream) columns, the Comptometer Supertotaliser looks much better.

At then end of this procedure and having done another machine earlier; we now have a jumble box of Comptometer keys in varying states of degradation. From almost ivory white to deep-brown.

Two of the green keys on the machine (circled in red) are already shrunk more than the rest; so a hunt through the box to find a potential replacement. There actually is a quite a bit of color variation in the green; the best quality 4-key is a distinctly different shade of green (dotted circle). For now, all green keys were left on the machine In case the keys really do fail in the future, there are some ok-replacements (circled white) in the box. 

As a final small step, the large engraved numbers '6' were removed from the typeplate. Or rather, the rusty keytop-donor Model J had exactly the same version of typeplate, so these were exchanged.

(Note that replacing typeplates on these machines carries a risk - the screws go into a threaded washer that is lodged in the cork-lining of the top-plate. If this washer should be pushed loose or lets go, it drops away into the mechanism and all keys need to be removed to take-off the top-plate to recover the washer and be able to re-fit the typeplate.)