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Enigma Corrosion Alert
Bad lamp films can cause serious damage - 16 February 2015

Most Enigma machines were historically supplied with a rectangular green filter that could be placed over the lamp panel in order to increase contrast in broad daylight. When not in use, the filter is usually stored inside the top lid of the wooden case, held in place by two metal clamps.

While restoring a number of historical Enigma machines, Researchers at have now discovered that some of the original acetate filters have a tendency to decompose into acetic acid, which may lead to oxidation and deterioration of the keyboard, the surrounding metal parts and potentially even the interior.

The image on the right shows Enigma machine with serial number 12430 of which the metal key tops have been corroded badly by acetic acid from a bad lamp filter [1]. The metal bracket that holds the spare light bulbs was also affected [1].

In this particular case, the machine had been stored for several years in a climate-controlled environment, which rules out any other possible cause for the corrosion. Meanwhile, more cases of corrosion have surfaced, which have triggered this urgent warning to existing Enigma owners.

It seems that the problems are caused only by a certain type of lamp filters. These bad filters can generally be recognised by their light-green colour, as shown in the images on their website. Based on the research of Enigma historians like Dr. David Hamer, Frode Weierud and Dr. Tom Perera, it seems likely that about 10% of the surviving Enigma machines are affected. If you know of any owners of a WWII Enigma machine, please help them by passing them this message.

Please note that the alert page on the website, is an evolving page that will be updated with new images and solutions as and when they become available. So please check this page regularly for new information. → More on the website

Seriously damaged key tops [1] Badly corroded spare lamp bracket [1] Good lamp film (left) aside a bad lamp film (right) [1] At the left an original 'proper' filter, at the centre the bad light-green filter. At the right a suitable reproduction. Fogging visible on the surface of the light-green filter The bad lamp filter shows some kind of 'fogging' on its surface Corrosion of the key tops caused by a bad lamp filter The same machine, just 3 years earlier showing no signs of corrosion yet

To show you just how serious the damage from these kind of filters can be, consider the rightmost two images above. The rightmost one was taken 3 years ago and shows the keyboard of an Enigma machine. As you can see the key tops are still shiny and there are no traces of corrosion yet. The second image from the right shows the machine just three years later.

The following actions are recommended
  1. Identify the type of filter
    The suspect filters have a light-green colour and are not very dense. When stored in the top lid of the wooden case, you can easily see the wood behind it. Only a minor part of the wartime Enigma machines were supplied with this type of filter. These filters often show some kind of 'fogging' on their surface.

  2. Check the machine for corrosion
    Carefully check the keyboard - and especially the metal rings of the key tops - for traces of corrosion. This corrosion will appear as green 'mold' or green 'rust flakes'. Also check the metal bracket that holds the spare light bulbs in the top corner of the lid of the wooded case and the two metal retaining clips that hold the lamp filter in place.
The image below shows the suspect light-green filter at the centre. It shows a large contrast with the original - proper - green filter at the left. As you can see, the proper is much more dense. The filter at the right is a reproduction filter that is available as a recommended alternative.

At the left an original 'proper' filter, at the centre the bad light-green filter. At the right a suitable reproduction.

If either of the above is true
  1. Remove the green filter
    If you have a light-green filter and/or corrosion around the key tops or the spare lamp bracket in the top lid of the wooden case, remove the lamp filter immediately and store it elsewhere, preferably in plastic bag.

  2. Ventilate the machine
    Leave the Enigma machine open in a well-ventilated room for at least one hour, to ensure that any residual gasses are dissipated.

  3. Report the serial number
    Please e-mail the serial number of the affected Enigma machine to us, or directly to This might help us to pin down the serial-number range of machines that were supplied with this type of filter. 1

  4. Replace the green filter
    You may want to consider replacing the lamp filter with a reproduction one. Proper lamp filters with the correct spectral range and density can be ordered from various places for approx. USD 250.

  5. Restore your Enigma machine
    If only minor oxidation is found, it may be sufficient to clean the metal parts. In that case, inspect the machine from time-to-time, to see if it has deteriorated any further. If your Enigma machine has been seriously damaged however, you should consider having it restored professionally. In that case, please contact us for further information.
  1. A database of serial numbers and photographs of Enigma machines that have been affected by corrosion of this type, is currently under construction. Please help us to expand our knowledge base, by supplying as much information about your machine(s) as possible. We will treat your information with the utmost confidentiality and will respect your privacy. Photographs of your machine(s) will only be reproduced here with your permission.

  1. Tom and Dan Perera, Urgent Warning to Enigma Owners: Corrosion Alert
    Website 12 February 2015
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