After learning that galvanised nails are a poor choice of metal to use for relay contacts, I adapted the relay so that the contacts are separate from the nails. Copper wire is used as the contact material.
This gives a lower contact resistance (less than 0.5 ohm) and a much more consistent performance.
In order to be able to assess at a glance how a relay is performing, I turn the resistance measurements obtained from the automatic test setup into an image. The top image below shows the performance of a relay with galvanised nail contacts. The bottom image is from the relay pictured above with copper wire contacts. In these images, time runs from left to right across a line in the image, and each image has eight lines. Each white rectangle corresponds to closure of the contacts for half a second. The taller the rectangle, the lower the contact resistance. You can see that the contact resistance for the galvanised nail contacts is variable, whereas for the copper wire contacts it is consistent and also lower. By producing 10 such images in succession, I found that the relay above performs consistently for over 1400 operations.