What is a "Set" Spring

Quite often when a clock comes into the shop that has not been in use for many years or has not been maintained, I will inform the owners that there is a possibility the piece may have a set spring.  Of course I get looks of not understand so I go onto explain.  When a car is put away or "mothballed" for extended periods, it is often put up on blocks to remove the weight off of the suspension and tires so that wear is reduced during its dormancy.  The same needs to be done for a clock.  If a clock is not to be in use or stored for extended periods, the spring needs to be let down so that as much tension as possible can be removed from the spring coils.
This picture is of a spring that is set from the clock being put away fully wound:

This is a spring for an 8 day movement and is 96" long.  However putting the clock in storage with the springs fully wound has caused this spring to "Set", that is to take the shape it held fully wound.  When the spring does not release its energy and be allowed to expand, this is the result.  The spring is ruined and needs replaced

This picture is of a spring that has not been allowed to set, notice how much larger the coils on the spring open up:

This spring appears to be in good condition and can be reused.  We will take a clean cotton rag and run the length of the spring,  if no flaws or cracks are found, the spring will be lubricated and placed back into the clock for continued use.

For reference, I have included the picture below with both springs side by side so that you may see the differences between a set spring and  spring that is still functional and useable.

Remember, when putting a clock away or mothballing the clock for extended periods, let the spring down to remove as much energy from its coils as possible so that the spring can remain viable and useful.