Set is the result, generally due to belly compression/compaction, of a bow’s inability to return to its original shape. For instance, if a bow was constructed from a bow stave with 3” of reflex and after tillering, shooting in, and unstringing, the bow then shows 1” of reflex, the bow is said to have taken 2” of set. It can be spread out over the entire limb, or localized in a smaller area.
What it's telling us is the wood is overstressed in the area the set occurs in. It can be due to pushing beyond the wood's ability to resist compression because of how it's designed.... too narrow, thick, short, etc. Or due to abuse.... like drawing it farther than it was designed to go. A bow can also take set because its moisture content was too high prior to flexing its limbs... or because it was drawn farther before a tiller correction was made. In other words... if there is a spot on a limb that is stiff and not flexing, the areas to either side will have to flex more to make up for it, and if they're not designed to be able to do so, set will result.
Even if everything is designed and tillered right, a bow can take excessive set if the wood wasn't seasoned properly. I've seen white woods like hickory, ash, and elm do this when improper care of staves caused the wood to degrade internally.
Is it a bad thing? Well, imo, it's a matter of degree. A wee little bit of set in an otherwise resilient and springy bow tells me I pushed the wood's limits just enough that it's not overbuilt. It will shoot fine and I'll be happy with it. More than a little, and I'm trying to understand why and may make some adjustments on the next one.
If I were going to heat treat, I'd do it before the set occurred... better as a preventative than a 'fix'.
Learn from set. Limb set should be viewed more as a mentor than an adversary. It simply points to where we could make improvements.
