While I can’t exactly give it a cause…
If it’s peeling like that, chances are it’s a shellac or something similar, though who knows what they’re using of late. Shellac dries fast, reasonably cheap, it’s what a factory would prolly use. If it came off the tools unevenly in a humid area ( like most garden sheds ) that may be a big part of the warping.
To replace it? Boiled linseed oil, 10 parts. Turpentine or mineral spirits paint thinner -8 parts or less and Japan Drier ( a boron compound available at a good hardware or paint store ) 1 part or less. Use pine tar ( 1 part) if you have it, mix well and apply with a rag after removing old finish and straightening ( if needed). You can start with a very thin mix ( more thinner or turps in the ratio ) so it’ll penetrate and add more linseed oil to the batch as you go along with the next few coats. Eventually it’ll build up to a semi-gloss or matt finish.
I kinda like using it with a little pine tar mixed in, acts as a wood preservative. This is likely the finish that good old tools were shipped with 50 years ago. Skip the pine tar, it makes a nice furniture finish. Without the Japan Drier, it will harden, but it can take a long time.
Word of warning: the rags you use to apply the stuff can spontaneously combust. The mix gives off heat ( like resin ) as it goes off. So, spread your rags flat on the ground outside to dry, don’t wad them up inside someplace. Also, don’t leave the tools outside in direct sun to dry, they may warp further.
Restoring - okay, shovel handles used to be made of steamed ash, which can be steamed back if need be, at least to a certain point, give or take the original grain of the wood and other stuff. Though that involves rigging a steamer:
Bad schematic shown below- unusually cobby even for me
Your burner should be capable of putting out lots of heat. I have used plumbers furnaces in the past with good results, but the inexpensive burners you can now get for deep frying turkeys and similar purposes are even better and cheap. Not shown, the propane tank, as you’d find on a gas grill or something.
Gas can, no more than half full of water. No gasoline, of course, unless you are tired of this world. Your steam box ( mine is simply pine boards nailed together and sealed with tar, pvc pipe will work in a pinch ) should be inclined a little so steam rises and condensate runs back down and into your gas can rather than escaping to the wild. Not shown, the cap on the low end with a pipe set through it and some sort of door at the high end that seals close but not absolutely airtight.
Steam for roughly 1 hour per square inch of cross section and be ready to clamp your wood to something to hold it when it comes out, tough ash may steam faster than the oak I’m used to.
Though first, try dampening the wood with some towels soaked in hot water and gently weighting things in the right direction - it’s a whole lot easier. But wet heat in any event.
that help any?
doc…sometime furniture and boat guy