The following was collected from another thread and moved here for relevance:
It's so dry out here in Colorado that your nose bleeds in the winter (no lie). Winter is also much dryer than summer and summer is definitely dry compared to where I grew up in North Carolina. Organic handle material of all kinds will shrink, sometimes with disasterous results. I decided to experiment with a rehydration method and by gawd it works!
I've done this with several knives now. The results have been excellent with zero complications. People have asked, "Will it rust the blade etc.?" Uhhh.... no.
A picture is worth a thousand words so see for yourself:
Ricardo Vilar's sheep horn handle shrank badly, to say the least. It is now almost exactly like it was when I bought it from him in Atlanta at BLADE about three years ago. You may not be able to fully tell by the picture but the pins are now flush with the handle and there is no gap at the front of the handle. There remains a small gap on the lower half of handle/butt cap junction that admits a very thin piece of paper. I will return this knife to my hydration chamber for another month and see if that goes away too. I bet it will.
Here's a crude drawing of my rehydration chamber:
My experience is that sheep horn takes longer than ivory of any type. Wood is variable with softer, more open grain woods being faster to rehydrate than denser tight grain woods. Bone rehydrates relatively quickly too.
My strategy has been to rehydrate a handle in need, then apply Butcher's (bowling alley) Wax to retard shrinkage. I can't say if this will make a difference long term since I have also attempted to add a bit of humidity to areas in my home where knives are displayed or stored. But to my way of thinking wax offers more protection than say, mineral oil. I could be wrong but as long as I'm able to control handle shrinkage I probably won't worry about which product is better.
In response to comments and questions...
I got no results at all from submerging handles in mineral oil for days and even weeks at a time. That's part of what drove me to this experiment. In some cases I know the handle took mineral oil into natural cracks (fossil ivory) or joints because upon rehydrating them using this method the mineral oil would be slowly squeezed out and collect on the surface of the handle!! Every couple of days I'd wipe down the handles and marvel at what was going on.
The water in the trash can is just regular tap water. According to the dates I included in the before/after photo above, it took 74 days to get the results you see there. That's the longest of the ones I've tried. The louvered door was enamel painted wood - metal might scratch the finish. Other than that I don't see a problem with metal.
How do you protect from rust and corrosion forming? As above, it seems this process will also promote rust.
My working assumption is that there's so little moisture in the air around here that the increased humidity under my plexiglass box is enough to rehydrate (most?) organic handle materials, but not enough to induce oxidation on the ferrous surfaces of the knife. Going beyond that, I'd bet the localized humidity my contraption produces is not even close to that of your ambient conditions in Virginia. If I lived back in NC where I spent the first 29 years of my dubious life, my knife collection maintenance routines would be much more oriented toward preventing rust than preventing handle shrinking/cracking.
...mineral oil and such will deter cracking, however I don't think it will reverse (dehydration caused shrinkage etc.)
I think you've hit the nail on the head - my challenge here is to reverse something that mineral oil, wax and other treatments are meant to retard.
When I first realized I'd been in denial about the risk to some of my knife handles, and that the problem had gotten completely out of hand, it made me kind of crazy for a few days. I'd lie in bed at night and imagine I could hear my ivory handles cracking!
Here's one thing I hadn't contemplated until it actually happened: Imagine a beautiful folding knife with creamy ivory scales and perfect black-lip MOP escutcheons (one on either side) sitting inside it's fleece-lined "Bill's custom case" inside my safe. Now, imagine the ivory slowly shrinking over a period of 8-12 months, tightening around the non-shrinkable MOP. The MOP escutcheons begin to bulge and then one day they crack! Now imagine the feeling in the pit of your stomach when you go to the safe.... You get the picture. (May God bless the maker, who took pity on me and fixed everything, including a new stabilization treatment for the ivory!) I'm a lot wiser now, and I think I've found a way to reverse some of the shrinkage I've experienced and go forward from here with an aggressive prevention routine.