Wooden instrument cases designed to absorb moisture & prevent rust

[LowEnergyParticle]

Measuring tools and instruments often come in wooden cases. I've seen mahogany, birch, poplar, pine, and several other woods used. The idea is that the case will absorb moisture from the tool and thereby keep it from rusting. Usually, these boxes are finished with a varnish on the outside, and less often, on the inside. When the inside is not varnished, it is usually left bare. I'm reasonably sure the original finish is a varnish, because I've tested for shellac with denatured alcohol, and these boxes generally predate polyurethane.

When I buy a used instrument the box is often in rough condition, and I like to put them back in good shape by regluing, sanding, etc. I've been leaving the inside sanded but unfinished (no coating of any kind) and spraying polyurethane on the outside.

1) to maximize the water absorbtion, should the inside of the boxes be left unfinished? 2) to maximize the water absorbtion, should the outside of the boxes be left unfinished? 3) Is clear gloss sprayed polyurethane a good choice for finishing, given that the water absorbtion issue is much more important than protecting the finish of the box? 4) For making a new box from scratch, what wood choice would maximize the water absorbtion? 5) I've seen some pretty old tools with ground cast iron faces arrive inside intact boxes without rust, so I'm kind of tempted to believe the whole water absorbtion idea. If anyone can substantiate this theory, or offer an alternative hypothesis, I'd be very grateful.

I've cross-posted to our brethern over in rec.woodworking since they have specialized knowledge of woods, as well as a shared deeply-held hatred of rust on tools!

Thanks very much! Dave

[Jim R Studdard]

I'm not too sure about the moisture absorbing qualities of wood but I think the main reasons wood was used to package precision tools in the olden times is that it was very much available cheaply and is a softer material that won't mar the tools, rather than for its moisture absorbing qualities. Wood is still being employed today for its traditional and aesthetic appearance but plastics are also becoming very common.

[Charlie Self]

There is no "water absorbtion issue."

Wood reaches equilibrium water content with its surroundings--most air, damp wood, dry air, dry wood--and most of what it absorbs comes from the air around it, not the tool stored inside. In other words, if you're storing a tool in a humid climate, the wood is going to reach equilibrium with that climate and will most definitely not absorb any more moisture to protect the wood. Wood as a rust preventer works well in Arizona deserts (no rust prevention needed). It's not helpful in Atlanta or New Orleans.

Wood has traditionally been used to protect measuring tools and similar instruments from physical damage other than rust. Wood doesn't absorb water well enough to work as a dessicant. Period. Buy a reheatable dessicant if you want water absorption. Old tools in wooden boxes that come through rust free were almost certainly stored in areas of low humidity.

[LowEnergyParticle]

Jim, JT, and Charlie,

Thank you all very much! I'm going to treat the wooden-box-as-rust- preventer story as hogwash and finish boxes inside and out with polyurethane. Furthermore, since I live down on the Gulf Coast near Houston, where humidity sometimes presents a small problem, I'm going to buy a case of those small cans of reheatable dessicant and clip one inside every box.

Thanks very much for the help. Dave

[Father Haskell]

If the original finish wasn't poly, I wouldn't use it, just for authenticity's sake. For new boxes, use whatever finish you like.

For ultimate slickness, install humidity gauges, or store tool boxes in a cabinet outfitted with one.

[HeyBub]

If I were refurbishing an antique, I'd try to match the original condition. If the interior were unfinished, I don't think I'd paint it with DayGlo enamel (or anything else).

[Nova]

... and all this time I thought the cases were to prevent the tool from getting banged, damaged and knocked out of alignment.

[stans4]

With all my old tools that have factory wooden cases, the cases are unfinished, nothing on them except some inked or burned-in labels. The "moisture-absorbtion" thing is bogus, if anything, they'd retain moisture and rust the tools even faster. Wood will track the relative humidity changes. Wood was used because it was cheap and easy to work and it was softer than the tools contained within. Same as plastic today.

If I had to put a finish on them, I'd use shellac inside and outside. It's relatively impermeable to moisture and can be readily removed and/ or touched up. Urethanes are hard to recoat and harder to strip. If you just do one side, you're going to warp the case due to differential moisture pickup. If you don't believe it, paint a thin slat on one side with your favorite varnish, wait until it dries and hold it edge on to a steaming tea kettle. Same reason both sides of a tabletop should be finished.

Stan

[Andy Dingley]

Lime (linden, basswood) is one of the best European options for inner liners. Even better though are some of the Japanese traditional timbers for cabinetry liners -- a culture with skilled woodworkers and a hard climate to store things in.

conservation site, lots of useful advice.