Oil's well that ends well.
September 2, 2011 12:37 PM   Subscribe

If I seal wood with some oil (say, olive oil, or mineral oil), does heating the wood add anything to this process?

I am making some cups! Due to the construction (bamboo), I have zero worry about leakage, so the sealing is more against long-term wear and flavors and such being picked up on the interior.

For some reason I have it in my mind that once I let the oil sit on the wood for a few hours, I should heat it in a low-temp oven for awhile. I.. have no idea if this is right or if it's actually doing anything.

Extra bonus sub-question: is this actually necessary at all? Pretty much all the wood-sealing-for-food-use stuff I've come across on the web during this little project is dealing with things like appearance, leaks, and splinters, none of which I have to worry about. Assume the cups would be used for a wide variety of liquids.
posted by curious nu to Science & Nature (10 answers total) 4 users marked this as a favorite
Bamboo? I'd avoid vegetable oils, due to the potential for going rancid and affecting taste. Mineral oil, maybe, very lightly applied, if the bamboo is looking particularly dry and prone to cracking. But I'd probably try to go without.

Anecdotally, I found dried bamboo to get fairly hard and wear resistant by moderately "toasting" the outer surface with a propane torch. Just enough so that it begins to change color, not quite blacken. It seems to cook out the moisture, and cook in the natural oils. Mind you, that's just my speculation of what happens. But the result seems worthwhile.
posted by 2N2222 at 12:52 PM on September 2, 2011

BTW, heating in an oven might aid in the penetration of the oil, in addition do driving out moisture. Perhaps this could be an alternative to the touchier method of using propane torch?
posted by 2N2222 at 12:54 PM on September 2, 2011

Anecdotally, I found dried bamboo to get fairly hard and wear resistant by moderately "toasting" the outer surface with a propane torch.

I'm pretty sure this is what they do to bamboo flooring in order to get it different shades of brown - heating it in a kiln. The grain is supposedly so tight that regular stains don't take as well (or so the flooring guy told us).
posted by jquinby at 12:55 PM on September 2, 2011

I'd try Carnauba wax:

* Melting point: 82–86 °C (180–187 °F), among the highest of natural waxes.
* Relative density is about 0.97
* It is among the hardest of natural waxes, being harder than concrete in its pure form.
* It is practically insoluble in water, soluble on heating in ethyl acetate and in xylene, practically insoluble in ethyl alcohol.

It's nontoxic enough to be used in candies and as a coating for pills.

I would try to buy it in powder form, get the bamboo wet and coat it with the powder, then put it into a slow oven-- I'm sure it'll wet the bamboo-- perhaps a couple of times, then finish it off with a hair dryer or a heat gun if you happen to have one.
posted by jamjam at 1:27 PM on September 2, 2011 [1 favorite]

My favourite finish for hard timber is tung oil. Use a mix of 1/3 tung oil and 2/3 gum turpentine (not mineral turpentine) for material as hard as bamboo you may want even thinner. Get the surface wet with the oil, leave, vigorously wipe off in 15 minutes or so. Leave 24 hours, sand with at least 600 grit paper and repeat. Buff to a fine polish if you like.

This oil mix cures very well, I've used it on chopping boards and a lot of furniture and found it very, very durable. The only down side is that it is hard to use on softwoods.
posted by deadwax at 3:43 PM on September 2, 2011 [2 favorites]

This is a known situation, and you don't need to develop new methods. Yes, heating the wood will help it absorb oils, but for most makers this is overkill. If you want to do this, go ahead.

You don't need carnauba wax, as other oils that are easier to apply (such as tung oil, mentioned by deadwax) will slowly air-harden (oxidizing into a solid). That is exactly why tung oil and boiled linseed oil, both traditional and modern (which is not actually boiled anymore, but processed and mixed with fast-drying volatiles) were developed. In fact, I suspect the oils harden better than the wax would, since the wax doesn't air-harden.

The rule of thumb I was told was to oil newly-made furniture "once a day for six days, once a week for six months"; that is, it absorbs more at first, and eventually less. This is how to saturate the surface with the finish oil; obviously wooden bowl manufacturers don't go to this much trouble, even when they do finish with food-grade oils.

As far as "food-grade oil" goes, BTW: it's another case of cautious overkill. I can't think of a single finishing oil that isn't food safe, with the caveat that alcohol (beer, wine) will still dissolve the oil /before/ it oxidizes (afterwards, I don't know how resistant it is to alcohol). But you do want a hardening oil, since that will fix itself into the grain: tung or linseed are traditional.

tl;dr: Follow deadwax' advice.
posted by IAmBroom at 8:20 PM on September 2, 2011

Best answer: "This is a known situation, and you don't need to develop new methods." QFT

You have two problems here: one is the inconsistent terminology used by finish manufacturers and the other is the collection of myths hovering around many finishes. I'll get to your questions in a moment.

First: what is called "oil" you find in the home stores is, 99.9% of the time, actually an oil/varnish blend or a thinned (wiping) varnish. That's a very different beast from what you're describing, a pure oil. So when many people give you advice about oil finishes, beware what they're describing because they're probably describing oil/varnish and not realizing it.

Second: non-curing oil finishes are incredibly ineffective at protecting wood but they do manage two things: making the wood look pretty and reducing a very small amount of water vapor exchange (not by acting as a barrier but by taking up space where the water would go). They do surprisingly little to prevent water damage because the oil doesn't form a film and is so easy to displace.

If you chose a non-curing oil finish, I do not recommend using a natural oil like olive oil. It will go rancid. The volatiles that the spoilage bacteria generate out can remain in the wood for a long time and leave a nice stink. Keeping ahead of the oil spoiling is mostly just a pain in the ass. Mineral oil is commonly used to make old chopping blocks look pretty and is easy to acquire from your local drug store (it's used as a laxative). Some people will tell you walnut oil won't spoil. That's another myth; go look up the shelf life data on it and see that it actually goes rancid faster than almost any other oil.

So the big answer: heating your oil it will reduce its viscosity so you get very slightly better penetration but, again, penetration doesn't really help protect. Don't sweat it.

Bonus question answer: yes, it matters. While many chopping blocks exist as bare wood and do just fine, they aren't submerged in water for hours at a time. (Ever forgot to empty a cup and left it overnight? Try putting your chopping block halfway in a sink-full of water a few dozen times with drying cycles in between.) Admittedly I don't know much about bamboo's drying issues. For all woods I know, many of the worst cases would leave your cups, unprotected, with eventual warps and cracks. I note that the two nice bamboo bowls I have here in my house both have varnish finishes of some sort.

Moving up to a curing (but still straight) oil like tung or linseed will offer a small amount of additional protection. Alas, the films that both form are thin and soft so are easy to displace (and then you're back to exposed wood). They're still better than non-curing oils, though.

Oil/varnish blends are the next step up the protection chain. They'll form a film for a moderate amount of protection. I typically never go softer than here if I'm doing food-exposed work. Depending on the surface and how it will look, I'll go to a full varnish. For chopping blocks, film finishes get compromised so fast (whack whack whack) that I don't bother; I'll put some mineral oil on them occasionally if I want them to look nice again. But they stay mostly dry and so mostly I can ignore the finish and just use them.

(Note that rubbing plain oil into wood is apparently a 20th-century invention. It became popular in the 60s here in the U.S. but wasn't used by much of anyone as a finish prior to that. Oils were always the cheap stuff used to make low-grade furniture look decent when you couldn't afford to shellac or varnish them. But no old craftsmen apparently ever thought of them as a protective finish.)
posted by introp at 9:45 PM on September 2, 2011 [4 favorites]

I'm mostly with introp - if you don't seal these (at least on the inside) they're going to do an expand/contract cycle every time you use them and eventually they'll crack.

Oils that polymerize as they oxidize (like linseed oil) will give you some protection but not a lot and if someone uses these for something hot...let's just say that while linseed oil looks a lot like hazelnut syrup, it doesn't go nearly so well with coffee. Don't ask how I know that.

I'd slather the insides with polyurethane, preferably a clean your burshes with thinner afterwards product, then I'd hand them upside down and let them dry a few days, then maybe give them a second coat. After that seemed dry I'd put them somewhere dry and warm and let them sit a week or three to make sure the finish was good and dry.
posted by Kid Charlemagne at 10:42 PM on September 2, 2011

Response by poster: Hmm, so the only way to get a really good seal is with something plastic-based? What did ye olde craftsmen use? Or were ancient wood utensils just routinely replaced?
posted by curious nu at 11:07 AM on September 3, 2011

Best answer: Ye olde craftsmen used shellac or varnish. Consider a curing oil to be a simple, albeit soft, varnish. (Old varnishes are effectively a plastic, just not in name.) I'm not an expert on ancient flatware but I believe cheap wood utensils were just expected to eventually warp, check, etc. Note that old wooden spoons, trenchers, etc. were very thick. This is for a number of reasons, but it would've also made the effect of surface expansion less problematic.

Both shellac and varnish are fine choices for utensils and eating ware. All modern varnishes are food-safe if properly applied and cured. Shellac is the only finish that is rated edible by the FDA before it cures (you can literally eat shellac flakes if you like) but you must not use it on vessels that will contain alcohol. Alcohol will redissolve shellac (this is one of the reasons it is such a dream to work with and repair).

Modern varnishes are usually much tougher, especially with respect to chemical resistance, but note that all varnishes aren't created equal. For example, water-based polyurethanes aren't actually terribly good at being a vapor barrier or resisting alkali soaps, despite the polyurethane label. They don't form continuous films at a molecular level so they're considerably weaker than traditional "polyurethane" varnishes. For many jobs, however, their lower noxious solvent content, high clarity (most are based on acrylic which is good enough for plexiglass), etc., can make them worthwhile.
posted by introp at 1:50 PM on September 3, 2011 [1 favorite]

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