Food safe and high temperature way of sealing wood.

August 14, 2011 10:14 PM

Woodworking filter: I need to seal a leak in a wooden vessel but whatever I use needs to be both food safe and high temperature resistant.

Long story short I am recreating an old Finnish brewing method of making something called sahti. When making sahti, you filter out your grains from the liquid using something called a kuurna, which is a long wooden trough typically made from a hollowed out log. The grains and liquids when poured into this kurrna will be just below boiling temp, around 80deg C (176deg F).

Since this was my first time hollowing out a log, I didn't know what I could and couldn't do. I got a bit over-zealous with the ax and cracked the log on one end. Thinking I could fix this easily, I continued.

Upon testing the kuurna with water yesterday, this thing poured water out like crazy. I need to find a solution to patch it if I am going to salvage my 1/2 of a weeks worth of work.

Here is a link to what I am working on. And on the last photo, you can see the crack.

Any thoughts? I would love not to scrap this and start over. Thanks!

Oh and I have tried using a high temp epoxy, but it doesn't fill the gsp. I need something that can fill.

posted by wile e to sports, hobbies, & recreation (17 answers total)

In composite manufacturing when you want to fill something with epoxy you mix in microscopic glass beads. The mixture forms a paste that will hold a bit of shape, useful for filling gaps and voids.

Of course being food safe isn't a requirement for aircraft panels, so I don't specifically know that it says that its not food safe, but given it's just plain glass, and of course totally encapsulated in the epoxy, I would think it'd be OK.

Surf board manufacturers use them too, and are usually a cheaper source.

The glass beads are seriously tiny - it's like super fine flour. Make sure you use breathing apparatus.

A really low-tech version is to mix saw dust into the epoxy. You probably already have a sawdust supply... Because the sawdust isn't as fine, the mixture won't be as smooth, but that doesn't look like it'd be a problem for you.
posted by trialex at 11:08 PM on August 14, 2011

Can you make a shim out of birch and drive it into the crack, then patch it with epoxy?

Were you working with green wood? That crack might be from the log drying.
posted by hydrophonic at 11:17 PM on August 14, 2011

Trialex- I thought about doing that with the sawdust but wasn't sure it would work. I might have to try it. Not sure I'd prefer the glass over the wood chips. Thanks!

Hydrophonic- The problem is the crack is not fully visible so a shim would only go so far. The wood was a little green yea, but the crack came from a good swing with the ax when cleaning out the inner wood.
posted by wile e at 11:37 PM on August 14, 2011

There are two solutions:
1. flexible glue; though GE #1 silicone sealant no longer says "aquarium safe" on the label, it's what people use. Don't use GE #2 as the mold inhibitors are probably not healthy for you. Unadulterated silicone sealants are supposedly good up to ~ 400 deg F. They'll certainly handle boiling water temperatures day in and day out.
2. do the woodworking fix and make a proper patch; this will be loads stronger, last longer, but take more work. Some common wood glues (Titebond II, most notably here) are labeled safe for indirect food contact, meaning don't put it in the food, but once dried can be in contact with food.

You'll likely end up with some hybrid of the two systems. Cut to make room for your butterfly, etc. Wedge and patch as best you can. Fill remaining small gaps with good silicone sealant.

Good luck!
posted by introp at 12:05 AM on August 15, 2011

Could you bind the log and soak it for a while? The wood will swell and may close the gap. It works for boats and dugout canoes, at least.
posted by hattifattener at 12:25 AM on August 15, 2011

I suppose the traditional method relies on the wood swelling and eventually sealing all minor cracks by itself. So a really neatly fitted patch, tightly wedged in from within (but without putting too much stress on the log so it doesn't go on cracking), so it doesn't pop out toward the outside, and a few times of use should fix the problem without any glue.
posted by Namlit at 12:27 AM on August 15, 2011

introp- Titebond II sounds like a great idea, I'm going to see if I can find it here in Finland. Thanks!

hattifattener and Namlit- The crack is too large for soaking it method, I unfortunately have tried already.
posted by wile e at 12:34 AM on August 15, 2011

Titebond II is not going to hold up to boiling water. Heat softens it, so you'll end up with (harmless) glue in your sahti, and the fix will fail sooner rather than later.

Epoxy would be my choice. I would smooth out the interior surface around the crack so you can see exactly where it is and how far it goes. Use a carving gouge if you have access to one, or coarse sandpaper if you don't. Blow any dust out of the crack.

Let the log dry thoroughly. DO NOT try to bind the crack together when gluing. On the contrary, I would tap a softwood wedge into the end and open the crack just a tiny bit before gluing, so that there will be no tension at all on the cured glue.

Finally, I would pack as much epoxy as I could into the crack. Wherever the crack is narrow enough to support the glue without fillers, i.e. the epoxy doesn't run through and fall on the ground, I would skip the fillers. Let cure 24 hours or so before using.

Before brewing a sahti in the repaired log, I'd pour a pot of boiling water over the epoxy and dump that out.
posted by jon1270 at 1:53 AM on August 15, 2011

Epoxy probably is a good solution, but there are many different grades of epoxy, so make sure the one you choose is rated for food/water contact at elevated temperature, both from a degradation/stability point of view, and a chemical safety point of view.

Much of the hysteria about bisphenol-A is not justified, but bisphenol-A migrates more quickly from epoxy into boiling water than cold water. Many (but not all) epoxy formulations contain bisphenol-A derivatives - depending on how you feel about bisphenol-A, this may affect your decision.
posted by firesine at 2:18 AM on August 15, 2011

How long is the crack? Could you just cut an inch or two off the end of the log, relocate the end cap further in, and wind up with a slightly shorter kuurna?
posted by Jinkeez at 4:20 AM on August 15, 2011

Food grade silicone sealant

Food grade epoxy resists high temperatures

I think you could safely mix sawdust with the epoxy to give more bulk without losing water resistance.
posted by Kirth Gerson at 4:27 AM on August 15, 2011

I hate to say this: I think any effort you spend trying to find a chemical solution to bonding (unseasoned) unmilled wood is going to be wasted. If you can figure out a way to plug this with natural materials, that would be good.

Personally, I wouldn't use epoxy on anything food related, high temp or not. That's just me.

Epoxy cures strong and stiff. If you glue it to something that's going to move a lot (aka a chunk of thick, uncured wood), the epoxy is going to stay stiff, and when the wood expands and contracts, the epoxy is going to rip part of the wood away with it, leaving you with another crack.

Could you maybe cover the whole thing with leather? Either inside or out?

I know it sucks to lose your work, but speaking from experience, when you screw up there are times to cut your losses and start from scratch.
posted by sully75 at 6:58 AM on August 15, 2011 [2 favorites]

jon1270 has a good point. I know that polymerized PVA glue (e.g., Titebond II) will not redissolve in warm water, but I've never tested it with boiling water. The image in my head was of a typical vessel repair: the water will never infiltrate the glue seam if you've patched it properly and tightly. However, because this is green wood, etc., you may not be able to make that guarantee. The Titebond data sheets don't spec max temperature so, out of curiosity, I've got four test pieces curing now. I'll test them in boiling water and see how they soften.
posted by introp at 10:45 AM on August 15, 2011

If it were me, I'd find some well-dried straw or grass (that you know to be non-toxic; maybe wheat straw or other livestock hay). I'd get some kind of pointy-ish metal tool, or maybe a dull butter knife, and just find a way to carefully jam the hay into the crack. When it gets wet, it should expand. Any smaller holes, such as on the end of the crack, you may be able to seal with, I don't know, maybe a stainless steel thumb tack driven well into the wood.

I second the idea of putting some kind of binding around the cracked end, though, especially at the small/starting end of the crack. Otherwise, the repeated heating/cooling of the log may cause it to expand.

You might also try finding a metalsmith who can drop some appropriate molten metal over the crack after you jam some stuff in there. Lead is an obvious low-melting-point metal, but that's a bad idea, obviously, since it's toxic. Maybe there's another metal that would work. Cooling it quickly would obviously be an issue.
posted by amtho at 11:11 AM on August 15, 2011

Three suggestions, one is to drop the whole shebang into a water trough for a couple days and see if it will swell up and seal off that crack.

Other is to buy a food grade platinum cure silicone rubber like this stuff: Linky and apply it to the area. But its going to be mind bogglingly expensive.

Third is to make a new one, now that you got that first one under your belt the first will go much much faster.
posted by Pink Fuzzy Bunny at 11:58 AM on August 15, 2011

cut overlapping tapered holes along the crack fit corks into the holes ,
posted by hortense at 2:59 PM on August 15, 2011

Ok it looks like my best option is to cut my losses and start over. Thanks for all the suggestions.

However introp I would be very interested in your results so please do post them here if you can. Thanks.
posted by wile e at 11:53 AM on August 16, 2011