Monday, December 30, 2013

End Grain Gluing

Do not zoom in if you have Trypophobia
Just a brief technical post, so be forewarned if you're not into the how-it's-made aspect of this blog.

I glue wood together all the time. I don't really like nails or screws that are visible. Sure you can plug a screw hole or put wood fill into a nail hole, but you can see that something is there. I think you can use plugs in a stylistically positive way, but only if you set out to add that to your design.

Gluing wood face-to-face has challenges, edge-to-edge is common, but anything-to-end grain presents a challenge...

Also, do not google image search "trypophobia"... trust me.
... the challenge is the holes. Expand the pictures above and you'll see them. Think of wood not as a board of lumber, but rather as a piece cut out of a tree. The end grain is the top or bottom of that slice you just cut out. Now think of what you know about trees: they suck water up from their roots and somehow distribute it throughout the tree. How do they do that?

No. Not teleportation. It actually has nothing to do with quantum superposition at all. It's the holes. The wood is just a whole bunch of tightly packed straws designed to suck water and nutrients up to the more important parts of the tree.

Why does this matter? Because the holes are designed to suck up water, they tend to suck up other fluids as well. Take a piece of wood and put it end down in a cup of mineral oil with the top end not submerged and in a few days the wood will suck up the oil and distribute it along the length of the board, all the way to the top (assuming there was sufficient oil).

This means that it will suck up the glue, taking the bond-forming fluid into the wood and away from the surfaces that you want to bond.

(Partial) Solution: Seal the end.

Pictured: A high res picture of wood glue mixed with water. Someone call the Pulitzer folks.
By mixing one part wood glue to one part water, you get a very fluid light glue. When you brush it onto the surface, you're priming the holes with something you want them to suck up.

Recently painted/sealed end grains
The glue-water mix dries, partially clogging the holes, making it more difficult for the glue to migrate into the wood. This leaves more glue on the surface, so that it can bond the two pieces together.

Is it perfect? Nope. Not by a long shot. There are other techniques that improve the strength and longevity of joints involving end grain. (Dowels, tongue and groove, many others).

Does it help in a pinch, recent recipients of the cutting boards would say so.

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