Glue and burn marks?

[Dick Bringhurst]

I'm gluing up some cutting boards. Some of the pieces have sporatic burn marks from the table saw. Do the burn areas have a significant negative on the glue joint? I'm using Titebond. Dick B.

[Bill Huber]

I don't think the burn marks will have any effect on the glue up but my question is why are you getting burn marks.

If the blade is sharp and the fence is set right you should not be getting any burn marks.

[Peter Quinn]

Popular wisdom and experience among those I work with is that burn marks burnish and seal the grain of the wood and reduce the strength of glue joints where these burns are present. I have had a bit of a miter here and there that wound up with burn marks when cut to final length for one reason or another, I'm told given the weakness of the joint to start with it is better to start over with fresh stock. I can't imagine a few burns reducing an edge grain glue up to the point of failure, though one pass over the jointer should eliminate any doubt. Are you talking about small pieces for an end grain board that can't be processed further? If not I would generally remove any burnt areas and try to address the reason for the burning in the long term.

[keith ouellette]

I would look a little closer at the cut edges. If you see burn marks you will probably also see some saw blade marks. You may be able to feel them. That would effect the glue up some.

Bill is right. You may need a saw tune up.

[Wade Lippman]

Burns will probably affect gluing a bit, but the main thing is that if you have burn marks you almost certainly don't have an edge flat enough for gluing.

[Chris Padilla]

I think burn marks and glue do not mix well. As stated, it burnishes the wood. Feel it with your fingers...it is likely very very smooth...a drop of water on it will bead up...glue will not absorb into the wood and it will cause a poor joint.

[Jamie Buxton]

Okay, your saw needs a tune-up. However, unlike the other posters, I don't think scorch marks will bother glue. A really smooth surface -- even one so smooth it feels burnished -- takes glue perfectly well. I can sand and/or scrape wood down to a glassy surface, and it will still glue up. Glue bonds at a molecular level, way way below anything your eyes can see or fingers can feel.

[Chris Padilla]

It would be interesting to see high magnification of wood that's been burned/burnished and highly scraped to almost a glass feel. I wonder if they are different at all?

I just don't think I'd feel comfortable gluing up "majorly" burned/burnished parts but I suppose minimal light scalding would be okay.

After all, burnished/burned wood can be considered a finish....

[Howard Acheson]

If you want to answer the questiton, do this experiment.

Support the edge so it is standing straight up. Put a drop of water on the burned area and another on an area that you have sanded slightly with 180 paper. See which drop is absorbed first.

The ability of a water based adhesive to penetrate into the wood surface is critical to the strength of a glue joint. Burnished or burned wood is not as absorbent as cleanly cut or lightly sanded wood.

[Wade Lippman]

If you want to answer the questiton, do this experiment.

Support the edge so it is standing straight up. Put a drop of water on the burned area and another on an area that you have sanded slightly with 180 paper. See which drop is absorbed first.



Do you sand before gluing panels? If not, it is an ill-conceived experiment.

[Chris Padilla]

I think the experiment is just fine, Wade. All it is showing is how absorptive burned/burnished wood is to non-burned/burnished wood. If the sanding part is bothering you, freshly joint the edge on a machine or with a plane.

[Jamie Buxton]

The ability of a water based adhesive to penetrate into the wood surface is critical to the strength of a glue joint.

Hmm. Can you back that up? My understanding is that the glue molecules bond to the wood molecules right at the surface. They don't need to penetrate past those surface molecules.

[Karl Brogger]

The water part has to go somewhere though, and you can't tell me that porosity(sp) doesn't have an effect.

[Craig Moulton]

If it absorbs the water at all (or at least faster than glue set time) it won't affect the joint. If water gets absorbed into the fibers in the burnt area, so will water-based glue.

A better experiment would be water drop on a burned and normal-appearance section that you're gluing. The difference in absorption and joint strength will likely be minimal, but that's what experiments are for. Of course, you're also supposed to begin an experiment with no pre-conceived expectations about the result, so forget what I said about how it ought to work, and try it.

[Howard Acheson]

Rather than trying to explain it here, it would be best to go to the source. The source is the Wood Handbook published by the U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. Look in Chapter 9 (www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch09.pdf).

This is an excellent publication paid for by our tax dollars. It answers almost everything you would want to know about wood and products directly used on or in wood.

[Howard Acheson]

Do you sand before gluing panels? If not, it is an ill-conceived experiment.

Perhaps I wasn't as clear as I should be. The sanding was to remove any of the burn/burnished wood material. If the board had been ripped using a good, sharp rip blade, no sanding or planing would be necessary before glue up.

The point of the experiment is to show that the wood surface that has been burned or burnished is different from a freshly cut non-burn surface.

[Wade Lippman]

The water part has to go somewhere though, and you can't tell me that porosity(sp) doesn't have an effect.

Interesting issue. Woodworking glue works on goncalo alves, which is essentially waterproof; but not on metal.

If porosity is necessary, it sure doesn't need much.

[Craig Moulton]

Wow, thanks Howie. Neat book, and site.

Looks like best practice would be to at least scuff the surface with some fine sandpaper. Going back to reading the handbook of wood now.