Hide Glue Center Seam

[Mandoborg]

Hey folks, i've been having this problem for a bit and finally decided to come and ask for help after coming up empty here

When i glue up my tops, i've been seeing a fine dark line at the glue joint. I plane the halves to where you can't see any light when holding them up to the light, as well as trying to slide a piece of paper along the joint when the halves are just resting on top of one another. I know they're true, but after gluing, there's that pesky line again ! I did a test on two scraps , one HG the other Titebond. The Titebond one the seam is invisible, the HG seam you can see easily. Any ideas ?? A local told me to only put the Hide Glue on one half of the top, does this make sense ??

The joint is structurally sound, can't break it if i try, but the line draws your eye, which means it's going to draw a potential customers eye as well.........

Thanks for any help !!

Jim

[sunburst]

I get those lines sometimes too, and I don't know why. I have one right now, a guitar top that I jointed using my freshly honed and set up #7 plane, glued and clamped the joint. No question that it was a good joint, but there's that dark line on the center seam. Sometimes it happens, sometimes it doesn't. One of the mysterious things about it is that it doesn't show up immediately, but shows up later. When the top is first carved or thickness sanded there is no line, but by the time the finish process is underway, there it is (but only sometimes!).
I hope you get a good explanation of what causes it, and maybe even some ways to avoid it, because I want to know too!

Oh, BTW, my glue is high clarity hot hide glue, not some dark brown stuff.

[multidon]

I don't truly know the answer, but I will speculate. Hide glue is organic and I would be surprised if any organic compound did not change given enough time. There is a thin layer of glue in between your two pieces of wood, no getting around that. No glue no bond. Perhaps being organic it is subject to some sort of oxidation process? Something that just changes the color but not the integrity of the bond? In any case, if it bothers you, why not just use Titebond for your center seams and HHG for everything else? I don't think your center seams would be any weaker with AR.

[Jim Hilburn]

There are times when you get 2 grain lines right on the seam and they can appear to be a dark line along with the center seam. May not be what your experiencing but it's happened to me before.

[testore]

The glue may be too thick or too cool when applied.

[sunburst]

Yes, two dark grain lines can "double up" sometimes, but that's different from the dark lines I sometimes get (though I occasionally get the double grain line too).
I don't use glue that is too cool or too thick, though that is a plausible explanation too. Only fresh batches of glue for center seams. I jointed three guitar tops at nearly the same time recently; two European spruce and one red spruce. Same glue, same temperature, all parts warmed before gluing, all joints planed and clamped the same. Only one dark line; the red spruce top.
Basically, I've considered all causes that I can think of and eliminated each, so now I have to find causes that I haven't thought of, so I'm optimistic that this thread will produce those!

[Dale Ludewig]

Well, I'm in the line for an answer also. Same situation. With Titebond, I never have a visible line. With HHG, mostly no, once in a while yes. I'm gluing almost all red spruce. I've not noticed if they show up immediately or later, after I carve into them. It's puzzling.

[Bluetickhound]

could it be that there is a chemical reaction that only manifests itself when the wood is cut closer to or further away from a grain line? Every piece of wood is individual so the chemical makeup of one piece of (insert type here) will not be completely the same.as the last, or next piece.

[CedarSlayer]

Hot hide glue is a different beast. The traditional brush for hide glue is free of metal. It is thought that the metal can taint the glue and then react with wood and make a dark line. I have no evidence either way, I have just read way too much about hide glue.

Another possibility is that hide glue transparency can cause this. Hide glue actually bonds with wood. That means that the crystal glue mixes into the cellulose and vise versa. Sometimes the dark glue line can be due to it being clear and leading into shadow. No good simple cure for this although you could rub warm wet titanium oxide into the glue and then perhaps sand it. I have heard of this method on furniture, but I would not trust myself to do it. I might try fine wood flour.

These are all methods that others have advised and I have no proof they are any good.

The method I use is an old one that is probably preventative. I try to barely 'spring' the joint. Sand the lightest amount from the center of the join before gluing so that when the hide glue pulls together it sucks the edges in tighter.

If I have issues in woodworking I generally I follow the Greene and Greene philosophy. Make the joint pretty and you can stop trying to hide it.

Bob

[sunburst]

Just checking the other tops I have in the works right now, I have the two euro guitar tops and the red spruce guitar, an engelmann mandola, red spruce mandolin, sitka mandolin, and red spruce mandocello. Only one (the aforementioned red spruce guitar top) shows the dark line. Others, like the engelmann 'dola, I can't find the joint. All the carved tops are rubbed joints, all the guitar tops are clamped. All the same glue, same brush, same plane, same everything.
Hmmm, I'll have to look for some brushes with no metal...

[David Houchens]

I've had this dark line as well. Can do invisible with titebond but always get a thin brown line with HHG. I always have recut,replane and glued back with titebond. Doing a dark suburst I probably could have left them. I would rather have the HHG in the seam. It makes any repair if ever needed a lot easier. I tought it was just me. Makes me feel better that I'm not the only one experiencing this. Next time I'll cut a little off the end and test. If its getting a burst I'll leave it.

[Austin Clark]

It happens to me sometimes too, and I use a brush with no metal, hand planed and rubbed joint and all manner of woods. I notice it sometimes when gluing on the riser block, too.
In both of these cases, I tend to think that it is a glue thickness issue. There is a big difference to how well a rubbed (or clamped) joint works if the glue is ever slightly too thick. It needs to run off the brush in a thin stream when pulled from the jar and not break up into droplets. The best consistency is thinner than one might think. Too thin is bad, but too thick of a glue also makes for a weaker joint. This is particularly important to be aware of when gluing in braces!

In the end, if in doubt, it is best to just saw it apart, rejoint and re-glue. It only takes a few minutes and you'll feel better about the end product.

[Mandoborg]

Wow, i'm really surprised to see the names of some builders i hold in very high esteem having the same problem i'm having. I usually ask those questions that end up with the Homer Simpson 'DOH' after reading the answers !! I use a squeeze bottle for the hot hide glue so that rules the brush out. I'm thinking along the lines of what Austin said about the thickness of the glue. This one wasn't runny, but it wasn't like syrup either, it was a good, workable viscosity. I'm thinking that's why I got the advice of only putting the glue on one side, maybe i put too much glue in there. The joint was also clamped, and i had plenty of glue squeeze out that was still liquid so i was like ' Success' !!!

Thanks for the comments.

Jim

[CedarSlayer]

This is an interesting issue. I use a copper container as is traditional to give a touch of preservative property to the glue. If copper does not cause an issue, then binding the brush with a copper wire should be fine.

There is a type of ink marker that Japanese Carpenters use that is made from bamboo. If you soak the end of a bamboo chopstick and then slice into the end and separate the fibers you can either ruin a chopstick or make a brush that can be shaped to precision and can be discarded easily. crushing the fibers will make it a bit less precise and a bit better for holding glue. These brushes have the advantage of not wasting a lot of glue held in the bristles. The downside is that it does not hold a lot of glue. Great for fine gluing, not great for fast gluing.

I think there may be an even better method available easily available to luthiers. I just remembered an old reference (in a knotmaking book as I recall) to a Native American who made superior quality ropes from basswood. So I did a bit of searching on Basswood brushes. These might actually make the best brushes possible for hide glue.

So I thought about what I know of basswood and realized that in the old world tilia goes by the name of linden. I looked up linden brushes and URL="http://http://www.howardcore.com/cgi-bin/shopper.cgi?search=action&category=TOOL&template=T emplates/Linden.htm&keywords=%22Linden%20Bast%20Glue%20Brus hes%22"]found this. [/URL]

Bob

[David Newton]

I don't get dark lines when I make a well-planed joint, but then I'm doing guitar tops, not mandolins.
I think it may have something to do with the "depth" of a mandolin-top joint.
Why you would see it only sometimes is a head-scratcher.

[Dale Ludewig]

As we pursue this issue, here's my situation. I melt my HHG in a glass container. Then it goes into little plastic squeeze bottle with stainless steel bolts and nuts (ala Mario Proulx) to hold the heat in. These go into a little crock pot. When I glue a joint with the HHG, no metal other than the stainless steel is ever in contact with it. I squeeze it onto the joint edge (heated) and if necessary I may quickly runt my finger down the edge. Then I clamp. Never a problem with Titebond. Once in a while with HHG.

FWIW, I've been in business since 1975 in the cabinet business. I've glued thousands of joints together. But, of course, the little dark line on the glue seam on an instrument top, shows up once every so often.... I think my glue's thin enough. Hmm.

[HoGo]

Have anyone of you inspected the black line really closely (microscope or at least strong lens)? I suspect there will be either tiny gap filled with clear glue that appears thicker than it really is. My glue is light brown caramel color and I've never seen this in well fitted joints. Whenever I could see a line I thought the problem with late camping or thick glue is on my side.
Two things that may make this happen is that either some pieces of wood will become more translucent when soaked with HHG or some pieces just act differently when gluing and the glue gels faster and prevents full closing of the gap....

[Skip Kelley]

Jim, I have had the same thing happen a few times! The line usually is very faint with the sunburst top.