Gluing Horn - Air bubbles
My first bow failed from a poor horn / core glue joint. Part of the problem was from poor grooving and the rest from air pockets & voids in the glue joint. I have solved the grooving problem but I am concerned about the air pockets. On my first bow I sized the horn and core with 3 coats of 10% glue and then heated up the parts and applied 35% glue and used C-clamps.
After some research I think applying approximately 20 coats of glue, with the first 10 coats using 10% glue and the next 10 coats with 15% glue. Then use 35% on the final glue up.
My problem is with very tiny air bubbles that develop in the bottom of the grooves. After each coat I lean the horn and core on edge to dry, but I am finding very tiny air bubbles. Just a few on the wood core but there are more on the horn. Should I be worried? Is their way to apply the thin coats of glue without the air bubbles? I have run a knife edge in the bottom of the grooves to open up the bubbles. What have others experienced?
Here you can see the maple core, note there are a few very small bubbles in the bottom of the grooves.
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Here on the horn it is hard to see but it has the same sized bubbles as on the core just more of them.
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After some research I think applying approximately 20 coats of glue, with the first 10 coats using 10% glue and the next 10 coats with 15% glue. Then use 35% on the final glue up.
My problem is with very tiny air bubbles that develop in the bottom of the grooves. After each coat I lean the horn and core on edge to dry, but I am finding very tiny air bubbles. Just a few on the wood core but there are more on the horn. Should I be worried? Is their way to apply the thin coats of glue without the air bubbles? I have run a knife edge in the bottom of the grooves to open up the bubbles. What have others experienced?
Here you can see the maple core, note there are a few very small bubbles in the bottom of the grooves.
[external image]
![[external image]](http://i334.photobucket.com/albums/m426/bheggeseth/turkish%20bow/IMG_1512-1.jpg){kind=link}
Here on the horn it is hard to see but it has the same sized bubbles as on the core just more of them.
[external image]
![[external image]](http://i334.photobucket.com/albums/m426/bheggeseth/turkish%20bow/IMG_1511-1.jpg){kind=link}
I am using 100% sturgeon air bladder glue heated to 145F (63C). My workshop is approx 65F (18C). For the first coat I did use a heat lamp to warm up the parts, I could very easily handle the horn and wood core.
put the coated pieces on the side as adam karpowicz says in his book,never had a problem and you might be clamping them too tight and starving the glue joint.
To me it sounds like you do everything right (or according to the book, hehe). The grooving looks good to me.
I think 3 coats before glue-up is too little though. You will probably not have any trouble with it if you go with more coats and perhaps a little thinner glue for the coating layers.
You might also consider not glueing in too much reflex in the bending parts of the limbs, and rather pull the limbs into reflex with the sinew layer. A straight horn/wood limb that is pulled into reflex by the sinew, should not delaminate as easily as one with a lot of reflex glued in between the horn and wood. I think Adam suggests a radius of 12 inches as a safe amount of reflex for the turkish design.
I think 3 coats before glue-up is too little though. You will probably not have any trouble with it if you go with more coats and perhaps a little thinner glue for the coating layers.
You might also consider not glueing in too much reflex in the bending parts of the limbs, and rather pull the limbs into reflex with the sinew layer. A straight horn/wood limb that is pulled into reflex by the sinew, should not delaminate as easily as one with a lot of reflex glued in between the horn and wood. I think Adam suggests a radius of 12 inches as a safe amount of reflex for the turkish design.
I added another coat of glue and now the problem with air pockets has really showed up on the horn. I have washed off the glue and will start over. I was tipping the horn at an angle while it dries, this time I will stand it completely on edge.
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![[external image]](http://i334.photobucket.com/albums/m426/bheggeseth/turkish%20bow/IMG_1513-1.jpg){kind=link}
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![[external image]](http://i334.photobucket.com/albums/m426/bheggeseth/turkish%20bow/IMG_1507-1.jpg){kind=link}
10% glue for sizing is too concentrated. You need something like 2% - very thin (hot) and able to wet the horn better. 15 coats, horns dried on edge, should be enough for a glossy coat, clear of the bubbles.
Adam
Adam
Another thing that is worrisome: you are using a heat lamp to dry the glue faster. Glue dried with heat and not allowed to cool to form the gel at all will be brittle when dried. You can use a fan, but no heat.
Good luck
Adam
Good luck
Adam
Adam, I have taken your advice, I mixed up some 2% glue and put the first coat on the horn. No bubbles. Now I am thinking I should start over on the maple core as well. I am a little concerned about the core getting too wet and softening the handle splices.
Also, I was using the heat lamp to heat the parts before the glue aplication, not for drying. My shop is at about 65F, is there even a need to heat the parts before applying glue?
Also, I was using the heat lamp to heat the parts before the glue aplication, not for drying. My shop is at about 65F, is there even a need to heat the parts before applying glue?
Below 60deg F it will be safer to warm the parts. And I would not worry about the core splices weakening from the thin glue.
Adam
Adam
Adam, thanks again, I will take your advice. In regards to the handle splice I was concerned about washing the sizing glue off of to remove any trapped air bubbles in the grooves.
I went ahead and washed off the glue from the core and horn and have started applying 2% glue without any heat. I will make sure the shop stays above 60F.
I went ahead and washed off the glue from the core and horn and have started applying 2% glue without any heat. I will make sure the shop stays above 60F.