Bubbles in sinew backing?

[Marin]

Hey guys,
This is an issue I have always noticed when I sinew back though I think it comes down to aestetics, since I haven't had a bow fail because of this.
I often find as my sinew backings dry small bubbles appear to form in the backing. They don't appear too deep or seem big enough to cause adhesion problems. I once thought it was because my hide glue (I use store bought knox gelatin) was too thick. Thus, I always keep my hide glue on the thin side, more like thin syrup/slighty thicker than water as I have heard it is best made, so I don't think it's this problem. I make sure to soak the sinew, usally for an hour or more. I also dip it several times in my glue pot or even lay it in there for a couple seconds to make sure it's fully soaked in glue. If I have to quickly add a new strip, I may only soak it for a couple minutes in water until it feels soft, and often chew it several times to aid in quick softening. I'm curious if this is the problem, but the bubbles appear to be in multiple areas regardless of how long the sinew strip is soaked. Maybe it has something to do with glue temperature? I usually keep the glue around the temperature of hot bath water; hot enough to steam slightly but not too hot that it burns my hand or ruins the sinew.
I have attached pics of what I am seeing. The bubbles are hard to see but if anyone has had this problem before, this might look familar to them.
Am I right in assuming that this is more of an aethtic issue and not something that will lead to future problems? If it is, is there a way to fix it?

[Phillip King]

I would Try Rolling them Out with a Burnisher Or Mini Rolling pin

[Pat B]

Try lightly rubbing your finger over the spot. Where the sinew has adhered well to the wood the sound will be muted but where there is a bubble it will sound hollow.
 How did you prepare the back of the bow just before adding the sinew? Were your hands clean? It doesn't take much oil from a finger or a drop of sweat to prevent adhesion. Your glue consistency sounds about right and your sinew prep sounds good so IMO it is possibly there was contaminants(oils) on the bows back at sinewing time.
 If it is in fact oil on the bow it might be difficult to repair. You might try heating the areas where the bubbles are and wrapping it until completely cooled.

[superdav95]

I can’t see the bubbles you speak of in the pics but so long as they are not air bubbles I would say they are more asthetic and shouldn’t be a performance issue.  How are you laying your sinew down?  Do you lay down smaller shorter sections of bundles or do you lay down longer more measured layers?   In the past When I’ve laid several smaller bundles of sinew I noticed irregularities in the surface of my backing that I believe were the some of the feathered ends of the bundles curling up or twisting underneath other bundles.   Hard to know for certain but a possibility.  It functioned fine but this may be the same thing you’ve seen in yours.  I now since have switched my sinew method to longer more measured layers.  FWIW

[Marin]

Try lightly rubbing your finger over the spot. Where the sinew has adhered well to the wood the sound will be muted but where there is a bubble it will sound hollow.
 How did you prepare the back of the bow just before adding the sinew? Were your hands clean? It doesn't take much oil from a finger or a drop of sweat to prevent adhesion. Your glue consistency sounds about right and your sinew prep sounds good so IMO it is possibly there was contaminants(oils) on the bows back at sinewing time.
 If it is in fact oil on the bow it might be difficult to repair. You might try heating the areas where the bubbles are and wrapping it until completely cooled.

I washed my hands quite often before and during sinewing. To prepare the back I used soap and water and then brushed acetone on top and let that evaporate. I did that twice just to be sure. And this is also not the first layer but the second/third so it’s not adhesion to wood issues.
I did have to move the bow while it was drying but after the glue had gelled so I don’t know if touching it during drying caused any issues.

[Marin]

I can’t see the bubbles you speak of in the pics but so long as they are not air bubbles I would say they are more asthetic and shouldn’t be a performance issue.  How are you laying your sinew down?  Do you lay down smaller shorter sections of bundles or do you lay down longer more measured layers?   In the past When I’ve laid several smaller bundles of sinew I noticed irregularities in the surface of my backing that I believe were the some of the feathered ends of the bundles curling up or twisting underneath other bundles.   Hard to know for certain but a possibility.  It functioned fine but this may be the same thing you’ve seen in yours.  I now since have switched my sinew method to longer more measured layers.  FWIW

I would say these were more “medium” sized bundles, around 5 or 6 inches.
I cut the ends of my bundles to square them off and avoid the “feathery ends” from doing weird stuff, so I’m not sure if it’s that.

[Marin]

Also I should note that these bubbles are so small I don’t hear any popping or cracking when I push along these areas.

[Marin]

I'll attach some more photos of a different section of the bow where it is more noticeable. I tired in two different light settings. The loghter colored parts of hte backing is where the air bubbles I am talking about are. Some seem to be on the surface while others appear a bit deeper. In some cases the "air bubble just seems to be where the glue retracted without taking the sinew with it, bhich makes me wonder if maybe my glue isn't hot enough or wrong mixture or something.

[Marin]

I should also note, these bubbles really only appear after the glue begins to harden. I'm planning on sanding the back a bit and thin layer of glue so if the bubbles really are that shallow, I hope this help fill them in.

[superdav95]

Looks to me that this may be an issue only with your last layer.  Are you doing one solid layer or several layers and letting each dry well before the next layer.  I like to scuff between layer personally and depending on the bow or project aim I’ll do 2 or 3 layers letting each dry well.   I put a good layer of glue prior to next layer.  It could be you’ve starved it of glue between layers perhaps??? Just a theory….  You sinew looks pretty clean to me this being said.  I don’t foresee any performance issues.  Here’s a couple pics of my latest sinew.  It’s been drying for a few months now just almost ready to start going on it.  It’s ready and dry now but I’m giving it lots of extra time.  You’ll see that it has some lighter coloured inclusions and not totally amber in Color.  This is normal for sinew.  Help this helps. 

[Marin]

Looks to me that this may be an issue only with your last layer.  Are you doing one solid layer or several layers and letting each dry well before the next layer.  I like to scuff between layer personally and depending on the bow or project aim I’ll do 2 or 3 layers letting each dry well.   I put a good layer of glue prior to next layer.  It could be you’ve starved it of glue between layers perhaps??? Just a theory….  You sinew looks pretty clean to me this being said.  I don’t foresee any performance issues.  Here’s a couple pics of my latest sinew.  It’s been drying for a few months now just almost ready to start going on it.  It’s ready and dry now but I’m giving it lots of extra time.  You’ll see that it has some lighter coloured inclusions and not totally amber in Color.  This is normal for sinew.  Help this helps.

Yeah this is an issue I have seen priamrily with this layer. It did occur a little bit here and there for the previous layers.
I do usually try to do the layers searate, but usally give myself about a day or two in between applications. Usually I have to wait longer since I need to process more sinew (I apply my layers thicker, with bundles of about 30 strands of sinew or more).
In the case of this "layer" this was actually the crowning layer (so a thick strip of sinew going down the center of the back) for the second layer on this bow, which was ideally supposed to be the last, and I had put the full second layer on only a couple days ago so it had some time to try. I do usually try to sand and roughen up layers before putting on a "sealing" layer of glue in between applications, but for this crowning layer/strip I did not do that, so perhaps that played a part.
I'm going to sand and put a final layer of sealing glue on it tonight and since it appears most of these bubbles are shallow, I'm hoping that fixes it. For the deeper bubbles, I'm just going to cross my fingers that these doesn't lead to future problems.
So as long as the bubbles don't appear or sound like they are compromising the backing (like with crunching or popping sounds when I press) I should be fine?

[Marin]

Also, do you guys think maybe adding too much glue could be a potential cause? I notice that I tend to add glue pretty liberally, not only dipping the bundle in glue and not squeezing it but dabbing my finger in the glue pot and rubbing that on the strip once it has been laid flat. I always thought it was better to be more liberal in the adding of glue so you can avoid not adding enough, but now I am wondering if that is the cause.

[superdav95]

I had heard a general rule of equal weight of sinew to glue.  I pound and separate my sinew dry and then comb it dry with wire brushes.  I weight it all in layers per limb.  I then soak it in warm water overnight in seperate bowls in the pre weighted bundles.  Next day I wash it with dawn soap really well and put it back in clean water.   I prepare my glue using a glue pot and use granular hide glue.  I’m not a fan of the Knox gelitan as glue.  I’ve had mediocre results with Knox glue.  I know a lot of guys have good luck with it but my personal opinion.  I use 30-40% by volume glue to water as my mix for laying down the bundles.  Prior to doing any of the above I size the back of the bow and score it before laying sinew.  Sizing is vitally important in my opinion.  I do about 7-8 or more size coats with very thin hide glue.  Pretty much water with a little glue.  This allows the watery thin glue to seep down into the wood pours of the back of the bow.  This ensures good adhesion.  When sizing is done and ready to lay the bundles I take my bundle out of the water it soaks in and squeeze out as much water I can with fingers then dunk it in the hot hide glue squeezing out the excess then dunk again doing the same and lay it down and smooth it out.  I also put a good fresh coat of glue down just prior to laying bundle down.  Hope this helps.  I’m sure there are lots of ways to do this but this is basically how I do it.  A little heat when sizing the bow is also good idea too. Best of luck. 

[Marin]

Thanks I'll keep this in mind,
I guess I have only been soaking the sinew but I never would have thought to wash sinew even with dawn dish soap.
So just to follow up on what you said earlier, you don't think these bubbles will provide any issues? Like are bubbles only of concern when they are pretty big and between the sinew and wood?
 

[Pat B]

If you read the info I posted while I was building "Elg Bue" the copy of an ancient Danish bow that I added a sinew backing I went through my process for prepping the sinew and the bow's back...basically I soak then wash the sinew in warm water and Dawn dish soap and rinse it with warm water. Then I scrub the back of the bow with Dawn and rinse with boiling water. I do these preps just before I start the sizing coats then continue by adding the sinew to the back of the bow. I like to start the sizing and sinewing while the back is still damp so I'm sure the sizing coats soak into the wood.