Heat treating bamboo backing strips?

[Eric Garza]

When folks make bamboo-backed bows, do you heat temper the bamboo backing strips? If so, do you heat-temper them before you sand them down for glue-up, or do you sand and shape them first?

[Hamish]

The traditional theory is don't temper a boo backing. Tempering is traditionally used for belly facing or laminated belly staves. Tempering increases the compression resistance of boo, but makes it more brittle in tension.

You can temper the boo, in full thickness or partially worked down. I wouldn't temper at final thickness as it will cup and you might get charring on the edges.

You can put boo backing in a hotbox to drive out excess moisture.

[superdav95]

Eric.  I have tempered my boo backing strips.  As hamish says mainly to get rid of moisture.  It also provides more strength but at a risk.  I have made many attempts and failures.  The biggest thing was figuring out the thicknesses.    I have heat treated my belly lams too.  I have a few bows done now with using this method and they can be hit and miss.  When they miss they go bang and when they hit they work very very well.  I would go easy with the heat on the backing strip.  I use my belt sander to get the strip down to about .150-.200” thick.  If using typical 2” wide strips from echo or elsewhere online or if making your own from poles select the very best quality of backing with the least amount of irregularities at the nodes.  Space out your nodes well to account for more even stresses on both limbs.  I use a radiant heat source to temper my backings and go until it darkens the belly of the strip a little. Just little toasted.  I do not toast the rind side at all on these builds.   You will see steam exit the ends as you do this.  You will also notice that the flat sanded surface of your strip will have rounded or crowned.  I also like to weigh my backing before and after temper to observe water moisture loss.  I then take this down from here to my final dimensions depending on build.  For a long bow that’s fairly narrow around 1.25-1.3” wide at fades tapered out to the tips I’ll make the thickness at handle area about .125” thick and gradually tapering out to the tips at around .090-.095” thick.  It’s important to keep it pretty thin otherwise you get a boo strip that overpowers the belly wood.  Once I get the tapers looking good I trace it out on my bow the way it will sit.  It’s best to choose good boo strips.  The less natural crown on your boo the better.  This way it will less chance of it overpowering you belly wood.  Another thing to remember is that boo adds a lot more draw weight then you think.  Always go thinner then you think when building and laying it out.  I use ea40 mixed according to can instructions.   I’ve posted a few of these build here on pa over last year or so and they all held a crazy amount of reflex and are very fast bows.  You need a stringer to safely string them but once they are strung and tillered well they make very fast bows.  I’ll see if I can dig up one of my earlier posts from this year.  Reach out if you need to.  I don’t mind sharing what I’ve learned with these bows.  Best of luck.  Here’s a link to one of these bows from few months ago. 

http://www.primitivearcher.com/smf/index.php/topic,72796.msg1021113.html#msg1021113

[Eric Krewson]

Years ago, there was guy in Texas that was selling what he called Tonkin bamboo, being younger and not as worldly as I am now, I bought some. This bamboo had been flame tempered, the back was dark brown and the tips of the nodes were slightly charred.

Compared to the standard Mosa I had use up to this point, this bamboo was very strong and made superior bows. I ordered more and the guy sent me junk slats with cracked backs. When I questioned him about what he sent he got nasty, I realized he wasn't an honest person and later found out that Tonkin cane never got bigger than 3" in diameter and there was no way this was Tonkin anything. I suspect these slats were flame hardened Mosa, the backs were almost flat so it came off of large diameter bamboo poles.

If you watch the Japanese bow makers on you tube you will see that they flame harden their bamboo and have been doing it for thousands of years so there must be something to it.

 

[Eric Garza]

Thanks for the responses! I have another question: Is there a benefit to using bamboo backing strips with fewer nodes on a bow, or does it not matter?

[superdav95]

Yes I think there is a benefit to less nodes.  The reason for this is that it’s a strong point on the back that can cause a flat on the belly.  This could lead to a hinge if not careful.  If done builds with both only one node on bending sections and others with 2-3 nodes just fine too.  Just something to be aware of though is that the concentrated strong point at the node on the back.  Also watch for the quality of the boo strips to get the least amount of ramping at the nodes and the flattest you can get if sorting through them.   I make my own slats from large moso poles for this very reason.  To ensure I get what I want in my boo strip.  To go to all the trouble of building a boo backed bow justifies the extra work imo. 

[Eric Krewson]

Early on I used any bamboo I could find; some slats had a lot of nodes. I didn't have a problem tillering a bow with more nodes but I did have a few backing failures at the nodes, probably because I used to sand down the nodes too far. I prefer the looks of a bamboo backing with fewer nodes.

I found a patch of Mandrake across town where I could buy whole culms for $15 each and never ordered bamboo ever again.

This stuff has widely spaced nodes and is very strong, unfortunately the land sold and the new owner bulldozed the patch.  I don't make many bows now but I have a lifetime of slats put back if I decide to make a bow.

[superdav95]

Ya that looks like good stuff there.  I’m assuming the green Color was when you first got it harvested?  Must be different color now I imagine.  Did much of it crack on you?

[Eric Krewson]

Yep, just cut, it turns tan with a hint of green when it dries, it is tan under the rind. I knock out the inner nodes with a hoe handle and let it dry intact before I cut the slats, I don't get any cracking that way. I get too much cupping if I try to dry it in slats.

[superdav95]

Yep, just cut, it turns tan with a hint of green when it dries, it is tan under the rind. I knock out the inner nodes with a hoe handle and let it dry intact before I cut the slats, I don't get any cracking that way. I get too much cupping if I try to dry it in slats.

Yup exactly what I do.  Sometimes I need to really pic through my poles as they have been mostly dry already from my supplier which can be a good or bad thing.  He punches out the holes in middle to avoid cracking but still happens.  Over the winter months I’ll go out to my shop and hear cracking from my bamboo due to drop in relative humidity.  I can usually work around the linear cracks to get usable slats and section for bows regardless. 

[Hamish]

I haven't found there to be a great deal of difference in shrinkage across the slat, when the culm is split when green. That might be due to where I live, and the temp, and relative humidity, and type of boo.

You guys could go a step further. After you have knocked out the inner nodes, you could split or saw down one side. That would provide stress relief, and probably stop any loss from uncontrolled splitting if left in the round.

[superdav95]

Ya hamish the guy I buy my poles from has an option for buying them in half rounds too.  I found that if this is done when green they can warp a bit.  I’ve done dry Half rounds and been fine though. 

[Eric Krewson]

It must be where you live. I live in NW Alabama, hot and humid in the summer. Because I cut my own bamboo, I cut it long so I have a node on the end of the cut, the node is less likely to split. I run a bead of superglue around the ends most of the time like one would seal the end of a stave.

Any checking I get is only at the ends and seldom runs down the culm more than an inch or two.

I don't store bamboo in the raw slat form, I run it through my bandsaw to thin it as much as possible, then make a few passes through my jointer or beltsander with a 36grit belt to level the belly. It looks like this when I put it on the storage rack.



I did run into something new the other day; I made a bamboo osage bow to donate to a charity, the charity never came after it so I put it on a bow rack near the ceiling of my shop where it has been for 3 years. I got it down the other day and found that the bamboo had cracked on the finished bow without it being shot. I will take off the finish, superglue and sand the cracks and refinish it.

[superdav95]

I don’t normally store raw slats either for this reason too.  My climate is the reason I believe for the checking.  We are very humid in warmer months and quite dry in cold with relative humidity.  This change in humidity becomes the issue.  I could store them inside heated space but will continue to do what I’m doing as it has been working fine.  I end up with some waisted bamboo but can usually use it for other projects.   

[Hamish]

Yes, I think local climates must have an impact on how the boo reacts. I can pretty much leave it, and it doesn't degrade from checking, or warping.
Green wood, even when heavily sealed is another matter, it doesn't like the hot Summers where I live. An hours drive away, going up in altitude, and dropping in temperature, my friend can just leave wood unsealed and gets minimal to no checking, in wood normally known for checking.