"Zatara," bamboo backed osage bow, part 3 - heat treating

As I learned from my experience with trying to heat my yew "fence post", you don't heat treat an already glued bow. So I'm heat treating the rough-tapered osage limb sections before I put the bamboo backing on. This should give me an opportunity to straighten the deflex out of them as well as hopefully improve the performance of these marginal leftovers.

All of the heat treating I've done before was done by simply holding the gun and waving it back and forth over the limbs until they changed color. It's tedious and time consuming. I expected the latter, but the former has got to be changed. So, I built a holding jig to keep the gun about 5 inches from the limbs, as you can see in the photo below. So as to evenly temper it, I'm heating only one inch of limb at a time. The first limb took about 5 minutes. At 29" of limb to heat, this is about 2.5 hours of time per limb! Fortunately, I had other things to do (clean the workshop), so it was not too big a deal to multi-task. Set the timer, then move the whole limb one inch to work the next section. Repeat.

 5 minutes seemed like a long time, but I was going by the color change. At some points 4 minutes seemed more appropriate, so I switched to that as needed. I think part of the reason it took so long was that the doors to the workshop were open, allowing some draft to pass through and rob the limb of its heat. However, I didn't want to overdo it, so I limited the time. The resulting limb looked somewhat splotchy, so I ended up treating it in a separate session later, at about 2-3 minutes per inch. I wanted the treating to really permeate the limb, and the result was a nice even deep brown.

Untreated handle on the left; treated limbs on the right.

For the second limb, I did the same routine of 4-5 minutes per limb -- but with the garage door closed, limiting the draft. Apparently this made a significant difference. After I nearly finished the limb I noticed that there were several small cracks throughout the wood! These appear to be small drying or stress cracks. Were they already there, exacerbated by the heating; or were they the direct result of heating?

Cracks can be seen in the limb on the right.

Upon inspecting the first limb, I discovered a few of those there as well, but localized to one small area as opposed to throughout the limb, as they are on the second one. As a result, I've decided that I should go back to the manual method of heating, or reducing the time if using the jig. I've also designated the "crackled" one as the upper (weaker) limb.

I don't know if these cracks will have much effect on the final bow. The cracks shouldn't pose a problem in tension, as the bamboo back will be doing the work there, but they may very well be a sign that the limb got over-treated and will crumble or fret in use. Or maybe not.  I could stop, but I've got nothing to lose but my time at this point, and I'll learn something in the process. I figure that once the bamboo back is glued on, I can hit the cracked areas with low viscosity super glue as a hedge against problems.

Time (and experience) will tell.

"Zatara," bamboo backed osage bow, part 2 - rough limb tapering

As mentioned in the last post, the outside of the stave section from which the limb section was taken was in pretty bad shape. My hope was that by turning the back of the staves into the belly of the limbs, the problem areas would come off in tapering and tillering. Since this is going to be a backed bow, I can really treat these sections as lumber and go ahead and give them a rough taper before glue up. I'll refine the taper/tiller after it's all glued up and already starting to bend.

The limb thickness of the osage is currently a nominal half inch (just under), and I want to taper it out about 1/8" along the limb. I'll start the taper just outside where the handle section will get glued over the splice. The pieces are long enough that I can get a full 66-67" nock to nock length. Since the overall length is close to 69" and my handle is 9", I'll plan to taper 29" for each limb, which will give me a half inch margin on either side of the handle as a "safety zone" to eventually bring down when tillering.

The ideal way to taper this dimensional stock would be to use a planer, which would allow me to take 4 passes of 1/32" and call it a day. I don't have a planer, so I'm going to go to the bandsaw instead. When I first started building bows, I took a bow class in which we built bamboo backed ipe longbows by first tapering the stock by cutting it on the bandsaw using a .005" per inch pre-tapered piece of wood attached to the stock. On 29", I calculate that this makes closer to 5/32" than 1/8" (.145" total taper, to be exact), but in practice I have found that it is close enough to what I want that it works just fine. It generally means that most of the refining I do to the tiller happens in the inner limbs first.

This job is going to be a little trickier, as the stave originally had some good reflex to it. Since I'm orienting the grain in the the opposite direction, I'm actually fighting the wood a bit. Here is one of the future limbs next to my pre-tapered stock. One can see the amount of the reflex pretty clearly. In order to keep it in place for tapering on the bandsaw, I'm going to use my favorite secret weapon: blue painter's tape!

Here you can see the limb section taped to the pre-tapered stock. You'll also notice blue clamps in place. I use those to hold the limb in place against the stock so it is as flat against it as possible. The taping actually uses a technique I picked up from lutherie: One stretches the tape while applying it so that it exerts some clamping/pulling force. How much does it stretch? Not much, but enough to create some reasonable force on the two pieces, especially when applied in quantity as it is here. How much stretching is too much? If the tape snaps, you've gone too far. It's a nice self-correcting exercise.

 

Because the limb section is already at its maximum thickness near the handle, setting up the bandsaw fence is relatively easy: I just put the taped up piece next to the blade just outside of where I want the cut to stop, then slide the fence over until the stock is slightly snug against the blade and the teeth of the blade catch the tape when trying to pull it through. This lets me know that the blade will effectively stop cutting at that point -- plus there's that half inch "safety zone" I mentioned.

All the prep work for taping it in place took a considerable amount of time, but putting it through the bandsaw couldn't have gone smoother. Because of the direction of flex in the piece, as each part is cut and is freed from the tape, the section actually pulls away after the cut, which means that the taper is pretty consistent. Luck also appears to be on my side, as the bad sections of the piece have come just away in the tapering process. You can see the before and after in the photos below.

Limb 1: before

Limb 1: after

Limb 2: before

Limb 2: after

"Zatara," bamboo backed osage bow, part 1 - salvaging

This series of posts is really a sequel to my "Driftwood" series describing the making of a sister bow from the salvaged offcuts of that project.

In the process of getting to a thicker growth ring and a usable length from the basket-case osage piece Jarrod gave me (which yielded the short selfbow, "Driftwood"), I removed a large, flattish section of the outside of the osage (including some sapwood) by using the bandsaw. I actually had to go down a little more than a half inch deep (if you remember from this post, the stave had already been worked down through several layers on one end), and the resulting offcut was  great than 1/2" thick and a little over 2" wide.  I couldn't very well throw the piece away, so I decided to see if I could get a bow out of it by recutting and splicing, and backing it with bamboo. Here's a bit of information about what I'm working with:

First, the 9" bow handle section was removed from one end of the offcut. One can see a trace of sapwood in the photo below. This is probably the best piece of wood out of the entire original stave! I thought it would be cool to be able to incorporate more of the stave back into the bow, so this section will make its way into the handle.

Also shown is the remaining section (original stave belly side, which will end up being the back gluing surface -- more in the next paragraph), already sawn into two pieces, about 38" in length and a little over 1" wide at the widest end (the narrower end tapers to 3/4"). You can probably also see the extreme coloring in this osage piece -- lots of red streaks. One of the major ones was bad in the section that eventually became "Driftwood," but these seem pretty solid. At any rate, they'll captured in the interior of the bow near the bamboo back, far away from the compression of the belly where they could do real harm.

The original stave, as I mentioned in the post "Driftwood part 1" was pretty well worked down in odd places on the back when I first got it. Normally I would try to orient flatsawn grain so that the original stave back would be next to the bamboo backing and the original stave belly would be the bow's belly. In this case, because there is so little wood to work with and because the the back was in such poor shape, I'll need to make the stave back into the new bow's belly with the hope that all of the bad spots will come off in the tapering and tillering. You can see the spots in the photos below. They are incredibly dark, as the original stave had been sitting around for some years and the exposed wood had oxidized. It's a beautiful color -- and the bright yellow of the freshly cut osage gives you a real appreciation for just how much the color changes over time!

It's probably not apparent from the pictures above just how marginal the wood is in places. The center photo shows a fairly deep cut that looks like the back of the stave encountered a radial saw! I'm really hoping that most of that will come out in the tapering and tillering -- but it's going to be close. The bottom picture shows a nice void in the middle of the stave.

Of note: Orienting the back of the stave toward the belly of the bow in the glue up does cause the rings to travel in the same plane as the eventual radius of the belly, creating the potential problem of having large sections of soft earlywood exposed, which could weaken the bow or cause hinging. However, I'm at least aware of the problem going in, so I'll know to look for it and correct for it wherever / whenever possible.

As a further appreciation for just how marginal this wood is, I've taken a couple of photos of the sides. These were the original sides of the stave, and they had all sorts of sections missing. I'm not sure that the photos do it justice. For example, what looks like 1 3/8" width on the sawn section is really reduced to 1" of usable wood once these wood voids are accounted for...

In case you are wondering about the scrawled "NOT" on the side of the left-most piece of wood in the first picture above, it was a note to myself when preparing to cut the original stave on the bandsaw. One side of the stave had some good relatively flat spots that traveled well on the bandsaw table and allowed the stave to be stable, whereas the other side had all sorts of dips that spelled trouble. Though it was fairly obvious which was which, I did not want to leave anything to chance, so I wrote the word "NOT" to remind me not to use that side against the saw's table. I hate to think that its vestige might be a sort of prophecy, telling me not to attempt to salvage this wood as a bow!

To conclude this post, I guess I should explain a little bit about the name. I was telling my friend Nick about the "Driftwood" bow I was working on and how I thought I might have enough for a second bow. I was casting about for a potential name.  "Firewood" had made the list, as it was a) appropriate! and b) reflected the spirit of the red streaks I kept finding, but Nick suggested "Zatara," the name given to Edmond Dantes in the movie version of The Count of Monte Cristo. The name was supposed to mean "driftwood" in the movie (a search on the 'Net reveals influences of Italian, Spanish, and Arabic, and it might actually mean "raft"), though there is this tidbit from one of the Yahoo! Answers pages:

Zatara is a surname. The most famous Zatara was a magician that died on the Titanic. Shortly after his death, ther were a few jokes that came about. The most famous that stuck was: The Great Zatara was performing on the Titanic. One day a man with a parrot was in the audience. This parrot started to heckle The Great Zatara. Parrot voice: “Browk, it is in his hat!” Parrot voice: “Browk, it is his hand!” Parrot voice: “Browk, it is his pocket!” That night the boat hits an ice berg and sinks. The Great Zatara wakes up on a piece of driftwood with the Parrot. Two days go by with the two of them just staring at each other – really glaring! Then the Parrot says, “Ok, I give, Where’s the boat?” The producers of the movie The Count of Monte Cristo heard that joke and gave tribute to The Great Zatara by giving the name to Edmond Dantes and changed it to mean driftwood. (There are ancestors of The Great Zatara that have kept the name. They are mostly in Ireland.)

Regardless of whether or not it is a real word, I liked the connection to the name of its sister bow, and that making a working bow out of this other piece of "driftwood" is going to take some magic. That, and I already promised Nick I would make him a bow, and this might as well be it. (Unless it doesn't turn out!)