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| Coals of douglas fir; the scraps from Surewood Shafts arrow shaft production |
Last night, I heat tempered an ocean spray (
Holodiscus discolor) bow over hot coals. This is the first bow that I have experimented with heat tempering on. The process itself is really enjoyable. It is reminiscent of attempting to slow-roast a marshmallow to perfection. I built the fire simply for the purpose of tempering, or heat-treating as the process is also known. The scrap pieces of Douglas-fir I had on hand ended up being ideal for getting a hot bed of coals established in no time at all. I know that a lot of bowyers heat-temper bow limbs using an electric heat gun, and while the heat gun I have is capable of doing the work, I can't see it doing the job nearly as well as this bed of hot coals. My fire pit is not very big, just large enough to do one limb at a time, and so the coals need to be stirred and built up between treating each limb. Even then, the treating itself was less than 10 minutes a limb. This particular ocean spray bow was not the most ideal candidate for heat-treating as it has a pithy center, and where this pith ran just under the surface of the limb's belly, a fissure would begin to open up during the heating. While these fissures are quite noticeable, they should only pose comsetic issues and some minor tiller touch up. While Marc St. Louis, Canadian bowyer that is expert in heat tempering, recommends avoiding such pithy centered sapling staves, I couldn't resist after reading that natives of the northwest would use fire-hardened ocean spray for everything from clam-digging tools to wooden nails. The wood has definitely hardened from the tempering process. I used a cabinet scraper to thin the limb tips a touch and found the wood to feel very dense. Also, it now has a very pleasant aroma. One step worth mentioning in the process that might differ from others is the use of shellac as a sealer on the bow prior to heat-treating. I applied several thin coats of shellac (fresh blonde shellac flakes dissolved in denatured alcohol) followed by several more thick coats. Shellac is reported to plasticize under high heat. My thoughts are that the thin cut of shellac will penetrate into the wood pores, and then during heat-treating will plasticize, possibly even binding with the lignan. If not it should still function to regulate the loss and re-gain of moisture during and after the heat-treating process. By the way, shellac is a really interesting and versatile natural material produced from the shellac bug. If you are interested in learning more on shellac, check out
Vijay Velji's video http://youtu.be/lQcQ0yuekZ0, where he travels to India to learn about shellac. I am looking forward to stringing and drawing on the bow to feel the effects of the tempering, but that will have to wait as the bow will need to rest in the garage and come to a stable moisture content after the heating session.
Well, how did it work out? (Re: heat treated ocean spray, Nov 2012)
ReplyDeleteThe heat-tempering of the ocean spray bow turned out great! It made a very quick shooting 50# longbow. The bow retained nearly all of its initial, natural reflex. Ocean spray is a dense wood to begin with and this heat-tempering made it even more dense. It did up the poundage of the bow abit, I would guess roughly 5#, but after touching up the tiller and finish sanding, I was back to where I was in poundage prior to heat treating. The benefit being that the bow took no set.
ReplyDeleteI have heat-treated several other bows since writing this, using both a heat-gun and the hot coal method. The heat-gun is convenient and always at the ready, but does take much longer, and requires close attention to get even tempering, while the hot coal method requires all the work upfront. It takes time to build up a good, deep bed of coals, but a thorough tempering can be accomplished with ten minutes or less per limb.