This spring we cut down two old laburnum trees on my land
Together with a log i bought from a landscaper last year, I have quite a cache of laburnum, although a lot of it is very full of character.
From reading here, and other places, there's not a lot of information on the properties of Laburnum, and what is there is often conflicting, apart from a seemingly universal opinion that it ranks among the very best bow woods, being often compared to yew, and perhaps more appropriately, osage. It is also a very pretty wood, with dark chocolate heartwood (when aged), and a yellowish-white sapwood.
The TBB just lists its density at 0.82, identical to osage, and I have seen several people write that it is very elastic, and should be build narrow and deep. Like osage, it has pale thin sapwood, that would make a nice contrast to the dark heartwood, however, people don't seem to agree on the the merits of the sapwood. Some people include the sapwood in the bow and claim that such bows hold up fine, while others claim that it is no good, and should be removed.
I think the sapwood/heartwood bows I've seen look gorgeous, but wanting to test things out for myself, decided to prepare some scraps for testing, and to do tests on the wood I have around, to compare the properties of the sap and heartwood, as well as of laburnum in general. I decided not to do the standard bend test from TBB1, since I was doubtful of my ability to make a piece with the neccessary precision of the dimensions. So instead, i did what Tim Baker recommends three books later, and coupled a specific gravity measurement with a break test to compare tension/compression strength
Specific Gravity:First, I selected three samples of wood, a piece of heartwood from one of the trees on my land, a heartwood piece from the log that I bought, and a piece of sapwood, also from the log. I then dried out the three chunks of wood in my kitchen oven at 100 degrees celcius, weighing the samples regularly and noting the reduction of weight on a quality scale with 0.1 g precision, untill they stopped loosing weight.
I then submerged each piece of wood in water in a measuring container, quickly noting the volume increase with ~5ml precision. Weight in grammes/volume in ml equals specific gravity (thanks to the wonders of the metric system
Heartwood piece from my tree SG: 91.7g/135ml = 0.68
Heartwood piece from log SG: 111.2g/145ml = 0.77
Sapwood piece from log SG: 55.6g/73ml = 0.76
At this point I was quite surprised that the sapwood was the same density as the heartwood - and hopeful that this would speak well for the properties of the sapwood.
Breaking test:To compare the properties of the sap and heartwood, I prepared three ~20inch lengths of wood, all three from the 0.68 SG wood:
One piece of only heartwood, worked down to a single growth ring on the back, one heartwood/sapwood piece with a few growth rings of sapwood on the back, also worked down to a single growth ring, and finally, just for the hell of it, a 'reverse' piece with a sapwood belly, and a heartwood back, worked down to a single growth ring.
All three was worked down to almost finished dimensions, following the grain on the back, and then stored in my bow room, which has a dehumidifier set to keep a constant relative humidity between 47.5-52.5%. The pieces sat there for 4-5 months, so should be thoroughly dry. Before the test, I made each piece (more or less) uniform in thickness, I rounded all edges with a scraper, and roughly sanded the surfaces with 150 grit sandpaper.
I then bent each one over my knee, increasin the pressure gradually until it snapped. I'll comments each piece separately, note dimensions are very approximate, since I didn't actually measure the pieces):
Reverse piece: ~20 x ~3/5 x ~2/5 inch dimensions (left in the side-by-side picture)
This one took less set than I expected (but still a substantial amount) before it broke with a clear tension break. However, it looks like the break started on the edges that were higher than the middle (do to the back being the 'inside' of the wood), so it might have held up better if I had scraped down the edges to be level with the middle of the back (would one call that crowning or decrowning?) Overall though, it did take substantial set before breaking, despite the weakened back, so I would probably not recommend building a laburnum bow with a sapwood belly.
Heartwood/Sapwood piece: ~20 x ~3/5 x ~2/5 inch dimensions (right in the side-by-side picture)
This one took some set before breaking in a clean tension break.
Heartwood only piece: ~20 x ~1 x ~2/5 inch dimensions (middle piece in the side-by-side picture)
This one was much harder to bend (being substantially wider), and took massive set (see shape of the belly in pic above) before exploding in a complex break. Note how the fibres have a sort of 'hairy' look, from being ripped apart.
Conclusions:It seems the heartwood, as indeed most wood, is stronger in tension than in compression. However, there were no chrysals developed, and the wood seemed very tough and springy, so maybe it is compression strong, relative to other bow woods (as is osage and yew). The sapwood seemed overall weaker than the heartwood, at least in tension, however, the piece with the sapwood back did take some set before breaking. Thus a bow with a sapwood back would probably hold up well enough if well-tillered, and provided it is not an overstrained design (as also proven by several bows on this and other forums). Overall, each piece of wood felt very tough and springy, and was surprisingly hard to bend (keeping my limited experience in mind), so heres my take-home message:
TL;DR:- Laburnum sapwood is probably fine in a bow, if in a safe design, and with proper tillering, but does not have any advantages over the heartwood, being as dense, except for maybe aesthetics.
- For an optimal safety margin, or in a over-strained design, a heartwood only bow is probably best.
I would really like if this sparked some comments or discussion, so feel free to comment your reactions, whether you are surprised by the results or not, and also any criticisms you might have!
\Nicolas
