Posted this 20 years ago, Jan. 30-01. New wood bowmakers might not be fully aware of the value of knowing the specific gravity of a new wood, so here's one from the vaults: 

"There is a fairly direct relationship between the density of a particular wood and how wide a bow should be if made from that wood, all else being equal. Knowing a wood's SG can eliminate two or three trial and error bows sometimes needed to evolve the best design for a new wood. But many woods don't have published SG numbers, especially a new potential bowwood you've discovered. And many staves within a species can be much more or less dense than average. Here's a way to determine SG at home.

From the wood to be tested fashion a sample ten-inches long by 1/2" square. Be careful to keep thickness uniform it's entire length. Dry the sample in an oven at 200f [make that 150f] until it looses no more weight. An arrowmaker's grain scale or similar is useful here. Scales rated at more that a very few ounces will not be accurate. Leave the sample in the oven and go about your business, checking it every couple of hours. When it's lost NO, absolutely No more weight for two hours it's dry.

Make pencil marks at 4, 5,and 7 inches. then float the stick upright, encircled by you fingers, a wire loop, or a clear tube. Make certain it's perfectly free to find it's level. If six inches float above the water the wood is .40 SG; four inches above and it's 60 SG, etc. .."

Now we need a chart for different styles of bows giving the widths of bows recommended for a particular SG and length of bow.

I know we already have plenty of charts, I just can’t recall one based on SG.

For example: I’m working on a birch bow now. Pretty sure it has a a low SG, but I can go test it.

My next decisions are how long and how wide to make it. Without some specific tried and true guidelines I’m thinking I’m going to go about 66’ and extra wide. But how wide is too wide?

Maybe there is no such thing as too wide, but then if that is the case why worry about SG in the first place?

Do you know which species of birch you have? SG ranges from white's .55 to yellow's .62

Once you've made a low-set bow of a known SG  it's easy to know the safe width for the same design from another wood, just make it narrower or wider per the % SG difference.

"Maybe there is no such thing as too wide, but then if that is the case why worry about SG in the first place?"

If wider than need be the extra limb mass slows the bow, and the extra work slows the bowmaker, and  the extra stave width increases cost or effort.

Tim had a list of suggested widths in TTBB Vol. I, IIRC

Birch,and Walnut can make a good bow once you have learned the properties of the wood in the building process, but I guess that can be said of any bow wood. I mention those woods, because they are not real popular with bowyers, and many consider them 2nd string wood.

My goodness, Tim. I remember that SG method. Welcome back. Jawge

George!!! Great to see you here. For ages I thought the leatherwall was head. MY last posting here appears to have been 1-30-01, shortly after the great white wood wars and the MoJam shoot-off. Looking forward to posting occasionally again, but mainly whittling bowmaking ideas with you.

I know SG is different than weight alone. Tim is there any correlation with the BTU of a wood. Generally the more dense or the heavier the wood the more BTUs it gives.

For instance Beech is very dense and weighs more than many woods and gives some of the most BTUs. Does weight of the wood correlate to being able to make a narrow short limb design?

So wood beech make a narrow limb bow? I’m not sure if it’s considered a white wood.

Tim thank you for all the research you did in the Bowyers bibile. That book got me where I am today. I don’t understand all the math but just the chapter on design and performance was Instrumental for my success. Thanks again. Arvin

Tim- thanks for the following statement as it will be helpful. “ Once you've made a low-set bow of a known SG it's easy to know the safe width for the same design from another wood, just make it narrower or wider per the % SG difference.”

Without making too many bows of a particular design, this is where it would be helpful for others to report and collate data on SG for their low set bows of specific style and length.

The recommended width charts in TBB series probably are close to what would be achieved by this method.

Steve Gardner's (badger) method of biding bows based on mass is directly related to the SG. of the individual piece of wood you are working on.

Altitude:

Beech is a little heavier than red oak, for example, so all else equal, yes, beech could be a little narrower or a little shorter.

But don't go by published SG numbers alone. Depending on growing conditions SG of a species can vary widely; a twenty point swing not too uncommon. Best clue is ring count: in conifers the more rings per inch the denser/stronger; the opposite for hardwoods. 

As for BTUs, not my field, but denser woods means there's more wood in the wood, so more BTUs

Tim

Thanks Tim. This is what makes wood bows interesting and challenging.

It appears to follow the BTU charts or tables pretty close. For the same reasons you gave. Density of the growth rings. And weight

Species Weight (lbs./ Cord) Green Weight (lbs./ Cord) Dry Heat per Cord (Million BTUs) % of Green Ash Ease of Splitting Smoke Sparks Coals Fragrance Overall Quality

Easy Moderate Good Slight

Apple 4850 3888 Species Weight (lbs./ Cord) Green Weight (lbs./ Cord) Dry Heat per Cord (Million BTUs) % of Green Ash Ease of Splitting Smoke Sparks Coals Fragrance Overall Quality Alder 2540 17.5 Easy Moderate Good Slight

BTUs. Apple 4850 3888 27.0 135 Medium Low Few Good Excellent Excellent Ash, Green 4184 2880 20.0 100 Easy Low Few Good Slight Excellent Ash, White 3952 3472 24.2 121 Medium Low Few Good Slight Excellent Aspen, Quaking 2160 18.2 Easy Few Good Slight Basswood (Linden) 4404 13.8 69 Easy Medium Few Poor Good Fair Beech 3760 27.5 Difficult Few Excellent Good Birch 4312 2992 20.8 104 Medium Medium Few Good Slight Fair Boxelder 3589 2632 18.3 92 Difficult Medium Few Poor Slight Fair Buckeye, Horsechestnut 13.8 69 Medium Low Few Poor Slight Fair Catalpa 4560 2360 16.4 82 Difficult Medium Few Good Bad Fair Cherry 3696 2928 20.4 102 Easy Low Few Excellent Excellent Good Chestnut 18.0 Good Good Coffeetree, Kentucky 3872 21.6 108 Medium Low Few Good Good Good Cottonwood 4640 2272 15.8 79 Easy Medium Few Good Slight Fair Dogwood 4230 High Difficult Few Fair Douglas-fir 3319 2970 20.7 103 Easy High Few Fair Slight Good Elm, American 4456 20.0

100 Difficult Medium Few Excellent Good Fair

BTUs Elm, Siberian 3800 3020 20.9 105 Difficult Medium Few Good Fair Fair Fir, White 3585 2104 14.6 73 Easy Medium Few Poor Slight Fair Hackberry 3984 3048 21.2 106 Easy Low Few Good Slight Good Hemlock 2700 19.3 Easy Many Poor Good Honeylocust 4640 3832 26.7 133 Easy Low Few Excellent Slight Excellent Juniper, Rocky Mountain 21.8 109 Medium Medium Many Poor Excellent Fair Larch (Tamarack) 3330 21.8 Easy-med Many fair Slight Fair Locust, Black 4616 4016 27.9 140 Difficult Low Few Excellent Slight Excellent Maple, Other 4685 3680 25.5 128 Easy Low Few Excellent Good Excellent Maple, Silver 3904 2752 19.0 95 Medium Low Few Excellent Good Fair Mulberry 4712 3712 25.8 129 Easy Medium Many Excellent Good Excellent Oak, Bur 4960 3768 26.2 131 Easy Low Few Excellent Good Excellent Oak, Gambel 30.7 Oak, Red 4888 3528 24.6 123 Medium Low Few Excellent Good Excellent Oak, White 5573 4200 29.1 146 Medium Low Few Excellent Good Excellent Osage-orange 5120 32.9 165 Easy Low Many Excellent Excellent Excellent Pine, Lodgepole 21.1 2610 Easy Many Fair Good Fair Pine, Ponderosa 3600 16.2 81 Easy Medium Many Fair Good Fair Pine, White 2250 15

Easy Moderate BTUs. Good Pinyon 3000 27.1 Easy Many Poplar 2080 Low Easy Many Fair Bitter Redcedar, Eastern 2060 13.0 Easy Low Many Poor Slight Fair Redcedar, Western 2950 18.2 91 Medium Medium Many Poor Excellent Fair Spruce 2800 2240 15.5 78 Easy Medium Many Poor Slight Fair Spruce, Engelmann 15.0 78 Easy Few Poor Slight Sycamore 5096 2808 19.5 98 Difficult Medium Few Good Slight Good Walnut, Black 4584 3192 22.2. 111 Easy Low Few Good Good Excellent Willow 4320 2540 17.6 88 Easy Low Few Poor Slight Poor Green weight is the weight of a cord of freshly cut wood before drying. Dry weight is the weight of a cord after air drying. Green firewood may contain 50% or more water by weight. Green wood produces less heat because heat must be used to boil off water before combustion can occur. Green wood also produces more smoke and creosote (material that deposits on inside walls of chimneys and may cause chimney fires) than dry wood. Firewood should therefore always be purchased dry or allowed to dry before burning. Dry wood may cost more than green wood because it produces more heat and is easier to handle.

A wood's dry weight per volume, or density, is important because denser or heavier wood contains more heat per volume. Osage-orange is a very dense firewood with limited availability in Utah. It's included here to show what a very dense wood is like. It contains almost twice the heat by volume of cottonwood, one of our lightest woods. In general it is best to buy or gather dense woods such as oak, hard maple, or ash. Hardwoods, or woods from broadleaved trees, tend to be denser than softwoods or woods from conifers. Some firewood dealers sell "mixed hardwood" firewood. This may or may not be desirable, depending on the proportion of low- density hardwoods such as cottonwood that are included.

The amount of heat per cord of dry wood is presented in Table 1. Heat content is shown as a percent of dry green ash, a fairly common, dense firewood. Values above 100 signify a higher heat content than green ash and below 100 a lower heat content.

Table 1 also contains 27.0 135 Medium Low Few Good Excellent Excellent Ash, Green 4184 2880 20.0. 100 Easy Low Few Good Slight Excellent Ash, White 3952 3472 24.2 121 Medium Low Few Good Slight Excellent Aspen, Quaking 2160 18.2 Easy Few Good Slight Basswood (Linden) 4404 13.8 69 Easy Medium Few Poor Good Fair Beech 3760 27.5 Difficult Few Excellent Good Birch 4312 2992 20.8 104 Medium Medium Few Good Slight Fair Boxelder 3589 2632 18.3 92 Difficult Medium Few Poor Slight Fair Buckeye, Horsechestnut 13.8 69 Medium Low Few Poor Slight Fair Catalpa 4560 2360 16.4 82 Difficult Medium Few Good Bad Fair Cherry 3696 2928 20.4 102 Easy Low Few Excellent Excellent Good Chestnut 18.0 Good Good Coffeetree, Kentucky 21.6 108 Medium Low Few Good Good Good Cottonwood 4640 2272 15.8 79 Easy Medium Few Good Slight Fair Dogwood 4230 High Difficult Few Fair Douglas-fir 3319 2970 20.7 Easy High Few Fair Slight Good Elm, American 445. 20.0 100 Difficult Medium Few Excellent Good Fair Elm, Siberian 3800 3020 20.9 105 Difficult Medium Few Good Fair Fair Fir, White 3585 2104 14.6 73 Easy Medium Few Poor Slight Fair Hackberry 3984 3048 21.2

106 Easy Low Few Good Slight Good Hemlock 2700 19.3 Easy Many Poor Good Honeylocust 4640 3832 26.7 133 Easy Low Few Excellent Slight Excellent Juniper, Rocky Mountain 21.8 109 Medium Medium Many Poor Excellent Fair Larch (Tamarack) 3330 21.8 Easy-med Many fair Slight Fair Locust, Black 4616 4016 27.9 140 Difficult Low Few Excellent Slight Excellent Maple, Other 4685 3680 25.5 128 Easy Low Few Excellent Good Excellent Maple, Silver 3904 2752 19.0 95 Medium Low Few Excellent Good Fair Mulberry 4712 3712 25.8 129 Easy Medium Many Excellent Good Excellent Oak, Bur 4960 3768 26.2 131 Easy Low Few Excellent Good Excellent Oak, Gambel 30.7 Oak, Red 4888 3528 24.6 123 Medium Low Few Excellent Good Excellent Oak, White 5573 4200 29.1

146 Medium Low Few Excellent Good Excellent Osage-orange 32.9 165 Easy Low Many Excellent Excellent Excellent Pine, Lodgepole 2610 21.1 Easy Many Fair Good Fair Pine, Ponderosa 3600. 16.2 81 Easy Medium Many Fair Good Fair Pine, White 2250 15.9

Easy Moderate poor Good Pinyon 3000 27.1 Easy Many Poplar 2080 Low Easy Many Fair Bitter Redcedar, Eastern 2060 13.0 Easy Low Many Poor Slight Fair Redcedar, Western 2950 18.2 91 Medium Medium Many Poor Excellent Fair Spruce 2800 2240 15.5 78 Easy Medium Many Poor Slight Fair Spruce, Engelmann 15.0 78 Easy Few Poor Slight Sycamore 5096 2808 19.5 98 Difficult Medium Few Good Slight Good Walnut, Black 4584 3192 22.2 111 Easy Low Few Good Good Excellent Willow 4320 2540 17.6 88 Easy Low Few Poor Slight Poor Green weight is the weight of a cord of freshly cut wood before drying. Dry weight is the weight of a cord after air drying. Green firewood may contain 50% or more water by weight. Green wood produces less heat because heat must be used to boil off water before combustion can occur. Green wood also produces more smoke and creosote (material that deposits on inside walls of chimneys and may cause chimney fires) than dry wood. Firewood should therefore always be purchased dry or allowed to dry before burning. Dry wood may cost more than green wood because it produces more heat and is easier to handle.

A wood's dry weight per volume, or density, is important because denser or heavier wood contains more heat per volume. Osage-orange is a very dense firewood with limited availability in Utah. It's included here to show what a very dense wood is like. It contains almost twice the heat by volume of cottonwood, one of our lightest woods. In general it is best to buy or gather dense woods such as oak, hard maple, or ash. Hardwoods, or woods from broadleaved trees, tend to be denser than softwoods or woods from conifers. Some firewood dealers sell "mixed hardwood" firewood. This may or may not be desirable, depending on the proportion of low- density hardwoods such as cottonwood that are included.

The amount of heat per cord of dry wood is presented in Table 1. Heat content is shown as a percent of dry green ash, a fairly common, dense firewood. Values above 100 signify a higher heat content than green ash and below 100 a lower heat content.

Sorry everyone for the above post. I thought I had it arranged in columns to show the BTUs.

The list shows the Favorites bow woods as having the highest BTUs which makes sense. The surprises for me were Apple, Pinyon, juniper,

??ok?

If you weight any size block where the volume is easily calculated, then you can just go by the weight and the weight of an equal volume of water. I cubic inch of water, is 253 grains.

So the volume of Tim's .5x.5x 10" is 2.5 square inches. So if that volume were water, it would weigh 632 grains. So if your sample weighed 379 on your arrow weight scale, it would have a specific gravity of .6 = 379/632.

If you can make cubes in a size of 1 cubic inch then you can just divide their weight by 253. Luckily, most of us have grain scales.

Tim, I still remember this method. The years have really passed. Jawge

George:   Our posts have been crossing for what, almost 25 years now? Over those years, of all the online bow-related voices, I think you've posted most steadily, and always with sound, reasoned advice and information, always with a civil tone. When they start handing out medals for this you should get the first one.

Thanks, Tim. Nice of you to say. Jawge

You, Tim, played a major role in the renaissance of selfbowyery. Thanks so much for that. Jawge

My cut offs, and Osage shavings burn hot. Lots of btu. I use it some times to roast the belly of some of my bows. I have found that if you make Blue Beech to narrow it can cause problems. What works for me is 62 inches long 1 5/8 at the fades to 1/2 inch at the tips 45lbs. for 25 inch draw, and most of my staves are dense. I have also recurved limbs tips with dry heat with no problems. Takes heat well. Not easy to find a clean knot free stave long enough were I live, but will make a nice short plains bow.

I have followed you both ( mostly quietly) off and on and want to thank you both for your civil and educational information. I hope to keep learning from both of you. Thanks Tim and George.

You are welcome, bentstick54. Jawge

Jawge is always civil.

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