Red gum

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peter.coombe
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Red gum

Post by peter.coombe » Tue Jan 12, 2021 10:10 am

Has anyone tried red gum (i.e. river red gum) as a back and side wood? Only thing I can find after a bit of a hunt around is it is too heavy, splits easily. I am looking around for an Ebony substitute for mandolin backs that is more sustainable and not nose bleed expensive. So, Ebony is heavier and also splits easily but sounds really great so that too heavy stuff is rubbish. The principle is to make a dead back so heavy = good, so tick. I do have a piece I bought 20 years ago and have just made into a back and cut some sides. It bends easily so another tick. It has relatively high internal damping (so does Ebony) and for a dead back that is probably another tick. It also is commonly highly figured, readily available and finishes beautifully, so more ticks. It actually is a very good substitute for bloodwood bindings. Bloodwood is a nightmare to bend so I think I will use it for bindings when the red colour works. It does tend to have gum veins and borer holes and being a Eucalyptus is not very stable so some crosses there. However, for a dead back it might be the ants pants. Won't know for sure until I string it up. Will report here again when it gets the strings on. In the meantime I would interested if anyone has used it.
Peter Coombe - mandolin, mandola and guitar maker
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kiwigeo
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Re: Red gum

Post by kiwigeo » Tue Jan 12, 2021 10:49 am

I've got a couple of back and side sets that have been sitting on my shelves for 20 years now. The grain in the stuff is about as rogue as you can get and I don't look forward to running plane over it. I may be pleasantly surprised when I finally come to use the stuff but right now these sets dont incite a great desire to turn them into an instrument.
Martin

blackalex1952
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Re: Red gum

Post by blackalex1952 » Tue Jan 12, 2021 12:34 pm

My experience with the sound absorbtion qualities of red gum is that I wouldn't use it for anything much,especially an instrument.-Ross
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peter.coombe
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Re: Red gum

Post by peter.coombe » Tue Jan 12, 2021 4:16 pm

This is where I have problems with explanations. What is meant by "sound absorbtion qualities". A piece of wood has mass, stiffness and Q (amount of internal damping, high Q has less damping). High Q (e.g. rosewood) woods ring on for a long time, low Q woods have a shorter ring. So I assume you mean it has a low Q? Comparing an Ebony back I have with the Red Gum back it has slightly less mass, less stiffness along the grain and about the same Q (it rings on for about the same length of time). The Chladni pattern that shows it is mostly only bending along the grain has a lower frequency, but other frequencies are not that much different from the other backs I have. Nothing really unusual about it. That can be corrected with a brace, but I don't know if that is really necessary. So, just from the physical properties I can't see why Red Gum would be all that much different from Ebony. Jarrah also has high mass and internal damping and there is nothing wrong with Jarrah as a back and side wood in terms of sound, I used it many years ago with success. A high Q wood does not necessarily sound "better". Different, but not necessarily better. I am predicting a warmer sound, so am using a stiff piece of Red Spruce for the top, same as the Ebony mandolins I made. This is a dead back system so the back is not contributing much to the sound any way, so i can't see any reason why it won't work. We shall see in a few weeks.
Peter Coombe - mandolin, mandola and guitar maker
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Re: Red gum

Post by kiwigeo » Tue Jan 12, 2021 6:42 pm

Where you want to minimize "leakage" of string energy from the top down the sides red gum would be the ticket. Not so good if you're after a live back. My main reservations would be it's density and the difficulty working the stuff.
Martin

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peter.coombe
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Re: Red gum

Post by peter.coombe » Tue Jan 12, 2021 9:22 pm

Martin, you probably did not read my mandolin thoughts thread. I have been making flat top mandolins with dead backs for around 2 years. You can make a dead back by more hefty bracing, making the back thicker, using more dense wood, or laminating. I was mostly making the backs thicker. Why? They sound better than a live back mandolin. Around 6 months ago I decided to go to extremes and make a back from Ebony and use really hefty bracing. Bingo! The sound is way better than any of the others I had made. I made another one and the sound is even better, would challenge any of my arch top mandolins in tone and is louder because the top is lighter. So, the more I shut the back up the better the sound. What actually happens, and you can see it in the Chladni patterns, is like a see saw. The top is about 1/3 the weight of the back so the fulcrum of the see saw is moved so that the top moves much more than the back. The energy is the same, and obeys the basic physics law of everything has an equal and opposite reaction. The node of the main top mode moves out so it sits right on top of the ribs. So the whole top is vibrating and the ribs are not. The effect is the same as adding mass to the ribs of a guitar, only it is even more effective. The tone is unlike any other flat top mandolin I have come across, and they are loud with an enormous dynamic range.

So, after all that. I want a dead back, a live back just does not have the sound. So, I do want something that is really dense. As for workability Red Gum is way easier than Ebony or Gidgee. Gidgee is heavier but it is brittle and hard as, and not readily available quarter sawn in the sizes I need. Making the center join is a challenge, and it is not the easiest wood to bend. Ditto for Ebony, but Gidgee is worse. Red Gum is a breeze in comparison, although I think I would avoid the widly figured pieces.

I am not getting what I wanted here. I really wanted practical experience with Red Gum, but so far no one has used it so it is all just speculation.
Peter Coombe - mandolin, mandola and guitar maker
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Trevor Gore
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Re: Red gum

Post by Trevor Gore » Wed Jan 13, 2021 8:19 am

Give Gerard a call.

iirc, what he told me was a) it is not very stable in guitar sized panels, so prone to cracking and b) whilst the grain in most woods is enhanced with finish, in red gum any figure seems to disappear under finish. But best to check with Gerard as I have one set, but have not built anything with it and probably won't.

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peter.coombe
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Re: Red gum

Post by peter.coombe » Wed Jan 13, 2021 10:11 am

Thank you Trevor. I never used it until now because the one piece I purchased twisted and cupped badly, but I am not so concerned about that in a flat top mandolin and for bindings it should not matter. So far so good and it has been though some big humidity changes with the braces on. Will try a bit of shellac and varnish on scrap and see what happens
Peter Coombe - mandolin, mandola and guitar maker
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peter.coombe
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Re: Red gum

Post by peter.coombe » Thu Jan 14, 2021 10:53 am

Not too shabby under varnish. I am happy with this. Will continue, more later.
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Peter Coombe - mandolin, mandola and guitar maker
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Mark McLean
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Re: Red gum

Post by Mark McLean » Thu Jan 14, 2021 9:02 pm

That does have a lot of life to it!

If it sounds as good as it looks.............

This is tantalizing.

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Trevor Gore
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Re: Red gum

Post by Trevor Gore » Fri Jan 15, 2021 8:52 am

peter.coombe wrote:
Thu Jan 14, 2021 10:53 am
Not too shabby under varnish.
Yep, that looks fine. Gerard would have been using nitro over a grain filler, which may well look different from an (oil?) varnish.

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peter.coombe
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Re: Red gum

Post by peter.coombe » Sat Jan 16, 2021 9:37 am

That is shellac, followed by Aquacoat pore filler, followed by varnish. The varnish is Target Coatings EM2000, which is an oil varnish in a water emulsion, so it is a water based finish but does not have any of the blue tinge you can see on darker woods that many of the water based finishes have. I like it, no need to wear PPE if brushed on, or worry about weather conditions, and I especially like the way it ages.
Peter Coombe - mandolin, mandola and guitar maker
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Bruce McC
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Re: Red gum

Post by Bruce McC » Sat Jan 16, 2021 10:44 am

Peter
I have neither experience with river red gum nor mandolins but I have found your posts
very informative. Thank you for your time and effort.
Bruce Mc.

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WJ Guitars
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Re: Red gum

Post by WJ Guitars » Sat Jan 16, 2021 12:41 pm

Peter, I also find your information and comments are interesting especially when your are testing out other alternatives of timbers on your mandolins. I noted that your want the back to be dead and not live to improve the sound quality.

Side tracking, would a carbon fiber mesh back construction achieve similar sound control results? I 'm not sure whether a carbon fiber mesh finish on the back of a mandolin would be visually acceptable. However, I have seen some McPherson acoustic guitars on his website that are completely made out of carbon fiber mesh.

Looking forward to seeing the Red Gum mandolin result.

Regards
Wayne
https://wjguitars.wixsite.com/mysite-1

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peter.coombe
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Re: Red gum

Post by peter.coombe » Sat Jan 16, 2021 5:42 pm

Carbon fiber has a very high stiffness to weight ratio, which is good for the top, but that is not what I want for the back, it needs to be heavy. Remember the see saw analogy. The top needs to be as light as possible, so a carbon fiber lattice bracing might be worth a try, but I would not use it on the back. Problem is, getting the carbon fiber lattice right so it sounds good is tricky. Others have tried, but the result has been very loud and obnoxious sounding mandolins because the lattice is too stiff. I think it is possible, but would involve a lot of experimentation and hence a lot of time.

There are other woods that may be worth a try. We have a lot of native dense woods available. Many are too small a tree to get the size needed, but I have tried Jarrah and grey ironbark. Jarrah was successful, but is a bear to bend. The ironbark was a disaster so won't use that again. Very loud and course sounding, and the back inverted to concave and then popped back out again.
Peter Coombe - mandolin, mandola and guitar maker
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Re: Red gum

Post by blackalex1952 » Sun Jan 17, 2021 10:06 am

back inverted to concave
..a custom build for a bloke with a beer gut and a skinny wife?-R
"Everything I say on the topic is based solely upon inexperience and assumption!"

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