What glue to use

[guy young]

I am building a 36 string harp, I will be gluing a spruce soundboard to the soundbox. My question is if I use hide glue, will it hold the 1200 lbs of tension? Or should I use plastic resin glue, or expoxy

[Dominic Carpenter]

Hi Guy,
Fine Woodworking did a nice test of several glue types, including epoxy, on bridal joints in several different woods species. They then calibrated the amount of lbs. required to break the glue joint or the wood itself. The test results are available online. Search for issue#192 of Fine Woodworking Magazine. That may help you.

[george wilson]

Lyon and Healey uses Knox unflavored gelatin for their refined hide glue. The joints that I have seen on natural finished maple joints are remarkably difficult to see. Unless there is something wrong with your design,like insufficient glue surface area,I'd go with hide glue. Get some Knox gelatin,mix it up,and experiment gluing up some wood with it.

Personally,I tried epoxy back in the 50's to glue classical guitar bridges on. After some time,they popped off. I think the epoxy kept hardening until it got real hard and lost its strength. In any case,you want a REVERSIBLE glue on instruments,so they can be repaired when needed.

Raw data on newly glued joints is not sufficient to judge long term strength. There are instruments made SEVERAL hundred years ago that are still holding together.

[guy young]

I check out the test report for glues as Dominic suggested. The epoxy was stronger at 1700 and the liquid hide glue at around 1400. I really would like to use the liquid hide glue because the sound board has a life expectancy of 10 to 15 years at best. With a 1/8" thick surface of spruce glued to a 1" wide glue surface on the outer perimeter, it seems impossible for glue to hold the 1200 lbs.

George, your a luthier, I believe so you must know the strength of hide glue. Do you think hide glue is strong enough. The surfaces should be perfectly flat as I will be finishing them in a surface sander.

OH, one last important thing, the spruce sound board can be remade, but I do not think I would be able to find any more Cuban Mahogany to replace any of the sound box should it get damaged if some of glue separates under the tension.

[george wilson]

Hide glue has been used for thousands of years. Certainly musical instruments have been glued together with it forever. Did you READ my post above about Lyon and Healey harps?

Initial glue strength is not the only consideration. How long does it keep its strength is another. Read what I said about gluing bridges on with it.

[guy young]

Yes, George, I did read your and appreciate you post. I have read everything I could about all the harp makers. I have viewed a full tour of Lyon and Healey building production. The builders normally say they use glue type "X", but a couple say I use type "X" because of no creep and I'll use glue "Y" for the sound board of neck or whatever. I have not found anyone to mention that they use hide glue for the sound board.

In your last post, you made a great point that I had not though about. "They used hide glue for thousand years". You're right, they did not have any other glue, and they built Harps back then to.

[Julie Moriarty]

George, when you did your restoration work with the museum, did the powers that be ever ask their restoration people to use new technology when doing repairs or restoration? Your point about hide glue lasting hundreds of years got me to thinking about the masters of the past who created instruments that outlived them many times over. We're conditioned to jump on the "new and improved" products without really knowing what long term results are. I had never really thought about that until I read your post here. You want to buy glue and everyone sings the praises of Titebond. Antonio Stradivari couldn't buy the stuff if he wanted and yet he seemed to do pretty well making violins. Sometimes we forget that.

[Ernie Miller]

I check out the test report for glues as Dominic suggested. The epoxy was stronger at 1700 and the liquid hide glue at around 1400. I really would like to use the liquid hide glue because the sound board has a life expectancy of 10 to 15 years at best. With a 1/8" thick surface of spruce glued to a 1" wide glue surface on the outer perimeter, it seems impossible for glue to hold the 1200 lbs.

George, your a luthier, I believe so you must know the strength of hide glue. Do you think hide glue is strong enough. The surfaces should be perfectly flat as I will be finishing them in a surface sander.

OH, one last important thing, the spruce sound board can be remade, but I do not think I would be able to find any more Cuban Mahogany to replace any of the sound box should it get damaged if some of glue separates under the tension.

Where did you get the idea that a soundboard will only last 10 to 15 years? Instruments have lasted hundreds of years with their soundboards intact.

With all due respect, I think you're suffering from analysis paralyses. There is a danger in over-thinking the building process which can lead to getting nothing done. As far as strength is concerned, any of the glues you mentioned will be more than strong enough. Take George's advice - use hide glue, which is reversible if necessary.

[guy young]

Where did you get the idea that a soundboard will only last 10 to 15 years? Instruments have lasted hundreds of years with their soundboards intact.

With all due respect, I think you're suffering from analysis paralyses. There is a danger in over-thinking the building process which can lead to getting nothing done. As far as strength is concerned, any of the glues you mentioned will be more than strong enough. Take George's advice - use hide glue, which is reversible if necessary.

I have read on a couple of harp makers sites that the spruce soundboard start cracking as they age. Where as the laminated sound boards hold up. I did not make the statement, I only repeated it from professional harp makers. Therefor I can only assume they know what they are talking about, heck, they definitely know more than I do.

[Ernie Miller]

I have read on a couple of harp makers sites that the spruce soundboard start cracking as they age. Where as the laminated sound boards hold up. I did not make the statement, I only repeated it from professional harp makers. Therefor I can only assume they know what they are talking about, heck, they definitely know more than I do.

While it's true that a laminated soundboard will not crack, there's more to the story. The life expectancy of a soundboard is not necessarily affected by a crack. Henry Steinway said "I am always amazed when I see one of my pianos that has no cracks in the soundboard". In spite of its tendency to crack, almost all upper level instruments use spruce for their soundboards. The reason is simple - sound quality. Precautions can, and should, be taken to minimize the risk of cracking - such as humidity control.

The builder can also take precautions in the building process. Installing a soundboard in humid conditions almost guarantees a cracked soundboard in the future. Many builders heat their soundboards immediately before installing them - using an electric blanket, a fireplace, placing the soundboard in the sun for several hours, and who knows how many other ways.

In your research, I'm sure you've found many conflicting opinions on this and other matters. Who should you believe - who should you trust? This is what I meant by "analysis paralyses". In the end, whatever decisions you, as the builder, will make will be applauded by some, and be deemed disastrous by others. This is where experience enters the equation. Until that experience is accumulated, the best you can do is look to those that have it, and follow their recommendations.

I hope my comments have been helpful, as this is the spirit in which I submitted them.

[guy young]

Ernie, I do appreciate you comments. I have been careful building this thing so far. As I live in South Florida I humidity is also something to think about. So before final cutting and gluing up I storage the lumber in my Dining room. After building the piece, so for the harp and neck, they get stored there also. As I have read, no one seems to have a different opinion when it comes to how dry the wood should be, 7% or less. I have been building the harp with Weldwood Plastic Resin Glue and made the neck and pillar in 4 layers making the grain about 15 to 20 degree opposite of each other. So after I do a glue up I let the piece dry out for at least a week in the house before I glued the next layer.