MIMF

I'm going to have a rather delicate carving for the rosette in the instrument I'm working on and I'm thinking I'm going to need something to back the spruce to give it a bit more toughness. I seem to recall that perhaps lute makers hand carve something similar and I'm asking for some advice to select the backing.

This is cut on a CNC so I do have a depth limit on how thick the backing is. In my test run, I used manilla paper from the art store that's probably about .010 thick and I'm not happy with it. For this test run I just used titebond to adhere it and once on, you could peel it away leaving just a super thin layer.

Any thoughts on what I should use? I'm almost wondering if I should use some sort of thin linen or something.

Rag or cotton paper might be worth a try. It is stronger than pulp papers.

What about linen? Strips of linen (or similar this material) are frequently used to strengthen the seams in bowl-back instruments. Soak the linen in hide glue and adhere to the back of the spruce.

I've found a source for .005" calf parchment that I'm going to try.

For lutes they would use thin little bars of spruce, stained black, that stretched across the back of the hole. Once the soundboard is on the instrument, the black stain makes them effectively disappear.

That is a very delicate spruce rose, and if you backed it with very thin parchment it would be much much stronger, but with the wood shrinking and expanding over time , with humidity changes, and the likelihood of a light bump from a finger, don't be surprised if it cracks someday- likely right where the points meet the soundboard.

Hide glue is the best way to bond parchment, for bond strength.

Yep, I was planning on using HHG to bond the parchment.

Hopefully it will last at least a little while. It's for my kid's instrument so I won't be surprised if some little fingers go poking in there just to see how strong it is. Plan B is to just cut all the points of contact and have an open hole if that happens or if it's damaged in the machining process.

Another idea from the advent of modern materials would be to use NOMEX sheet. It comes in varying thicknesses and is very strong for it's dimension. There is a version called Yellowstone that is being used for banjo heads with great success. Jon Whitney knows more about it as he has used it several times now.

The back of the soundboard usually historically had paper glued to it, but it was hand-made paper that had long randomly oriented fibers unlike not modern machine-made paper that has short fibers that are more or less aligned. You can still get hand-made paper from places like this:
http://handmade-paper.us/page/CPO/CTGY/tre
A builder on the early guitar ning forum uses the Japanese "kozo" paper and likes it. Others use watercolor paper with a high rag content. This is in addition to the small bars that are dyed black and glued across.

Mark Day wrote:Others use watercolor paper with a high rag content.

I saw that at the art store and almost bought some - next time. I've already ordered the parchment so I'm going to give that a shot. Cross your fingers for me.

Michael Lewis wrote:Another idea from the advent of modern materials would be to use NOMEX sheet. It comes in varying thicknesses and is very strong for it's dimension. There is a version called Yellowstone that is being used for banjo heads with great success. Jon Whitney knows more about it as he has used it several times now.

If you are interested in getting a sample of the Nomex sheet, go to banjohangout.org and search for a member named Mark Ralston. I think he bought a huge roll of the stuff (apparently the only way you can get it) and is selling it off one banjo head at a time for a very reasonable price. He owns a jewelry company called "Yellowstone" hence the nickname for the banjo head material. I think it would do a great job of reinforcing your soundhole rosette without being subject to shrinking or expanding with humidity variations.

How does the nomex sheet glue? Can you use hide or do you have to resort to a synthetic glue? Also does it have a high tear resistance? I see it is noted as an electrical dielectric and "durable" but I've never handled the stuff so I don't know if tears easily or not.

It has a bit of "grain" to it, like paper does. It tears with difficulty in the direction of the grain, and darn near not at all across the grain.

I've just glued a scrap of Nomex to a scrap of wood with Titebond to test that out. I'll let you know how it works. I don't have any hide glue mixed up or I would have tried that, but I think it would be similar to Titebond.

Has any one tried Tyvek? The house wrap material?

I've had disappointing results trying to glue the Nomex sheet with Titebond, CA, and Plastic Welder glue. The Plastic Welder (a type of epoxy) pulled off a few surface fibers, the others peeled off cleanly. So Nomex doesn't seem too great for lamination purposes.

Jon, did you mean Nomex or Tyvek in your last post?

I meant Nomex. Tyvek may be similar but I have glued that stuff to Mylar for banjo heads, with 3M Spray adhesive - I can't remember if it was the Super 77 or the 90. That is more of a rubbery bond than I think would be required for reinforcing thin wood.

I would avoid any non-rigid material, just seems the wrong way to go. Perhaps you could widen the members and lighten the design with light chip carving or side bevels?

Too late! Parchment is applied and the box is closed. I'm hoping to have the finish on by the end of the week. I'll be cutting the rosette after buffing the top, hopefully in about three weeks!