Guitar backstripe question

[Istvan]

I read where some D-18's have a celluloid backstripe laminated between the two back halves. Does anyone know what glue they would be using, or does it matter as long as you have a solid wood support strip on the inner side?

[pops1]

The backstripe would not be the thickness of the back. It would be set in maybe half way into the back. Several glues could be used, don't know what Martin would have used.

[j. condino]

Just because they do something in a production environment where they need to make 100,000+ instruments a year does not mean you should copy every single thing in a small shop.

'Much better to take a few extra minutes effort & make a perfect joint between the two halves; keep it simple rather than introduce another material with different expansion / contraction rates.....

[MikeEdgerton]

Just because they do something in a production environment where they need to make 100,000+ instruments a year does not mean you should copy every single thing in a small shop.

'Much better to take a few extra minutes effort & make a perfect joint between the two halves; keep it simple rather than introduce another material with different expansion / contraction rates.....

Blasphemy!

[sunburst]

The backstripe would not be the thickness of the back. It would be set in maybe half way into the back. Several glues could be used, don't know what Martin would have used.

The Martin back stripes I've had occasion to examine were indeed the thickness of the back, but were not celluloid. I don't know if that feature changes from year to year or model to model, but the wood purfling strips in D-28 backs, at least some of them, are glued in between the two back halves at full thickness.
I agree with James. We don't have to copy everything done in factory produced instruments (yes, Mike). I think it is better to have a good quality wood to wood joint in the back center.
I am currently building a guitar with a one-piece back. No need for a spruce back graft inside the guitar, so I did not put one there (like Gibson did on one-piece backs).

[MikeEdgerton]

Just because they do something in a production environment where they need to make 100,000+ instruments a year does not mean you should copy every single thing in a small shop.

We don't have to copy everything done in factory produced instruments (yes, Mike).

I concur.

[Richard500]

Just because they do something in a production environment where they need to make 100,000+ instruments a year does not mean you should copy every single thing in a small shop.

'Much better to take a few extra minutes effort & make a perfect joint between the two halves; keep it simple rather than introduce another material with different expansion / contraction rates.....

Sometimes, an expansion joint is a good idea. In furniture panelling, for example, the edges are not glued (panels float in frames). Something like a flat guitar, where the edges are fixed to a rigid structure would be a candidate for splitting or buckling. Another good reason for arched structures. Of course, flat guitar backs do actually survive given proper material and craftsmanship. Good thing engineers don’t design them.

[sunburst]

Guitars are full of "no-no"s from the vantage point of a wood worker. Cross-grain glue joints, wide areas of wood glued so that they can't move, etc. That is one of the reasons that they must be so well cared for in terms of relative humidity (and one of the reasons engineers and trained woodworkers shake their heads when they find out how guitars are built).
Most "flat top" guitars have enough induced arch to give them some room to move. The braces are arched and glued to the top and back forcing the thin wood of the top and back into an arch. As the wood takes on moisture in high humidity, the arch gets a little higher (and tuning goes wacky). As the wood looses moisture the arch flattens (and tuning goes wacky). (That is one of the reasons players get frustrated with tuning problems when they don't understand wood movement.)

[Buck]

D-18's have a thin rosewood rosewood strip (never celluloid) between the back halves. Martin center strips are always the full thickness of the back, rather than being inlayed part depth. Because the mahogany is dark stained it's sometimes hard to see the rosewood strip, but it's there on all D-18's regardless of period.

I took a quick look and didn't see a great closeup of that on my D-18's. Here's a photo my Suda, best closeup I had. It's constructed similarly to a D-18.




This is not a closeup, but if you look at the worn area, you can see the narrow rosewood strip. This is a 1943 D-18.




This is a section of a modern HD-28 back showing the full thickness center marquetry. Nothing to do with D-18's, but you can see the full thickness method Martin uses. Very early Martins often differed in construction features, but everything at least from the late 19th century forward is built with full thickness strips like this one.