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If the humidity is high at the time that the guitar is built (specifically, on the day the braces are glued on, or the day the top is glued on the box) then the wood is relatively swollen at that time, and becomes fixed in position by the glue while in that swollen state. Later, when the guitar dries out and the wood shrinks across the grain, the top of the guitar wants to get skinnier - but its edges are fixed in place where they are joined to the rims. The only way that the dimension of that panel can change is for the arch to collapse. If it gets too bad it will crack along the grain or the centre seam will separate. This is the reason why it is essential for a luthier to control the humidity in their workshop and avoid doing the critical parts of the build in conditions of high RH, and it is important for the owner of the guitar to avoid exposing the instrument to low RH (such as when you north americans heat your houses during winter).

Author:	DanKirkland [ Wed Dec 09, 2020 5:00 pm ]
Post subject:	Concave top?
Had a retopped 37 Martin D28 in my shop recently and the top was distinctly concave shaped, had no radius whatsoever. It sounded fine it just was a bit shocking to look at it. Was just curious what could cause a concave shape to the top like that?

Author:	bluescreek [ Wed Dec 09, 2020 5:55 pm ]
Post subject:	Re: Concave top?
low RH

Author:	DanKirkland [ Thu Dec 10, 2020 10:12 am ]
Post subject:	Re: Concave top?
bluescreek wrote: low RH Well today I learned something. Is it low RH when it was built or is it just from what it's been exposed to over time?

Author:	bluescreek [ Thu Dec 10, 2020 10:39 am ]
Post subject:	Re: Concave top?
over time if you made this and glued the top at 55% and it is not 35% that 20% change causes the grain to pull together and shrink across the grain. As this pulls on the braces you build up stress and to relieve it the braces make the top pull into itself and causes it to disk and if you go the other way the top will cup up and rise

Author:	Mark Mc [ Thu Dec 10, 2020 3:24 pm ]
Post subject:	Re: Concave top?
If the humidity is high at the time that the guitar is built (specifically, on the day the braces are glued on, or the day the top is glued on the box) then the wood is relatively swollen at that time, and becomes fixed in position by the glue while in that swollen state. Later, when the guitar dries out and the wood shrinks across the grain, the top of the guitar wants to get skinnier - but its edges are fixed in place where they are joined to the rims. The only way that the dimension of that panel can change is for the arch to collapse. If it gets too bad it will crack along the grain or the centre seam will separate. This is the reason why it is essential for a luthier to control the humidity in their workshop and avoid doing the critical parts of the build in conditions of high RH, and it is important for the owner of the guitar to avoid exposing the instrument to low RH (such as when you north americans heat your houses during winter). It sounds like the person who did the retopping job might not have had good RH control. That is quite a tragedy on a pre-war D28! It is a classic story that it becomes a problem at this time of the year when the owner has recently fired up the furnace of their central heating system. Have you rehumidified it yet?

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Author:	TerrenceMitchell [ Thu Dec 10, 2020 3:51 pm ]
Post subject:	Re: Concave top?
What they said. I had a parlor body recently where the top when from convex to concave and back to convex in the period of a week. It started at a 40' radius... so very slight across a narrow body. But, we had a wild swing in RH, and it did what wood does.

Author:	doncaparker [ Thu Dec 10, 2020 3:55 pm ]
Post subject:	Re: Concave top?
Mark Mc wrote: If the humidity is high at the time that the guitar is built (specifically, on the day the braces are glued on, or the day the top is glued on the box) then the wood is relatively swollen at that time, and becomes fixed in position by the glue while in that swollen state. Later, when the guitar dries out and the wood shrinks across the grain, the top of the guitar wants to get skinnier - but its edges are fixed in place where they are joined to the rims. The only way that the dimension of that panel can change is for the arch to collapse. If it gets too bad it will crack along the grain or the centre seam will separate. This is the reason why it is essential for a luthier to control the humidity in their workshop and avoid doing the critical parts of the build in conditions of high RH, and it is important for the owner of the guitar to avoid exposing the instrument to low RH (such as when you north americans heat your houses during winter). This is spot on, and it is a lesson I had to learn the hard way. We have hot, humid summers and cold winters here in Wild, Wonderful West Virginia. Building a guitar in a high humidity summer (without proper RH control in my shop) and having the top crack in the dead of winter (without adequate humidification) was a heartbreaking experience. Shoot for building at 40% in the workspace, and you are in the middle of the range. The guitar can likely survive both extremes (high RH and low RH) if it is built in the middle. But building in high humidity and sending the guitar to live in low humidity is begging for problems.

Author:	DanKirkland [ Sat Dec 12, 2020 10:35 am ]
Post subject:	Re: Concave top?
Mark Mc wrote: It sounds like the person who did the retopping job might not have had good RH control. That is quite a tragedy on a pre-war D28! It is a classic story that it becomes a problem at this time of the year when the owner has recently fired up the furnace of their central heating system. Have you rehumidified it yet? The customer brought it by to show it off so it wasn't anything I was going to work on to begin with. It was a good looking retop and it sounded good despite the humidity extremes. The shop he bought it from were making claims about who had done the retop but after reading this and knowing what caused it I don't think the person they claim did it actually did it.

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