A week ago I glued three bars on a guitar back and achieved a nice dome across the back of about 3mm. Yesterday I noticed that the lower bout area of the back had gone almost flat, and the waist and upper bout areas were completely flat.
Not the end of the world; the back hadn't been glued in place. I can plane the bars off and start again. But why has it happened? Will it happen again? And what can be learned from what has happened?
A bit more information..
My workshop is a "posh" shed in the garden. I don't keep wood in there. All guitar wood and work in progress is kept in my dry central-heated house.
The back was 40 year old Indian Rosewood, 2 piece, cut dead on the quarter. It has stayed dead flat ever since I bought it. The bars were Honduras Mahogany which I have had for more than 20 years. 5 years ago it was cut into 10 x 20mm strips and these have stayed dead straight.
Six months ago I joined the back, cut it into a guitar shape, but left it 5mm thick, storing it in the house, flat against a board. It stayed flat throughout.
A week ago I planed it down to thickness - about 2.7mm. I noticed that it was very stiff across the grain. My background is in making lutes and early guitars, and I don't have much experience with Rosewood. It was certainly a lot stiffer across the grain than Sycamore, and other European hardwoods that I am more used to working with.
After planing it to thickness I glued on a central re-enforcing strip of cross-grain Mahogany, using a caul to try for an end-to -end-dome of about 3mm. Not sure if this is normal in Classical guitar construction, but I wanted to avoid the back coming out hollow from end to end. When the glue was drying I planed the back bars down to 8mm thick, planed a 3mm dome on the longest, then used this as a template to mark out the other two.
Then I glued the bars in place, using cam cramps, and a thin flexible softwood caul behind each bar.
Next day the back was still domed, but the end to end dome had gone flat. Now, a week later, the end to end dome is back, but the dome across the back had almost disappeared across the lower bout, and completely gone at the waist and upper bout.
The back has been in the house for the whole of the week. I left it flat for a day, and when all seemed ok I stored it upright, on end, snug in a cardboard box, and resting against a flat surface.
I look forward to any comments that might shed light on the problem.