Here were the boards before I started. The rosewood should look even prettier once it's sanded and French-polished. They looked quite flat from this angle.
Here's an attempt to show the problem:
What's worse, if you rotated the same board through 90 degrees, the warp was in the opposite direction:
My plan to flatten the boards was:
- Put the boards back in the press with several sheets of damp paper either side of them and leave them for several hours to absorb enough moisture to reactivate the hide glue.
- Heat the boards up to about 60C for long enough to soften the glue all the way through the laminate.
- Press them between dry paper as before, but this time applying more weight and leaving them for longer than two weeks.
Step 1 seemed really promising. Moistening the boards for six hours in the press caused them to go completely flat and slightly floppy. In hindsight I may have added too much water, but I really had very little feel for how much would be enough.
Step 2; my first idea was to wrap them in tinfoil and put them in a low oven. I was concerned though as to how I could accurately control the temperature, particularly as the minimum setting on the oven thermostat is something like 140C. My second idea was to scale up the glue pot double boiler idea: seal the boards in plastic bags and place them in a metal bucket of water on top of an electric hob, using a thermometer to ensure it didn't get too hot:
I'm filing this one under, "it seemed like a good idea at the time." The tapered shape of the bucket proved awkward, and the plastic bags both leaked, causing the boards to soak up extra hot water. The result was that the outer veneers wrinkled and started to de-laminate, so I decided to cut my losses and dismantle the boards straight away while they were already hot and damp and I had a bucket of hot water available:
Most of the veneers came apart OK and the remaining glue washed off without a problem, though two or three pieces got so soggy that they partially split.
I'm slightly disappointed, though there's no permanent harm done and the wood is mostly still usable (and I have lots more glue). I have two or three different ideas for what I can try next.
One suggestion that has been made to me is to get hold of some 2.5mm thick board of a cheaper hardwood like pear wood and then put a single layer of veneer either side of it (at right-angles to the grain of the core so you get some of the advantages of plywood even though there are only three layers) and around the edges.
Another possible idea I'm considering is to get hold of some relatively cheap plain hardwood veneer and build up a seven layer laminate using the following process: laminate together three layers of the cheap stuff, wait for it to dry (hopefully flat), add two more cheap veneers to the outsides of the three layer boards, wait for them to dry, add two more veneers on the outside of that (one of which would be rosewood), then finally put a rosewood finishing strip around the edge. By building up the layers in pairs that should avoid the uneven shrinking that can cause a board to warp if you only veneer one side of it.