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The only problems I've had bending mahogany have been rippling in the waist (too much water) and compression in the waist (not hot enough/applying pressure too soon). I pretty much follow John's method, and even for very tight waists I haven't had problems. I bend my sides a little thicker than you do for guitars--maybe .08 to .085, but thinner on ukes. Are they cracking in the waist?

Dave

Are the sides quarter sawn? That would help a lot if they aren’t.

The failure to properly retain the bent shape and scorching both indicate that any moisture in the bending stack was cooked off well before you accomplished the bend. This was likely due to your over-long cooking of the side prior to bending. Mahogany - particularly figured mahogany - hates excess water, but really needs a strong blast of steam to bend properly (minimal cross-grain ripple and retention) of mold shape after bending.

Other issues:

- A light spritz of the side is a hit-or miss method to calibrate the amount of water available. Using wetted kraft or butcher paper allows surprisingly accurate calibration (brown kraft gives a matte appearance at minimum water for figured maple, a silky/satin appearance at the correct amount of water for figured mahogany, figured anigre, and figured ash, and a glossy appearance for rosewoods where the dang, steam-saturated paper does a nice job of capturing heat-mobile resins from the wood.

- The only reason to use foil in the stack is to isolate spring steel slats from the water in the kraft paper or on the wood. It may be safely left out of the stack when bending with stainless steel slats on all but rosewoods, where the foil barrier seems to eliminate resins from collecting on the slats.

- Envelopes around the wood may produce a number of problems, to include puckering and printing, re-condensing of water vapor and relaxation of the bend, and watermarking. Separate sheets of kraft paper and foil work as well, and avoid these issues. Ideally, all free water in the stack is exhausted by the time the post-bend cook is accomplished, and - indeed - that is the reason for the practice.

- Figure mahogany and other figured woods may need some help with uniform plasticizing of wood fibers. SuperSoft 2 works well here if used according to instructions (apply generously to all sides of ready-to-bend side, allow to fully dry, then bend within 24-36 hours of application). Ammonia works as well, but in the experience of the build/repair shop where I worked, was less effective for highly figured, difficult-to bend woods like curly anigre, plum pudding/fiddleback mahogany, and highly figured fiddleback ash). A gallon is effectively a few year's supply for a prolific small shop builder, and is far cheaper than the cost of one replacement side for a premium set of back and side wood.

- Rosewoods are like honeybadgers; they just don't care. Even without water, most rosewoods are happy to bend with heat only, but will likely scorch a bit and still require some serious resin cleanup.

- Acacias such as Tasmanian blackwood, bay laurel, and koa hate aluminum, picking up a blue undertone where waterborne aluminum ions react with the wood. This is the one time we replaced aluminum foil in the bend stack with non-silicone parchment paper.

- Brown kraft paper works well for everything except holly or the whitest of white timbers. White kraft is a decent substitute, but needs to be wetted out well to hold the same amount of water as brown kraft.

This gets us to method for figured mahogany and other hard benders:

1. SuperSoft 2 the sides and allow to fully dry before bending; add blue tape tabs to index the waist if using a Fox-style bender

2. Build the sandwich with (from top to bottom): slat/blanket/slat/foil sheet/wet craft paper sheet/dry wood/wet kraft paper sheet/foil sheet/slat. DO NOT PRE-WET THE WOOD - the kraft paper is your water reservoir for the bend.

3. Load the stack in the bender without delay, using spring clamps to maintain contact between all elements (this means at least one pair of clamps on the upper bout and two pair on the lower after the bending cauls are mounted)

4. Set the temp for maximum (fastest rise) and turn it on. If using a router speed controller, this is the full-on position on the rocker switch.

5. Watch for the first signs of steam generation in the area near what will be the widest part of the upper and lower bouts. Once those early tendrils of steam are seen, START BENDING...DO NOT DELAY! Bring the waist down to 1/4" to 3/8" off the mold, then lower bout, then upper bout/cutaway area.

6. Upon completion of the bend (this should take no more than 4-5 minutes total even with a cutaway ram), watch for exhaustion of water in the stack (no more steam… yet another good reason to eschew envelopes), then turn the temperature control down such that max temperature in the stack does not exceed ~300 for mahogany and closer to 260-280 for rosewoods. Really, you can leave the router control set to something like the 2/3’s mark just short of the yellow and run that for 30 minutes if you do not have a thermocouple to monitor temp.

7. Turn off and unplug the blanket, then give the stack 12 hours to cool and dump any residual moisture trapped in the waist area.

There are several YouTube videos on bending... look for the one on bending figured mahogany by my old boss for what is a lengthy, but somewhat useful exposition.

Ken Cierpolowski advocated bending the waist on a pipe before putting the bending sandwich in the Fox bender. I've done that for a half-dozen projects with no cracking at the waist. Am I getting the protection that lucky accidents offer or doing something right?

Thanks!

It does surprise me a bit that, when bending comes up, everybody assumes that we are all using a Fox bender with electric blankets. I own some Fox benders, but I prefer hand bending. To each their own.

Here is a link to my favorite bending iron:

https://luthiers-bench.com/

It heats up faster, and keeps its heat steadier, than the ones we are all more familiar with.