So what is the best option here? I want to bond a 3/8" slab of highly figured burl to a piece of 3/8" thick tempered glass. The slab ranges from 5-12" in width, and approx 72" in length. The adhesive will not be visible from the back side, so it wont matter if it doesn't dry clear.

Epoxies vs. structural silicones?

I'm curious as to whether the silicone will have enough strength to hold the burl flat when it tries to cup and twist?

And also whether the epoxy is too rigid and will cause extensive cracking of the slab?

The finish that will go on the exposed side of the burl is still up for discussion, but I figured an epoxy would be appropriate if I used epoxy to bond the other side to glass.

It will be exposed to the outdoors, and thus seasonal fluctuations, but not much direct sunlight & it will be a vertical surface.

Disasters waiting to happen...or it could work....anybody's guess. I wouldn't bet on it working if it were a paying job.
Glass moves, wood moves, it'll be in the weather....a concoction of evil.

I'm with Chris. You'll be sorry, if you try this.

No adhesive can stop that 3/8 thickness from moving, so it will self-destruct.

Also, clear epoxy breaks down quickly in Sunlight.

There is a reason you don't see this. It is because it won't work.
There is a chance it may work - thin burl, properly dried and prefinished burl, sheltered exposure, no epoxy, flexible adhesive, thick glass to resist wood movement, etc, etc. As was stated above - not for paying work.

Why would you want to do this? Doesn't make sense to glue such dissimilar materials together, especially when one breaks. Can you find another design that wouldn't require gluing? Fasten the burl with Z clips or something along that line.

The "why" I want to do this is to make a paying client happy. I didn't go into specifics on the application because I wanted feedback strictly on the adhesive, without getting into all the other details.

I am pretty confident that a sheet of 3/8" tempered glass will not fail due to the stresses imposed by seasonal movement of a 3/8" x 7" x 7' piece of wood. So when you guys are referring to disasters waiting to happen, and saying it will not work, or will self destruct, I assume you are referring to the wood cracking.

The original question was basically trying to get some feedback on the bonding strength of a more flexible adhesive, such as structural silicone, vs the flexibility of a modified epoxy. I highly doubt a piece of wood that is well dried and finished on all sides, and placed in a bed of silicone will self destruct. What I don't know is if it will be able to warp or twist at all....likewise I know that a piece of wood properly epoxied to something rigid will not be able to lift up off the substrate, but how much cracking is to be expected? depends of course on the wood and the conditions...

Has anyone tried anything like this and watched it fail? Or are the responses based on gut feelings related to general best practices, other scenarios that may be similar, but not exactly?

I believe you will have problems with both of the options you are asking about. I have used silicon as a fastener in various projects. It is incredibly strong and of course flexible. The chalange with silicon is that you can not get an extremely thin layer. So any movement will more than likely tear the wood off the glass in the middle of the glue line. Epoxy would be a better option for a rigid glue line that would be very thin. The problem is that I have not had any luck getting epoxy to stick well to glass. Maybe if you scuff the glass with a diamond grinder it may stick which will be nearly impossible with tempered glass unless it is done prior to the tempering process. However wood veneer glued to any subsrate and put outside has next to no chance of surviving more than a few months. I tried it with a patio table "to please a client" and when I built the third one to replace the first 2 I told them it will be solid wood or not at all. If you must do this try a thinner veneer. Veneers in any surcumstance over 1/8" will fail due to wood movement. Now put that outside and it will happen even faster. Have you given any thought to epoxy coating the wood and forget the glass. Maybe if you share a little on what you are trying to do someone may have an idea on what may work.

You know, there isn't a person on this thread who hasn't seen the "rules" broken with no consequences. A buddy of mine years ago epoxied 1/4" thick redwood burl to 1/2" steel plate with no ill effects; my supplier reminds me every day that my loud proclamations of failure of the maple countertop he screwed fast to a cabinet were wrong.

Still, those are exceptions, flukes. The rules exist because of millennia of collective experience, backed by cold hard science. With respect to your plan, I would expect the burl to crack with either adhesive. I don't think there is enough meat there to twist or lift.

But if you have to do it, and it sounds like you do (been there, batting about .250 :) ) I would want to use the adhesive that has the most open time, and the one I had the most control over in terms of application. I would trowel it on with a shopmade trowel with 1/16 notches. I think your idea of sealing (not really a 100% thing, of course) all six sides before glue-up is smart. I would want to be thinner than 3/8, too. Then I would do a ton of testing. When I have weird stuff, I make up as many samples as I can and throw them in the fridge, the freezer, on the roof, out in the grass.

Personally, I'd never do it for money. But I also understand the need to go for it sometimes. And maybe the customer is willing to sign off on liability. So if you do, at least report back and let everyone know what happened. If it works out, I for one would like to stand around with you and scratch my head over it.

If you can cya, I'd go for it.
I'd consider vacuum infusing epoxy--the thin clear variety-- after treating the burl with something to make it more hydrophobic and minimize seasonal movement.

I've heard tung oil is hydrophobic and vacuum impregnation as a pre-treatment would tame that burl--which has a lot of endgrain to convey both oil and epoxy inside. I would consider grooving the back of the burl to help negate some of the tension from movement as is done with flooring.

Sunlight? The glass will filter most of that; but give it as much help as you can-- there are epoxies with uv blockers I think-- or should be.

The problems seem well understood and you can find ways around them. Besides, I'm curious.