I work construction and often find partial/almost empty double cartridges of epoxy which are deemed either too much hassle to deal with for the next time they may be needed or may have too little left to be worth saving....(obviously, the 3-Rs don't get much attention at some of these construction sites...)
I've found that often, there is more material left in the tubes than what you get when you buy the little 2-tube epoxy packs from the hardware store for... (I'm not even sure what they cost anymore, it's been so long) $5?
Admittedly, most people will buy those packs and use, at most, maybe a chocolate chip's worth of each at a time for what their needs are. Well, let me tell you some of the things possible if you have several tablespoons worth available.
Remember that most epoxies of this type are usually mixed in a 1:1 ratio. 1 small blob of part A and an equal size blob of B. Many of them come in (2) different colors for each part so mix them until you have a uniform color with no streaks of one or the other.
Also remember that once they're mixed, you've started down the road of no return...you need to use it because it can't be stored for use later...
Step 1: Mirror, mirror on the wall...
You may have noticed that many older mirrors start to lose the silvering around the edges first. This use of epoxy gives new life to an old mirror.
Carefully remove the mirror from the wall (or ceiling, as the case may be...heh heh). If you have even the slightest concern about the possibility of chipping glass, remember SAFETY first. Wear glasses and gloves. *Remember that there is NO project you can do which is worth the loss of an eye (or any other irreplaceable body part). So you suffer a 5 minute delay while you try and find your safety glasses....pretty minor inconvenience no?
Lay it flat on a smooth surface (table or whatever) and work out a design for a glued-on frame using any left-over tile, beach glass or whatever you'd like to see on there.
Once you have your mental picture of what you want, clean the area well and maybe wipe it down with rubbing alcohol to remove any oils. Allow it to dry while you mix your epoxy.
Mix less than you think you'll need each time, 'cause you can always mix more but can't un-mix what's done. I have an artist's paint spatula I use, but an old table knife, popsicle stick or some such would work.
With the application, again, until you get used to how it squishes out, use less than you think you'll need. It doesn't take a lot of glue to hold a 2" X 2" tile on and it prevents having to clean up any material that squeezes out. Once you have it set down, you can kinda see the reflection of the back to see the coverage of the epoxy.
Another reason to mix less than you think you'll need is that if you mix too much at one time, you may still be arranging tiles and your glue has already hardened...
Leave it all alone for a few hours, anyway and let it all set up well before remounting the mirror.
A word of caution: I've found that before the material sets up, it has a tendency to flow so if your mirror is not sitting flat, the tiles may slowly slide one way or the other depending on the slope...