After cutting and adjusting the size (up or down) wrap the stone
portion of the ring in toilet paper and wet it to protect the
stones. I generally use a commercial preparation called Heat
Shield. Synthetic sapphire will take the heat if it is even, but
why take the chance.
I use paper towel instead of toilet tissue, it doesn’t fall
apart so easily.
Regarding corundum, it can definitely be heated, but be aware
that boric acid or flux will etch corundoms when heated. You can
heat it, but you must make sure there is no flux or boric acid on
it. What is really funny here is that I’ve had jewelers argue
with me on this point, I’ve also repolished sapphires that have