So I made a Frozen cake for a customer and added tall shards of sugar glass, inserting them into my fondant cake. It looked super cool! ... Until I woke up 6 hours later to find it had wilted! Bent! My sugar glass was now on a slant! I swore it was solid as a rock when I inserted it. I literally smashed it with a hammer to break it up. Did I not boil it long enough? Did it absorb moisture and become soft? I ended up replacing the tall pieces with unused short pieces which didn't look terrible but definitely didn't have the same effect. :(
Well, depending where you are, some hours is enough to make the sugar glass melt. I live in the tropics (very high humidity!) and thin sugar glass like for a frozen cake lasts a maximum of 9 hrs. It even starts to get sticky way before that.
Sugar is hygroscopic, which means that it absorbs water really easily. When I put it on cakes I wait until the last minute and I try not to refrigerate it because it's usually okay but I'm paranoid about the changes in humidity in and out of the fridge. You might not have cooked it long enough because when you're coking sugar you're basically removing the water from it, but it might just have absorbed moisture from the air.
I'll add that putting broken shards of sugar on a cake is pretty dangerous, because it can be sharp enough to cut someone. I've sliced myself with sugar enough times to know that I would never put broken pieces on a cake for a customer...But if yours melted the edges probably softened up too so that's a moot point right now.
they use boiled sugar for the big 'break through the window' scenes on movies and the stunt people & actors can/do get cut sometimes
Yes I put in the contract that the sugar glass is decorative only and is not to be consumed, and also that it is to be removed by an adult only before serving the cake.