I have melted hard candy in the windows of my gingerbread house, two days later the candy is cracked. Tried it again, using more candy, but it cracked again. The gingerbread wall with the windows was baked and sat for two days before the hard candy was melted.
Using melted hard candy, you are at the mercy of the ingredients of the hard candy manufacturer. I like to make my own sugar glass. Here is my recipe for sugar glass:
- 1 C Water
- 1.75 C Sugar
- .5 C Corn Syrup - the clearer the better
- .25 t Cream of Tartar
I always supplement it with cream of tartar and corn syrup. The cream of tartar stabilizes the mixture and the corn syrup helps the glass stay clear. Sucrose (table sugar) is a disaccharide (two sugars) of glucose and fructose. Most cracking occurs when when there is a structural defect due to crystallization. Applying acid (cream of tartar) causes the sucrose to break down into its component sugars and stabilize.
Also remember that heat and humidity are the enemy of sugar glass. Do the best you can to control the environment of the sugar glass.
I don't consider ExpertChef's answer directly applicable, but it shows an interesting experiment possibility. It could be that glueing a tough, flexible film to your sugar windows will absorb some physical tension or jolting effects, and/or prevent drying out which leads to cracking. (The second shouldn't be a problem with traditional hard candy, but we don't know what your recipe is made of).
You could make such a film from high concentration gums in water (don't bother flavoring, at that concentration aroma isn't really noticeable, and the lack of sugar probably won't register against the sugar window). There was some molecular gastronomy recipe for packing butter pieces in a transparent "hull", if you can find it (I couldn't :( ) whichever hydrocolloid they used will probably work well for you. I don't know if it is better to brush the wet hydrocolloid on the window and let it set, or to make the film first and then glue it, this needs experimenting.
As an alternative to a gum film you could try glueing rice paper or oblaten to the inside of the windows. It is probably the easier way out. But it should be a more or less strong bond, just sticking it with jam or similar won't keep the sugar part from experiencing physical stress.
These are all untried ideas, if somebody has applied them, I would like to hear results.
Skip the melted candy windows! It's not worth the stress of wondering about the amount of humidity in the air and when the windows will start melting and running in riverlets down the side of your beautiful gingerbread house and then disappear completely, leaving open holes where your windows were. Use sheets of gelatine instead. They look awesome, stay good FOREVER and actually look like old fashioned leaded windows with the little diagonal diamond shapes. I make gingerbread houses every year for my confectionery and find the finest, tiniest strings of battery operated lights which I fix to a fondant Christmas tree standing in the "garden" before threading it through under the wall to the inside of the house. So beautiful.
When I make glass windows for my ginger bread houses I just crush hard candies, put the sides of my house on a cookie sheet with a thin smear of oil where the cut out windows are. Then I put the crushed candy in the cut out windows of the sides and front of my house.
I put it in the hot oven, watch it closely, and wait for it to melt. Then I have candy windows. When it's cool they come right up without cracking because I put a small amount of oil under them.
Then I put my ginger bread house together. I also put a light under my houses so it looks like someone is "home".