I am making a halloween wedding cake and have a few questions about royal icing. I have the cakes covered in fondant and are in the fridge. I have bats that need to be glued on with RI and a few spider webs to make also with RI. Would it be ok to do these onto the cold cakes or would there be a condensation issue that might cause the RI not to harden? Also can the cakes go back in the fridge after I'm done decorating without anything bad happening to the RI? I've never used it on cold fondant or put it in the fridge, always just room temp and left to air dry.
Condensation aka moisture and royal icing don't get along. The moisture can cause the royal icing to melt.
If it were me, I'd wait until the cake came back to room temperature before piping with the royal icing. If the cake condensates when it comes out that could be disasterous for the royal icing. I agree fully with the above poster.
Also, don't put the cakes back in the fridge after applying the RI.