Originally Posted by mamawrobin
What is your fat to sugar ratio? I use 1 1/3 cups of shortening to 2 pounds of powdered sugar. I've seen several say that your fat/sugar ratio shoud be 1:1. I don't agree. If your buttercream isn't crustiing add less fat than sugar. I'd say 1 cup to 1 1/4 pounds of sugar.
I am one that will tell you that 1 to 1 is a good place to start. It doesn't mean that this is the only way the icing will crust, but when I am looking at recipes I do use it as a guide.
I know, that for me, if a recipe has 1 cup fat to 1lbs sugar it will crust. I also know if it has 2 cups fat to 1lb sugar chances are it will be slow to crust, if it happens. And if it is less fat to the 1lb sugar it will crust faster.
I think your local weather does play a factor in to it. The past few months I've been tempted to tell my students to add more fat than the 1 cup to 1lbs when we do rose because there is something in the classroom at the store that just sucks the moisture out of the icing and causes it to get dry.
I also personally think it depends on if you are using all Crisco vs butter or a combo of butter and crisco. The butter does have more liquid in it it that may cause the crusting time to be a little slower.