I made cupcake and iced half of them, with buttercream, last night. I put the letter "K", made out of royal icing, on the top of each one. Got up this morning to finish and found out half of my letters have deteriorated. I stood them up on top and they have fallen and broken into.

Anyone have any clues why this happened and what I can do to keep it from happening again? They were stored in a cupcake box.

Thanks!

Your buttercream was more than likely the problem. Royal is very sensitive to oil. Did you let your letters dry really well before placing them on the cupcake or did you pipe them onto the cupcake? You can put dry royal on buttercream and it be safe for a short period of time - maybe 24 hrs (not sure about how long).

I piped the letters about a week in advance.

You have been very helpful! Thank you very much!

I agree with mcalhoun that maybe they weren't dry?? and that the bc may have affected them. Also if you don't beat the RI for the right amount of time it will 'melt'.

I have heard of the shortening breaking down the royal icing. I made some cupcakes last weekend where I put royal icing flowers on top of buttercream. The flowers had been in my fridge for several months. Four days after I made the cupcakes, the flowers still had not broken down.

Grease is RI worst enemy, that's all it happened, no matter how much you let it dry. try making the letters in gumpaste or cover your cupcakes in fondant then ad a fondant or a RI letter
a fondant cup cake with fondant letter is on my pictures

It could also be from the moisture in the icing. Moisture will break down the royal icing.