I have made royal icing a couple of times now and I dont think I am doing it right.
I have used a couple of recipes and it never dries glossy (even though the recipes come from people with beautiful shiny cookies). I also cant understand how anyone could compare it to "super glue". My royal icing seems brittle and powdery. It wouldnt take very much at all to crush into a fine powder.
YOU have to keep royal as airtight as possible. Even when working with it or mixing it, I have often draped a damp towel around the top of my mixing bowl. I make it "flood" consistency for cookies and put it in a bowl, cover with saran and let it sit on the counter overnight to get most of the air bubbles to the surface. Flood consistency should be enough water that when you are mixing it, if you drop a small bit off the spoon, it will disappear in the count of 10. I mostly used Wilton's but recently switched to Antonia's. It's on here. Wiltons dries very hard. Antonia's is hard enough on the surface to stack but not rock hard when you bite it. If you are coloring it or working with it, be sure to keep it as covered as possible.
Also to prevent RI from breaking down it must not come in contact with any grease while you are making it. Be sure your bowl, beaters, color cups, etc. are totally grease free. I go so far as to wipe mine down with white vinegar and paper towel....
I also weigh my C&H powdered sugar....
To keep things totally grease-free, I use separate tupperware-like containers that I use only for royal icing.
If it's brittle and powdery, you're over-beating it. Use the paddle attachment, not the whip, and beat for 7 - 10 minutes.
If you want your royal icing extra shiny, you can put a little bit of corn syrup in it.