Oh, I forgot. Tartaric acid is used for fine lace work. If you want to do long dropped lines, extension work with your royal icing, than I read that acetic acid is what you add. There is a book on cake decorating I was reading today at the book store and it had two recipes for the tartaric and the acetic acid. I don't even know what the second acid is. I guess I am off to google it. But, there could be other royal icing recipes out there that we have not tried yet. Maybe one will turn up to be a charm for us.
A bit more to think about: Cream of tartar is a weak salt derived from tartaric acid when it is half-neutralized with potassium hydroxide (hence it's other catchy names: potassium bitartrate and potassium hydrogen tartrate). Cream of tartar is therefore an offspring of tartaric acid.Tartaric acid is not called for very often in the home kitchen – when it is, it is primarily to provide extra tartness to foods. If you don't have tartaric acid on hand (which is much harder to find and more expensive than cream of tartar, you could probably use two teaspoons of cream of tartar in place of one teaspoon of cream of tartar and one of tartaric acid.