Yeah, Bunnahabhain, like most potters I use red iron oxide FE2O3. Unlike most potters, I use "red iron precipitate" which has to do with the way the iron oxide is processed so that it is truer to that formula.

...but lemons is a good idea too. In fact, I looked it up in my "The Potter's Dictionary of Materials and Techniques". It says that "rutile" is just another name for "lemon rind". Who knew?