I was wondering if menthol crystals could be added to lotion which has already been made, or must it be added when the lotion is made? I have a batch of prepared lotion and some menthol crystals and a crick in my neck! I would love something with that heating effect to rub on my shoulders and neck. I have never worked with the menthol crystals before. I bought these a while back thinking I would try something with them but just didn't really know where to start. Is there a guideline as to how much you add when using it?