I bought an ounce of crystals a year ago. To make a high concentrate of menthol liquid, I crushed half of it (about 14 grams) and put that in a 30ml bottle. It pretty much fills it. then put in 15ml of PG. That will fill the bottle. Shake vigorously til the crystals are mostly dissolved. Then get you a sink of hot tap water and drop the bottle in there. The warmth will finish dissolving the crystals. Let the sink come back to room temperature and take the bottle out. All the crystals should be dissolved. I find this method keeps the menthol from re-crystallizing and it makes for a very potent batch of menthol. Only a drop of two per ml is all you need for flavoring. I'm only half way
through that first bottle of XXX menthol and still have another half ounce of crystals to mix.