Can we measure freezing point of glycerin by freezing it.

Some research articles says that freezing point of glycerin is appx 18 degC. I tried to freeze USP grade glycerin (99.5% glycerin content) but it doesn't solidifies even at -5 degree Celsius. Does glycerin forms crystals after solidification or it just remain liquid state? If yes then what is the exact freezing point of USP grade glycerin (99.5% glycerin content). I have kept glycerin in humid environment but in a tightly closed container for freezing, does that affect on crystal formation?