Two ways to add menthol to a snuff you already have. Crystals in the tin give a mild result. Dissolving them in alcohol gives a stronger, more even one, which is how manufacturers are said to do it.
Overnight for the crystal method. About 30 minutes of airing for the alcohol method.
Notes
Don't leave any crystals in the tin, because a pinch of pure menthol is painful. For a stronger cooling snuff, add eucalyptus oil up to 4.5% of the weight. Keep menthol at 5% or below, and let the alcohol evaporate completely before you take a pinch.
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