When the potpourri smells and looks good, it is time to consider preserving and strengthening its scent. Orris, the dried root of the Florentine iris, is the best fixative, and should be used chipped, not powdered. It has no scent of its own, but helps others to last. Drying has evaporated some of the fragrant oils in the flowers, it is best to replace these with essential oils. Rose is the most versatile of these, blending well with nearly any other fragrance.
Lavender is the strongest and tends to dominate others.
Bay, balsam, cedar, orange, lemon, gardenia, and carnation are also favorites, but since good oils are quite expensive, it is best to begin with just a few and add more if you find you enjoy creating potpourri. Mix your ingredients in whatever amount you have or like, and add 1 or 2 tablespoons of orris root per pint of flowers. Depending on the intended use for the potpourri, use 4 to 8 drops of oil to a pint. Mix well and seal in a jar with plenty of air space. Shake or stir it daily for 2 weeks to allow it to blend and ripen.