In short
Diffusion is how far a smell carries through the air. Substantivity is how long it stays on a surface. They are separate: a material can cling for days without carrying across a table, while another fills a room and then disappears. Knowing which you need changes which material you reach for.
What you will learn
- Distinguish diffusion from substantivity and strength
- Run an informal room test and a surface test
- Choose materials by the property you need
Before this lesson: Balance Volatility Across a Formula
Keeping the words straight
| Term | The question it answers |
|---|---|
| Odor strength | How strong is it at the source? |
| Diffusion | How readily does it spread through the air? |
| Tenacity | How long is it perceptible on a strip? |
| Substantivity | How well does it stay on skin, hair or fabric? |
| Sillage and projection | How noticeable is the wearer to others? (A wearer's question) |
Level 8 tests these in a controlled way. This lesson gives you enough to use them while composing.
What seems to help
Trade experience attributes diffusive lift to several materials, such as Hedione, Iso E Super and dihydromyrcenol. Substantive behavior is often associated with musks such as ethylene brassylate and Galaxolide. Treat these as reported tendencies. The effects depend on dose, formula and substrate, and research on them is often proprietary or hard to apply directly. So test your own.
A rough room test
- Choose a room without other smells, and leave it to air out.
- Prepare a strip of the material at a known dilution. Have someone place it at one end while you wait outside.
- Enter, and walk slowly toward it. Note where you first detect it.
- Air the room for a long time, and repeat with another material at the same dilution.
Only large differences mean anything. Draft, temperature, your memory of the first test and your own adaptation all affect this.
A rough substantivity test
Apply a measured small amount, such as 0.05 g of a 10% dilution, to clean squares of unscented cotton and paper. Do not use skin for raw materials. Let them dry, label them, and store them apart. Rate them at 1 hour, 24 hours, 3 days and 7 days. You may find that a material lasts far longer on one than on the other, which tells you the substrate matters. Fabric can stain, and many materials are flammable before they dry, so keep away from flames.
Using this when composing
- The fragrance is close to the skin, and you want more presence: test a diffusive material by ladder.
- It carries well, then vanishes: check the low band, and add something substantive that belongs to the idea.
- It clings but smells flat: lift it with something brighter in the middle.
- It is loud at first, then quiet: reduce the top, rather than boosting the rest.
Beware of chasing performance at the cost of the composition. A fragrance that lasts forever and smells of little is not a success.
Practical exercise: Test carry and cling separately
- Choose three materials that you expect to differ.
- Run the room test with a helper, if possible, and repeat it on another day.
- Run the fabric and paper test over a week.
- Write a short table: expected, observed, and what the test cannot show.
- Choose one material to add to your current formula because of what you found, and test it by ladder.
You have done this well if your table separates carry from cling, and you state the limits of what a home test shows.
Common mistakes
- Treating strength as diffusion. A strong smell at the source may not carry.
- Testing raw materials on skin. Use fabric or paper here.
- Overinterpreting one run. Repeat, and report large differences only.
Key terms
- Diffusion
- How readily a smell spreads through air and is noticed away from its source.
- Substantivity
- How well a material stays on a surface such as skin, hair or fabric over time.
- Tenacity
- How long a material stays perceptible on a substrate such as a smelling strip.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.