Two clients with the same chemical history can respond in opposite ways to the same product. What sets them apart is almost never the hair type — it is porosity: how open the cuticle is and how much water the fiber absorbs and loses.
Porosity is not something you find out by looking. You find it out by testing, and the most reliable chairside test is also the cheapest one: a glass of water.
The test, step by step
Take a loose strand
Use a strand that already came loose in the brush, from mid-length to the ends. A strand pulled from the root comes with its bulb and sinks from weight, not from porosity.
Fill a glass with room-temperature water
Hot water opens the cuticle and pushes the reading too high. Cold water does the opposite.
Lay the strand on the surface
Don't push it under with your finger. Let the surface tension hold it and watch for two minutes.
Read the timing, not the final position
What matters is how long it takes to sink. Almost any strand sinks if you leave it long enough.
What each reading means
Where the strand sits after two minutes splits the service into three different paths — and this is where the test stops being a curiosity and starts changing the dosage.
The three readings
Stays floating — low porosity
The cuticle is closed and repels product. The hair takes time to get wet and the mask sits on the surface. Give it heat and time: the active needs help to get in. Overdosing here only builds up residue.
Sinks slowly — medium porosity
This is hair in balance. It absorbs well, dries in a normal time and responds to the dosage on the label, with no adjustment.
Sinks fast — high porosity
The cuticle is open: it takes in everything and loses everything. Start with half the dosage and shorten the processing time. Saturated porous hair turns heavy and stretchy, and it breaks under the brush.
The most common mistake
Treating high porosity with more product and more time. The logic sounds right — the hair is damaged, so it needs more — but it is exactly the opposite: an open fiber has no way to hold what it receives, and the excess turns into weight.
With high porosity, less, more often works better than a lot at once. Two split applications a week apart deliver more mass to the hair than one double application.
Run the test again
Porosity changes. After a bleaching service, a summer in the pool or three months of good maintenance, the same hair gives a different reading. Repeating the test at every visit costs two minutes and keeps you from repeating a protocol that no longer fits.
