Understanding Hair Porosity
Porosity refers to the hair's ability or inability to absorb water or chemicals into the cortex. All hair is naturally porous and permeable to water, but the degree of porosity usually varies by the individual and the condition of the hair's cuticle layers.
1) Porosity is a physical condition that belongs to an individual hair - or a portion of a hair. It is the amount of surface area with porosities - gaps and holes through which water and other things can move.
2) Porosity is a behavior or your individual hairs and your hair as a whole - how quickly does water move through any porosities present.
3) We want to use porosity to determine what to do to our hair - but all the things we do can modify our hair's porosity behavior or even mask actual porosity. For example, sun-damage and therefore porous hair may feel different than bleach-damage and therefore porous hair. Or hair which is easily weighed down or low-density may or may not be able tolerate lots of conditioner and heavy oil applications that we usually think porous hair needs. Different porous hair has different needs.
“Why does porosity-behavior matter? Let's say you use an oil on porous hair and it becomes water-repellant, it doesn't seem to get wet and dries very quickly. That is a behavior of non-porous hair. The same thing can occasionally happen with protein treatments, coconut oil or butters.
Has that oiled hair truly become less porous? Yes and no. The porosities that were there before still are there. But the movement of water through those porosities has been delayed by the oil residue. The hair's behavior is less porous, but the hair still has porosities - it just takes a lot longer for water to move through them. With coconut oil and hair-penetrating oils, the oil repels the water, causing non-porous behavior in normal porosity to porous hair”.
Why the Float Test Is Unreliable
The float test for hair porosity is one in which you try to float a strand of hair in water. It is supposed that a porous hair will immediately begin to absorb water, become heavy, and sink. But there are some flaws in the design.
First- consider the surface tension of water. The molecules at the surface of water stick together where air meets water. Like skin on the water. If you place a light-weight object on the surface of the water gently, it will float. You can make sewing thread float on water this way. So let's say you have a very lightweight, but porous hair and you drop it in a glass of water. The surface tension may well override the weight and porosity of the hair strand and it floats - even after many minutes have passed.
Second - specific gravity. Hair and water have a similar specific gravity. Things of similar or lesser specific gravity to a given liquid will float in that liquid. Dry hair is buoyant in water - like ice or driftwood.
Third- What is on the hair? Is the hair heavier due to the weight of a product? Is that product repelling water? Is that product a wetting agent (like hair conditioner) which will cause the hair to become wet more quickly and sink.
As you can see there are too many variables. And for porosity, your own observation is going to take many variables into account which will give you plenty of relevant information.
One of the most accurate ways of measuring hair porosity is using gas sorption determine total pore volume (holes in the surface) - which is expensive and may not be accessible to everyone. It measures how much of a given gas can be taken into hair - hair which has more pores will take on more gases. Hair can also be examined under a microscope to determine how much chipping, wearing away, or lifting there is in the cuticle. This is all visual.
For more info. about gas sorption click the link here: http://www.beauty-review.nl/wp-content/uploads/2014/06/True-porosity-measurement-of-hair-A-new-way-to-study-hair-damage-mechanisms.pdf
Why Does Porosity Matter?
Porosity matters because it determines how much protection your hair needs (what you'll apply to it) and how quickly it will lose water. It also determines how well or quickly your hair takes color. Porous hair loses water more quickly than hair which is not especially porous. Dehydrated hair is less pliable, breaks more easily, has less shine and does not hold a shape (like a curl or wave).
When your hair faces a process like relaxing or permanent color, its protein structure is attacked and its protective cuticle shielding, if you will becomes tattered and torn. Hair in this condition is said to be porous.
To better understand porosity, picture your hair like a shingled roof (I couldn't think of anything else to compare it with at the time I was writing this so bare with me). When the roof is new, it's able to shield the house it covers. It can stand up to the rain, sun, all the elements, etc. As it ages, the roof begins to become brittle/dry and the roof's protective barrier can be breached.
Overtime, holes/gaps in the roof can occur because of missing shingles. The roof has become weathered (porous). Our hair functions like the roof. As it ages, the hair's protective cuticle layers begin to peel and lift away. This change in the cuticle's disposition make the hair less able to absorb and hold the moisture it once could. Older hair is more porous (ex. your ends) or has higher porosity than newer hair (your roots). Porosity increases as we move from the roots to the ends of the hair because this represents age progression along the fiber.
Hair with low porosity does not readily absorb moisture and can resist chemical treatments. This hair is generally healthy and has not been exposed to cuticle degrading treatments. Also, with low porosity hair, the cuticle ridges along the hair shaft are tightly "closed". (P.S. I put the word closed in quotations because the cuticles don't really close, they just lay flat and smooth. And in case someone more technical will try to critique my wording). Anyways, the regular daily mechanical manipulation, styling, any chemical processes eventually cause the cuticle scales to lift and lose their tightness over time.
The Hair's PH and Porosity Characteristics are Connected
Low pH products and styling treatments reduce the hair's porosity by constricting the cuticle and causing it to tighten. And high pH products have the opposite effect and increase the hair's porosity by swelling and lifting the cuticles scales.
**The more damaged the cuticle has endured, the greater the porosity and the more water and moisture it tends to absorb.
Highly porous hair absorbs more water when wet but loses even more as it dries. When it is fully dried, porous hair feels swollen, puffy and "sometimes" rough to the touch due to the lifted damaged cuticle layers which have contributed to an inherent moisture deficiency. **If damage is repeated too often (ex. back to back coloring, frequent relaxing, heat abuse), the cuticle layers may never fully "close".
Low porosity should focus more on moisture than protein. Hair already has high levels of protein, for the most part.
Medium Porosity should focus more on maintaining already balanced proportion of moisture/protein. Alternate every week; one week do a 30 min. deep conditioning treatment after shampooing then the next week, do a protein treatment after you shampoo. Of course, always remember this will vary from person to person.
High porosity should focus on both moisture and protein. The reason why you have to focus on both is because the lack of protein in your hair makes it almost impossible to hold in moisture. Your hair's lack of protein makes it difficult for hair to hold in moisture/water. Try to make sure your regular conditioner contains hydrolyzed protein as one of the top 5 ingredients. With this hair type, to determine whether to add more protein or more moisture, it's really important to LISTEN to what your hair is telling you. Start by doing one or more protein treatments a month or vice versa with the deep conditioning treatments. This hair type doesn't necessarily need to use the extra heat when deep conditioning for treatments to penetrate. The use of a plastic cap for 30 mins. should suffice.
Source: Hair and Hair Care (Cosmetic Science and Technology). Sciencey-blog by Wendy. ncbi.nim.nih.gov. The Science of Hair Care. International Journal of Trichology.