Okay, up next: hair elasticity and density. Let's see how coherent I am after my Steelers' big Super Bowl win last night :)
Elasticity is the ability of your hair to stretch and then return to its original length without breaking. It is an indication of how strong the side bonds are in your hair, which are the bonds that hold the individual protein chains of the hair strand into place. More than any other property, elasticity is what dictates your hair's ability to hold its curl, whether natural, or created by a wet set or perm.
Hair with normal elasticity can be stretched when wet to up to 50% of its original length and will easily return back without breaking. Hair with low elasticity is brittle, will not return to its original length when released, and usually snaps or breaks easily when stretched. Hair with low elasticity also will not hold a curl from a wet set or a perm.
Previous overprocessing and excessive heat styling (flat irons, etc.) are the biggest reasons for low elasticity. I hear a lot of this from clients who are just starting to let their curls go natural: "I've flat-ironed for a long time and I think my hair has forgotten how to be curly." Hair cannot forget how to be curly; only your genetics and medical condition can determine your wave pattern. But if you flat-iron or blow-fry your hair over a long period of time, you compromise the health of the hair by killing the elasticity in your hair strands. No elasticity = dry damage, high porosity and less curl.
To check the elasticity of your hair, wet a single strand, then stretch it to 50% of its original length. If it bounces back to its original length without breaking, you have normal elasticity. If it does not return to its original length or breaks, you have low elasticity.
Hair density is the number of hairs on your scalp per square inch, and is classifed as low, medium or high -- we usually describe it in layman's terms as "thin, medium or thick." Typically, natural redheads have the least amount of hair strands on their head: about 80,000. Black hair averages 108,000, brown is 110,000, and blondes average the highest density at about 140,000.
Density is important for two reasons: cut and product types. A good stylist will take both your hair texture and your hair density into consideration when determining the most appropriate cut for your hair. On straight hair, for example, someone with fine, thick hair would take a heavily-texturized razor cut well, but using a razor on someone with fine, thin hair will be unflattering since they need more weight, not less of it.
For curly girls, choosing product type is where knowing your density can be quite helpful. If you have very thick hair, gels -- which are volume-minimizing -- are often the best choice in creating a more structured, less "poofy" look. Girls with thin hair, however, will most likely get much better results by using a volumizing mousse, which creates the appearance of more hair.
Now that we've discussed the hair properties, next I'm going to touch a bit on hair analysis, which determines the behavior of the hair. This includes density and texture, which we've already covered, but also wave pattern, hairlines and growth pattern.