& Focus on Water & Beard Hair
The first research paper focused on shaving was published by Lester Hollander and Elbridge J. Casselman. Their “Factors Involved in Satisfactory Shaving” published in The Journal of the American Medical Association, started the science of shaving in 1937.
In it, they cite previous research in dermatology, hair growth, detergents, and historical information on barbering. For our purposes, the important scientific research they cite was done by the textile industry, notably J.B. Speakman's work on wool fibers. Speakman wrote at length about the wetting behavior of hair. His emphasis was on textiles, so the important properties included stretching & retention of shape, the ability to hold dyes, fraying, etc. Cutting never entered into his work.
One important fact established by Speakman is that hair absorbs water and expands when wet. Much confusion surrounds this simple observation. The confusion is easily explained by observing that most research on hair is focused on the chemistry and physical attributes of styling and cleaning products for hair on the head. In 1959, M. Feughelman proposed “A Two-Phase Structure for Keratin Fibers” (Textile Research Journal; 29, 223). In it, he describes keratin as composed of a water-resistant “microfibril” embedded in a water penetrable matrix. Water easily enters into the space between the fibers, even if it isn't easily absorbed by the fibers themselves, nor does the water stick to the hair fibers. For a hair styling product, such as a conditioner or shampoo or coloring, the stickiness (wetting ability) is important. For the purposes of shaving (i.e. cutting the hair), the water is important.
The long lasting impact of “Factors Involved in Satisfactory Shaving” can't be underestimated. They report on a variety of issues studied over 6 years in the first shaving clinic established at The Mellon Institute. The report covers the basics of how to get a good shave, including necessary prep time, proper blade angle and how adjustments affect the shave, how to reduce irritation, factors that affect blade life, ingrown hairs, and a variety of other issues. They even quantify irritation by measuring the quantity of skin removed during a typical shave.
While Hollander & Casselman did quite good science, and set the established procedures for decades, the technology to measure certain things just wasn't available at the time, so they were forced to make some assumptions that turned out to be false.
Hair was originally thought to have only one important direction of physical strength--length--so only the tensile (stretching) properties were studied. It's from this research that we get the notion of hair being stronger than copper wire. Copper wire simply breaks sooner than hair because hair stretches more while still maintaining much of its original strength. But tensile strength has nothing to do with cutting, except that it can determine the shape of the cut in certain failure modes (where the blade splits the hair across its length).It may be assumed that facial hair is substantially softened under the same conditions that produce 0.85 of its full stretch under load.
This focus on tensile strength leads down the wrong path. In the two-phase structure, the microfibril structure maintains much of the strength of the hair. Think of the microfibril structure as the trusses of a bridge, and the matrix as a loose pile of string that holds water. This structure can be pulled, much like a plastic mesh bag. Certain properties of this structure are dependent on the chemistry of keratin. So we end up with all kinds of factors, such as heat and pH, that matter if we're stretching hair, but are almost meaningless for cutting.
In 1976, Donald E. Deem, and Martin M. Rieger published "Observations on the Cutting of Beard Hair". They measured the force to cut (ftc) beard hair against various factors:
- cross-sectional area: There appears to be some correlation.
- rate of cutting: ftc increases significantly as the rate of cutting is increased
- washing with SLS: No significant difference. The rate of hydration is not altered by the removal of surface lipids.
- Force vs. hydration time--dependency on temperature: After 2 minutes, ftc drops from 5g@24C to 3g@55C, but an anomaly appears at 30C.
- pH: pH has little or no effect on the f-t-c or on the rate of hydration of beard hair.
- temperature, dry hair: ftc of dry fibers is lowered by raising the temperature.
- temperature, wet hair: A roughly linear drop in force occurs from 4.5g@25C to 2g@80C.
Other conclusions are:
- hydration time: Beard hair fibers appear to have been completely hydrated by exposure to water at room temperature within about 2 to 3 min.
- Even the most severe chemical (covalent bond) damage, which is known to lower the tensile modulus drastically, has almost no effect on the force required to cut beard hair.
- The fact that the cutting force is less dependent on relative humidity than the shear or tensile modulus suggests that these moduli-even at a rate of 0.5 in./min-are not the predominant factors in beard hair cutting. Instead, the f-t-c might be more closely related to stress propagation or to the creation of new surface area, than to the viscoelastic properties of the fiber.
The last conclusion shows that bending and pulling are not involved in cutting. They suggest that stress propagation through the hair is the important factor for cutting. In fact, this is true for cutting of most materials.
A more sophisticated test was reported in “Cutting Characteristics Of Beard Hair” (S. M. Thozhur, A. D. Crocombe, P. A. Smith, K. Cowley, M. Mullier; 2005-2007).
They reach the following conclusions.
- Moisture reduced the cutting stress through a combination of a reduction in the cutting force and an increase in the cross-sectional area of hair due to swelling.
- In the presence of moisture, the peak cutting stress was brought down by almost 30%.
- Variation between the subjects could not be distinguished from the test scatter.
- Blade angle did not affect the cutting stress noticeably.
- From the cutting force data, it was found that moisture reduced the force in the cutting direction more than in the tip-bending direction.
They also studied the way hair actually breaks when cut. They catalog them into four basic fracture mechanisms. A particular fracture pattern tends to occur depending on the distance between the blade and the “skin”.
The fracture mechanisms are:
- Partial penetration and bending followed by longitudinal splitting of hair towards the base
- Partial stable penetration followed by bending of hair and rapid fracture
- Slipping of the blade on the hair surface, followed by partial penetration and then skiving leading to gradual fracture
- Partial penetration followed by extreme skiving leading to propagation of blade along the hair length
It should be noted that this is a very short survey of the literature. Other reported tests give different results. For example, Thozhur et. al. reported the force to cut hair is reduced by about 30% due to moisture, while others have reported 65%. However, they didn't report any actual data. Other questions may arise due to the differences between force, stress, energy, etc.
So here's the way I look at shaving.
An analogy should illustrate what matters for cutting. An empty bag--let's use a sausage wrapper or tea bag or even a rolled up sheet of bubble wrap--should be easy to bend. If the bag is filled, say with jelly, it becomes harder to bend. Now think about cutting it. Empty, it should be pretty hard to cut. But filled, it should cut easily, with very little force. Of course, you'll also need to cut through a longer thickness. To complete the analogy, the bag is made of the hair's hard microfibril, and the filling is the rest of the hair filled with water.
It's important to distinguish here between softer and stiffer. Dry hair is hard, just like a dry bag would be. But filled with water (or jelly), the bag feels softer--more pliable--but it also becomes stiffer, at least in terms of being resistant to bending when a blade is pushing into it. It becomes more plastic and less elastic. This allows it to be cut easier.
So, fully soaked hair is easier to cut, or rather requires less force. A smaller normal force produces less friction, which means less irritation and less loss of skin.