A lot has been said about the effect of pH and water temperature on shaving. The outer layer of hair, the cuticle, is usually said to be hydrophobic or water resistant. The idea is that the cuticle is made mostly of keratin. Keratin is water resistant, but becomes less so under long exposure to high pH, or (to a lesser degree) in the presence of emollients and surfactants. Since the layers of the cuticle are made of keratin, shaving cream (with its high pH) supposedly helps to open or damage the layers of the cuticle, allowing water to penetrate. Hot water is also supposed to weaken the H-bonds in keratin, making it easier to absorb water.
But the scientific literature specific to shaving doesn't support these conclusions. At best, they suggest that these factors have little impact on shaving.
In A Short Survey of Scientific Literature on Shaving, I gave a brief overview of the important scientific research about shaving, specifically the effect of water. Here's a summary of the important points.
The effects of high temperature and high pH can have an effect on shaving, but it's very small compared to the simple absorption of water by the non keratin areas of hair. You can decrease the force-to-cut more than half by raising the temperature from 25C to 80C (77F to 176F), but no one could stand those temperatures for the two minutes it takes to fully saturate the beard, so this is impractical for shaving. The same is true for pH, where the alkaline strength and/or soaking time needed to significantly open or damage the hair's keratin is impractical for shaving.
It should be noted that "Observations on the Cutting of Beard Hair" does test these harsher conditions using solutions meant for permanent wave. The reduction in FTC was as high as 13%, but leaving it to soak longer, the damage to the hair was so bad that the hairs couldn't be cut at all, but would bend and split down the length. In normal hair treatments, such as used for hair coloring & waving, the damage, which is temporary, is often reversed by a second procedure. Obviously, reversing such temporary damage would also undo any benefit to shaving.
...
The following data shows that fully saturated keratin softens even more than the other internal parts of the hair. There's also a hard lipid layer, but no data on softening.
Springer Handbook of Nanotechnology; ed. Bharat Bhushan
Relevant page on google books
The numbers suggest that anything which softens the keratin will make shaving easier. The chemistry of (usually ground and processed) keratin is pretty well studied, but it's quite unclear that keratin can open or soften under the mild conditions of shaving. If anything, the tests show that much harsher conditions, such as chemical bleaching, are required for satisfactory results. Perhaps the shaving soap affects the lipid interface that separates keratin fibers, allowing water to penetrate more deeply into the hair.
On the other hand, studies on shaving show that saturation in plain water results in about a 30% reduction in the force-to-cut beard hair. This is matched closely by the softening of the medulla from 0.35GPa to 0.26GPa. Since hardness is typically defined by the force to cut or indent, this data suggest that the softening of the medulla (and probably the cortex) is responsible for the bulk of softening of the beard, and is the main factor in using less force to cut your beard.
One other conclusion is important from "Observations on the Cutting of Beard Hair"
This suggests that washing has no effect, and that leaving lather on the face for two minutes should give the same results as keeping plain water in contact with the face for the same period. Given the small sample size of their tests, I want to keep one reservation open. It may be that some people have excessive oils on their beard. For these people, perhaps cleansing would aid water absorption.
A second consideration is the effect that the emollients and surfactants in lather have on the contact angle of water. These would makes it easier for water to stick to or get close to the lipids and keratins, parhaps making it easier for water to penetrate between the mesh of keratin fibers. Since there were no tests with lather, only water, no conclusions can be drawn.
Finally, the terms hydrophobic and hydrophyllic are relative, not absolute. This is particularly true for the hair, whose outer surface is 90% hydrophobic and 10% hydrophyllic. If you include the space between the fibers, you have ample room for water to penetrate to the inside of the hair. The surfactants and hydrophyllic areas will make it easier for water to stick to the fibers, which in turn allows more water to seep through.
Discussion
It's often said that shaving cream with high pH breaks down, damages, or opens the overlapping layers of the cuticle cells. High pH and high temperature also help break hydrogen bonds. These all allow water to penetrate into the keratin, making it softer and easier to cut. But the scientific literature on shaving, hair, and keratins gives little support for these mechanisms. Instead, the literature suggests that much harsher conditions are necessary for these to have more than minimal impact on shaving.
Instead, the mechanism of beard softening is largely due to simple absorption of plain water by the parts between the keratin matrix. I believe this conclusion was already made in my "Introduction & Focus on Water & Beard Hair". What I hope to have added here is supporting data from research on keratin. Studies done on cutting beard hairs show a 30% reduction in force-to-cut, which is consistent with simple absorption of plain water by the medulla and cortex, as shown above.
Other studies have shown a 60% reduction in force to cut, which would be consistent with fully wetted keratin. These may have been done on hair treated to harsh conditions, such as the harsh environment of chemical permanent-wave solution, near boiling temperatures, and a soaking period as long as 24 hours. None of these are practical for everyday shaving.
My Personal Views
A measly 30% reduction in force-to-cut doesn't seem like enough to account for the effect of a nice long prep. It implies something else is important. Something that's not been measured. Perhaps stiffness and torsional behavior. My suspicion, which I can't back up at all, is that the effect of prep on skin is just as important, if not more so, than the effect on hair. After all, skin is similar to hair, both being made of largely the same keratins, lipid layer, etc.
Most people use hot water which does have a slight impact. A lather also has a slight impact. But adding these all up probably doesn't amount to even 40% reduction in force. If you start accounting for people who prefer cold shaving, or low pH creams, these additional factors don't seem to be relevant. Besides, if you look at the actual force numbers (less than 1 gram-force in some tests), it's even harder to believe that a 30% reduction matters at all. And that implies that something's going on that noone's really accounting for.
Additional Thoughts...
I had one other thought. I believe there's a larger difference in softening when wet across the diameter of hair than through it's length.[sup]1[/sup] To check, I wet a puck of soap, rubbed it on my dry face, and shaved immediately after it dried (with a single light pass), taking care to use little pressure. The blade gunked up thickly and immediately along its edge with soap, which made it hard to shave, and I can't say how this affected the shave. Otherwise, the most noticeable issue was that the blade bounced off the face instead of cutting through the hairs. So perhaps the important mechanism of softening is how it makes it relatively easier to cut through the hair when wet, while it's easier to cut down its length when dry. This has been shown previously in pictures where a wet hair is cut cleanly while a dry hair is irregularly split and fractured through it's length.
Technically, it's not completely a matter of softening, but a change in the fracture or cracking pattern. And this difference between length and width has a lot to do with H-bonds. If this is the mechanism, it would explain how the pH and surfactants of soap can have the impact on shaving we all know to be the case. In the future, I'm going to follow up this path, as well as look into how lather & water affect the face, friction, etc.
1. “Cutting Characteristics Of Beard Hair” (S. M. Thozhur, A. D. Crocombe, P. A. Smith, K. Cowley, M. Mullier; 2005-2007). From the cutting force data, it was found that moisture reduced the force in the cutting direction more than in the tip-bending direction. (But this is a derived result, not a complete explanation.)
But the scientific literature specific to shaving doesn't support these conclusions. At best, they suggest that these factors have little impact on shaving.
In A Short Survey of Scientific Literature on Shaving, I gave a brief overview of the important scientific research about shaving, specifically the effect of water. Here's a summary of the important points.
- Beard hair fibers appear to have been completely hydrated by exposure to water at room temperature within about 2 to 3 min.
- Moisture reduced the cutting stress through a combination of a reduction in the force to cut (ftc) 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%.
- 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.
- temperature, wet hair: A roughly linear drop in force occurs from 4.5g@25C to 2g@80C.
- pH: pH has little or no effect on the f-t-c or on the rate of hydration of beard hair.
The effects of high temperature and high pH can have an effect on shaving, but it's very small compared to the simple absorption of water by the non keratin areas of hair. You can decrease the force-to-cut more than half by raising the temperature from 25C to 80C (77F to 176F), but no one could stand those temperatures for the two minutes it takes to fully saturate the beard, so this is impractical for shaving. The same is true for pH, where the alkaline strength and/or soaking time needed to significantly open or damage the hair's keratin is impractical for shaving.
It should be noted that "Observations on the Cutting of Beard Hair" does test these harsher conditions using solutions meant for permanent wave. The reduction in FTC was as high as 13%, but leaving it to soak longer, the damage to the hair was so bad that the hairs couldn't be cut at all, but would bend and split down the length. In normal hair treatments, such as used for hair coloring & waving, the damage, which is temporary, is often reversed by a second procedure. Obviously, reversing such temporary damage would also undo any benefit to shaving.
...
The following data shows that fully saturated keratin softens even more than the other internal parts of the hair. There's also a hard lipid layer, but no data on softening.
Springer Handbook of Nanotechnology; ed. Bharat Bhushan
Relevant page on google books
The numbers suggest that anything which softens the keratin will make shaving easier. The chemistry of (usually ground and processed) keratin is pretty well studied, but it's quite unclear that keratin can open or soften under the mild conditions of shaving. If anything, the tests show that much harsher conditions, such as chemical bleaching, are required for satisfactory results. Perhaps the shaving soap affects the lipid interface that separates keratin fibers, allowing water to penetrate more deeply into the hair.
On the other hand, studies on shaving show that saturation in plain water results in about a 30% reduction in the force-to-cut beard hair. This is matched closely by the softening of the medulla from 0.35GPa to 0.26GPa. Since hardness is typically defined by the force to cut or indent, this data suggest that the softening of the medulla (and probably the cortex) is responsible for the bulk of softening of the beard, and is the main factor in using less force to cut your beard.
One other conclusion is important from "Observations on the Cutting of Beard Hair"
- Washing with SLS: No significant difference. The rate of hydration is not altered by the removal of surface lipids.
This suggests that washing has no effect, and that leaving lather on the face for two minutes should give the same results as keeping plain water in contact with the face for the same period. Given the small sample size of their tests, I want to keep one reservation open. It may be that some people have excessive oils on their beard. For these people, perhaps cleansing would aid water absorption.
A second consideration is the effect that the emollients and surfactants in lather have on the contact angle of water. These would makes it easier for water to stick to or get close to the lipids and keratins, parhaps making it easier for water to penetrate between the mesh of keratin fibers. Since there were no tests with lather, only water, no conclusions can be drawn.
Finally, the terms hydrophobic and hydrophyllic are relative, not absolute. This is particularly true for the hair, whose outer surface is 90% hydrophobic and 10% hydrophyllic. If you include the space between the fibers, you have ample room for water to penetrate to the inside of the hair. The surfactants and hydrophyllic areas will make it easier for water to stick to the fibers, which in turn allows more water to seep through.
Discussion
It's often said that shaving cream with high pH breaks down, damages, or opens the overlapping layers of the cuticle cells. High pH and high temperature also help break hydrogen bonds. These all allow water to penetrate into the keratin, making it softer and easier to cut. But the scientific literature on shaving, hair, and keratins gives little support for these mechanisms. Instead, the literature suggests that much harsher conditions are necessary for these to have more than minimal impact on shaving.
Instead, the mechanism of beard softening is largely due to simple absorption of plain water by the parts between the keratin matrix. I believe this conclusion was already made in my "Introduction & Focus on Water & Beard Hair". What I hope to have added here is supporting data from research on keratin. Studies done on cutting beard hairs show a 30% reduction in force-to-cut, which is consistent with simple absorption of plain water by the medulla and cortex, as shown above.
Other studies have shown a 60% reduction in force to cut, which would be consistent with fully wetted keratin. These may have been done on hair treated to harsh conditions, such as the harsh environment of chemical permanent-wave solution, near boiling temperatures, and a soaking period as long as 24 hours. None of these are practical for everyday shaving.
My Personal Views
A measly 30% reduction in force-to-cut doesn't seem like enough to account for the effect of a nice long prep. It implies something else is important. Something that's not been measured. Perhaps stiffness and torsional behavior. My suspicion, which I can't back up at all, is that the effect of prep on skin is just as important, if not more so, than the effect on hair. After all, skin is similar to hair, both being made of largely the same keratins, lipid layer, etc.
Most people use hot water which does have a slight impact. A lather also has a slight impact. But adding these all up probably doesn't amount to even 40% reduction in force. If you start accounting for people who prefer cold shaving, or low pH creams, these additional factors don't seem to be relevant. Besides, if you look at the actual force numbers (less than 1 gram-force in some tests), it's even harder to believe that a 30% reduction matters at all. And that implies that something's going on that noone's really accounting for.
Additional Thoughts...
I had one other thought. I believe there's a larger difference in softening when wet across the diameter of hair than through it's length.[sup]1[/sup] To check, I wet a puck of soap, rubbed it on my dry face, and shaved immediately after it dried (with a single light pass), taking care to use little pressure. The blade gunked up thickly and immediately along its edge with soap, which made it hard to shave, and I can't say how this affected the shave. Otherwise, the most noticeable issue was that the blade bounced off the face instead of cutting through the hairs. So perhaps the important mechanism of softening is how it makes it relatively easier to cut through the hair when wet, while it's easier to cut down its length when dry. This has been shown previously in pictures where a wet hair is cut cleanly while a dry hair is irregularly split and fractured through it's length.
Technically, it's not completely a matter of softening, but a change in the fracture or cracking pattern. And this difference between length and width has a lot to do with H-bonds. If this is the mechanism, it would explain how the pH and surfactants of soap can have the impact on shaving we all know to be the case. In the future, I'm going to follow up this path, as well as look into how lather & water affect the face, friction, etc.
1. “Cutting Characteristics Of Beard Hair” (S. M. Thozhur, A. D. Crocombe, P. A. Smith, K. Cowley, M. Mullier; 2005-2007). From the cutting force data, it was found that moisture reduced the force in the cutting direction more than in the tip-bending direction. (But this is a derived result, not a complete explanation.)
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