To get to the punchline, yep, as of right now, that’s how it looks. It will all make sense by the time you get through this.
Recap
Here are the results so far, for factors you can control that may affect razor blade or shaving cartridge life:
- Softening your beard with shaving cream greatly extends razor blade life.
- Stropping stainless blades seems to do nothing.
- Failing to dry off your blades makes them dull.
Or, restating that last one, drying off your razor blade helps keep it from going dull. That’s based on one test.
But why? Why does drying a stainless-steel blade keep it sharp longer? Or, rather, failing to drying it make it go dull faster.
Water spots or lime scale.
Last time, I did a one-off test using a blade for which one edge had been repeatedly dipped in tap water and air dried. After a week of that, I did a “blind” shave. That is, I shaved with that blade, not knowing which side of the razor blade had been dipped-and-dried, and which had been kept dry the entire time. The dipped-and-air-dried edge was noticeably duller than the always-dry edge. I’d almost go so far as to say unusably dull.
Today I repeated that test using a blade for which one edge had been kept continuously wet for one week. Unlike the dipped-and-air-dried edge, it was hard to tell much difference between the continuously-wet edge and the always-dry edge. Possibly, the continuously-wet edge gave a slightly rougher shave. Possibly not. In any case, the always-wet edge was, at best, only slightly different from the new, always-dry edge.
Why? Why that difference? And why do you need to dry a blade to prevent it from quickly going dull.
If you look into the question of why you should always dry your razor blade, the overwhelming majority of shaving websites attribute that to “rust”, “corrosion”, or “oxidation”. Which, when we’re talking about steel, is pretty much all the same thing. And it’s why some recommendations take that further, adding twists like keeping the blade stored in oil, keeping the blade in the freezer, and so on. All that extra effort is to keep oxygen and water away from the blade, and to slow down any rusting/oxidation that may be occurring.
But I don’t buy that. Mostly, stainless doesn’t rust under bathroom conditions. But in addition, the blade edges that I had kept wet for a week, or repeatedly wet and air-dried, looked absolutely the same as a new edge. And now, I have a single test showing that a keeping a blade edge wet for a week did next-to-nothing to reduce is shave-ability.
The alternative explanation that I have come across is that drying prevents the formation of “mineral deposits” from tap water. But I could not for the life of me figure out how that could matter. That said, there is a patent claiming that a routine, quick dip in 12% to 20% citric acid will extend razor blade life life at least five-fold, by preventing the formation of “mineral crystal buildup”.
I finally found a (semi)-scholarly website that sketched out the details: Knifesteelnerds. (Although, I note that their claim that Wilkinson introduced stainless steel blades to the market in 1962 is arguable, in that chrome steel blades had been available for decades. Chrome steel is, eh, kind of a rust-resistant steel, compared to rust-proof stainless.) They quote a scholarly paper on the following putative process by which letting razor blades air-dry dulls them.
The edge of a razor blade is microscopically thin, about 0.2 microns. (For comparison, human beard hair runs around 100 microns thick, or so.) Calcium carbonate precipitates out of the water as it dries, and coats that micro-thin edge. The much thicker coated edge is dull by comparison. And, apparently, the calcium carbonate deposits are hard enough remain stuck to the blade during shaving.
It was only after I realized that “calcium carbonate” is “lime scale” that this finally fell into place. Most would be familiar with that from (e.g.) buildup on shower heads and other plumbing fittings.
What they’re talking about is water spots. They are describing the same mechanism that generates water spots on glassware. So, apparently, the equivalent of water spots on your razor blade will dull it. That’s why, if you want to avoid that, you wipe off your glassware. And your razor blade.
That website describes a simple regimen to preserve blade life, consisting of rinsing in hot water, followed by careful drying. Consistent with mineral deposits as the suspected culprit, they do not go on to suggest ways to keep atmospheric moisture off the blade. Atmospheric moisture or shower steam or whatnot will not contain minerals.
This also puts into place the U.S. patent cited above. Calcium carbonate is vastly more soluble in acidic solutions. The recommended 12% to 20% citric acid solution would have a pH of around 1.6 (based on this calculator). That’s about 10x as acidic as household vinegar (typical cited at pH 2 to 3).
(Note that the left scale is a log scale, so solubility increases hugely with acidity (with falling pH number). No surprise there, as pH is itself a log scale. The underlying relationship between acidity (on a linear scale) and solubility (on a linear scale) would be quite close to linear.
Source: Application of Carbon Dioxide to reduce water side lime scale in heat exchangers, February 2012, Pulp and Paper Canada -Ontario- 1(2):67-70, by Peter W. Hart, Gary Colson,Jeffery Burris
So, while it might take an all-day soak in vinegar to remove a thick layer of lime scale from (say) a shower head, plausibly a quick dip in 20% acetic acid solution is all it would take to remove the water spots from razor blades. Parenthetically, the main active ingredient in Lemi Shine, a commercial product for preventing water spots on glassware, is citric acid.
In any case, there are some easy and obvious tests to distinguish “oxidation” from “water spots” as the cause of razor blade dulling if not dried.
First, I should be able to restore the already-dull blade by a quick dip in a strong acid, or even by using commercial water-spot remover. That’s easy enough to try. And for sure, if rust is the problem, dipping it in acid ain’t going to improve it.
Edit 2/1/2023: Confirmed. Dipping the calcium-carbonate-dulled blade in acid restored the edge. I took the blade with one edge that had been repeatedly dipped in tap water and air-dried. The always-dry edge shaved like new, the dipped-and-dried edge was virtually unusably dull. The plausible cause for the dullness was calcium carbonate deposits, from the tap water. The same stuff that causes water spots on glassware.
I swished that blade around in vinegar for about five minutes, then shaved with it. I made sure that I could not see which side was which. Both sides shaved adequately. It was almost a toss-up as to which edge was which. (I did, in fact, guess correctly when guessing which was the dipped-and-dried edge.)
Second, I can repeat the dip-and-air-dry experiment with two new blades. One will be dipped in tap water. One will be dipped in distilled water. If the calcium carbonate in tap water is the principal culprit, then at the end of one week, the tap water blade should be unusable, the distilled water blade should shave like new.
Edit 2/4/2023: Mostly confirmed. After repeatedly dipping new blades in water, and allowing them to air-dry, I tried shaving with them today. The blade that had been dipped in distilled water shaved like a new blade. The blade that had been dipped in tap water was usable, but clearly duller, and required additional passes. This was not a blind test, but the difference was pronounced enough that I’m pretty sure I’m not just kidding myself. I will now continue to alternate shaves with these blades, continuing to dip them and let them dry as before. I expect the difference to become more pronounced as the blades wear.
This rules out “rust”, “oxidation”, and so on, as the reason that stainless steel blades seem to dull faster if you don’t dry them off. Repeatedly dipping a blade in distilled water and allowing it to dry had no impact on the blade. Only the blade dipped in tap water dulled, even though it had never been used. That clearly points to the mineral deposits from tap water — water spots — as the culprit.
Footnote on stropping. Calcium carbonate is soft, coming in around 3 on the Mohs scale. That’s much softer than the abrasives in the green “stropping compound” used to strop razors. Plausibly, if you don’t dry your blades, stropping might have an effect by removing the water spots from the blade edges. But for my 12+shave, well-dulled blade, stropping did nothing. This shows that those blades didn’t die due to mineral buildup, but to one of the other (“wear and tear”) processes that also dull razor blades.
How thick is a water spot?
I need one more check on the plausibility of this. I’d like to know how thick the average water spot is. Or, at least, how thick a water spot is, using my tap water.
Oddly, I cannot find this critical information addressed anywhere on the internet.
So I’m going to have to gin up an estimate.
Time to strap in, mathematically speaking.
In broad outline, I’m going to estimate the weight of calcium carbonate in a water drop, based on the hardness of my tap water. Then, based on the density of calcium carbonate, and the diameter of a raindrop, I’ll estimate how thick the resulting film would be if I allowed a drop of water to evaporate completely.
This is crude enough that I’m not going to give citations as to source.
My town’s annual water report lists my water at 5 to 10 grains of hardness. That is, I believe, principally calcium carbonate. And 10 grains works out to be 640 mg of calcium carbonate per gallon of water.
A water drop is approximately 0.05 ml of water, or 1.32 x 10^-5 gallons. So one water drop, of my Town’s water, contains about (640 x 1.32 x 10^-5 = ) 0.0085 mg calcium carbonate.
A water drop is hereby deemed to be 2 mm in diameter, taking a typical figure and just ignoring the highly controversial estimate of the “splat” factor once it hits something. That’s a total area of (pi*1*1 =) 3.14 square mm.
The density of calcium carbonate is 2.7 grams per cubic centimeter, or 0.0027 grams per cubic mm. Or 2.7 milligrams per cubic mm.
Therefore, 0.0085 mg of calcium carbonate would occupy a volume of (0.0085*2.7 =) 0.023 cubic mm.
And therefore a cylinder with a base of 3.14 square mm would have to be (0.023/3.14 =) 0.007 mm thick, in order to contain that estimate weight of calcium carbonate.
So, best guess, a water spot is 0.007 mm thick. Now, a mm is 1000 microns. So a water spot is about 7 microns thick.
That’s an order-of-magnitude thicker than the reported 0.2 micron thickness of the edge of a razor blade.
So, yes, this seems entirely plausible. Assuming all the data are correct (regarding razor blade thickness), and assuming I didn’t slip a decimal point somewhere, if you allowed a water spot to form on the edge of a razor blade, its thickness would dwarf the thickness of the very edge of the razor blade.
That doesn’t prove that this is right. But it shows that it’s plausible
