Why the hanging hair test doesn't work
When searching the internet for straight razor tips one of the first things to appear is a magical test to check whether your razor is sharp enough or not – the hanging hair test.
What is the hanging hair test?
The idea is simple enough:
- Take a human hair
- Hold it facing down
- Take your razor and without putting any tension on the hair then see if your razor will cut the hair
Sounds brilliant right? No equipment necessary and the whole point of a straight razor is to cut a hair so if it can cut a hair then it must be sharp enough.
First of all, I have an engineering background – so my expectations of the accuracy and repeatability of any tests are very high. The hanging hair test relies on an extremely non-repeatable piece of hair with an enormous range of thickness and strength.
Here are some of my key issues with this test:
1) All hairs are different
If you can take the same razor and run the test with your hair and then repeat it with my hair and get a different result, then it shows the weakness in the test. Not a problem with the razor.
The width and strength of everyone's hair is different and gives it different properties and strength.
There is also a significant difference in hair between an extremely short beard hair and a long hair on your head.
2) Facial hair is very different
Facial hair is generally much shorter, stiffer, and thicker than hair on your head. So testing a head hair under no tension is not comparable to a stiff facial hair.
Another issue is that all wet shaving enthusiasts will either moist their facial hair (hence 'wet') or apply a thick layer of lather – giving another big difference between the requirements for this test and the real-world application.
Proving that a razor won't cut a head hair under no tension does not prove that it won't give a good shave of stiff and softened facial hair.
3) The blade angle makes a huge difference
If you pass your blade across the hair at a slightly downward angle then there is a high chance that an imperfection/nick in the hair will get caught by the blade and cut it. Does this prove anything? I don't think so.
So, if the hanging hair test is no good – what test should we use?
Thankfully, the only real way to test a razor is even easier – just use it. Have a shave. If there is no pulled hair feeling and you get a close shave then it is sharp enough.
If you get a pulling feeling and have rough stubble left over then it is not sharp enough (assuming you have used correct technique) and would be worth passing it over your honing stone a few times.
If you are really advanced (and to be honest I have only seen a handful of people go down this path), get yourself a USB connected microscope. These are cheaper than you would expect and give you the ability to inspect the edge of your blade. It is remarkable how many imperfections you will be able to observe after a few shaves. This is a great addition to your honing setup and helps you to know when you have made enough passes over the stones.