Care must be taken when testing the edge of a straight razor. A blunt razor is much sharper than most knives.
There are a number of ways for testing the edge of a straight razor.
You can feel the edge with your thumb, this takes a well trained thumb pad to really tell if the razor is sharp enough to shave with.
You can test shave arm or leg hair to get a feel for how sharp the edge is.
A more definitive test is the Hanging Hair Test or HHT. A hair is held with the follicle end away from the fingers. The hair is lowered onto the razor in a downward motion with no pulling of the hair or razor. The razor should contact the hair at least 10mm from the finger tips. The hair should cut cleanly with a bit of a pop or no noise at all to be shave ready after honing.
There are 5 levels of HHT:
1. Violin: The hair does not cut but violins across the blade making an audible noise.
2. Split: The hair is dragged across the blade it catches and splits the hair length ways.
3. Catch and pop: When the hair is dragged across the blade it eventually catches and pops the hair in half. The severed piece of hair jumps away from the razor.
4. Pop: The hair immediately pops when it touches the blade. The severed piece jumps away.
5. Silent slicer: The hair falls silently as soon as it touches the blade.
Some people can shave with a razor that is at the violin stage or below with no ill effects. Others will begin to get some irritation long before the razor gets to that level. Personally I start to get a some irritation and a tugging sensation if the razor is at 2. or below.
This test is very subjective. A coarse hair is much easier to cut that a very fine one. Its important when testing the blade in this way to have a repeatable method that is as consistent as possible.
The gold standard test for a razor is to test shave with it and see how it performs. Here at Cutler Road, all vintage razors we hone are shave tested and sterilised before sale. Customers razors are honed and HHT tested to at least the Pop level (#4).