The Paper Towel Roll Test 


To really be considered a good bladesmith, you have to go to the core of what a knife is. And in most blades, the number one purpose, the core, is not aesthetic, it is function. A good knife is one that is most perfectly suited for it's designed job. And how do you know if it will perform well? By testing it, of course. 

There are a huge number of ways you can test a knife. You can test its toughness and edge retention by chopping through 2x4's then bending it to a certain angle, but as I mentioned in the previous article, edge geometry is as important as heat treat when it comes down to performance. And for edge geometry, my personal favorite is the paper towel roll test, which I learned from knifemaker Shawn Hatcher

The test is simple. Once a roll of paper towels has been used up, take the cardboard core and place it vertically on a flat surface. Then take a swing at it with the knife. If it makes it all the way through, it passes. If it fails, it will only cut partway through the roll and knock it across the room. 

So why is this my favorite test? What does it really show in a blade?

The primary elements it shows are sharpness and edge geometry. I think it's either intuitive or I've explained by now how important both these are. 

If the blade isn't sharp enough, it won't cut through the towel roll. Now, it has to be very sharp, more than you'd think, because firstly, the towel roll is not braced against anything. A sneeze could knock it over. Secondly, partly due to the first, is you don't have time to do much of a slashing motion. It has to be very nearly what's known as a "push cut": a straight push directly through it (On a side note, this is why many slashing blades are curved (katana for instance), so that the swinging motion would allow as much edge length as possibly to slide and cut through). Slicing or slashing cuts are more effective, so a blade has to be extra sharp to make up for the lack of slashing it can do. 

As for edge geometry, too thick of an angle will cause the blade to become wedged in and stuck only halfway through the roll, and the roll will fly across the room rather than separating. Too thin of an angle and the blade would be fragile in other uses. As I've mentioned before, it's far more common for a blade to be too thick rather than too thin. The goal here is to be as thick as possible of a grind (again, varying according to the tasks the blade is designed for) yet still pass the roll test.