HOW TO SHARPEN
Evaluating the Knife
Before sharpening begins, one needs to identify the current condition of the knife and edge. A note on safety: Knives are extremely dangerous tools. Before picking up a knife, be aware of your surroundings. Know how your knife works, and have a plan.
In good lighting, I observe the edge from both sides, and feel the edge. Then making sure no one is close to me, I look at the knife from tip to heel. This allows me to see if there are any warps or bends in the blade. I'll also hold the knife edge up to see if any light is reflecting off of a rounded edge.
I would encourage you to stop multiple times through the process to evaluate the knife. Observing scratch patterns, and feeling for burrs is integral in creating a quality edge.
Choosing the Right Stone
After observing and evaluating the condition of the knife, it's time to figure out where to start. Is there a lot of damage that needs to be fixed? Has the edge just rolled slightly? Basically, how much time is it going to take to put an edge back on the knife. If there's a lot of work to be done, you'll want to start on the coarsest stone to get most of the work done. This will save time, even though you'll use more stones.
If there isn't a lot of work that needs to be done, you can start at an even higher grit, as there may not be as much material that needs to be removed. In taking less time between each sharpening, and using a higher grit to start out on, the less material that will need to be removed overall, leading to a longer lasting knife.
What stone you finish on will give you your knife a certain "tooth." This is in reference to the micro serrations at the very edge of the knife. a coarser grit will leave larger serrations than finer grit. The larger the serrations, the more likely the very edge will roll when hitting something with force. The finer the serrations, the more likely the edge is to slip on vegetable skins. There's often a good balance in between a super polished edge, and a very course edge.
A Note on Hardness
In the kitchen knife world, there are different thoughts on what makes a great kitchen knife. Often, when bringing out the best trait in one aspect of the knife, another detail must be compromised. The main example being the relationship between hardness and toughness. The hardness of a steel refers to how the steel resists change. The harder the steel, the more crisp of an edge that can be created, allowing for incredibly sharp edges. The trade off with a hard steel, is that it's not as tough. Too hard, and it can be as brittle as glass.
With the advent of industrialization in the western world, the design of the steels characteristics have been tapered towards being able to take a lot of abuse. So, while the steel is still hard, it has been dialed back from the extreme hardness, to a tougher working hardness. You'll find most common blades are hardened between 48-56 rockwell c. These numbers refer to the amount of deformation left after piercing the steel under a controlled force with a diamond tipped cone. The higher the number, the harder the steel. A rockwell rating of 100 c would be the hardness of diamond.
On the contrary, Japan has taken its once booming sword production, and turned the industry into a knife making mecca. Sword smiths with generations of experience, have had to reinvent themselves into the kitchen knife making industry. In general, with more highly priced blades, one will find harder steel. These blades have been crafted more for the professional that takes more care with their tools, and also focus on more detailed cuts.
Choosing an Angle
When talking about the angle in terms of sharpening, it is important to note that the angle is measured by the angle created between the edge, spine, and stone. The typical range for most kitchen knives will fall in the area of 11-20 degrees. There are special exceptions, of course, dealing with single bevel knives, and meat cleavers as two main examples. The inclusive angle of the knife, will add the angle sharpened from both sides to get one sum. This "inclusive" angle that the sharpener is trying to create will affect how well the knife cuts, how fragile the edge is, and will largely depend on not just the type of steel, but the heat treatment of the steel as well.
Angle selection is going to depend on a couple factors. First, what task is the knife going to tackle? Next, how hard and what steel is the knife made of? The most important thing to realize before starting is holding a consistent angle. You may not feel the difference between 15 and 18 degrees, but if your angle is wobbling, it will affect the end result.
Raising a Burr
Once the knife touches the stones, is to work on one side of the knife only. The goal is to sharpen at your chosen angle until a burr forms. A burr is small bits of steel, still attached, that fold over to the opposite side one is sharpening on. One can feel a well built burr with finger pads, and even finger nails. This burr indicates that you have reached the edge of the knife, and are ready to flip the knife over, and start sharpening that side. The evenness of the sharpening initial steel removal can be felt by how even the burr has been formed.
Flipping the Burr
Now that we've raised a burr on one side, it's time to flip the knife over, and start sharpening the other side. The same process is used, maintaining an angle, and using a back and forth motion, rubbing the knife against the stone until a burr can be felt on the opposite side. It's also important to feel the side you're sharpening to make sure there is no burr left. Once the burr has been flipped totally and consistently, we can move on to the next step.
Reducing the Burr
With the burr created, and flipped, we can start working on reducing the burr. In a perfect world, the burr will pop off, leaving the edge crisp and strong. This process is harder on coarser stones than on fine stones, and some sharpeners don't worry about removing the burr completely until they reach their final stone.
The process of reducing the burr is similar to sharpening. Yet, with the bevels already set, flipping the burr back and forth will get easier and easier. To get a good understanding of the process, think about bending a paperclip back and forth over and over again. At some point, the paperclip will break. This is what we're trying to do with a burr. Without creating a new burr, we are flipping it back and forth, over and over again. If you're able to hold a consistent angle, the burr will break off right at the edge, and leave a very crisp, sharp edge.
Stropping is the final act of sharpening. An experienced sharpener will strop on a stone in their progression, while for beginners, it is useful to strop on a softer medium. Strops leave a much finer edge than normal stones. Each different material will leave a different finish, and there are some man made pastes that can be applied to strops to increase their efficiency. Stropping is one stroke per side, flipping back and forth after each stroke. The less pressure used at this stage the better.
Testing the Edge
There are countless ways to test the sharpness of your edge. Probably the best way to test the edge, is to use the knife at its intended purpose, and see how well it performs. Probably not the most efficient way to test an edge quickly, but if you can accomplish the goal you want to tackle, then what else really matters?
Quicker methods for testing an edge include cutting paper, or even just feeling the edge with your fingertips. I hesitate to recommend shaving hair, as there is such a variety of coarse and fine hair, that it may not give consistent feedback as to how the edge is actually performing. Regardless, the knife should not snag, but should feel sturdy, smooth, yet toothy against the skin, and cut paper smoothly without catching anywhere along the cut.
For more precise, and measurable results, there are machines that measure the force used to cut a specific material of designed shape (like a tight wire). These machines give specific numbers that can be compared directly with different results.
The sharper the knife, the less force it takes to cut!