The final step in bladesmithing is the sharpening of the blade. This is the subject that most people will do the most judging of a knife's quality; the sharper it is, the better they perceive it to be. Now the quality depends more on the steel heat treatment, construction, and geometry, but after all, the number one use of a knife is to cut.
So to start off, there are a couple of basic terms. The first is secondary edge, also known as grind, slope, or bevel. This is the slope from the back of the blade to where the cutting edge is; if you look at the blade from the very tip, you see a narrow triangle with the secondary edge being the two longer angles, the spine forming the third. The secondary edge is usually around a ten degree angle, give or take.
The second term is the (obviously) primary, or cutting, edge. This is the final angle, the tip of which does the actual severing through material. Before sharpening, this angle, the primary edge, does not exist; it's just a (although thin) flat surface on the secondary edge.
Sharpening then, adds a primary edge onto the secondary edge. A dull knife is one that has a flat surface, a sharp one does not. The ideal sharp knife would be only one atom thick on the edge, and though this is impossible with the properties of steel, it's what you should be shooting for.
Understanding the science behind the concept of "sharpness" helps troubleshoot, hone your skills (pun intended), and understand what the difference between sharp and dull is.
The Science behind Sharpness
The line between a “sharp knife” and a “dull knife” is finer than you’d think. When talking about how sharp a knife is or how it all works, you have to delve in almost microscopically to the very edge of the knife. “Cutting” is literally separating a fused substance into two parts.To do this, the very edge of the knife, which you can consider as an edge, is pushed forward into the substance, say a tomato. The edge is pushed against the surface of the tomato, and if you look at it microscopically, the foremost atoms of the knife edge slip in between and push aside the molecules of the tomato, so they are no longer bonded together. To imagine a two-dimensional image of a perfectly sharp knife, edge up, point facing you, would be like a triangle made up of pixels, which are the grains of particles of steel. The edge, at the top of the pyramid, would be the smallest possible size, one particle, or one pixel. This single pixel is stacked and bonded to two more just beneath it, which are bonded to three beneath them, and so on. Imagine a cloud of tomato pixels or particles, descending down onto the point of the triangle. One tomato pixel comes down and touches on top of the topmost steel pixel. The tomato pixel is pushed by more pixels behind and around it, and it cannot be pushed back upwards, so it takes the path of least resistance, to say the right of that steel pixel, breaking the bond with the adjacent tomato pixel, which pushes to the left of the steel pixel. As the cloud of tomato pixels continue to push down, the tomato pixel just above the first, also comes into contact with the topmost steel pixel, and separates to one side. Because the bonds between the steel pixels are stronger than those of the tomato pixels, the very first tomato pixel is pushed to the right of the second steel pixel, rather than left and in between the first and second steel pixels, pushing it further away from the original partner tomato pixel, on the other side of the steel triangle. This continues, pushing the cloud of tomato molecules into two parts, until it has reached the end of the cloud, at which point the cloud of tomato molecules is to completely separate parts. This is an ideal perfectly sharp knife.
Now imagine the same situation, the triangle of steel pixels with the cloud of tomato pixels descending on it once more, but this time with instead of one single pixel at the top of the triangle, there are two (with three below them, four below those, and so on). When the cloud descends down, the nearest tomato pixel pushes against the top two, nestling in the valley between, but not slipping in between them because the steel pixels are too close and will not separate due to extremely strong bonds. The tomato pixels behind and around the first are not separated, rather they just push onto the triangle like a finger against a balloon. Eventually, they only separate from the first tomato pixel because they are stretched so far down the sides of the triangle. However the above situation would be rare, as the chance of that tomato pixel coming down perfectly on top of the two steel pixels is extremely minute. More likely the tomato pixel would separate to one side or the other, especially as tomato particles do not pack as closely together as steel particles. But imagine now if you had five steel pixels as the top layer. In this case they will almost definitely push against the cloud until the tomato bonds break because of stretching. This is an example of a dull knife. Quite often, depending the strength of the material being cut, such as bread, the bread particles will just be pushed upwards, breaking all bonds with the particles to either side. A dull knife cutting is just squishing whatever material is in it’s path, away.
Serrated knives on the other hand, cut by digging away a trench, like a row of shovels. When non-serrated knives are dulled, it is because those few topmost particles on the triangle have been moved to the side, bent out of the way because of tough resistance. Some particles will remain in the original space, while others bend to either side, causing jaggedness, like small serrations. This is why when people accidentally cut themselves with a super sharp knife, they sometimes don’t feel it, or don’t bleed immediately, even though the cut is fairly deep, as opposed to when they cut themselves with a dull knife. The sharp knife has not destroyed tissue, it simply separated the skin into two parts, which touch and are closed again as soon as the knife is removed. A dull knife rips away the tissue in its path, leaving a gaping gash, through which blood will flow freely.
So with the science of it out of the way, we can get to the process of sharpening itself. The basic things you need are two stones, a coarse, and a fine. Hardware stores will often sell them in these two categories. Technically, anything with a flat, abrasive surface will work, including concrete, though I don't recommend that for a fine edge (it'll be sharp, but there will be jagged edging that rips out chunks of what you're cutting rather than slicing through). Sharpening stones come in two categories, either water or oil, which serve to clean out gunk from in between the abrasive particles. Either type works, but water is easier and cleaner.
Start with the coarser stone secured in front of you, and dab on some water (or oil, depending on the stone type). Hold the knife in your right hand, cutting edge on the stone and raised 20 degrees or so; the more acute the angle, the more efficient the cut, but also the longer to sharpen and more likely the secondary edge getting scratched by the stone. Rub back and forth, keeping the angle steady as best you can. Go slow; your rate can speed up with muscle memory. As the stone gets dry, put some more water on, slathering it generously. Get all areas of the cutting edge, keeping the angle constant.
After a little while, flip over the blade and switch hands, doing the other side.You can flip back and forth all you like, just try to get an even rate. Every once in a while stop and feel the edge with a few fingers; not across the edge, as rugged burrs will still catch your finger, making it feel sharp, but very gently along the edge. The goal here is to establish the Primary Edge, completely eliminating any flat surface on the edge. This test will show if you've accomplished your goal or not: if you've eliminated the flat surface, the edge will catch and grab lightly onto your skin. This is the feeling of it actually cutting into your skin, albeit only a layer or two, and barely as deep as your fingerprint; the edge is thin enough to separate the skin cells rather than push them in. If you haven't eliminated the surface, it will simply slide across the fingers, feeling smooth. If this happens, keep going.
Many people skim over this step and go straight on to the honing, then are surprised when the knife doesn't cut well. If you do not properly establish the primary edge, then you will not end up with a sharp knife. Once the entire cutting edge has been established, you can move on to honing.
The burr is a tiny lip of steel curling away from the edge on the opposite side the stone is. A burred blade seems sharp, but once this burr is removed it reveals a once again flat (dull) edge. Detecting burrs is pretty easy; just slide your finger down the bevel of the blade and past the edge; you'll feel the burr catch against your skin. To get rid of the burrs, from time to time (I de-burr every time I flip sides) slide the edge twice along scrap leather or wood; the thin and weak burrs get rubbed off this way.
Honing does the final straightening and elimination of the last bit of flat edge. A pretty simple process. Use the coarse stone, and do five dragging strokes (dragging in direction; away from the cutting edge, as opposed to cutting direction, pushing in with the cutting edge) on one side, de-burr, and five on the other, and de-burr. Check for burrs, then do the exact same again but on the finer stone. Assuming you kept the angle constant (tough to do the first few times you sharpen), properly established the cutting edge, and were thorough all over the blade, it should be paper-slicing sharp, ideally, razor (hair shaving) sharp.
Sharpening is an art; getting the most efficient work by modifying under a millimeter of blade width is something that has been studied and done for millennia. Everybody has their own method, some more effective, some less, some are equal in yield but vary in how they do it. This is the way I do it. I am in no way a professional, but I have gotten many knives hair-shaving using the above process. Find what works for you.
So. You finished your knife. Take pictures, use the knife, post about it on a knifemaking forum, and ask for critique. Don't lose it, you'll really want to have it later on. Write down what you could have done better: was the grind uneven? Is there a gap between the guard and blade? Is the handle oddly shaped? Write down whatever the flaws are (ideally, write them on a board and hang it up in front of where you work), and of course, make another knife, and make it better than the last one. Then when you're done with that, make another!
Chances are, if you're reading this, your goal is to become a better bladesmith. That's what these articles are for. Merry Christmas and good forging!