How We Sharpen Knives – PostKnife

How We Sharpen Knives

Knife sharpening has been around for a very long time. Going back 500,000 years, stones were being sharpened to be used as knives. In 10,000 BC in the Fertile Crescent, people were trading sharp obsidian (volcanic rock) for cutting meat and harvesting grain. Every group of people in human history has had a tradition of making and sharpening knives, some for tools of war, but much more so for the everyday preparation of food. Over time, humans developed many different ways to sharpen knives.    

What is commonly referred to as 'sharpening' is better understood as two distinct processes: 1) maintaining an existing edge and 2) shaping a new edge. Maintaining an existing edge involves realigning and straightening an edge that is out of line or folded to one side or the other.

The primary method used for maintaining an existing edge is honing on a honing steel. There are a wide range of honing steels (also that are called honing rods or sharpening steels), but the types that maintain an edge are made of steel. With use, a fine edge can ‘roll’ or bend over and begin to feel dull. Periodic strokes on a honing steel realigns and greatly extends the life of an edge, but eventually, the blade will wear and a new edge will need to be shaped.

Shaping an edge involves removing material from a blade with abrasives. This process can be done manually, by using a series of flat stones, or through varying levels of mechanization employing grinding and other abrasive machinery. Regardless of the sharpening equipment, the fundamentals are the same.                                                                                               

The first step in shaping an edge is determining how much material needs to be removed from the knife to achieve a blade shape and edge shape that is suitable for the particular piece. A blade that hasn’t been abused, and appears to be very close to its original dimensions, should not require the removal of very much material to shape a fresh edge. Alternatively, a knife that has been heavily used, is misshapen, and has been sharpened many times, will require the removal of substantially more material to achieve a properly shaped edge.

At PostKnife, we sharpen a lot of knives, and need to be able to shape a professional edge efficiently. Our process is water-cooled to retain the temper of the steel. If the temper is lost, the integrity of the steel is lost, and no matter how well the edge is shaped, it will not be durable. The first and most aggressive step in our sharpening process is called ‘hollow grinding’. Hollow grinding is a thinning process that simultaneously removes steel from both sides of a blade. Most knife rental companies use hollow grinding in the sharpening process. The motion of inserting a blade and drawing it back out between two circular grind stones that are very close together (or touching slightly), while both grind stones are rotating in the same direction, rapidly thins a blade. The grind stones are 40 grit which is very aggressive. Hollow grinding requires skill to ensure that material is removed consistently and at an adequate rate along the length of the blade. After a blade has been hollow ground to the proper thickness, it is ready to be edged.

The ‘edging’ process at PostKnife is achieved by free hand flat grinding on a stand-up grindstone. This is the traditional method of edging that has been used in our family knife service for four generations. The grindstone is 60 grit, and starts off at a diameter of 36 inches; this stone is often referred to as a ‘big wheel’, or the moleta. With water cooling the blade, it is held flat and drawn from heel to tip across the grindstone. Knowing which angle to grind a particular knife at requires skill and experience. Different knives are sharpened to perform different jobs, which is an important consideration when shaping an edge. While edging, a ‘burr’ or ‘wire edge’ forms at the very ‘front edge’ of the knife. The burr is excess material that must be removed later in the sharpening process. A stand-up grind stone can efficiently shape a precision edge on the vast majority of knives and blades, however, grinding with this equipment is a skill, and the operator must be experienced to use it effectively. To be skilled at grinding, one must grind thousands of knives, and even then, some people will not acquire the ability to do it properly; an unskilled operator can do a lot of damage to a knife. Most other knife rental and knife exchange services use an edging machine to shape the edge. An edging machine can shape a very sharp edge, but a hand shaped edge done by a highly skilled craftsman will be superior. An edging machine relies on the blade being extremely thin in order to shape its edge, and the excessive thinning leads to an edge that deteriorates rapidly.

Once the edge has been shaped, we move on to the buffing wheel. Buffing removes some of the deeper grinding marks/scratches, helps de-burr the edge, and contributes to shaping a finer edge overall. Buffing can be used to improve the blade cosmetically, and later in the process can be employed using finer grits (2k, 5k, 10k, 30k, grease buff, etc.) to achieve a finer finish.

The final step in our process is honing. Some knives are honed by hand on a flat stone while others can be finished in a honing machine. Honing is the least aggressive step in the process with stones at 440 grit. The burr which is created by shaping the edge, is completely removed while honing, and the result is a durable yet fine, razor sharp edge.

A 440 grit honing stone does not need to be the final step. In our shop we have flat stones and buffers that put a mirror finish on a knife (reaching a level of grit that is essentially removing no material at all, only shining); most knife service knives and knives that are being used productively in a kitchen do not require that level of finish, but it certainly has its place and can be useful.

As noted earlier, there are many different ways to sharpen knives. Some begin the sharpening process with 1000 grit flat stones, and working all the way up to 30,000 grit. Alternatively, some processes involve hollow grinding at 40 grit, and then finish by combining edging and honing at 180 grit. At PostKnife we employ skills that have been handed down for generations to shape an edge that we and our customers have concluded works best. Our knife sharpening process has evolved over the years, but we have maintained traditional methods that contribute to creating a durable and finely finished edge.

Being able to sharpen knives is a valuable and rewarding skill to have. At PostKnife we love knives and knife sharpening, and we completely understand why a lot of other people do too. With that said, our knife service exists because commercial kitchens are busy places that require a volume of food prep that can make sharpening knives in house an expensive chore, that’s inconsistent, and often times not done. PostKnife manages the knives for you, so you can focus on the food.

Though knife service and knife subscriptions like ours have existed for generations, we're but the most recent chapter in the long and living history of knife sharpening.

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