Knife sharpeners may be simple or complex, manual or electric. Most make use of some kind of sharpening stone. Sharpening stones may be whetstone carborundum or corundum. Whetstones are also known as water stones or oil stones.
Carborundum and corundum are artificial or man-made stones. Carborundum is made of silicon carbide. Corundum is made of aluminium oxide. The advantages of the artificial stones are that they sharpen the blade faster than the natural stones.
Natural stones are still produced in some quarries. They are more expensive, but are sometimes preferred because they remove less metal. They can be used for polishing after a blade has been sharpened.
Stones and sharpening steels were used for many years. The process was something of an art. Both the blades and the sharpening utensils had to be held just right or the edge would be dulled rather than sharpened. The traditional sharpening steel does not sharpen it maintains the edge by unfolding it. This straightened edge is still weak and quickly folds again. Eventually the edge breaks off or folds so tightly that it cannot be straightened with steel and must be reshaped. Using steel requires significant skill and practice. To be effective at all, the steel must be used after every 10 to 50 cuts before the edge folds over too much to straighten. True sharpening removes the old weak edge and reshapes a new stronger edge.
Today, knife sharpeners include guides that make sharpening an easier process. Both electric and non-electric sharpeners are available. They allow you to sharpen knives of different shapes and sizes accurately without a lot of practice.
Sharpening blades with serrated edges such as those designed to cut bread can be difficult without a specially designed knife sharpener with spring-loaded guides to keep the blades at the correct angle. Inexpensive electric models will not work on serrated edges.
Budget manual models are available for around $20. They can be used on any blade that is not serrated.
Electric knife sharpeners without the necessary spring-loaded guides for serrated edges are available for as little as $40. Those with the spring-loaded guides start at around $50.
There are other features that add to the price. Multiple guides, replaceable guides and training manuals are examples of features that do not really make one product better than another but do affect price. Chefs and other experts have opinions about which brands are better but the average homeowner would find any of them useful once he or she has learned how to use the knife sharpener. It does take a little time to learn to use electric knife sharpeners. From that perspective manual knife sharpeners are easier.
Modern knife sharpeners are relatively low in price and easy to use and can sharpen various kinds of knives. You can save both time and money by sharpening all your knives yourself.