Archeological digs in Tanzania and Kenya indicate that people have been making knives for over 2 1/2 million years. For just as long we've been struggling to keep them sharp. The problem lies in the hardness of the blade. If a blade is too hard, it is impossible to sharpen. If a blade is too soft, it will lose its edge as soon as it is used.

When you look at a properly sharpened cook's knife under a microscope the metal on the cutting edge looks like the teeth on a comb, clearly defined, separate and straight. As the knife is used, the metal is bent back and twisted by repeated contact with the surface of the chopping block.

The metal can be straightened out by the use of a sharpening steel. However, it is essential to use a sharpener that is harder than the blade of the knife, or you end up sharpening the sharpening steel. Sharpening takes place when the harder material wears away a small part of the softer substance.

In addition to the hardness of the metal, there are four other factors to consider when choosing a sharpening steel: grain, magnetism, shape and guard. Steels come in various grains. Butchers will use a rough steel first to work up an edge and then finish off with a fine grain. For home use a medium rough grain will do an all around good job.

It is possible to find a steel that has been magnetized. A magnetic steel will help realign the molecules in a blade giving it a better structure. Unfortunately, the strength of the magnet diminishes with time, and it is a special assitance only with carbon-steel blades rather than with high-carbon steel, the most common type of knife metal used in America.

Sharpening steels come in three shapes: cylindrical, oval and flat. the flat design gives a greater surface to work on, and there are a number of models with a coarse grain on one side and a fine grain on the other. This style costs a bit more, but I feel it's worth it. However, if you have a knife holder with a round hole for the cylindrical steel you may prefer that configuration. Storing a sharpening steel is not a problem since most models have a ring on the end of the handle allowing them to be hung in a convenient place.

The fifth consideration, the guard, is the most important. A sharpening steel must have a guard or hilt between the base of the steel and the top of the handle. When you use a sharpening steel, the cutting edge of the knife is repeatedly brought toward you. Without a guard there is serious danger to the hand holding the sharpener. If there is no protective barrier between the handle and steel, reject the tool. I cannot understand why these models are made, but they should never be purchased or used.

Knives should be sharpened with a steel before each use. Professional quality sharpening steels with easily identifiable brand names are made by Henckels, Wusthof and Forschner. Prices range from $18 to $65.

The last few years have seen the development and introduction of knife sharpeners that are smaller, less costly and easier for the home cook to handle.

One way of being certain your sharpener is harder than your knife is to use a Zip-Zap, created by Alfred Zanger. It is made of a special ceramic, harder than any knife steel I have been able to find . . . and I have been looking for over 10 years. The Zip-Zap is held between the thumb and fore-finger at a natural 20 degree angle with the knife which is ideal for sharpening. You don't want to disturb the finish put on the blade by the grinder.

Sharpening should take place only on the very cutting edge. Holding the sharpener at a 10 to 20 degree angle will assure that only the cutting will be affected. To use a Zip-Zap, hold the knife stationary and draw the Zip-Zap across it.The smaller and lighter tool is always moved while the heavier is held in place. The Zip-Zap motion is therefore the opposite of that used with a larger sharpening steel.

The only drawback to the Zip-Zap is that because it is made of ceramic it is breakable. Zip-Zap are 5 1/2 inches long and retail for about $3.50.

Case Cutlery makes a tool called a Crock Stick. It consists of two ceramic sticks mounted in a wooden block at a V-shaped angle. It has always looked to me like a table top monument to Winston Churchhill's victory signal. The objective behind the design was to present the sharpening surface to the blade at just the proper angle to assure that only the cutting surface is sharpened. The knife is held vertically and moved down along one stick and then along the other. There is virtually no danger of cutting yourself, it is easy to use and retails for $10.75.

ROWOCO is distributing the Criss-Cross sharpener made by Diogenes of Solingen, Germany. The city of Solingen is the knife-making center of Germany and a focal point for new product development in the cutlery industry. gThe Criss-Cross sharpener looks like a miniature horse shoe with the tips bent over each other the knife is held in one hand and inserted between the V of the grinding rods formed by their crossing. This tool is light, easy to use and can bring up an edge on standard and serated knives and scissors. Its retail price is $2.25.

Sharpening steels, Zip-Zap, Crock Stick or Criss-Cross sharpeners are excellent tools for bringing up an edge between each use, but when the cutting edge is really dilapidated use a Carborundum or Whetstone.

The finest whetstones are mined in the Ouachita Mountians of Arkansas. They are usually sold in boxed sets of two stones, one coarse and one fine. A few drops of oil are put onto the stone and the knife is drawn across the surface at a 20 degree angle. a few years ago a diamond whetstone was introduced and I have found it to be the best product of its kind. It is made of a honey comb sheet of perforated steel molded to a plastic base and set in a natural wood box with rubber feet that keep it from sliding on a table surface while in use. The perforated steel is covered by tiny diamond particles embedded in electro-deposited nickel. A sprinkling of water is used in place of oil. The blade is then drawn over the diamond surface at the traditional 20-degree angle. The more force you put on the knife, the coarser is the sharpening. With less force you get a finer abrasion.

With most whetstones there is a slight burr or feather-like scar on the blade after sharpening because the stone plows aside the metal. With the diamond stone there is no scar. Each diamond tends to cut off chips of metal rather than just pushing them aside. The DMT diamond six-inch whetstone retails for $34.

All of the knife sharpening instruments I have discussed are hand-operated.

I feel the electric sharpening devices sold in most cooking equipment shops or attached to electric can openers are horrendous. Each blade is different and you must get the feel of it as you sharpen. Unless you are a professional knife sharpener, it is highly unlikely you will be able to get a precision edge with an electrical wheel. They shorten the life of a good knife by inevitably wearing away more than is needed and by distorting the edge.