The Story of Cutlery Metals

Byron Bitar, Ph.D.
Owner, A Cook's Wares
Professor of Philosophy
Geneva College 

         Good kitchen knives are works of art as well as superb functional pieces of cooking equipment.  As useful as Cuisinart food processors are, knives are the real food processors in a kitchen.  Yet it is hard to get good information on the metals used in knives, why they are chosen, and their advantages.  To remedy the situation, I have written three discussions on knife metals. 

Part I
Iron and Steel in the Past

Early Knives

         The first knives were made from stone, particularly flint, and also volcanic glass, bones, and shells.  The cutting edge of flint knives was formed by chipping off pieces.  As a result, the edge was serrated, which as you know is good for cutting.  The Egyptians glued serrated flint pieces into slots in wood shaped for cutting and sawing.  Volcanic glass, bones, and shells were sharpened by rubbing in the hollow of a stone, a method still used by aboriginal peoples in central Brazil, Australia, and New Guinea.

Copper and Bronze

         When we think of knives, we think primarily of metal blades.  The metals first used for blades were two:  copper and iron.  There are copper axes that are over 6,000 years old from the Balkans in Europe.  However, copper and iron are soft and will not keep an edge for long.  To get a blade that has sufficient strength, the copper and iron have to be mixed with some other element.  To do that, heat has to be used.  The copper or iron has to be heated to the point it will absorb another chemical as part of its constitution. 

         Copper melts at lower temperature than iron, specifically 1,300º F, and as a result was the first to be heated to form what is called an “alloy,” a metal with more than one element in its structure.  The element used to strengthen copper was arsenic.  Yes, arsenic.  Why arsenic, you ask?  Is not arsenic a poison?  The answer is, “Yes, it is a poison; even the fumes can cause poisoning; but it has tin in it, up to 3%.”  And it is the tin that is important.  Copper and tin form bronze, and bronze is a lot stronger than copper and tin alone.  Pure bronze tools seem to have been made around 3,000 B.C. in the region that is now Iraq.  The Greeks used bronze to produce knives.  Bronze was also used to make scissors.

                              Copper melts at 1,300º F
                             Bronze:  copper and tin

Iron

         Iron is one of the most common elements found on the surface of the earth, but its melting point is quite a bit higher than copper.  It melts at 2,800º F.  It takes a roaring fire to get iron ore that hot.  So getting pure iron was a problem.  It did not happen in early times.  That is why the Bronze Age preceded the Iron Age.  When iron ore was heated, the result was a spongy impure mass.  Fortunately the impurities had a lower melting point, so most became liquid and ran off the iron glob, while the iron remained hot and soft.  The iron was then pounded to get more of the impurities to leach out.  The mass of iron was called a “bloom;”  the impurities were called “slag.”  Iron obtained by repeated heating and pounding was called “wrought iron.”  Iron was in use around 2,000 B.C. in what is now western Turkey, known as Anatolia.  By 1,200 B.C., weapons, particularly daggers, were fashioned in Anatolia from iron.

Steel

         Pure iron is soft, about as soft as copper.  To give iron strength, carbon needs to be added to it.  Just a half of a percentage (.5%) of carbon will increase the strength of iron immensely.  But to do that, iron has to be heated.  Fortunately, charcoal is rich in carbon, so when iron is heated in charcoal by the use of furnace drafts, it naturally absorbs carbon, producing steel.   Steel is iron and carbon, specifically 2% carbon or less.  The Romans made knives out of steel.  The Romans also continued the development of iron and used it to make nails, beams, chairs, harness fittings, chariot tires, fireplace equipment, axes, and razors.

                                Iron melts at 2,800º F
                                Steel:  iron and carbon

         The ancients also discovered that steel could be made harder by heating it and then cooling it quickly in water.  The cooling is called “quenching.”  It makes the steel hard, but also brittle.  However, by heating the steel again to a rather low temperature, around 1,200º F, the steel will gain elasticity.  This second heating is called “tempering.”

                              Tempering:  heating steel to 1,200º F

         After the Roman age, tough durable swords were made by forging iron and steel into a single blade.  The core would be iron, the outer blade steel.  The iron gave the blade toughness; the steel exterior provided hard sharp edges.  Such swords became the basis of legends, the most famous being that of King Arthur’s sword Excalibur.

Brass

         There is one more alloy that was important in the ancient world:  brass.  Brass is a combination of copper and zinc.  It was developed later than bronze and steel, around 30 B.C. in Egypt.
        
                             Brass:  copper and zinc

                    Three Metal Alloys Important in the Ancient World

                          Bronze:  copper and tin
                          Steel:  iron and carbon
                          Brass:  copper and zinc

         The finest steel was produced in India using a process known to the Egyptians.  Pieces of wrought iron were put into a clay container with wood chips and heated until the iron absorbed the carbon from the wood to get an even composition of 1 to 1.6% carbon.  The steel was known as wootz steel.  It was used to make the famous Damascus swords possessed by medieval Arab warriors.

         In the ancient world, cast iron was developed in China.  Cast iron has a high carbon content, over 2%.  It is very strong, but also very brittle.  There are Chinese cast iron swords and agricultural tools that date from the Fourth Century B.C.  The Chinese also developed the first blast furnaces, which enabled a greater production and a higher quality of iron and steel. 

         The development of steel in the modern world depended primarily on two things:  (1) better furnaces so iron could be heated efficiently and (2) a supply of fuel, which finally was found in coke derived from coal.

Part II
The Metallurgy of Steel

         Iron, like copper, is a soft metal.  Iron needs carbon to make it strong.  When iron is mixed with carbon in amounts up to 2%, it is called steel.  It takes a lot of heat to melt iron so that it will absorb the carbon evenly, 2,800º F to be precise.  However, too much carbon, specifically over 2%, results not in steel, but brittle cast iron.  Also steel is tempered by heating it a second time to a low temperature and cooling it rapidly.  Thus steel developed in locations where timber was plentiful for fuel and water for tempering.  From the 13th century on cutlery manufacture began to settle in London and Sheffield in England; Thiers and Paris in France; and Solingen in Germany. 

Iron Furnaces and Fuel

         In Part I, we noted two problems that had to be solved for steel to develop in the modern era:  (1) iron furnaces needed to be made more efficient and (2) a reliable source of fuel needed to be obtained.

         The fuel problem was solved by coke obtained from coal.  In 1709, Abraham Darby I converted coal to coke by partial burning in open pits.

         The furnace problem took some time.  In the 15th century, blast furnaces were being used in Europe.  Air was blown on the charcoal to make a very hot fire.  The result was pig iron, a form of cast iron, not steel.  The pig iron had to be heated again to remove excess carbon to get steel that could be formed into tools.  About 1740 in Sheffield, England, Benjamin Huntsman made a clay vessel, called a "crucible," that was strong enough to melt steel for removing impurities.  As result, Sheffield steel became the preferred material for fine tools and cutlery until the end of the 19th century.  In 1784, Henry Cort in England devised a process to speed up the reduction of pig iron to lower carbon iron (wrought iron), called the “puddling process.”  Steel was still expensive, though, because it could only be produced in small quantities.  It became mass-produced in the 1860's after Henry Bessemer in England began to blow air right into the melted steel to raise the temperature, and the Siemens brothers in Britain and the Martin brothers in France invented the open-hearth furnace.  It used hot gases from a prior smelting of iron to heat the next cycle, thus saving fuel.

The Chemistry of Steel

         A good knife blade needs three features:  (1) hardness, (2) corrosion resistance, and (3) tensile strength.  Hardness is the ability of the metal to resist pressure; it is known as “compression strength.”  The thin edge of a sharpened hard blade will resist pressure to bend and flatten it.  Knife blades become dull by the edge bending back upon itself, not by loss of metal from the edge.  Rockwell hardness is a measure of compression strength; so is diamond pyramid hardness (DPH).  Corrosion resistance is the ability of the metal to resist primarily oxidation or rust, but also deterioration of the metal by acid.  Tensile strength is the ability of the metal to be pulled without tearing or rupturing it; good tensile strength means it will spring back into its normal shape.  Tensile strength includes the flexibility of the blade.

Hardness:  the ability of the metal to resist pressure;
known as “compression strength.” 
Corrosion resistance:  the ability of the metal to resist
primarily oxidation or rust, but also deterioration of
the metal by acid.   
Tensile strength:  the ability of the metal to be pulled
without tearing or rupturing it; the metal will spring back
into its normal shape; the metal is flexible.

         Metal, then, needs to be hard so it will hold an edge.  Carbon is the main element that gives the iron hardness.  The more carbon, the harder the metal.  Iron atoms naturally form crystals.  At room temperature, an iron crystal will have nine iron atoms in it.  Picture it as a cube with one atom at each corner and one in the middle (body-centered cubic arrangement).  The crystals easily move against each other, causing iron to be soft.  When heated up to its melting point (2,800º F), the iron crystals will absorb carbon atoms.  The carbon atoms will make the metal very hard.

         However, if the metal cools slowly, the carbon atoms will leak out of the iron crystals, resulting in a soft metal.  As a result, the metal is cooled rapidly, called "quenching," which keeps the carbon atoms inside of the iron crystals.  When cooled with extreme rapidity, large amounts of carbon are kept within each iron crystal.  This is called "solution hardening."   The solid steel with the carbon atoms stuffed inside each iron crystal is called "martensite."  Martensite is extremely hard, with a diamond pyramid hardness of 1,000, but also very brittle.  The iron crystals are too stuffed with carbon atoms for the metal to have tensile strength and flexibility.

         Solingen cutlery companies call the rapid quenching "ice-hardening;" Henckels calls it "Friodur."  Usually liquid nitrogen is used.

         Because the resulting metal is so brittle, it is then tempered.  In tempering, the metal is heated but only to a moderate temperature (600° -1500° F), and then again cooled quickly.  Some of the carbon atoms escape from the iron crystal and form very small carbides.  Carbides are compounds with carbon and another metal, such as iron.  The carbides fill the space between the large iron crystals.  The carbides give the metal both additional hardness and new tensile strength or flexibility.  The formation of very small carbides is called "precipitation hardening."

         Fine steel blades are hardened by both solution hardening and precipitation hardening.  Fine knife blades usually have .5% carbon.   All of A Cook’s Wares®’ knife lines do except Chef'sChoice Trizor Professional and MAC knives.  Both Chef'sChoice and MAC knife lines have 1% carbon in the blade metal, twice the carbon of most knife metals.  That makes for a very hard steel and long lasting edge.  However, the edge of these knives cannot be realigned by a honing steel.  When dull, the edge must be reground by a ceramic or diamond sharpening device.

Part III
Alloy Metals

         The important elements in a knife blade other than iron and carbon are these: chromium, molybdenum, and vanadium.  In the international code for Solingen knife steel, you can see the reference to these three elements: X 50 Cr MoV 15

Chromium

         Chromium is used to prevent corrosion, particularly oxidation of the iron, which we know as rusting.  Chromium causes a very thin layer of oxide to form on the surface of the steel.  No oxygen can penetrate this layer, so additional corrosion is stopped.  The chromium content has to be at least 13% to be effective in resisting corrosion.  Chromium also increases the compression hardness and tensile strength of the metal.

         Both German Solingen cutlery and French carbon steel knives have carbon, .5% to be exact.  However, the Solingen knives also have chromium to resist rusting.  French carbon steel knives lack the chromium.  That is why the French carbon knives are softer than the German Solingen knives, and stain and rust easily.  As a result, the French carbon knives both dull quicker and sharpen easier.

Molybdenum

         Molybdenum has two functions.   First, it gives the metal tensile strength and flexibility.  Tensile strength is the amount of force that can be exerted on the metal, stretching it without breaking it.  Second, molybdenum dramatically enhances the effectiveness of chromium in protecting against corrosion.  1% molybdenum is worth 10% chromium in enhanced corrosion resistance.  As a result, steel with molybdenum is twice as corrosion resistant as the same steel with just chromium.

         Molybdenum is expensive.  The typical knife metal has .2% to .3%.  Solingen steel has .4% to .6%.  Chef'sChoice Trizor has a molybdenum content of 3%, which is about 6 times that of the other cutlery.

         In addition, molybdenum in sufficient quantity enables steel to be cooled slowly and still form a hard martensite structure.  In other words, quenching and ice-tempering are not needed.  Chef'sChoice Trizor knives have enough molybdenum to be able to skip the quenching process.

Vanadium

         Vanadium is named after Vanadis, the Scandinavian goddess of beauty and youth.  Vanadium has two effects.  It (1) creates a fine grain in the metal and (2) forms compounds with carbon (carbides).  This makes the metal strong and hard, with improved resistance to shock, and also more resistant to corrosion.  With vanadium, then, the metal will reach maximum hardness easily.

         Other elements in knife steel are manganese, phosphorus, silicon, and sulfur.

Manganese

         Manganese is important in steel making.  Used in the low concentration of 1%, it reduces the harmful effects of impurities such as oxygen and sulfur.  It combines readily with sulfur.  In so doing, it converts low-melting iron sulfide in steel to high-melting manganese sulfide.  Produced without manganese, steel breaks up when hot-rolled or forged.  The net effect of manganese is increased strength, toughness, and hardness in steel.

         Phosphorus, silicon, and sulfur are impurities and do not contribute to the function of the metal.  In addition, nickel is not ideal in knife metal because while it resists chemical corrosion, it also softens the steel.  Nickel is appropriate for pots and pans, and utensils that do not need honing and sharpening.

Schaaf First Class and Goya Cutlery

         Schaaf First Class and Goya Cutlery have the following metal element percentages, in accord with the international formula for Solingen steel:  X 45 Cr MoV 15.

         Carbon:  .42 - .5%
         Chromium:  13.8 – 15%
         Molybdenum:  .4 - .6%
         Vanadium:  .1 - .15%
         Manganese:  1% maximum
         Phosphorus:  .05% maximum
         Silicon:  1% maximum
         Sulfur:  .03% maximum

         Over a hundred years of experience has shown that this formula is one of the finest for cutlery.  The knife-edge is hard and durable, but can be realigned with a honing steel.  One does not have to take off metal with a ceramic or diamond device to restore a dulled edge.  The knife also has superb corrosion resistance and tensile strength.

Diamond Pyramid Hardness and Rockwell Hardness

         It is common for cutlery companies to give the Rockwell hardness of their knives. Lamson is 55; Schaaf is 56-57; Chef'sChoice and MAC are 60.   What do those numbers mean?

         The numbers measure the compression hardness of the metal, not the tensile strength or flexibility.  In other words, the numbers measure the ability of the metal to resist force pushing into it, trying to compress it.  The higher the number, the more the metal resists compression.  With both a Vickers Diamond Pyramid Hardness tester and a Rockwell Hardness tester, the metal is subjected to the pressure of a diamond pyramid or tip, usually a 50-kilogram force.  The force is stopped and the metal allowed to spring back.  The depth of the impression is then measured.  The amount of the force is then divided by the depth.

Hardness = Force exterted on the metal ÷ Depth of the impression

         The greater the depth, the softer the metal and the lower the number; the shallower the depth, the harder the metal and the greater the number.