Hot Stuff – metal, ceramics & glass

Hot metal - crucible furnace for melting metal

How hot is HOT?   

   While physicist agree upon an absolute lowest temperature (absolute zero – where even subatomic particles don’t move) there is no consensus or formally defined limit for a maximum temperature.  The best approximation of maximum temperature might be Planck temperature (1.4168 × 10^32 Kelvin).  That’s about 100 million million million million million degrees in other words.  Within a thermonuclear bomb a temperature of 50 million °C is needed to initiate the fusion of a deuterium and tritium tamper.  The temperature at the core of our sun is assumed to be about 15 million °C.  The fission bomb “Little Boy” dropped on Hiroshima generated a heat of about 299,726 °C at its core.  The surface of the sun and the earth’s inner core are both much cooler at about 5,778 K (5,505 °C) each.   We have no instruments like thermometers or thermocouples to physically measure even these relatively low temperatures but instead must rely upon idealized thermodynamic theory to extrapolate these numbers.   

Hot Iron and Steel (first)

   Through chemical decomposition, oxidization and other natural processes happening over geologic time, few metals are found physically in metallic form.  Most of the earth’s retractable metals are dispersed as small flakes or inclusions within an ore of some type.  Since gold, copper, silver, and metals of the ‘platinum group’ are not very reactive chemically, early man was occasionally able to find bits of these “native metals” just lying upon the ground.  Mankind likely first encountered metallic iron however, in the form of a deposited meteorite.

Humankind’s technological advancement through the early ages is typically categorized by its tool making progress.  An archeologist considers tools of the ‘Neolithic Age’ to be more complicated than those of the general “Stone Age” – but less so than those of the ‘Bronze Age’ (some may distinguish between Copper and Bronze ages because the latter infers the more sophisticated smelting of alloys).  It took primitive civilizations about 3,000 years to progress beyond the Bronze Age to the “Iron Age” alone.  Also associated with or occurring concurrently with a period’s tool making technology were changes in religion, artistic styles, agriculture and societal structure.

Beyond about 9,000 years ago most cultures had no reliable method to initiate fire.  It would take humankind another 88 centuries to develop an easy method to start a fire (as in the 19th century phosphorus friction match) but that is another story.  As humans experimented with the heat of fire they even cooked rocks and dirt – and significantly thereby, created or discovered metals, ceramics and glasses.   Metal, ceramic and glass can be used to manufacture trade items which rank right up there with other achievements (like plant and animal domestication, division of labor and written language) to define what civilization really is.  This half-baked discourse intends to explore some simple metallurgy.

  Smelting is the separation of metal from its ore.  The reduction of aluminum using electrolysis instead of heat can also be called smelting.  Smelting with heat is often assisted by adding a reducing agent and a flux.  When smelting iron, coke or charcoal are added to the crushed ore within a traditional blast furnace and act as a reducer in the redox (reduction-oxidation) reaction.  Carbon monoxide is produced as the oxygen is striped from the iron ore.  Limestone, carbonate of soda, potash and lime might be used as a flux or slag forming agent to absorb impurities into a slag that can be separated from the liquid molten metal.  With the low grade copper ores available today, soap bubbles and pine oils are frequently used as reagents to detach the metal from its crushed ore slurry.  The cyanide process (cyanidation) can be used to extract (smelt) gold, copper, zinc or silver from their low-grade ores.  Mercury dissolves gold and can form amalgams with several other metals as well.  Easily separated from its crushed ore the gold can further be separated from the amalgam (in small samples) by squeezing it through a rag of chamois leather or by baking it in a potato.

  Before turning the discussion to simple blacksmithing some melting points (°F or °C) of some familiar materials are listed in ascending order below.  The temperature of molten lava depends upon its chemical composition.

Tin 449° F 232° C Sn, #50
Lead 621° F 327° C Pb, #82
Zinc 787° F 419° C Pb, #30
Antimony 1,167° F 630.6° C Sb, #51
Magnesium 1,202° F 650° C Mg, #12
Aluminum 1,220° F 660° C Al, #13
lava 800°C
Silver 1,763° F 962° C Ag, #47
Gold 1,947° F 1,064° C Au, #79
Copper 1,984° F 1,084° C Cu, #29
lava 1100° C basalt
Silicon 2,577° F 1,414° C Si, #14
Nickel 2,651° F 1,455° C Ni, #28
glass 2,700° F 1,500° C soda lime
Iron 2,800° F 1,538° C Fe, #26
Titanium 3,034° F 1,668° C Ti, #32
Platinum 3,215° F 1,768° C Pt, #78
kaolin 3,275° F 1,800° C porcelain
Vanadium 3,470° F 1,919° C V, #23
glass 4,200° F 2,300° C silicon-
Molybdenum 4,753° F 2,623° C Mo, #42
Tungsten 6,192° F 3,422° C W, # 74
 * Carbon C, #6

* Allotropes (forms) of carbon have the highest thermal conductivities of all known materials and they don’t melt.  Carbon undergoes sublimation at about 9,980 °F (5,530 °C) which is to say that the element transitions from a solid to a gas without passing through a liquid phase.  Carbon is also the fourth most common element in the universe by mass, forms more recognizable compounds than any other element and is the chemical basis or building block for all known life. 

In the previous table antimony, vanadium, molybdenum and tungsten are used in small amounts to make alloys and are only included for the sake of curiosity.  Antimony is not a metal but a metalloid.  Like gravel compliments the integrity of concrete, antimony combined with tin, hardens lead for bullets or linotype (the lead alloy historically used for typesetting).  The biggest use for antimony today is in the production of lead acid type automotive batteries and to harden the lead wheel weights used when mounting and balancing new automobile tires.  Vanadium, molybdenum and tungsten serve mainly as steel alloys or as catalysts.  Vanadium is useful in tool steels like drill bits, where it facilitates higher possible working temperatures without sacrificing temper (hardness).  Molybdenum improves steel by restricting its expansion and softening at higher temperatures and was commonly used in artillery pieces and tank armor.  Molybdenum also improves the corrosion resistance and weld-ability of steel.  Tungsten is a rare element but having a very high melting point found use in light bulb and x-ray tube filaments.  Used in cutting tools and abrasives, tungsten –carbide tipped implements are almost three times harder or stiffer than plain steel.  Chromium is yet another metallic element which is often found alloyed within steel.

*  House fires and even forest fires can sometimes reach impressive heats.  Stones in masonry chimneys have been known to explode like bombs when the attached cabin or dilapidated house is burned down.  The pressure probably comes from steam created by moisture trapped within the rocks.  Uniform Building Codes (UBC/IBC) stipulate that steel beams if used to support the roofs of modern wood framed homes and buildings, need to be shielded from possible flame.  Without flame and heat protection steel girders might quickly soften, sag and collapse, leaving potential victims with no exit from the building.  At their flame front wildfires can heat the surrounding air to 1,470 °F (800 °C).  If fed by wind the internal temperature of a wildfire might surpass 2,192°F.  That’s a temperature high enough to substantially soften steel or liquefy several other types of metal.

Cupola Furnace source: Released into the public domain (by the author)

Cupola Furnace
Released into the public domain (by the author)


A “bloomery” was the earliest form of furnace capable of smelting iron from ore.   Having a channel for air flow at the bottom the simple bloomery structure was typically sacrificed to retrieve the metal.   Early blacksmiths often worked with iron wrought from a bloom.   A ‘bloom’ (cruder than ‘pig iron’ from a blast furnace) is a porous, impure mass of iron and slag (video links one & two).  The hot bloom was hammered, reheated, pounded, twisted and pulled to squeeze out the slag.  Wrought iron is the almost pure iron product produced by all that excess labor (another video).   Wrought iron is very rare today – its main source being from antique structures or implements.   In its place modern blacksmiths use malleable and ductile low-carbon or mild steels.   Low carbon steel contains about 0.05–0.15% carbon while mild steel is about 0.15 –0.3% carbon.  Further carbon proportions quickly become harder and more brittle.  High carbon steel might contain between 0.6–2.0% carbon.

Although not as old as bloomeries the modern blast furnaces used to smelt ore today are merely embellishments of a design used in the Middle Ages (or since the 1st century AD in China).   Fed from the top by conveyor belts of ore, coke or coal and limestone (flux) the big chemical reactor called a “blast furnace” works continuously, year after year without being shut off.   It might take an individual atom of iron 8 or 9 days to work its way to the bottom of the furnace.  The name “blast furnace” reflects the fact that air (hot air in modern times) is forced into the bottom.  Crude “pig iron” is the product produced from a blast furnace and this is processed later to become steel.  The “Bessemer process” for economical industrial steel making was patented in 1855 and was the prevalent steel making method for about a century afterward.  The process involved re-melting the pig iron and removing impurities by blowing air through the molten iron.   Following WWII regenerative “open hearth furnaces” began displacing previous Bessemer converters.  Using exhaust gasses to preheat incoming fuel and air, open hearth furnaces operated much more slowly thereby offering more control over the process, allowing the refining of scrap metal along with pig iron and reducing the amount of undesirable nitrogen introduced to the reaction.  By the 1990’s most industrial open hearth furnaces were themselves displaced by the Basic Oxygen Furnace (BOF) and non inductive Electric Arc Furnace (EAF).  The BOF is in essence a refined Bessemer converter where pure oxygen rather than air (which is about 78% nitrogen) is injected into the molten metal.   Perhaps situated next to a blast furnace, the BOF accepts already molten pig iron, mixes in perhaps 20-30% scrap steel and injects oxygen at supersonic velocities.  Great heat is created, the scrap steel is melted and carbon and silicon are oxidized.

Iron ore is basically iron oxide so producing iron metal necessitates removing the oxygen.   This “reduction” is accomplished by using carbon.  At elevated furnace temperatures the strong chemical iron-oxygen bonds in ore are swapped for even stronger carbon-oxygen bonds.  Coke is analogous to charcoal or char cloth and all three are products of “pyrolysis” (the act of driving off volatiles with heat in the absence of oxygen).  Char cloth (from fabric) and charcoal (from wood) are created in an oxygen deficient environment.  Likewise coke is made in a ‘coke oven’ where coal is heated in the absence of air to produce a hard porous material of almost pure carbon which will burn twice as long and produce twice the heat as the original coal.  Coke won’t burn by itself without the forced air or oxygen blast from a blower.  While coke is unanimously preferred over coal for steel making it is also important to make it from coal selected for low sulfur content.  That nasty, odorous and very effective wood preservative used on telephone poles and railroad sleepers is actually a byproduct of the coking process known as coal tar “creosote”.

Blacksmithing 101

Before the appearance of acetylene cutting torches, arc welders, electric drills and saws the principle utilitarian metalworking tools were forges, hammers and anvils.  It is possible occasionally to find portable forges still being used occasionally by cowboys to heat branding irons or by farriers to bend horseshoes.  Forges still have practical applications in this day and age because iron and steel become almost docile and easy to work with when hot.  Looking like backyard charcoal grilles the portable blacksmith forges labeled b & c in the following image probably served in just that capacity on several occasions throughout the last century.


Introduced in the late 1870’s forges resembling images a, b & c replaced the traditional bellows with a geared turbine or blower.  In examples a & b the blower is powered by lever, in example c the geared blower is cranked by hand.  In example d a shop vacuum is used to blow a strong stream of fresh air up through the bottom of the forge.  The concept first published in Popular Mechanics magazine in 1941, example d incorporates a kitchen sink.  One bay of the sink is lined with a cementatious refractory or firebrick while the other can be filled with water for quenching hot metal.  The airflow from the shop-vac or other blower is split between the bottom of the pit and a tube which creates an upward draft in the hood and chimney flue.  A PVC ball valve between the vacuum hose and metal drain pipe adds control to the airflow at the bottom of the forge.


A London pattern anvil
– a design perfected some 300 years ago

Appreciating the many complications of metallurgy takes high science but a rural blacksmith can somewhat refine iron or steel by understanding just a few basics.  By heating metal until it is soft, the blacksmith can easily bend and shape it, cut it, weld it and punch better holes than he can cut with a drill.

The temper of hard steel can be ruined and lost by overheating it.  High-carbon / hard steel must be worked at a lower temperature that mild steel would normally be.  Annealing or softening of carbon steel is accomplished by getting it red hot and then setting off to the side in the ashes to cool slowly.  Annealing might be useful to relieve stresses inside a bent piece of steel before it is to be hardened.  Hardening of carbon steel is accomplished by cooling it quickly – usually by dunking the item in water.  Steel hardened this way can become too brittle sometimes. To temper a piece of steel to a desired compromise between brittle and tough the hardened item is reheated once again – but this time to a lower temperature.

From a microscopic perspective mild steel has a fibrous or stringy structure while hard steel has a fine granular structure.  The blacksmith can distinguish between grades of steel by observing the sparks thrown off when grinding it.  Sparks from mild steel are red or yellowish and fly in straight lines.  Sparks from hard steel are lighter/ brighter in color, sprangled in flight and seem explosive.  The blacksmith develops the ability to judge temperature by observing the color and glow of the underlying heated metal as well as the color of the oxide or scale formed on its surface.  Ranging anywhere between dull red and bright white the glow should be judged in the shade, not in direct sunlight.

Wrought iron or mild steels are forged at yellow heat and using sand as flux between pieces, welded at white heat.  High carbon steels are forged at a lower red to low orange heat and are generally not welded by the blacksmith method.  Overheating tool (hard) steel is likely to destroy the grain structure.  The critical temperature for tool steel is indicated by a dark red color and ranges somewhere between 1,300 and 1,600° F depending upon carbon content.  Heavy hammering a piece of steel upon an anvil at a little above the critical temperature has the effect of reducing the grain size and refining the steel.  The hammering strokes preformed by a blacksmith are not thoughtless or random but are instead precise and calculated.  Light hammer strokes are to be avoided while medium, heavy and extra heavy strokes have their appointed applications.


In addition to a forge and a good heavy anvil that won’t bounce around a blacksmith might have a vise, a pair of tongs, an assortment of cross-peen hammers and a few hardys.  A hardy is an accessory which fits in the hardy hole; it has a square base so it won’t rotate by accident.  Hot metal is cut on the table (or chipping block) which is supposedly made of softer metal than the face – which is made of hard steel and should be kept free of mars and scratches.  Hot chisels are for cutting hot metal and are made of mild steel.  Cold chisels are made of hard steel and should be used for cutting cold metal only.  Holes in hot metal are initiated by punching them upon the face from both sides with a round punch, before being moved over the pritchet hole for completion.

As fuel for the forge the blacksmith can use hardwood, charcoal, coal or coke.  A high quality soft coal free of sulfur is considered the best choice.  Charcoal which comes from wood and coke which comes from coal are both produced by the process of pyrolysis.   Although the volatiles are driven off of the constituents from which they are made, the resulting charcoal and coke have higher carbon contents and therefore make more efficient fuels.  The problem with coke in a blacksmithing furnace is the fact that a steady stream of air from a bellows or fan is needed to maintain its combustion. * Charcoal can be made by digging a hole in the ground, filling the hole with wood and then igniting it.  Once the wood is burning furiously the hole is smothered by sheets of roofing tin perhaps and dirt is spread over that.

The fire in a forge is kept small and tidy, its size proportionate to the work required.  Clinker or slag is removed periodically, fuel added as needed and perhaps water even sprinkled around the rim of the fire to keep the combustion from spreading to an area larger than necessary.  Normally a uniform heat needs to be applied to a piece of metal and so the item is laid horizontally in the fire, not pointed down into it.

Note to self: Not only were the redoubtable subjects of ceramics and glass not discussed but important workable metals like bronze & brass deserve honorable mention also.  These subjects need to be attended to in subsequent post…



29 thoughts on “Hot Stuff – metal, ceramics & glass

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