What fuels are used for the forge?


Charcoal mound

Charcoal was the original forge fuel. A beginner will immediately ask the question here "can I use bar-b-que charcoal?" The answer is no, not bar-b-que charcoal. Charcoal is made from plant material (usually wood and/or straw and clay or some other filler material) that has been heated to drive off volatile matter with almost pure carbon left behind.

charcoal kiln
A charcoal kiln that is no longer used near
Walker, Arizona

By a process of distillation in which wood is heated hot enough to burn, but starved of oxygen, most of the compounds in the wood are driven off in the form of vapors or smoke leaving carbon behind. Before the industrial era this was accomplished by stacking logs in a pile and burying with dirt. A small fire was built at the bottom of one end and a hole opened in the top of the mound to vent the vapors or smoke. The air supply was restricted to allow the fire to burn hot enough to literally char the wood into 'coal', but at the same time starving the wood pile inside the mound from getting enough oxygen to burn completely. The idea was to put out the fire after the wood was converted to charcoal. After the burn most of the wood was recovered in the form of charcoal with only minor losses due to some wood being closest to the fire.


anthracite coal
Antracite Coal

Coal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly sulfur, hydrogen, oxygen and nitrogen.

coal strip mine
A coal strip mine - one of the environmental
consequences of using coal

Coal begins as layers of plant matter accumulate at the bottom of a body of water. For the process to continue the plant matter must be protected from biodegradation and oxidization, usually by mud or acidic water. The wide shallow seas of the Carboniferous period provided such conditions. This trapped atmospheric carbon in the ground in immense peat bogs that eventually were covered over and deeply buried by sediments under which they metamorphosed into coal. Over time, the chemical and physical properties of the plant remains (believed to mainly have been fern-like species antedating more modern plant and tree species) were changed by geological action to create a solid material.

Between approximately the 15th and 18th centuries, blacksmiths gradually began the change to coal as their primary source of forge fuel. Not all coals are suitable for forge fuels, and the lack of access to a source of good coal and the lack of success with coal slowed its adoption as the primary fuel for blacksmiths. Even today blacksmiths must be very picky about how and where they obtain their coal, and most smiths locate good sources by word of mouth. In some regions of the world, coke is easier to obtain.

As far as coal goes. There are grades to each type of coal, anthracite and bituminous. I find that the anthracite doesn't coke very well and burns with a hot flame - out of the fire. By that I mean there tends to be a large flame above the fire. That said, I have used Anthracite in a rice size and had very little flame. Bituminous will form coke and you should ask for the 'coke button index' or the 'free swelling index' - (two names for the same thing). This will tell you the ability of the coal to form coke.

acid rain forest
Forest decimated by acid rain from the sulphur in
coal - another environmental consequence

Whichever fuel source you select, it needs to be a 'metallurgical' grade with low sulphur and phosphorous. Both of these chemicals affect the steel adversely.

   (From Wikipedia, the free encyclopedia)

raw coke

Coking Coal

Coke is usually produced from coal; the process is called coking. Volatile constituents of the coal (including water, coal-gas, and coal-tar) are driven off by baking in an airless furnace or oven at temperatures as high as 2,000° F. This fuses together the fixed carbon and residual ash. Most modern facilities have "by-product" coking ovens. Today, the volatile hydrocarbons are mainly used, after purification, in a separate combustion process to generate energy. Non by-product coking furnaces or coke furnaces (ovens) burn the hydrocarbon gases produced by the coke-making process to drive the carbonization process. Bituminous coal must meet a set of criteria for use as coking coal, determined by particular coal assay techniques. These include moisture content, ash content, sulfur content, volatile content, tar, and plasticity.

The greater the volatile matter in coal, the more by-product can be produced, but too low or too high a level of volatile matter in the coal results in inferior coke produced in respect to coke quality properties. It is generally considered that levels of 26-29% of volatile matter in the coal blend is good for coking purposes. Thus different types of coal are proportionally blended to reach acceptable levels of volatility before the coking process begins.

Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Since smoke-producing constituents are driven off during the coking of coal, coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. Coke may be burned with little or no smoke under combustion conditions, while bituminous coal would produce much smoke.

The Chinese first used coke for heating and cooking no later than the ninth century AD. By the first decades of the eleventh century, Chinese ironworkers in the Yellow River valley began to fuel their furnaces with coke, solving their fuel problem in that tree-sparse region.

In 1603, Sir Henry Platt suggested that coal might be charred in a manner analogous to the way charcoal is produced from wood. This process was not put into practice until 1642, when coke was used for roasting malt in Derbyshire. Coal cannot be used in brewing because its sulfurous fumes would impart a foul taste to the beer. In 1709, Abraham Darby I established a coke-fired blast furnace to produce cast iron. Coke's superior crushing strength allowed blast furnaces to become taller and larger. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution.

In England in the first years of steam railway locomotives, coke was the normal fuel. This resulted from an early piece of environmental legislation; any proposed locomotive had to "consume its own smoke". This was not technically possible to achieve until the firebox arch came into use, but burning coke, with its low smoke emissions, was considered to meet the requirement. However, this rule was quietly dropped and cheaper coal became the normal fuel, as railways gained acceptance among the general public.

Historic coke burning
Coke ovens being reclaimed by nature

In the late 19th century, the coalfields of western Pennsylvania provided a rich source of raw material for coking. In 1885, the Rochester and Pittsburgh Coal and Iron Company constructed the world's longest string of coke ovens in Walston, Pennsylvania, with 475 ovens over a length of one and a quarter miles. Their output reached 22,000 tons per month.


Coke oven at smokeless fuel plant, South Wales

Coke oven at smokeless fuel plant
The bulk specific gravity of coke is typically around 0.77. It is highly porous. The most important properties of coke are ash and sulfur content, which are linearly dependent on the coal used for production. Coke with less ash and sulfur content is highly priced on the market. Other important characteristics are the M10, M25, and M40 test crush indexes, which convey the strength of coke during transportation into the blast furnaces; depending on blast furnaces size, finely crushed coke pieces must not be allowed into the blast furnaces because they would impede gas dynamics. A related characteristic is the Coke strength after reaction (CSR) index; it represents coke's ability to withstand the violent conditions inside the blast furnace before turning into fine particles.

The water content in coke is practically zero at the end of the coking process, but coke is often water quenched to reduce its temperature so that it can be transported inside the blast furnaces. The porous structure of coke absorbs some water, usually 3-6% of its mass. In some more modern coke plants an advanced method of coke cooling is by air quenching. Bituminous coal must meet a set of criteria for use as coking coal, determined by particular coal assay techniques. The volatility of coke reaches minimum levels at the end of the coking process.

Gas Forges

Natural Gas vs Propane?  Blacksmiths today now have access to gas-fired forges and furnaces as well as coal and coke forges to heat the iron. Gas forges offer the convenience of not having to worry about where to buy good smithing coal. Gas forges are not as hot as coal forges and take longer to heat the iron to a forging temperature. In all cases the lower heat value of gas means longer heat times and more oxidation. However the size and capacity of the gas forge allows a much larger number of straight un-worked, or nearly straight bars to be placed in the fire at one time, and therefore can heat more un-worked bars over a longer period of time than the coal forge. This last point is the reason why a business needing forgings is more likely to have a gas forge than a coal forge.

On the other hand the coal forge is still king when higher heats on larger and heavier bars are needed, and fewer bars are to be heated for work, and for heating work of more complex shape which cannot be placed inside the limited interior area of the gas forge. Each type of forge (coal or gas) has its advantages and disadvantages, and this is why each shop must choose what type of setup works best in their situation. Many shops employ both gas and coal forges and use them each for specific tasks such as-coal for heavy bars and gas for large numbers of small work.

Most factory-made gas forges cannot reach welding heat, and those that can, will heat the iron much more slowly. During the last 25 years, a new welding flux was introduced specifically for allowing gas forges (those forges that are actually capable of reaching welding heat) to be used for welding. Special fluxes are sometimes needed for fire welding with gas forges. Since gas forges will take longer to heat iron, more oxidation will develop during the extended heating period. Special fluxes are used to deal with the additional scaling which results from this oxidation. The lower temperature and slower heating associated with the gas forge is actually helpful to most beginner smiths and those with poor fire skills because, a cheap gas forge will not heat the iron to a sizzling white heat- suddenly destroying the iron. Instead the iron will waste away (slowly burning) in the fire over a long period of time, but the inexperienced smith need not worry about suddenly destroying his iron by accidentally leaving it in the fire too long. On the other hand...

Note: My personal experience is that gas forges can and do reach welding heat and the only flux you need is borax (buy 20 Mule Team Borax in the grocery store laundry product aisle to save money). Caution though, the melted borax flux will destroy the bottom of a forge.

If you want to go the gas forge route, I recommend visiting Ron Reil's website to see how to design the hotter custom-made forges at http://ronreil.abana.org/design1.shtml. Ron has compiled a large collection of designs both of his own and those sent to him by friends. Lots of designs of burners, insulation, most are inexpensive. These guys keep adding more stuff.

The Question: Coal Forge Vs. Gas Forge?

To which the correct answer is: It all depends. Each has its pros and cons. Availability: natural gas, you either have it from a pipeline or you don't; propane, readily availble in most areas from the people who fill gas grill bottles; coal/coke can be a problem to get a dependable supply and it's a lot harder to start and maintain your fire.

Okay, here are a few pros and cons for coal vs gas forges:

Type of Forge Pro Con
  • coal forges get hotter than gas forges
  • coal forges are more traditional
  • coal forges with blowers can use hand cranked blowers, meaning that you can forge in places that don't have electricity available
  • coal forges present less of a CO hazard than gas forges might. It's not a zero hazard, however.
  • coal forges don't require pressurized gases
  • coal forges don't make any noise when idling and they make very little noise when being blown
  • coal forges don't use much fuel when idling
  • coal can be hard to find, especially in a short time and of good quality for smithing
  • coal forges, when run improperly can be smoky and maybe smelly. Could produce a problem with neighbors
  • coal forges take more expertise to run. We teach fire management first because of this
  • coal forges indoors require a well designed chimney (10 inch min diameter)for proper operation
  • coal forges are dirtier to operate
  • coal forges have to be cleaned out and have clinker, coal and coke separated after each use
  • gas forges are relatively clean to operate
  • gas is readily available from a variety of stores
  • gas forges don't take a lot of training to use
  • gas forges are more neighbor-friendly
  • gas forge fuel, usually propane, is all contained in a pressurized cylinder so no mess
  • gas forges usually don't need a chimney; but they DO need ventilation
  • gas forges don't require constant maintenance while they are running

  • gas forges are noisy, whether in use for heating or when idling, because they usually (but not always) idle at full power
  • gas forges radiate a lot more heat at the operator and into the room than coal forges do
  • gas forges have been around a long time in industry, but the "public" doesn't see the gas forge as the tool of the "smith under the chestnut tree"
  • gas forges can be sources of carbon monoxide poisoning, so ventilation is always a vital concern
  • gas forges can be cantankerous. One can forge weld in some of them but not in others

    Gee Mom, look what else I've found!

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