Introduction to The Forge
A forge is (usually) a type of hearth which is used specifically for heating metals in order to work them in some way. While furnaces and kilns can be used to heat metal as well, the hearth distinguishes itself by being more of what we would consider to be a fireplace; used to hold fire and generally open-aired in order to provide heat by flame. Furnaces and kilns usually rely on a closed, oven effect to compound and thermally insulate heat inside a chamber, often for the purpose of changing the actual state of matter. By contrast, when we use a hearth (like a fireplace or fire-pit) for heating up metal enough to work it in some way, we start calling it a forge. We’re not necessarily talking about heating metal to the point of melting, this would usually require the heat of a furnace in a foundry. Rather, the forge serves the purpose of heating the metal just enough so that it can more easily be shaped by force (forging).
Since the forge is essentially something that is used in order to facilitate forging, it is handy to establish what forging actually is. Forging is a metalworking process which is used to shape metal into tools and objects that are stronger than wood or stone counterparts. Someone who forges is called a smith. Smiths forge metal by striking it with a hammer in order to literally pound it into another shape. Today these hammers can be giant and mechanized in modern factory forges, but historically they were handheld by the smith themselves. While there are two kinds of forging (hot forging and cold forging), only hot forging requires an actual forge for heating. Cold forging is the act of deforming metal without heat and often under great pressure and stress. Cold forging is also synonymous with a term called work hardening, and this is what happens to metal after it has been shaped by squeezing, bending, drawing or shearing. The particles of metal are crystal in nature; when the little crystal particles get mashed together, they become roadblocks for the movement of other particles. As a result, cold forging forces metal to become harder, and work hardened, meaning you can only cold forge a metal so many times until it becomes too hard to manipulate further. Heating a metal causes its particles to re-crystallize so that they can be moved again by force. This heating is what hot forging is all about, and hot forging is what we are talking about today.
Types of Forges
The Coal Forge typically uses coal, charcoal or industrial coke burned as fuel in order to heat metal. This is the type of forge that you would see medieval (or otherwise traditional) blacksmiths using with an open hearth fire pit. Air is blown into the fire to add more oxygen, which will burn the fuel quicker and make the fire burn hotter; hot enough to alter the state of metal.
This blowing is facilitated by a bellows, which can be a fan or pump, so long as it forces air into a tuyere. Think of the tuyere as a pipe which delivers the blown air into the fire, which is held by the hearth. This hearth holds your coal/charcoal/coke fuel, which when burning and blown, heats the metal workpiece. While you can technically get a normally-aspirated fire to burn hot enough so that it will heat metal to a workable state; a true coal forge consists of a hearth, tuyere and bellows.
A tuyere can be positioned beside, or under a hearth in order to deliver a flow of air into the fire. Managing this process is the job of the smith, who must make adjustments as necessary in order handle the fire and to achieve a size of fire or level of heat necessary for a specific workpiece of metal. A skilled metalsmith, such as a blacksmith, will be comfortable and knowledgeable with the management of the consistency of the combustion of their fuel. Knowing when to feed more air, or how to reshape the heart of a coal fire in order to accommodate different metal workpieces, is absolutely key in the operation of a forge which is powered by coal, charcoal or coke.
Coal forges can vary greatly due to necessity and circumstance; they can be the size of a building, or it can simply be a hole in the ground with a pipe feeding air into it.
The Gas Forge makes use of combustible gas (such as propane or natural gas) in order to heat a chamber which metal workpieces can be placed within. Apart from requiring a supply of flammable gas and oxygen (in order to facilitate combustion), gas forges are much more simple to operate and manage than a coal forge. Essentially all that is required is a chamber and means to feed gas into said chamber. The burning gas along with pressure in the chamber can heat the air within it to temperatures capable of re-crystallizing metal. With a gas forge, a smith needs only to regulate the flow of gas and oxygen in order to manage the process, and they don’t have to reshape or handle fuel as would be required in a coal forge. While this makes the gas forge more simple to operate with a more consistent heat chamber; the gas forge can also be less versatile, allow less customization and prohibit the heating of small, specific sections of a workpiece. What goes into a gas forge is usually heated in its entirety.
Gas forges can also vary greatly in size and shape, and can direct a single flame into a chamber or multiple flames in order to compound their effect. They can be built with complicated designs for maximum efficiency, or simply carved out of a brick with a blowtorch stuck into it.
A Finery Forge is a type of coal forge which is used specifically for refining low-quality pig iron into higher quality (and strength) wrought iron. While the principles of function and design vary minimally from the concepts and parts of a coal forge, the finery forge is distinguished by its purpose and process. This kind of forge is used specifically to melt and liquefy iron in order to remove carbon from it, so its construction will differ from most common coal forges. That being said, many smiths construct coal forges which serve multiple purposes and are capable of heating metal for forging, or melting it down completely for refining. Finery forges have been made obsolete by the advent of the puddling process; a cornerstone of the industrial revolution which uses furnaces in order to refine greater quantities of fine iron faster and more efficiently than a forge can.
As a crucial component in forging, knowledge of forges are important for anyone looking to manipulate or form metal. While many components of forges have been standardized for commercial sale (some available here), you can also build gas and coal forges relatively easily, and we can teach you much more about that and other things here:
- How to build a Gas Forge
- How to build a Coal Forge
- Introduction to Forging
- Introduction to Blacksmithing