Hot Iron Work: Forge, Anvil and Heat Treatment

01

What does drawing out actually do to the metal?

Hot iron work is the set of operations a smith performs on steel while it is above its recrystallisation temperature: drawing out, upsetting, bending, punching, forge welding and, afterwards, heat treatment. Each operation moves metal in a direction the hammer and anvil allow, and each depends on a fire that delivers the right heat to the right length of stock. The equipment decisions, anvil, fire, tongs, follow from those operations rather than preceding them.

The vocabulary is old and mostly literal. A smith who writes about the craft in the Hudson Valley, where water-powered ironworks once ran along the river, uses the same terms as a smith in Sheffield or Essen, because the behaviour of hot steel does not change with geography. A journal such as The Forge Ledger records that vocabulary alongside the surviving iron of the Capital District, from Burden Iron Works to the cast-iron facade of the Watervliet Arsenal.

Drawing out, also called stretching, reduces the cross-section of a bar and increases its length. The smith places the stock across the anvil and strikes with the face or the peen, letting the metal flow away from the hammer in the direction of least resistance. Because the anvil acts as a second hammer, the metal between them thins along the axis the smith chooses.

The operation is directional. Striking flat on the anvil spreads the bar in two directions, wider and longer, unless the smith turns the work a quarter turn between blows. Turning the bar keeps the width constant and sends the displacement into length. A fuller, a blunt-edged tool struck between hammer and work, localises the reduction so a taper or a shoulder forms where it is wanted.

Temperature governs how much can be done in one heat. Carbon steel at a bright orange, roughly 1,100 to 1,200 degrees Celsius at the surface, moves easily; below a dull red it begins to resist and then to crack. A smith who keeps working a cooling bar risks a split that no later heat will close.

02

How is upsetting different from drawing out?

Upsetting is the reverse: the smith shortens the bar and increases its cross-section. The stock is stood on end on the anvil face and struck on the top, so the metal thickens at the point of impact. The technique is used to form a head on a rivet, to thicken a section before bending a corner, or to build mass where a joint will be made.

Upsetting is harder to control than drawing out because a long bar buckles before it thickens. The usual remedy is to work only a short length at a time, keeping the rest of the bar cool, or to confine the hot section in a tool such as a monkey tool or a bolster plate. The fire must be deep enough to heat the end of the bar evenly; a shallow fire heats the surface and leaves the core cold, which produces a fold rather than a thickening.

03

Bending, punching and the limits of a single heat

Bending is the simplest operation and the one most likely to be done badly. A bend made over the anvil’s horn or a bending fork is a controlled deformation; a bend made by eye is a guess. For a right angle, the smith upsets the corner slightly first, because bending thins the outside of the curve and the added mass compensates.

Punching opens a hole without removing metal, which distinguishes it from drilling. A hot punch is driven through the stock over the pritchel hole or a bolster, and the displaced metal is pushed aside rather than cut away. The punch is cooled in water between blows so it does not soften and bend. On thick stock the smith works from both faces, meeting in the middle, so the hole stays straight.

Forge welding joins two pieces by bringing them to a welding heat, a white or near-white colour, and hammering them together. The surfaces must be clean; borax or another flux keeps oxide from forming at the moment of contact. The weld is made with quick, light blows first to set the joint, then heavier blows to consolidate it. A weld that looks sound but was made below temperature will show a cold shut, a seam that opens under stress.

04

How is carbon steel heat treated after forging?

Heat treatment has three stages. Annealing is the softening step: the steel is heated to its critical temperature and then cooled slowly, often buried in ash or vermiculite, so the internal structure relaxes and the metal can be filed or drilled. Normalising is similar but the steel cools in still air, which refines the grain.

Hardening, or quenching, is the opposite of annealing. The steel is brought to its critical temperature, judged by colour and by the loss of magnetism, then plunged into oil or water. The rapid cooling traps a hard, stressed structure. Quenching alone produces a brittle tool, so tempering follows: the hardened piece is reheated to a lower temperature, judged by the oxide colours that run from pale straw to blue, and held there before cooling. The colour indicates the temperature, and the temperature determines the balance of hardness and toughness.

A smith working alone judges all of this by eye, which is why the craft is taught by repetition. The colours are not decorative; they are the only thermometer available at the forge.

05

Choosing an anvil and building the fire

An anvil is chosen for mass, face condition and the tools it carries. A working anvil has a hardened steel face, a horn for bending, a step or shelf for cutting, a hardy hole for bottom tools and a pritchel hole for punching. Mass matters because the anvil must absorb the blow rather than move; a light anvil on a loose stand wastes much of the hammer’s energy. A face that is chipped or swaybacked will mark the work, and a face that has been refinished too aggressively may be soft.

The stand should be solid and the anvil fixed to it. Height is set so the knuckles of a relaxed hand rest on the face. A smith who has to stoop or reach loses control of the hammer and, over a day, of the work.

The fire is built in a forge pan or hearth around a tuyere, the pipe through which air reaches the fuel. Coal, coke and gas behave differently. Coal cokes up as it burns and produces a hollow fire that suits welding; coke burns hotter and cleaner with less smoke; gas gives a controllable, consistent heat without the sulphur and ash of solid fuel. The fire is shaped with a rake and a shovel: a deep, narrow fire concentrates heat for welding, a broad, shallow fire suits long sections that need even heating.

Tongs are selected to fit the stock, not the other way round. A pair that is too large will not grip; a pair that is too small will not close. The jaws are adjusted by the smith, often by forging them, so that the work is held securely at the angle the operation requires. Poor grip is the most common cause of a dropped workpiece, and a dropped workpiece at forging heat is a hazard as well as a loss.

06

What the surviving iron shows

The techniques described here are not historical curiosities. The cast-iron building of the Watervliet Arsenal, the Monitor’s armour plate at Rensselaer, the Paley grilles at the Capitol and the Ross Valve foundry in Troy are all products of the same operations: metal drawn, upset, bent, punched, welded and heat treated. Reading them as forged objects rather than as monuments is one way to see the craft as it was practised, and as it is still practised by smiths working today.