# Fusion welding

> Joining metal by melting it

- Date: c. 600 BCE
- Era: Antiquity
- Domain: [Tools & Making](https://lastcodex.org/en/domains/tools/)
- Rebuild stage: IV (Science & industry)
- Difficulty: 4/5
- URL: https://lastcodex.org/en/inventions/welding/

Fusion welding joins pieces of metal by melting their edges together, usually with some added metal, so that they cool into a single piece. Bronze founders were already doing it in the 7th or 6th century BCE, pouring molten bronze between the cast parts of large statues in South Arabia and then in Greece. The heat later came from a gas flame or an electric arc: Benardos and Olszewski patented carbon-arc welding in 1885, Fouché and Picard built an oxy-acetylene torch around 1903, and coated electrodes, then gas-shielded arcs, spread welding through industry.

![Fusion welding · Joining metal by melting it](https://lastcodex.org/_astro/plate.ChdQNzri_1Ijn7h.webp)

## Legend

1. Welding set
2. Electrode holder and coated electrode
3. Earth clamp and return cable
4. Arc and molten pool at the joint
5. Weld bead
6. Hand shield with a dark filter

## What it is

Fusion welding joins two pieces of metal by melting them where they meet. The edges and, most often, some filler metal run together into one pool, and when the pool freezes the two parts are a single piece of metal across the joint.

![Two pairs of steel plates lying on a steel slab, their cut ends showing: on the left two overlapping plates with a thin film of solder between them, the plates themselves unmelted; on the right two plates butted end to end and melted together into one continuous bead](https://lastcodex.org/_astro/v1._lBQUNSx_1wGNNH.webp)
_Left, a soldered joint: only the solder melts. Right, a weld: the plates themselves are fused._

Two other ways of joining metal look similar and work differently. In [soldering](/en/inventions/soldering-iron/), a filler that melts at a much lower temperature wets the surfaces and sets between them, while the parts themselves stay solid. In forge welding, the old method of the blacksmith, two pieces of iron are heated in the fire and hammered until they join, without melting. Fusion welding needs a heat source intense enough to melt the parts locally, today usually an electric arc or a gas flame.

A typical arc-welding set of the mid-20th century is built around a [transformer](/en/inventions/transformer/). One cable runs from the set to the electrode holder, which grips a metal rod with a coating, the electrode. A second cable ends in an earth clamp fixed to the work. The arc burns between the tip of the electrode and the joint, over a small pool of molten metal, and leaves a weld bead behind it as the welder moves along. The welder watches through the dark filter of a shield. In the finished butt joint, the bead fills the gap and the fused zone reaches into both plates.

## How it works

When the electrode is brought close to the work, current jumps the small gap through a column of hot, ionised gas, the arc. The arc is hot enough to melt the tip of the electrode and the edges of the joint. Drops of metal from the electrode cross the arc into the pool, so the electrode is used up as the weld grows. Its coating melts as well and gives off gas and a layer of slag that cover the molten metal and keep air away from it; the slag is chipped off the bead once it has cooled. The circuit runs from the set through the holder, the arc, the work and the earth clamp back to the set.

![An oxy-acetylene outfit: two upright gas cylinders chained on a trolley, a regulator on each, two hoses coiled on the trolley and running to a torch hung on it, unlit](https://lastcodex.org/_astro/v2.BwXkIS6U_1mm9mC.webp)
_A gas-welding outfit: oxygen and acetylene are mixed in the torch._

Gas welding uses a flame instead. Oxygen and acetylene come from two cylinders through regulators and hoses and mix in the torch. In 1895 Henri Le Chatelier obtained a flame of about 3000 °C by burning the two gases together. The welder melts the edges of the joint with the flame and feeds a filler rod into the pool by hand. Unlike the arc, gas welding needs no electricity.

Later arc processes protect the pool with a gas instead of a coating. In TIG welding, perfected in 1941, an electrode of tungsten that does not melt makes the arc under a stream of inert gas, and the filler is added separately. In MIG welding, developed in 1948, a wire fed continuously through a gun is both electrode and filler.

Every form of welding is dangerous work. The arc gives off ultraviolet light that burns the eyes and skin, which is why the shield carries a dark filter and welders wear gloves and covering clothes. The fume of melted metal and coating causes cancer: the International Agency for Research on Cancer classed welding fume as carcinogenic to humans in 2017, and the British Health and Safety Executive now expects it to be extracted at the source. Arc sets carry the risk of electric shock, gas cylinders and hoses can feed fires, and sparks and hot metal set alight anything nearby. Welding equipment is used by trained welders, with fume extraction, screens and fire precautions.

## History and evolution

Benoît Mille has identified flow fusion welds on Hawtar'athat, a large bronze statue from South Arabia that its inscription dates to the late 7th or first half of the 6th century BCE. In Greece, the oldest fusion welds identified so far are on large bronzes. Statues of life size or more were cast in several pieces by the lost-wax process and then joined, and Aurélia Azéma and her colleagues date the earliest Greek welds to the late 6th or early 5th century BCE, the time of the first large bronze statues. The founders poured molten [bronze](/en/inventions/bronze/) into a channel cut along the joint or into a gap left between the two parts, so that it melted their edges and fused with them.

![The bronze torso and arm of a large ancient statue on a stone block, with a line of oval weld basins round the joint of the arm and the shoulder](https://lastcodex.org/_astro/v3.Df5AjoWe_KFQ1F.webp)
_Oval basins along the join of an arm, where molten bronze was poured in._

On some statues, such as the Riace bronzes, the welds show as a line of oval basins along the joint. Angelini, Artioli and their colleagues have analysed the lead isotopes of samples taken from the welds of the Riace bronzes. In laboratory trials, Azéma's team could not make a weld longer than a few centimetres, while some ancient welds are up to a metre long, and they suspect that the founders used an additive that acted as a flux. Smiths had joined metal long before without melting it. Miller and TWI both mention small gold boxes of the Bronze Age with welded joints, which Miller says were made by pressing lapped edges together. Hot [iron](/en/inventions/iron-smelting/) was joined by forge welding, hammering two pieces until they bond: Nurcan Küçükarslan reports traces of it in Middle Bronze Age iron objects from Kaman-Kalehöyük in Anatolia, and TWI notes that it remained the usual way of welding iron until the 19th century.

The electric arc made new kinds of welding possible. Humphry Davy showed an arc between carbon electrodes early in the 19th century; TWI dates it to 1800, Miller to 1809. In 1881 Auguste de Méritens used the heat of an arc to join the lead plates of storage batteries. Nikolai Benardos and Stanislaus Olszewski patented welding with a carbon electrode in Britain in 1885 and in the United States in 1887. Nikolai Slavyanov in Russia in 1888 and C. L. Coffin in the United States in 1890 used a metal electrode that melted into the joint.

Gas welding grew alongside. After Le Chatelier's work on the oxy-acetylene flame, the French engineers Edmond Fouché and Charles Picard developed a torch to burn the two gases, dated 1903 by Metinvest. Miller places a torch suitable for low-pressure acetylene around 1900. Between 1907 and 1914 the Swede Oscar Kjellberg invented the coated electrode. The demand for armaments in the First World War spread welding through industry, and by the late 1930s shipyards were using automatic arc welding under a layer of flux.

## Key facts

- Early fusion welds on a bronze statue: Hawtar'athat, South Arabia, late 7th to first half of the 6th century BCE
- Carbon-arc welding patents of Benardos and Olszewski: Britain 1885, United States 1887
- Oxy-acetylene torch of Fouché and Picard: France, about 1903
- Welding fume: Classed as carcinogenic to humans by the IARC in 2017

## Requires

- [Bronze](https://lastcodex.org/en/inventions/bronze/): Copper and tin, harder together (c. 3,300 BCE)

## Sources

- [An experimental study of the welding techniques used on large Greek and Roman bronze statues (A. Azéma, B. Mille, P. Echégut, D. De Sousa Meneses, Historical Metallurgy 45.2, 2011)](https://www.hmsjournal.org/index.php/home/article/view/129), Historical Metallurgy Society
- [Bronze Guidelines, visual atlas, figure 202: preparation for flow fusion welding](https://www.getty.edu/publications/bronze-guidelines/visual-atlas/202/), Getty Conservation Institute
- [The history of welding](https://twi-global.com/technical-knowledge/faqs/the-history-of-welding), TWI
- [The history of welding](https://millerwelds.com/resources/article-library/the-history-of-welding), Miller Electric
- [Methods of welding metals without using electricity](https://metinvestholding.com/en/media/pdf/sposobi-svarki-metallov-bez-ispoljzovaniya-elektricheskoj-energii), Metinvest
- [IARC Monographs volume 118: Welding, Molybdenum Trioxide, and Indium Tin Oxide (2018)](https://publications.iarc.who.int/569), International Agency for Research on Cancer
- [Change in enforcement expectations for mild steel welding fume (STSU1, 2019)](https://www.hse.gov.uk/safetybulletins/mild-steel-welding-fume.htm), Health and Safety Executive
- [Artistry in Bronze: The Greeks and Their Legacy, conservation and analysis abstracts: Angelini, Vidale, Artioli, Guida and Micheli, lead isotopes of the Riace bronzes A and B, including welding samples](https://www.getty.edu/publications/artistryinbronze/conservation-and-analysis/abstracts/), J. Paul Getty Museum and Getty Conservation Institute
- [Essay by Dr. Nurcan Küçükarslan: iron objects of the Early and Middle Bronze Age from Kaman-Kalehöyük, traces of forge welding](https://pml.misasa.okayama-u.ac.jp/pages/news/NurcanKucukarslanPhD.php), Pheasant Memorial Laboratory, Institute for Planetary Materials, Okayama University
- [The Casting Techniques of Antique South Arabian Large Bronze Statues](https://www.researchgate.net/profile/Benoit-Mille/publication/340635269_The_Casting_Techniques_of_Antique_South_Arabian_Large_Bronze_Statues/links/5e965705299bf130799ac4f0/The-Casting-Techniques-of-Antique-South-Arabian-Large-Bronze-Statues.pdf), B. Mille, 2012

Fact-checked on 2026-10-19.

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