If you have ever picked up a length of old railing from a scrap heap and compared it with a bar of fresh stock from the steel merchant, you will already have felt the difference. Both look like iron, both rust, both will move under a hammer — but they behave quite differently at the forge. Knowing which is which, and why, is one of the first genuinely useful things a metalworker can learn.

What the names actually mean

Wrought iron is iron that has been worked — the word "wrought" is simply an old past tense of "work". Traditionally it was produced in a bloomery or later a puddling furnace, and the process never fully melted the metal. The result is a very low-carbon iron, typically under 0.1 per cent carbon, shot through with thousands of fine threads of iron silicate slag. Those inclusions give wrought iron its characteristic fibrous grain, and they explain almost everything about how it behaves.

Mild steel, also called low-carbon steel, is what most of us simply mean by "steel" in the workshop. It carries roughly 0.1 to 0.25 per cent carbon, along with small amounts of manganese, silicon and sulphur. It is made in enormous quantities, it is remarkably consistent from bar to bar, and it is cheap. You will find it sold as black — hot-rolled, coated in dark mill scale — or bright, which is cold-finished, accurate in size and clean in appearance.

Both are ferrous, both forge, both rust. Beyond that, the similarities fade quickly.

How they behave under the hammer

Wrought iron is soft, forgiving and extraordinarily ductile. Bring it to a bright yellow-orange and it moves sweetly under the hammer, tolerating bending, twisting, drawing out and upsetting that would make many steels complain. It also fire welds more easily than any modern steel; the slag rises to the surface at welding heat and helps protect the joint rather than hindering it.

Mild steel forges in broadly the same range, roughly 950°C to 1250°C. Below that it turns stiff and is liable to crack at the edges; above it you risk burning, permanent grain growth and a bar that crumbles under the hammer. Worked at the right heat it is a pleasure, whether by hand, under a power hammer or in a press. Forge welding mild steel is perfectly achievable, but it is less forgiving — cleanliness, correct heat and usually a flux such as borax are essential.

One practical habit worth forming early: watch the colour, not the clock. Wrought iron gives you a little more warning before it goes too far. Mild steel does not.

Strength, corrosion and hardening

Neither metal will harden by quenching. There simply is not enough carbon in either to form martensite, so a punch or chisel made from mild steel will mushroom rather than hold an edge. Small parts can be case hardened by packing them in a carbon-rich compound and heating, but for tools the sensible route is a proper tool steel.

In straight tensile strength, mild steel wins comfortably and its uniformity makes it easy to specify for structural work. Wrought iron's strength lies elsewhere: it is notch-tough, fatigue-resistant and bends rather than shatters. That combination made it the material of anchors, chains, rivets, railway tyres and agricultural implements for well over a century.

On corrosion, wrought iron has the edge. Its slag layers interrupt the path of rust and old ironwork often settles into a stable, protective patina. Mild steel rusts readily outdoors and needs help — galvanising, paint, oil or wax — to survive a British winter.

What each one is used for

• Wrought iron: restoration of historic gates, railings and architectural ironwork; decorative scrollwork where a fine, sinewy texture is wanted; weather-exposed pieces; rivets and traditional joinery details. Genuine wrought iron is now scarce — new production has essentially ceased, so stock is salvaged from old gates, farm machinery and structural ironwork, and priced accordingly.
• Mild steel: almost everything else. Gates, railings, brackets, fire baskets, hearth equipment, tongs, jigs, tooling and general fabrication. It is available in every size and section from any steel stockholder, welds readily by MIG, TIG or stick, and machines and drills without drama.

Choosing between them

For everyday blacksmithing, mild steel is the sensible default. It is predictable, affordable and available, and the vast majority of modern decorative ironwork is made from it. Reserve wrought iron for repairs to historic pieces, where matching the original material matters.

Mixing the two in a single piece of restoration is best avoided. They expand, move and corrode at different rates, and a repair that looks neat on the day can stand out badly within a few years. If you must use mild steel in a wrought iron repair, keep it small and place it where it is not on show.

Learning to tell them apart is straightforward with a little practice. Take a scrap of each, nick it deeply with a chisel and bend it until it breaks. Wrought iron shows a fibrous, silky, splintery fracture, grey in colour and often visibly layered. Mild steel snaps with a brighter, smoother, more crystalline break. A file test helps too: wrought iron files and hacksaws noticeably softer than mild steel of the same section.

Get into the habit of keeping the two in separate racks and labelling them. Your future self, halfway through a restoration job with a hot bar in the tongs, will be grateful.