鐵鍋知識室
Pure Iron vs Carbon Steel Pans: What Does 99.9% Actually Mean?
碳鋼鍋是好品類,純鐵鍋是它的更純版本:一般碳鋼鍋約含 1-2% 的碳與其他成分,純鐵鍋把非鐵成分壓到 0.1%。做鍋職人講清楚兩者差在哪、碳的強度問題怎麼用窒化解決,以及怎麼挑。
Quick answer
Carbon steel is a good category: light, fast-heating, no coating. A pure iron pan is its purer version: typical carbon steel contains about 1 to 2% carbon and other elements, while pure iron brings non-iron content down to about 0.1% and gets its strength from a nitrided hardened layer instead. When choosing, look at three things: the purity number, the surface treatment, and the test reports.
In the world of cookware, carbon steel is a good category: light, fast-heating, uncoated, and it gets better the more you cook with it. Renowned European makers use it, and the classic Chinese wok is carbon steel too. So why does Tetsu Kuroyanagi insist on calling its pans "pure iron", and put "99.9%" at the heart of the brand?
Because between those two names sits a gap you can measure in numbers. Here it is, from the family that makes the pans.
What carbon steel is: iron, plus carbon
Carbon steel is an alloy of iron and carbon. In typical cookware-grade carbon steel, carbon and other trace elements make up about 1 to 2%, and the rest is iron. Well-known European makers often label their carbon steel pans "99% iron, 1% carbon", which is already a clean recipe within the category.
The carbon is there for a reason: carbon makes steel harder and stronger. A thin-walled pan body needs strength, and the traditional way to get it is carbon. So a carbon steel pan is not a shortcut. It is a mature, honest engineering choice.
What pure iron is: pushing non-iron content down to 0.1%
A pure iron pan takes a different path: it pushes carbon and other non-iron content down to around 0.1%, so the iron itself reaches 99.9% purity.
From 99% to 99.9% looks like a gap of just 0.9. But run the math another way: the amount of "not iron" in the pan body drops from 1% to 0.1%, roughly ten times less. For an uncoated pan that touches your food every day and releases a trace amount of iron into it, the meaning is direct: the trace iron you take in comes from a purer material.
Tetsu Kuroyanagi pan bodies are made from 99.9% pure iron from Taiwan's leading steel mill. That is not a marketing number, it is an incoming-material spec: the purity has a number, PFOA, PFOS, and PTFE test at zero detection, and our pans are tested for food-contact safety in five countries: Germany (LFGB), Japan (JIS), the USA (FDA), France (DGCCRF), and Taiwan (SGS). Behind every one of those statements sits a report you can check.
How much impurity is that? One quick calculation
Picture a 100-kilogram batch of iron:
- 96% pure iron: 4 kilograms of it is not iron
- 99% pure iron: 1 kilogram of it is not iron
- 99.9% pure iron: only 100 grams of it is not iron
Going from 96%-purity iron of unknown origin to 99.9%, the impurities differ by 40 times. Compared with a typical 99% carbon steel pan, it is still about ten times less.
What are those non-iron parts? Carbon, sulfur, and phosphorus that were never fully removed during smelting. And if the raw iron came from an unknown source, nobody can tell you what other metals are mixed in. A pan touches your food every day, and an uncoated iron pan releases a trace amount of iron into your dishes, so the fewer impurities the better. That is not marketing, it is common sense.
Cheap iron pans usually will not tell you three things: where the iron came from, how pure it is, and what it was tested for. We lay all three on the table: the iron comes from Taiwan's leading steel mill at 99.9% purity, the pans are tested for food-contact safety in five countries, and heavy metals test at zero detection. You do not need to trust adjectives. Just look at the numbers.
The honest question: without carbon, where does strength come from?
This is the engineering question every pure iron pan has to answer. Carbon steel gets its hardness from carbon. Take the carbon away and the base material is on the soft side, so traditional pure iron pans really were easier to scratch and easier to warp.
The modern answer is nitriding: at high temperature, nitrogen atoms diffuse into the surface of the iron and form a hardened iron-nitride layer. Surface hardness and wear resistance rise sharply, and so does rust resistance. In other words, carbon steel buys strength by adding carbon to the alloy, while pure iron buys strength through surface metallurgy and keeps the pan body itself high-purity iron. Tetsu Kuroyanagi adds a carbonizing treatment on top, which helps the seasoning bond to the cooking surface. (For the full story, see What is nitriding?)
That is why we call the pure iron pan the "purer version" of carbon steel, not its opposite: it keeps everything the category does well, light weight, fast heating, no coating, a seasoning that builds with use, and then takes one more step on material purity.
The whole picture in one table
| Aspect | Typical carbon steel pan | Pure iron pan (nitrided) |
|---|---|---|
| Iron purity | About 98-99% | 99.9% |
| Non-iron content | About 1-2% (mostly carbon) | About 0.1% |
| Source of strength | Carbon (in the alloy itself) | Nitrided hardened layer (surface metallurgy) |
| Rust resistance | Relies on the seasoning and upkeep | Iron-nitride layer, strong rust resistance, pre-seasoned at the factory and ready to cook out of the box |
| Weight, heat response | Light, fast | Light, fast (shared category strengths) |
| Seasoning upkeep | Needed | Also builds with use and gets more nonstick |
How to think about choosing
If you have been going in circles through "best carbon steel pan" search results, our advice is simple: the category is right, now look at purity. The carbon steel family, pure iron included, is the right answer for uncoated cookware. Within the family, check three things:
- Is the material purity written down anywhere? A maker willing to print the number can usually stand behind it
- What is the surface treatment? Nitriding means rust resistance and hardness are accounted for, and the pan comes pre-seasoned at the factory, ready to cook out of the box
- Are there test reports? Food-contact testing for heavy metals and coating substances is the baseline for peace of mind
Common questions
Is carbon steel a bad choice?
No, it is a good one, and the mainstream answer for uncoated pans. Pure iron and carbon steel belong to the same family. The difference is material purity: pure iron pushes non-iron content from about 1-2% down to about 0.1%.
If pure iron is that soft, will the pan wear out easily?
Traditional pure iron pans really were on the soft side, and that is exactly why nitriding exists: a hardened iron-nitride layer forms on the surface, hardness and wear resistance rise sharply, and steel wool and metal spatulas are both fine to use.
Do pure iron and carbon steel pans cook differently?
Heat response, weight, and wok hei are shared category strengths, so the everyday feel is very similar. The purity difference shows up mainly in how much you can trust the material, and in how pure the source is when an uncoated pan releases its trace amount of iron.
How do I check the iron purity of a pan?
See whether the product page states a purity number and the source of the steel, and whether food-contact test reports are available. Tetsu Kuroyanagi pan bodies are 99.9% pure iron from Taiwan's leading steel mill, with test reports from five countries on file.
References: de Buyer's official carbon steel material page (99% iron, 1% carbon), Tetsu Kuroyanagi product knowledge base
Related reading: Pure iron, cast iron, or nonstick: how to choose a pan, What is nitriding?, Can cooking in an iron pan really add iron to your diet?
Tetsu Kuroyanagi
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