Conversion of cast iron to wrought iron according to Tian gong kai wu 天工开物

Below is Song Yingxing’s description of a fining process in his Tian gong kai wu, printed in 1637, together with an English translation. Perhaps the most striking aspect, here and throughout the Tian gong kai wu, is the sheer pleasure that Song Yingxing evidently takes in writing clear descriptive prose, and a large part of his purpose with this book was surely to entertain readers whose interest was not practical but that of a dilettante. While technology appears to be his primary concern, he also makes excursions into natural philosophy, economics, and geography.

He calls the process chao 炒, ‘fining’, but he describes a process that is quite different from any of the fining or puddling processes we otherwise know of, ancient or modern, in China or elsewhere.

Molten iron tapped from the blast furnace is allowed to flow into an open hearth; a substance called wu chao ni 污潮泥 is spread on it; and several workers stir it with willow poles. The description and the illustration (the left side of the illustration) are so precise that there is no real doubt that Song Yingxing, or the author of his source, had seen something very like this process. But it is very difficult to explain. Anyone who has worked with molten cast iron, as I have, will immediately object that the molten iron from the blast furnace, flowing into such a large open hearth, without thermal insulation, fuel, or any sort of air blast, will solidify before any significant amount of carbon has been removed.

Most translators and commentators seem unaware of this objection. They explain the passage and illustration in terms of modern open-hearth steelmaking processes, and state that the curious wu chao ni would contain iron oxide, FeO, to help remove carbon by the reaction FeO + C = Fe + CO, but that would not solve the problem, for this reaction is endothermic and would hasten solidification.

The only commentator, as far as I know, who has been aware of the problem was one of the first, the German metallurgist Adolph Ledebur (1885: 192), more than a century ago, and he also proposed a solution. A Japanese friend had shown him a copy of Tian gong kai wu and translated the metallurgical sections for him. In his article about it he suggests that the wu chao ni spread on the molten iron contained saltpetre (potassium nitrate, KNO₃). It is a powerful oxidising agent (this is its function in gunpowder), and might very well be able to accelerate the oxidisation of the carbon sufficiently to keep the temperature up until the carbon is exhausted and the cast iron is converted to wrought iron. Several uses of saltpetre in puddling, including the Heaton process, are described briefly by Wedding (1874: 264–5; 1901: 254); see also Gruner (1869).Ledebur adds: ‘That a quick conversion of pig iron to wrought iron can be effected by the use of saltpetre was demonstrated two decades ago by the Heaton process.’

Wu chao ni 污潮泥 means (or can mean) something like ‘filthy wet loam’, and the label in the illustration has the variant chao ni hui 潮泥灰, ‘wet loam and ashes’. In pre-modern China and Europe, saltpetre was manufactured in ‘nitre beds’, in which the raw materials were dung, earth, urine, and wood ashes (Williams 1975; Needham 1980: 188ff). In China, nitre beds were described as so foul-smelling that birds avoided flying over them.

In the nitre bed fermentation produces calcium nitrate. Treating a solution of this with ashes gives potassium nitrate and a precipitate of calcium carbonate. It is likely that the terms ‘filthy wet loam’ and ‘wet loam and ashes’ describe something like such a nitre bed. The saltpetre used as a component of gunpowder must be very pure, but perhaps, in the fining process described by Song Yingxing, some much less pure product, containing a small but significant amount of saltpetre, might have been useful.

What is the relation between author and illustrator in this case? The illustration fits the text in almost every detail, so that one definite possibility is that the illustrator simply followed the text without having seen anything himself. One detail tends to contradict this hypothesis: the label chao ni hui, ‘wet loam and ashes’, where the text has wu chao ni, ‘filthy wet loam’. This might imply that the illustrator had some information about the process without reference to the author; for example he might have heard the term in conversation with the workers. This is not, of course, the only possibility: the text and illustrations were no doubt prepared at different times, and Song Yingxing might on the two occasions have chosen differently among the terms he knew for this substance, whether he knew them from first-hand experience or from written sources.

One point suggests that Song Yingxing relied on written sources for this process: the curious wall mentioned in the text and seen in the illustration. The text states unambiguously that the workers stand on this wall, but why should they do this? If on the other hand they stood behind a light waist-high wall it would provide some protection from the flying sparks and intense radiant heat from the hearth. I suggest as one possibility that Song Yingxing misunderstood a written source that mentioned such a protective wall, and the illustrator blindly followed Song Yingxing’s text.

Chinese text

The full text of Song Yingxing’s description of iron, together with a translation into modern Chinese, is here: www.8bei8.com/book/tiangongkaiwu_15.html 凡造生铁为冶铸用者，就此流成长条、圆块，范内取用。若造熟铁，则生铁流出时相连数尺内，低下数寸筑一方塘，短墙抵之。其铁流入塘内，数人执持柳木棍排立墙上，先以污潮泥晒干，舂筛细罗如面，一人疾手撒，众人柳棍疾搅，即时炒成熟铁。其柳棍每炒一次，烧折二三寸，再用则又更之。炒过稍冷之时，或有就塘内斩划成方块者，或有提出挥椎打圆后货者。若浏阳诸冶，不知出此也。

English translation

If they are producing cast iron to be used in foundrywork, [the molten iron] flows into moulds for bars and round pieces for use.

If they are producing wrought iron, when the cast iron flows out, it is led into a rectangular pool which is constructed a few chi 尺 [feet] away and a few cun 寸 [inches] lower, up against a short wall.

When the iron flows into the pool, several persons holding willow poles stand in a row on the wall. Earlier they have taken wu chao ni 污潮泥 and dried and sifted it in a fine sieve so it is like flour. One of the men quickly spreads this while the others quickly stir with the willow poles and it is immediately fined into wrought iron.

After fining, when the iron has cooled slightly, some [ironworks] simply chop the iron in the hearth into square pieces; others lift [these] out [while still hot] and hammer them into round bars before marketing [the iron]. Some ironworks, such as those in Liuyang 瀏陽 [Modern Liuyang, Hunan, just east of Changsha] do not know how to produce these.

References

Last edited by DBW 25 February 2023
Stylistic changes 4 June 2024