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This article is a revised
version of a talk I gave at the one-day Symposium on Cast Iron in
Ancient China, in Beijing, 20 July 2009. Many
thanks to the organizers and to several participants who made useful
comments.
Click on any
illustration to see it enlarged.
What is
Cast Iron?
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Participants in the
Symposium were all
familiar with the Iron–Carbon Equilibrium Diagram, which gives the
atomic state of iron–carbon
alloys in relation to temperature. But it seemed worthwhile to remind
them of the implications of the Liquidus Line, emphasized here. This
can also be called, somewhat imprecisely, the “melting point” of an
alloy. Practical casting temperatures lie 50–100°C higher than this.
We can see that casting steel, with up to about 1.5% carbon, requires
temperatures well over 1500°C. Such temperatures were often reached
in
early times in various parts of the world, but the refractory materials
necessary to manipulate molten steel and cast it into useful artefacts
were not developed until the late 19th century, in Britain.
With a higher carbon content, 3–4%, practical casting is much easier,
requiring temperatures around 1300°C. Iron with this carbon content
is
called “cast iron” because it is easy to form by casting. It lends
itself well to large-scale production, which has always been important
in ancient China. It was undoubtedly the use of cast iron in ancient
China that made it possible for every peasant to have iron implements.
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Some Ancient Cast Iron
Artefacts
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Etched with Nital, scale bar 100 µm.
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The use of cast iron makes
it possible to mass-produce
implements
cheaply, but for most implement types it has inferior mechanical
properties.
All pre-modern Chinese cast iron has a very low silicon content,
usually under 0.5% Si, and therefore solidifies as “white cast iron”,
in
which the carbon is present as cementite,
Fe3C, which is extremely hard, harder than quartz, and this
makes the iron brittle.
An example is this mattock-head. The micrograph shows that it is white
cast iron. Its hardness may be an advantage with such an implement, for
it would be very abrasion-resistant. But the worker would have had to
be careful, for if it hit a rock it might shatter.
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2nd–1st cent. BC
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AD 1102–1106
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AD 1851–1861
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Coins were usually
made of copper alloys, but sometimes shortages of copper made it
necessary to cast them of iron. Here are three iron coins, from the
Western Han, Song, and Qing periods.
Mike Wayman and Helen Wang studied 37 iron coins of the Song period in
the British Museum, London. They are all of white cast iron, and their
article has a great deal of detail on the types of white cast iron
found here.
An interesting aspect is revealed by X-rays of some of the coins: these
show that there are sometimes quite large casting bubbles in the coins,
making them feel distinctly light in the hand. It seems possible that
the coin-founders intended these bubbles, in order to save material.
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AD 1208–1224, X-ray
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 AD 1211, Etched with
Nital, scale bar 50 µm
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(Mike Wayman &
Helen Wang, Historical Metallurgy, 2003, 37.1, pp. 13, 14)
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This is a cast-iron
mirror – probably Han-dynasty (206 BC – AD 220), probably white cast
iron.
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Diameter 13 cm.
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An interesting
characteristic of traditional
Chinese metallurgy is that
cast iron is often combined with other materials. The legs of this
vessel are cast iron, while the body is
bronze. I have not seen a metallurgical investigation of a vessel of
this type, but the legs are most probably of white cast iron.
The vessel is a ding 鼎
from a grave excavated at Yutaishan in Jiangling, Hubei, dated to the
4th century BC.
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Malleable
Cast Iron
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These
crossbow-bolts
have bronze tips and iron shafts – presumably because bronze has better
casting properties than iron, while iron is cheaper than bronze.
Lian Haiping 廉海萍 in Shanghai has studied some examples
of this type of artefact and found that the shafts are of cast iron
which has been decarburized in the solid state – this type of iron is
called “whiteheart malleable cast iron”. She reported this at the BUMA
conference in Beijing in 2006.
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Image taken from
ebay.com
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Hoes and spades of wood with iron caps were common in ancient times and
continued in use well into modern times, as can be seen here:
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From a grave in
Changsha, Hunan, 3rd–4th century BC
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Etched, scale bar 250 µm
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This implement-cap is
“blackheart
malleable cast iron” – it was first cast, then annealed at a high
temperature, probably around 950°C, for a period of days. This
treatment caused the carbon in the iron to precipitate as graphite,
making a material which has much better mechanical properties than
ordinary cast iron (white or grey).
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These gads from a
copper-mine site show how tough the ancient Chinese cast iron could be.
They are obviously subject to very hard punishment, being hammered into
cracks in the rock.
But they are of cast iron – again annealed at a high temperature for a
period of days, this time decarburizing the iron at the surface and
precipitating graphite farther in.
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Tonglüshan
铜绿山 copper-mine site, 4th–3rd century BC
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Etched,
scale bar 50 µm
(Hua Jueming 华觉明, 自然科学史研究, 1982, 1.1,
pl. 1)
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Scissors, 1st–2nd
century AD
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Etched, scale bar 40 µm (Hua Jueming 华觉明, 自
然科学史研究, 1982, 1.1, pl. 1)
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These scissors are also of
cast iron.
They were cast, then annealed in an oxidizing atmosphere to decarburize
to a uniform carbon
content around 1%, with a few very small grapite nodules, then bent
into shape by a smith.
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This
sword
from Gansu appears to be made by casting it and then decarburizing to a
uniform 0.6% carbon.
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Cast Iron Sword,
2nd–1st century BC
Ethanolic
picral
etch
(David
A.
Scott & Qinglin Ma, Historical Metallurgy,
2006, 40.2, p. 109) |
Grey Cast Iron
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Here is a cast-iron mould for casting an iron implement.
That in itself is surprising enough, but note that the iron is very low
in both carbon and silicon, and nevertheless the structure is grey cast iron.
Normally such an alloy would solidify as white cast iron – only very
slow cooling would allow it to solidify as grey cast iron.
This must have been cast in a massive heated ceramic mould and allowed
to cool very slowly, over a period of days.
The only ancient artefacts I know of which are grey-cast are moulds,
and I believe the reason is that the moulds must be very tough, to
tolerate the thermal shock when molten iron is poured in, but they
could not be made of malleable cast iron, because the
annealing process tends to warp castings slightly, so that it would be
difficult to fit the parts of the mould together.
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A Cast
Iron Wall
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(满城汉墓,
1980, vol. 1, pp. 216, 218, 220; vol. 2, pl. 154, 155.1)
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Finally here is another
amazing use of cast iron in
ancient China. This is the entrance to a tomb cut into rock in
Mancheng, Hebei, dated shortly before 104 BC.
Two brick walls were constructed, after which molten iron was cast
in between them to seal up the tomb.
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How Iron is
Cast – the Cupola Furnace
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The cupola furnace was the usual furnace
for casting iron from ancient times to the
middle of the 20th century.
Here is a small iron foundry in Denmark in 1890. The cupola is a shaft
furnace. Iron and fuel (in this case coke) are
charged in the top, and air is blown in near the bottom. There is a
steam engine inside the building at the left.
The fuel burns in contact with the iron, the iron melts, and molten
iron is tapped out at the bottom.
Notice the workers. The one whose job is to open and close the taphole
is protected by a screen from the high radiant heat from the molten
iron. |
(Torben Witt, Aalborg
og Fabrikkerne, 1980, p. 21)
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(Gottwald,
Technische
Mitteilungen
Krupp,
1938,
6.4: 110–111)
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Left: Cupola
furnace in Shanghai in the 1930’s.
Below: Cupola
furnaces from the 19th and early 20th centuries.
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Courtesy of Martyn
Gregory Gallery, London
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Courtesy of British
Museum, Oriental and India Office Collections (see here)
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Hommel, China at work, 1937,
p. 27
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Click to see a 50-second
film-clip by Yang Ruidong 杨瑞栋 of the casting of iron
in Huize 会泽, Yunnan.
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This is a
reconstruction of a large cupola furnace at a Han-dynasty ironworks
site at Wafangzhuang in Nanyang, Henan 南陽瓦房莊. Height 3–4 m.
Note that the blast goes through a pipe over the top of the furnace, so
that the air is somewhat heated before it enters the furnace.
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(考古学报, 1978.1: 12)
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(Li
Jinghua 李京华, 中原古代冶金技术研究,
1992, p. 147)
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This is a
reconstruction of a cupola furnace for melting bronze, from the Chunqiu
period (777–476 BC).
The furnace is composed of three sections. It is charged copper, tin,
and
fuel, air is blown in through the top (or sometimes through a hole in
the side), and the bronze melts and settles in the bottom. The two
upper
parts are then removed, and the bronze is poured from the bottom
part.
This type of furnace has had a long history . . .
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(René Antoine Ferchault de Réaumur, [雷歐姆], L’Art de Convertir le Fer
. . ., 1722)
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. . . Here is
the same sort
of
furnace, used by Gypsy ironfounders in Europe. The illustration is from
the 18th century.
The Gypsies are a minority people in Europe who traditionally have no
fixed
domicile, but travel around. They are also called Romanies, Travellers,
Zigeuner, Tsiganes, and many other names. They seem to have come to
Europe from
India in the 13th century, but they certainly had this type of furnace
from China. It has sometimes been suggested that they were the first
ironfounders in Europe.
Here again we see a cupola furnace in three parts. When the iron has
melted, the upper sections are removed and the iron is poured from the
bottom section.
A description of a slightly different Gypsy cupola furnace can be seen here
(quotation in English in a Danish article).
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At
Huangkiao
we
witnessed
the
operations of a Chinese foundry.
. . .
The blast was stopped, the bellows disconnected, and the upper and
middle sections of the furnace taken off and laid aside. The surface of
the molten iron being skimmed of its slag, it was well covered with
rice husk ashes. This protected the face of the man who next had to
handle it from the intense heat that would otherwise have radiated from
the molten iron. This man’s duty was to clasp the crucible in his arms,
literally hugging it to himself, and to fill the molds arranged around.
. . .
(American Manufacturer, 1899, 64:
125)
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I do not have an
illustration of this type of furnace in China, but one very interesting
description, from 1899. Here is a brief extract; the whole description
can be seen here.
It is amazing to imagine the worker clasping
the
crucible
in
his
arms, literally hugging it to himself.
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More Uses for
Cast Iron in China
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Woks
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Diameter 65 cm, thickness 3.5 mm
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Here is a wok – that is the usual English
word, which comes from the Cantonese pronunciation of 镬,
which
in
Mandarin
is
pronounced huo.
It is broken, so it was possible to measure the thickness at its
thinnest part.
How they were cast in the 19th century can be seen here. They are still being cast
today, probably by different methods.
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Here are a few
illustrations related to woks.
It is interesting to note that cast-iron woks are now being imported to
Europe in large numbers and used as portable fireplaces.
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Woks were expensive,
and
they could easily break, as can be seen in a photograph further
above.
This is an itinerant tinker, who mends broken woks using molten cast
iron. There is more on how the tinkers worked here. |
Gouache by an anonymous Chinese
artist in Guangzhou, mid-19th century. (The Royal Library, Copenhagen,
Denmark)
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Monumental
Castings
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Photo by Tang
Huancheng 唐寰澂
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Another major use for cast
iron was for statues and other large monuments. These are the famous
bridge anchors at Pujin 蒲津, Shanxi, cast in AD 724.
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Iron Rhinoceros (铁犀), cast AD 1446, village of Tieniu 铁牛, near Kaifeng
开封, Henan
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In this case we have a
very interesting phenomenon. The photograph shows what looks like a
repair to the statue,
but in fact it represents a repair to the mould.
Something happened to the mould – it broke and had to be repaired. The
bumps which can be seen are probably the heads of spikes that were used
to hold the repair in place.
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Nanjing Museum (南京博物院), 1987
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Here is again an example of
a combination of materials – this anchor is made of several pieces of
wrought iron, joined together by casting iron at the join instead of by
forge welding, which was also practised in China.
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And here is the famous
Iron Lion of Cangzhou, which also uses a combination of materials.
The lotus seat, and a bronze Buddha which originally sat on the seat,
were too heavy to be supported by cast iron alone, so wrought-iron
supports were incorporated into the casting. Part of the wrought-iron
reinforcement can be seen extending from the centre of the rightmost
photograph to the lower right corner.
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Thomas T. Read, Mining and Metallurgy, August 1937,
p. 383. Photographed ca. 1910.
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Inside the statue,
photographed from below, 1987. Note modern repairs.
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There
is more on Chinese monumental iron castings in my article in Journal
of
East
Asian
Archaeology,
2000,
2.3/4,
pp. 199-224, which is available for purchase from Ingenta.
The
illustrations
of this article can
be seen for free here, and a
Chinese translation can be downloaded here.
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Cannons
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Cast Iron Cannon,
Ming Dynasty (1368–1644). Military Museum, Beijing (中国人民革命军事博物馆),
2006.
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The ancient founders’
experience in casting statues and such came in very handy when they
began casting iron cannons.
The first iron cannons in China come at the very beginning of the Ming
Dynasty, in the 1370’s.
The cannon above is probably a combination of materials – wrought iron
and cast
iron – as we can see at the right: two cannons with wrought iron inside
and cast iron outside. George Banks sketched these at the Dagu Forts,
in modern Tianjin, in 1860. He believed that these were ‘evidently very
old’, from the +17th century or before; while this dating is quite
plausible it is not clear what evidence he could have had for it. He
does not mention any inscriptions.
He described the guns as follows:
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1 had a piece broken from the muzzle, which enabled me to see how it
was made. The inner part or bore was made of longitudinal bars, one
inch wide and half an inch thick [2.5, 1.25 cm], welded together, and
forming a lip where they terminated at the muzzle. Round these, and
binding them together, were rings, one inch thick and three inches wide
[2.5, 4.5 cm], also welded. Outside these, again, is a layer of cast
iron, two inches and three-quarters [7 cm] thick at the muzzle, and of
course much thicker at the breech, giving shape to the gun. The faint
lines on the surface are caused by the crevices between the bricks of
which the mould was built in which the casting took place. This piece
is 9 feet 6 inches long, 23 inches diameter at the breech, and 15
inches diameter at the mouth [291, 60, 39 cm]. No. 2 is a similar gun,
but with only rings welded together and encased in cast iron. It is
very singular that both these guns should be broken in the same way. It
is 9 feet 7 inches long, 2 feet 1 inch diameter at the breech, and 16
inches at the mouth [respectively 292, 64, and 41 cm]’ |
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George Banks, Illustrated London News, 6 April
1861, p. 325
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Cast Iron in Europe
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The earliest
iron casting in Europe
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This is part of a
description of iron casting, in a German manuscript dated 1454.
15th-century German is not easy to read, but it says roughly that if
you want to cast iron, you should use the same sort of furnace as is
used to cast bronze bells, and add certain materials which seem to have
the effect of adding phosphorus to the iron, thereby lowering its
melting point.
This would seem to be the earliest technical description of iron
casting in the world.
The original manuscript was in the Zeughausmuseum, Berlin. It was one
of a large number carried off as war booty by Soviet troops at the end
of World War II, and is now presumably lost.
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David E. Potter, The Bells and Bellringers of York Minster,
1987
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To show the type of
furnace which the German manuscript refers to, here is a picture of
bell-founding from around that time, the famous Bellfounders’ Window in
York Cathedral, England.
With a little imagination one can see the cupola furnace, the bellows,
and molten bronze flowing out.
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This boundary post in
the Märkisches
Museum, Berlin, has been considered to be the earliest extant iron
casting in Europe, dated by Otto Johannsen to 1345-1364 and by some
as early as 1251. It turns out to be somewhat later, probably 15th
century. This is still very early, and the artefact remains
interesting, but it is no longer the earliest in Europe.
It can be seen that a large amount of phosphorus has brought down the
melting point considerably, just as the 15th-century manuscript
suggests.
The boundary post is mentioned in a document dated 1364, a renewal of a
boundary settlement of 1251. The document has been published in Codex diplomaticus Brandenburgensis,
Reihe
A,
Bd.
24, 1876, pp. 71-76, and also, together with a German
translation of 1613, in Johann
Carl
Conrad
Oelrichs
Beyträge zur Brandenburgischen Geschichte, 1761,
pp.
57-75. The editor of the Codex,
A.
Fr.
Riedel,
states that the document is clearly not genuine, but
that it is an important source for the boundary controversies of the
15th century. This information was kindly supplied to me by Dr Uwe
Winkler of the Stiftung
Stadtmuseum Berlin.
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Analysis
of
the
boundary
post
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Carbon
Silicon
Phosphorus
Melting point
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2.31%
0.43%
6.20%
ca.
950°C
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(Otto Niezoldi, Die Giesserei, 1942, 29.8, pp.
136–137)
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From
the museum's inventory journal:
566. 15.8.1876.
Grenz-Pfahl von Eisen, 84 cm hoch, wovon die zugespitzte Länge von 54
cm in der Erde gewesen, während die übrige Länge von 30 cm, welche
4eckig und 14 cm im Quadrat stark ist, außerhalb der Erde (frei) stand;
die horizontale Fläche zeigt ein vertieftes Ordenskreuz. Geschenk des
Gutsbesitzers Landsky in Tempel bei Schermeisel. Solche Pfähle wurden
im Jahre 1251 in grösserer Anzahl zur Bezeichnung der Grenze zwischen
Polen und der Neumark gesetzt. cfr. Wedekind, Geschichte
der
Neumark,
Anhang fol. IX. X, sowie Acta
M II.
1508 C.B. 1876. Diese Pfahl wurde gefunden an der alten Straße von
Schwiebus nach Landsberg, auf der Grenze des Märkischen Dorfes Tempel
und des Posen'schen Dorfes Neudorf. Er ist wahrscheinlich identisch mit
dem von Wedekind, pag IX unten, bezeichneten.
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Cast iron objects were
always very simple until Europeans began casting cannons and other
artillery.
This German bombard, AD ca. 1400, seems to be the earliest extant
cast-iron gun in Europe.
It seems that in the course of the 14th century European iron-founding
quickly became quite advanced.
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Otto Johannsen, Geschichte des Eisens, 1953, p. 203
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Joseph Needham, Science and civilisation in China,
vol. 5, part 7, p. 286, from Tushu
jicheng 圖書集成.
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In China, on the other
hand, cannon-casting had a long background in
the casting of other large objects, and I am sure that the European
development owed a great deal to China.
Here is a Chinese bombard of much the same type around the same time.
The drawing is from the 18th century, but it is a copy of a copy of a
copy, and Joseph Needham believes the original would have been from the
14th or 15th century.
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Later developments
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Later cast iron was
used for all manner of useful products, like this stove in Denmark, ca.
1900 . . .
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Ebbe Johannsen, Danske Antikviteter
af Støbejern, 1982, p. 59
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Ebbe Johannsen, Danske Antikviteter af
Støbejern, 1982, p. 59
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. . . and
these kitchen
pots, flatirons, etc. (Denmark, ca. 1850).
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Cast-iron manhole
covers are found in every modern city in the world. These are in
Beijng. The one on the left is dated 1953; the one on the right may be
considerably older, since the text reads right-to-left rather than
left-to-right.
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Here is a flatiron
(England, 19th century?), which is interesting because it is one of the
very few examples I have seen in Europe of the use of a combination of
cast iron and wrought iron. The bottom part is cast iron, the handle
wrought iron.
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Anchor Chain, Wrought Iron with Cast Iron Reinforcement, Bristol, 19th
Century?
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And here is one more
example. These chain links are of wrought iron or steel, but the
reinforcements
are of cast iron.
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Cast iron became very
important in the Industrial Revolution of the 19th century. This
enormous steam engine (England, 1840) could not have been built without
cast iron. In particular, the tilt arm is 11 metres long, and at this
time could not possibly have been made by smithy methods.
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Williams’ Perry
Foundry Co., Catalogue, ca. 1875, p. 7
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Malleable
Cast Iron in Europe
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Prince Rupert
(Rupprecht von der Pfalz), 1619–1682
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I have talked about
malleable cast iron in acient China – the first malleable cast iron in
Europe seems to come more than 2000 years later, in the 17th or 18th
century.
Prince Rupert doesn’t look like a scientist or a soldier, but he was in
fact both.
He patented a process for softening cast iron, but the patent was kept
secret, and it now seems to be lost, so we do not know what the process
was. But undoubtedly it was a kind of malleablizing annealing.
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René Antoine
Ferchault de Réaumur [雷歐姆], 1683–1757
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Réaumur was the
first person to make a scientific study of cast iron.
His
book was published in 1722. It has been translated into English,
and it would be a good idea to translate it into Chinese. It is still
useful as an introduction to the early European techniques of iron
casting.
He describes very clearly the process of making malleable cast iron,
and says he learned of it from artisans in Paris.
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Here are the sorts of
things he wanted to make of malleable cast iron – mostly decorative
objects which would otherwise have been made by a
smith.
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Illustrated catalogue of malleable iron
castings made by the California Iron and Steel Company, 1884.
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Later we find other
objects of malleable cast iron which earlier had been made by smiths
and now are being made cheaper of malleable cast iron.
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Today malleable cast
iron is used for a wide variety of fittings of various complex shapes
which need to be cast but don’t need the strength of steel.
These happen to be made in China, but I could have shown similar
products from almost anywhere in the world.
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Shijiazhuang Jizhong
Malleable Iron Co., Ltd.
(石
家莊冀中瑪鋼有限公司)
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The Fate of the
Ancient Chinese Techniques
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w
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European
Knowledge
of
Chinese Malleable Cast Iron
Emanuel
Swedenborg,
De Ferro, 1734, p. 194:
There is also a
tradition that the Chinese and the Japanese know an art of bringing
[cast] iron to a high degree of softness, so that it can receive
impressions of figures as easily as lead can; . . .
(translated
from the Swedish translation of H. Sjögren, 1923)
John
Barrow,
Travels in China, 1804, p. 299:
Their cast-iron wares
appear light and neat, and are annealed in heated ovens, to take off
somewhat of their brittleness.
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It is very curious that
the latest known Chinese artefacts of malleable cast iron seem to be
from the 9th century AD, but European travellers as late as the 18th
century told of Chinese and Japanese malleable cast iron.
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And here are some
19th-century Japanese vessels of cast iron which have been
surface-decarburized so that they can be engraved.
J. J. Rein also gave a very interesting description
of
the
Japanese
process.
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Literature
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There is more to read in some of
my publications:
Toward the reconstruction of ancient
Chinese techniques for the production of malleable cast iron
(East Asian Institute occasional papers, 4). Copenhagen: East Asian
Institute, University of Copenhagen, 1989.
Iron and steel in ancient China
(Handbuch der Orientalistik, IV:9). Leiden: E. J. Brill, 1993.
Science and civilisation in
China. Vol. 5: Chemistry and
chemical technology. Part 11: Ferrous
metallurgy.
Cambridge University Press, 2008. 512 pp.
“The
cast iron lion of Ts'ang-chou”, Needham
Research
Institute
newsletter, no. 10, June 1991, pp. 2-3.
“Chinese
monumental
iron
castings”, Journal
of East Asian archaeology, 2000, 2.3/4: 199-224. Colour
Illustrations.
“Zhongguo
gudai
de
daxing
zhutieqi”, tr. by Li Yuan, in Wenwu
keji
yanjiu (“Scientific and
technical research on cultural heritage”), vol. 5, Beijing: Kexue
Chubanshe, 2007, pp. 68-82 + colour plates 10-15. Tr. of “Chinese
monumental iron castings”, above.
“The
casting of iron woks in Guangdong, China, in 1840”. Poster
presentation, Founders, smiths and platers: International Conference on
metal forming and finishing from the earliest times, Oxford, 20-24
September, 1999.
Chinese
tinkers.
“Støbejerns
metallurgi
og
lidt
om
kinesisk støbejern”, 53 pp. in Jern:
Fremstilling, nedbrydning og bevaring. Fortryk af forelæsninger
til
Nordisk Videreuddannelse af Konservatorer, København, 17-28
august
1987. København: Nationalmuseet, Bevaringssektionen, 1987. (“The
metallurgy of cast iron, with some notes on Chinese cast iron”, two
lectures for museum conservators). The Google
translation is not really a translation at all, but can
nevertheless be quite useful.
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