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?
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.
Some Ancient Cast
Iron Artefacts
White
Cast Iron
Etched with Nital, scale bar 100 µm.
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.
2nd–1st
cent. BC
AD
1102–1106
AD
1851–1861
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.
AD
1208–1224, X-ray
AD
1211, Etched with Nital, scale bar
50 µm
(Mike Wayman & Helen Wang,
Historical Metallurgy, 2003, 37.1,
pp. 13, 14)
This is a
cast-iron mirror – probably Han-dynasty (206
BC – AD 220), probably white cast iron.
Diameter 13
cm.
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.
Malleable
Cast
Iron
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.
Image
taken from ebay.com
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:
Mawangdui,
2nd century BC
Ancient
well, 3rd century BC
Han dynasty, Sichuan
Drawing by
William Alexander, 1793
From a grave
in Changsha, Hunan, 3rd–4th century BC
Etched, scale bar 250 µm
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).
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.
Tonglüshan 铜
绿山 copper-mine site, 4th–3rd
century BC
Etched, scale
bar 50 µm
(Hua Jueming 华觉明, 自然
科学史研究, 1982, 1.1, pl. 1)
Scissors,
1st–2nd century AD
Etched, scale bar 40 µm (Hua Jueming 华
觉明, 自 然科学史研究,
1982, 1.1, pl. 1)
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.
This sword from Gansu appears to be made by
casting it and then decarburizing to a uniform
0.6% carbon.
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
Cast iron mould for casting an iron
mattock-head, 3rd century BC
Etched with
Nital, scale bar 200µm 2.31% C 0.21% Si (文物, 1976.8,
p. 54)
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. 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.
A
Cast Iron Wall
(满城汉墓, 1980, vol. 1, pp.
216, 218, 220; vol. 2, pl. 154, 155.1)
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.
How Iron is Cast – the
Cupola Furnace
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)
Below: Cupola furnaces from the
19th and early 20th centuries.
Courtesy of Martyn Gregory Gallery, London
Courtesy of British Museum, Oriental and
India Office Collections (see here)
Hommel, China
at work, 1937, p. 27
A
50-second film-clip by Yang Ruidong 杨
瑞栋 of the casting of iron in Huize
会泽, Yunnan.
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.
(考古学报,
1978.1: 12)
(Li Jinghua 李京华, 中原古
代冶金技 术研究, 1992, p.
147)
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
. . .
(René Antoine Ferchault de Réaumur, [雷
歐姆], L’Art de Convertir le Fer
. . ., 1722)
. . . 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).
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)
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.
Here is a poor photograph of this type of furnace in Laos in 1900.
Some Tai
Neua people [tribesmen from Northern Thailand] have settled at
the entrance to the village to make plowshares.
They place old cooking pots on a small charcoal stove. Bellows similar
to those we saw so often in China blow in air.
While the iron is being melted, a Tai Neua man prepares the mold of hardened clay
and smears it with soot mixed with water. This layer will prevent the content
from sticking. In a few minutes, everything is ready. The molten metal
is poured into the mold, and the perfectly made plowshare is sold for six
salung, or about 60 cents, to the potential client who oversaw the operation.
Alfred Raquez, Laotian pages: A classic account of travel in Upper, Middle and Lower Laos, Copenhagen 2019, p. 140. Translated from Pages laotiennes, Hanoi 1902, p. 133.
More
Uses for Cast Iron in China
Woks
Diameter 65 cm, thickness 3.5 mm
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.
It is interesting to note that cast-iron
woks are now being imported to Europe in
large numbers and used as portable
fireplaces.
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)
Monumental
Castings
Photo by Tang
Huancheng 唐寰澂
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.
And another
quite famous cast iron statue, one of four at
the Zhongyue Temple in Dengfeng, cast in AD
1024.
By studying the mould-seams on statues like
this one and the next we can learn a lot about
how these statues were cast.
Zhongyue Temple, Dengfeng, Henan (登
封中岳庙), 1987
Iron Rhinoceros (铁犀), cast AD 1446,
village of Tieniu 铁牛, near Kaifeng 开封,
Henan
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.
I think it
is only in China that bells have
been made of cast iron.
This broken bell is interesting because we can
see in the fracture that something has gone
wrong. Those air bubbles at the surface would
(I believe) have had a bad effect on the tone
of the bell.
Probably the founders added sulphur to the
iron to assure that it would solidify as white
cast iron, but added too much, so that the
bubbles formed.
Shaolin Monastery, Dengfeng, Henan (登
封少林寺), 1987
Nanjing Museum (南京博物院),
1987
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.
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.
Thomas T.
Read, Mining
and Metallurgy, August 1937, p.
383. Photographed ca. 1910.
Inside the
statue, photographed from below, 1987.
Note modern repairs.
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.
Cannons
Cast
Iron Cannon, Ming Dynasty (1368–1644). Military
Museum, Beijing (中国人民革命军事博物馆),
2006.
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:
‘No. 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]’
George
Banks, Illustrated
London News, 6 April 1861, p. 325
Cast Iron in Europe
The
earliest iron casting in Europe
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.
David E.
Potter, The
Bells and Bellringers of York Minster,
1987
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.
Cast Iron Boundary Post,
Trzemeszno
Lubuskie, Poland, said to be dated
AD 1345–1364
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.
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.
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.
Otto
Johannsen, Geschichte des Eisens,
1953, p. 203
Joseph
Needham, Science and civilisation in
China, vol. 5, part 7, p. 286,
from Tushu
jicheng 圖書集成.
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.
Later developments
Later cast
iron was used for all manner of useful
products, like this stove in Denmark, ca. 1900
. . .
Ebbe Johannsen, Danske Antikviteter af
Støbejern, 1982, p. 59
Ebbe
Johannsen, Danske Antikviteter af
Støbejern, 1982, p. 59
. . .
and these kitchen pots, flatirons, etc.
(Denmark, ca. 1850).
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.
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.
Anchor Chain, Wrought Iron with Cast Iron
Reinforcement, Bristol, 19th Century?
And here is
one more example. These chain links are of
wrought iron or steel, but the reinforcements
are of cast iron.
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.
Williams’
Perry Foundry Co., Catalogue, ca. 1875, p.
7
Malleable
Cast
Iron in Europe
Prince Rupert
(Rupprecht von der Pfalz), 1619–1682
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.
René Antoine
Ferchault de Réaumur [雷歐姆],
1683–1757
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.
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.
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)
Their cast-iron
wares appear light and neat, and are
annealed in heated ovens, to take off
somewhat of their brittleness.
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.
And here are
some 19th-century Japanese vessels of cast
iron which have been surface-decarburized so
that they can be engraved.
“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.
“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.
