Conservation of Iron ObjectsAuthor(s): R. M. OrganSource: Historical Archaeology, Vol. 1 (1967), pp. 52-54Published by: Society for Historical ArchaeologyStable URL: http://www.jstor.org/stable/25615075 .

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52 HISTORICAL ARCHAEOLOGY

CONSERVATION OF IRON OBJECTS

By R. M. Organ

Let us consider some of the reasons which lie behind the choice made by a conservator of a specific method of treatment when he is faced by a particular iron object. We shall consider - four reasons, beginning with its

shape. When an archeologist brings a piece of

rusted and flaking iron to me and asks me to conserve it, I am correspondingly discour

aged. I always ask him: "What do you mean

by 'conserve'?what particular feature makes

it valuable to you?" The lower object in the

figure is a piece of iron which is supposed to be late Iron Age in date. When I ask the

archeologist its value, he replies, "You can see that it is a Merovingian scabbard, or a

plate off a Roman ballista," or whatever it

may be. However, often I cannot see, not be

ing an archeologist, but once I have realized what is wanted I know that my task as a con servator is to preserve the shape. The upper part of the figure represents the same object after conservation, still having much the same

appearance. In view of the slightness of the change in

appearance caused by treatment, you may won

der why we cannot take the piece of rusted

iron, merely put it on a self in a museum and

thereby have it conserved for all time. But, as

those of you who are practical archeologists know full well, one cannot do this. If one

takes a piece of iron fresh from the ground where it may have been wet or at least moist, and saturated with the salts present in the

soil, and then set it on the shelf in a museum, after perhaps 6 months of storage it will crack and flake and eventually one has left only a

pile of fragments. You also know that if you excavate a cannon ball from under the sea,

even by the time you have it out of the sea

and on to shore it may be beginning to be come hot, later to crack and probably fall to

pieces. These things happen because of cor

rosion processes which have been taking place slowly inside the buried iron and which change their nature and develop much more rapidly

when the iron is brought into contact with the

oxygen and fluctuating relative humidity of

the atmosphere. It is often the task of the conservator to choose some method of treat

ment which will stabilize the shape of the

piece of rusty iron for the benefit of future

archeologists. Now let us say in this particular instance

the archeologist recognized the shape and knew

immediately where the value of this piece lay. In another instance he may not have been able to do so, for example, if rust had devel

oped over the entire surface of the original object or had formed beneath it and raised and distorted it, then little evidence of the

original shape may remain. Consider an ex

ample of a European purse-mount, much cor

roded and cracked, which can barely be

recognized but which may contain valuable information. Now there is a practice in

Europe, where it is perhaps more important than here, of making radiographs of almost all corroded iron objects that come into the

laboratory for conservation. A radiograph shows clearly this object has been decorated with a very elaborate inlay of a different

metal, perhaps silver or copper. This infor mation would have been lost if the object had not been radiographed; but because the con servator takes this precaution, he is then in a position to select a method of treatment which will best restore the original shape and

beauty of the object. Now consider iron that has not yet been

changed. In the centre we see a thick crust of oxide or other corrosion products that have

replaced and obliterated the original surface of the iron object. However, in our example the inlay that is present can serve as a marker for the original surface and we can grind away the oxide toward its level. When the greater part has been ground off we can pick away

carefully the remaining thin cover. In this

way we can restore the hitherto hidden and

decorated surface of the very much corroded

object. The oxide crust is not always so very thick.

In burials of more recent date the oxide layer may be thin but still obscuring. Where the

oxide is very thin, the conservator can choose to dissolve it away by chemical methods. A

reagent consisting of a 2 per cent aqueous solution of di-ammonium citrate can be em

ployed to dissolve iron oxide with scarcely any

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CONSERVATION OF IRON OBJECTS 53

effect on the iron itself. Alternatively, a thin

layer may be removed mechanically. In the examples of a 19th century (Asian)

Indian iron box with silver decoration the reason for treatment was the restoration of

something which has been lost to view as a result of corrosion processes. Sometimes the

composition of the iron may prove to be at least equally important. For example, it may happen that your find could either have been

imported from Europe or have been made on the site. If you wish to decide between these

possibilities then you may have to bring in a scientist who will wish to analyze the iron and to determine whether his analysis corresponds

with European material or with local mate rial.

Now consider how the analyst takes a sam

ple. He normally does this by drilling through the oxide crust, discarding the oxide, which is not necessarily representative of the iron beneath and therefore of no value to him in the analysis, and preserving the drillings from the uncorroded iron which lies underneath.

The analyst is interested in a great many ele ments: first of all carbon, then silicon, man

ganese, sulphur, phosphorus, arsenic and so on. I want to draw your attention particularly to the phosphorus. Suppose the quantity in volved is quite small, less than 1 per cent. Now the presence of phosphorus in a recov ered iron artifact need not necessarily result from the presence of phosphorus in the metal: it has been found as a phosphate corrosion

product resulting from the reaction of iron with phosphates present in the ground in the

vicinity of decaying bones. Phosphates may also be introduced by the conservator who

incautiously employs a commercial de-rusting fluid containing phosphates. In this event a film rich in phosphates will remain upon the surface of the cleaned artifact. See Fig. 8.

Sampling of the object will then result in an inaccurate analysis unless the conservator has

kept careful records and used them to warn the analyst of the situation. Failure to do this

may lead the archeologist to make a false attribution.

Another valuable feature of the material in the object might be not its analysis but in formation about the method of fabrication. For example, was it blacksmithed? Was it cast? The next figures indicate some differ

ences revealed by metallographic methods. Consider a cross-section through an iron ob

ject. In the centre area can be seen brightly gleaming metal that has been polished by the metallographer and etched in order to reveal its structure. This area is bordered above by a thick crust of dark-coloured oxide and below by an area where the dark oxide has penetrated into the bright metal. Now the structure of that metal is clear to the metal

lographer as substantially plain iron: it has not the expected appearance of the black smithed Roman iron that it was supposed to be.

In contrast, we may see the structure pres ent in a piece of steel. This is a cross-section

through an (Asian) Indian sword, a beautiful watered blade. In the section we see the cause of the so-called watering of the surface. It originates in the lined-up globules of iron carbide which are so beautifully distributed in the metal as the result of an advanced process of manufacture.

Now you might believe that whatever a conservator does to an iron artifact he cannot

destroy these internal structures which give us so much information. Such a belief would be erroneous: many of these structures can

be altered by the application of heat. Yet there are conservators who, in order to remove

rust, will take a blow-torch to it. And there is another process, not uncommon in Europe, aimed at stabilizing the rust coating by volatilization of iron chlorides formed inside the rust layer during burial. This process con sists of heating the artifact in hydrogen gas in an oven at an elevated temperature. Both

of these heat-treatments can alter the struc

ture of the metal and obviously should not be used by conservators if the object is likely to be examined by a metallurgist sometime in the future.

The fourth reason for choosing a specific treatment is that some evidence of its origin

may remain on the surface of an ob

ject in the form of decoration or a

trade mark. For example, one may have a trade axe of either French or En

glish origin. But if its origin cannot be deter mined merely from the shape of the axe, there

might yet remain an indication on its surface if only it could be seen beneath the rust. In this case the task of the conservator is one of

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54 HISTORICAL ARCHAEOLOGY

simple cleaning for which he may select one of the chemical or mechanical methods already described. As an interesting example for which slides happen to be available the screen shows not a trade axe from N. America, but an ob

ject that was brought to us in London, Eng land, by a factory manager engaged in

crushing asphalt which was being imported from the south of France. His crushing mill

stopped unexpectedly and this object was found to be the cause of the stoppage.

Wherever its surface was not obscured by asphalt the colour of bronze was visible. He

brought it in and asked whether it was in fact a bronze celt because if so it must be the earliest ever found, relating as it did to a

geological stratum containing asphalt. A su

perficial inspection revealed that the metal consisted not of bronze but of either iron or

steel that had been coloured yellow by contact with sulphur-bearing asphalt. Next, part of the surface was cleaned of asphaltic deposits in order to reveal any marks that might have been present. A mark actually found is shown on the screen and proves conclusively that the so-called celt was made in Birmingham, Eng land, in modern times. Clearly, this particu lar task of cleaning yielded worthwhile results.

In the above instance the trademark has been preserved in a metallic surface. More

commonly such a work will be found to have

partially corroded away or to have been re

placed entirely by oxide. In this event an

X-radiograph may reveal it if the iron hap pened to be thin. Occasionally it may be found that the trademark on the surface of the

object has been replaced and preserved as a mass of black iron oxide overlaid by the softer red oxide. In this event it is sometimes pos sible to clean away the overlying incrustation

by electrolytic reduction, without loss of the detail preserved in black oxide. For example, take a group of trade files recovered from the Winnipeg river, some of them still bun dled in a cloth wrapping, but all rusted and their teeth obscured. After treatment the

cutting teeth that survive are now all ex

posed, some of them no longer made of iron but of iron oxide.

In conclusion we may summarize this dis cussion of the several features that influence a conservator in selecting a suitable treatment as follows. Firstly, discussion with the ar

chaeologist in order to determine the prin cipal feature of interest. Secondly, selection of a treatment that will emphasize this par ticular feature without lessening the value of the object to scholars from other disciplines.

This last consideration usually results in se lection of a treatment in the cold that will not

alter the structure of any metallic iron that

remains and the use of chemical reagents that

do not add to the treated object zinc, phos phates,

or other substances that are poten

tially confusing to a future analyst.

FINANCING EXCAVATIONS

Financing of Archaeological Work at Historic Sites on Federal Lands

Paul J. F. Schumacher

The Federal Antiquities Act of June 8,

1906, makes it a Federal offense to appropri ate, excavate, injure or destroy any historic or

prehistoric ruin or monument or object of

antiquity on Federal lands. This very impor tant legislation authorizes the President of the

United States to establish National Monu ments on Federal lands by proclamation, and

places the regulating of this Act in the hands

of the Secretaries of the Interior, Agriculture and Army. The Army has since turned juris diction of its lands for Antiquities Act pur

poses over to the Secretary of the Interior.

The National Park Service, a bureau of the

Department of the Interior and under the di

rection of the Secretary of the Interior, is the

Federal agency responsible for preserving pre historic and historic American sites. Under

the terms of the Historic Sites Act of August 21, 1935, the Park Service is specifically charged with the responsibility for the evalua

tion and preservation of the historic sites and

buildings in the United States and its posses sions. Included under the Act are archaeo

logical sites as well as the purely historic ones.

The over-all Park Service policy is to pre

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