Guidelines on the X-radiography of archaeological metalwork

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Guidelines on the X-radiography ofarchaeological metalwork

2006

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Contents

1 Introduction . . . . . . . . . . . . . . . 3

2 Why X-radiography is necessary . . . . . . . . . . . . . . . . 3

3 When to X-ray . . . . . . . . . . . . . 4

4 What to X-ray . . . . . . . . . . . . . . 5

5 What X-radiography can show . . . . . . . . . . . . . . . . . . 5

6 How to make informative X-radiographs . . . . . . . . . . . . .10

7 How to view the X-radiographs . . . . . . . . . . . . . 13

8 How much should it cost? . . . 13

9 Where to get help . . . . . . . . . . 14

Preface

Archaeological investigations frequentlyyield numerous metal finds. These shouldbe X-rayed as part of the post-excavationprocedures to assist in the identificationand interpretation of the finds, andthereby help understand the site. Thisprocedure will also provide a record of thefinds in the conditions in which they wererecovered.

Government policy on planning issues inarchaeology is stated in Planning PolicyGuidance Notes PPG 15 (Department ofthe Environment 1994) and PPG 16(Department of the Environment 1990).These documents provide guidance tolocal authorities and others who arerequired to make planning decisions andto prepare development plans. Localauthority planning archaeologists arerequired to advise on archaeologicalaspects of the planning decisions andbriefs, of which the X-radiography ofarchaeological metalwork forms a part.

These guidelines on the X-radiography ofarchaeological metalwork advise on goodpractice, including when to schedule thework and when to cost for it. They will beuseful to local authority planningarchaeologists when providing advice orbriefs, to field project directors writingspecifications, and to managers over-seeing excavation or post-excavationprojects. The guidelines will also be usefulto anyone directly involved with finds

work, whether recording, conserving,researching or curating finds fromevaluations, excavations, museumcollections or the Portable AntiquitiesScheme.

Within the next few years there will beadvances in digital imaging and this willaffect our expectations and output of anyX-radiography programme. Althoughthese guidelines give advice on goodpractice in the production of an ‘X-rayarchive’ using conventional film X-radiography, they should not restrictthe development of additional or differentsystems of archiving in the future.

These guidelines concentrate on the X-radiography of archaeologicalmetalwork, which is one of the principalmaterial categories to benefit from its use,providing a record of the material andassisting in a range of investigations andclassification. Other archaeologicalmaterials are commonly X-rayed for avariety of reasons, such as the study ofbone pathologies and analysis of soilsediments, but it is beyond the scope ofthis brief document to discuss these otherinvestigations in detail, although referenceto them is made where appropriate.Equally, other more specialised methodsof radiography, using different techniques(such as micro-focus or stereo) ordifferent ionizing radiations (such asgamma rays) are not covered here and soall references to radiography relate only tothe use of X-rays.

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information that cannot begained by any other method.Technological details can berevealed without the need forinterventions. In addition, the X-radiograph itself provides a long-term visual record ofinherently unstable andpotentially deterioratingartefacts. While much can bedone to slow down thedeterioration processes ofmetal artefacts following theirremoval from burial,X-radiography should beinitiated as soon afterexcavation as is practicable.A good quality X-radiographmay provide the informationnecessary to identify, classify,date and illustrate an object that hassubsequently disintegrated beyondreconstruction.

X-radiographs of metalwork are anessential component of the site archive(English Heritage 1991, 30, A3.1.1) and, where necessary, the researcharchive (English Heritage 1991, 37,A6.1.1) and are a requirement of the deposition of those archives. Theguidance now in place from PPG15,PPG16 and MAP2, makes it clear that

an X-radiographic archive is an integralpart of the transfer of an assemblage ofmetalwork when the project archive isfinally deposited.

The benefits of X-radiography may besummarised as follows:

● visual record of shape, technology and condition

● aid to identification● non-interventive● non-destructive● cost-effective● long-term record of deteriorating

objects

1. Introduction

AimsThese guidelines providerecommendations on the minimumrequirements for the X-radiographicscreening of metalwork fromarchaeological projects. They complementand expand the advice for best practiceoutlined in Management of ArchaeologicalProjects (English Heritage 1991), hereafterreferred to as ‘MAP2’. The guidelinesoffer advice on what to X-ray, when X-radiography should be undertaken, thestandard of X-radiograph necessary, andhow best this can be achieved. They donot provide practical instructions on X-radiography or describe the basicprinciples involved. These topics arecovered elsewhere (eg Lang andMiddleton 2005). They will be of use tothose who commission, manage ormonitor post-excavation projects involving the recording and analysis ofmetal finds, and to those who produceand use radiographs in the course of such work.

Why the guidelines came aboutThe need for guidelines on the X-radiography of archaeologicalmetalwork was recognised at a meeting atthe Museum of London in February 2003(‘All may be revealed – X-radiography and archaeological artefacts’) organisedjointly by the Archaeology Group of theInstitute of Conservation, the FindsResearch Group AD 700–1700, and the Roman Finds Group. The meeting was held to stress to those commissioning,managing and undertaking archaeologicalprojects the necessity for high-quality X-radiography to enable the satisfactoryassessment, recording, analysis andconservation of archaeological material.At the conclusion of the meeting it wasagreed that guidance on the basicstandards was required.

2.Why X-radiography is necessary

X-radiography is an invaluableinvestigative technique that is non-destructive, quick and cost effective.It enables the form and structure of anobject obscured beneath corrosion layersand burial accretions to be viewed without any physical intervention to thatobject (eg Figs 1 and 2). In somecircumstances, such as when an ironobject is heavily or even completelymineralised, an X-radiograph can provide

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Fig 1 Coins can sometimes be dated from their X-radiographs.The two Romancoins, shown as excavated and their X-radiographs, are (upper) a sestertius ofNero dated AD 54–68 and (lower) a dupondius of Domitian dated AD 95–6.

Fig 2 Good quality X-radiography reveals features of the four keys illustrated (2a–d) that are obscured by corrosion and notvisible even under magnification.Three of the four (2a, 2b, 2d) are plated, two (2c, 2d) have decorative mouldings at the neck,one (2c) being finely grooved.The nature of the key bit is clarified on three of the examples. 2a has multiple clefts, 2d hasopposing channelled clefts. 2b is a door key as, having a symmetrical bit, it could be used from either side of the lock. It has asolid stem projecting beyond the end of the bit and a collar above.The stem of 2c is hollow and the piped stem fitted over alocating pin in the lock. Length of keys: 2a, 65mm; 2b, 175mm; 2c, 98mm; 2d, 58mm.

a

b

cd

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of Field Archaeologists 2001). By thisstage in the project, the core teammembers will have contributed to theproject design and the need for X-radiography should have beenrecognised and an appropriateorganisation contacted.

The initial X-radiography should beimplemented at the ffiieellddwwoorrkk pphhaasseeof a project so that the site archive,the product of the fieldwork, can becompleted. Decisions should be madebetween the appropriate core teammembers and any other relevantspecialists about what components of the finds assemblage will be X-rayed(see Section 4).

A basic record of a metal object – asspecified in Roman Finds Group and FindsResearch Group AD 700–1700 (1993, 3) –cannot be undertaken without consultinga good quality X-radiograph. Whensubmitting metalwork for radiography,data about the objects to be X-rayedshould be supplied with them, preferablyin an electronic form so that theradiograph numbers can be addeddigitally. The data should include the sitename and the site identifying number (if allocated), context, unique identifyingobject number (if allocated), material typewhere known, box number (if allocated),and any relevant associations or otherinformation (available at that time).

The metalwork component of the archivecannot be aasssseesssseedd ffoorr ppootteennttiiaall ffoorraannaallyyssiiss without reference to X-radiographs. Provision of high qualityX-radiographs will provide sufficientinformation for the majority of theassemblage to be recorded and studied asnecessary for archive and analysis. TheseX-radiographs will normally includemultiple exposures of many types ofartefacts in order to show the fullvariation in morphology (see Section 6).

Additional X-radiography of selecteditems may be required during theaannaallyyssiiss pphhaassee of the project, forexample to clarify certain features or toinvestigate particular aspects of anassemblage. This requirement will usuallybe identified during assessment and willbe costed with the analysis phase.Recommendations for further examinationand analysis should be made followingconsultation between the finds specialists,conservator, excavator and any otherrelevant contributors.

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3.When to X-ray

It is important to X-ray metal objects assoon as possible following anyarchaeological investigation in order toprovide an archival record of the items andtheir present condition as, under certaincircumstances, deterioration may quicklyset in. The early identification and dating

of the finds from a range of interventions,including evaluations, may contribute tothe interpretation of the site and thusinform subsequent action. (Table 1)

Financial provision for X-radiographyshould be made at the ppllaannnniinngg ssttaaggee ofa project, when the costed project design iscompiled (English Heritage 1991; Institute

Table 1 X-Radiography: Guidance for Project Planning

MAP2 phase Actions and outcomes

1 Project Planning • Project Manager and Contractor undertaking X-radiography identify the likely requirements (this will depend on factors such as site type, size of excavation, specific needs of receiving organisation, etc). In order to inform project budgeting,establish factors such as likely volume of material for X-radiography, possibility of large items such as soil blocks, and if large-scale facilities will be needed.

• Estimate costs for X-radiography based on above• Identify core team members and principal contacts• Liaise over proposed timetabling• Prepare costed project design

2 Fieldwork • Decide materials and categories for X-radiography• Compile list of finds for X-radiography• Transfer material and list to contractor (eg laboratory)

undertaking the work.This should occur during, or at the end of, the evaluation or excavation.

• Confirm costs based on assemblage received• Produce initial X-radiographs in archival quality envelopes,

and supply X-ray data to allow for completion of the basic record* before the site archive is completed

• If no formal assessment is to take place, transfer the site archive**

3 Assessment of • Results of X-radiography to inform assessments and Potential for Analysis contributions towards the finds and conservation assessment

reports• Establish further X-radiography requirements through liaison

of appropriate specialists and core team members to inform updated project design and additional project costs

• Update records accordingly• If review of assessment report shows that an analysis phase is

not required, transfer the site archive**

4 Analysis & Report • Produce additional X-radiographs as agreed during thePreparation assessment, or for other requirements identified during analysis

• Update records accordingly• Transfer the site archive**

5 Dissemination • Site publication• Advocacy of project through other agreed media

Notes:* The basic record of an object or group of objects forms part of the site archive, as specified by the RomanFinds Group and Finds Research Group AD 700 – 1700 (1993).

**Project Manager transfers the site archive (finds, X-radiographs and records) to the project archive fordeposition with agreed receiving organisation. The transfer of the archive can occur at three different stages depending on the project type and complexity: after the fieldwork stage (English Heritage, 1991,13, 5.6), after a review of the assessment (English Heritage, 1991, 18, 6.15), or after analysis and reportpreparation (English Heritage, 1991, 23, 8.2).

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4.What to X-ray

The majority of metal artefacts and metalcomposite artefacts should be X-rayed.These will include ferrous and non-ferrous metals and alloys, including coins.

There are, however, several categories of metal finds that might not necessarilymerit radiography, depending on thenature of the archaeological project.Examples of these include:

● lead alloys and heavily-leaded copperalloys, where these will not yieldinformative X-radiographs (for example, thick and chunky items suchas melted roof lead, and leaded copperalloy cast handles)

● some copper alloy finds from waterfront sites, where these are free of accretions and X-radiography will not reveal additional technologicalinformation (for example some sheetmetal and wire)

● thick pieces of metal, where these willnot yield informative X-radiographsusing the facilities available (for example industrial artefacts such as very large bars and blocks)

● items that are obviously modern andeasily identifiable, such as many findsfrom plough soil, and that have not yet been critically sorted for discard (for example, parts of agriculturalmachinery, gun cartridges and bullets,modern household items)

● unstratified finds where these are clearly of no archaeological significance(English Heritage 1991, 33, A4.3)

● very large assemblages of clearlyidentifiable nails are sometimes sampled for X-radiography where there is no academic value in examining them all

● large architectural and structural items, such as components from post-medieval industrial complexes

Decisions on materials to be excludedfrom the X-radiography programmeshould be made between the core teammembers of the project, and it may be relevant to record the reasons in thesite archive.

In the assessment report it is normal torecord the proportion or number of findsX-rayed as a statement of the means ofcollecting the data (English Heritage1991, 32, A4.1.2), as well as stating theadditional X-radiography requirementswithin the statement of potential.

Where large or substantial artefacts meritX-radiography and suitable archaeologicalfacilities are not readily available, large-scale industrial facilities should be sought.

5.What X-radiography can show

Object identificationAccretions can be so dense that theoriginal shape of the object is obscured.This happens particularly with ferrousartefacts, which are more susceptible tothis extensive form of deterioration

(Fig 3). A less-encrusted item might bereadily identifiable when complete butsubject to mis-identification if broken and only partially surviving. Implementsfor writing, leatherworking and textile-processing may be indistinguishable frombroken nails when corroded, for example.Similarly, coins with surface detailobscured by accretion can be identified by X-radiography in some instances (see Fig 1). When this cannot be done,the radiograph informs decisionsregarding the prioritising of subsequentaction (eg Brickstock 2004, 24).

Other identifications may depend on moresubtle variations in the radiographs, whichrely on rigorous techniques of both imagecapture and viewing in order to draw outthe information. An example of suchevidence is the survival of organic materialthrough mineral replacement, for examplethe presence of organic sheaths andscabbards associated with swords, daggersand knives (Fig 4). This kind ofpseudomorphic evidence can be extremelyfaithful (to the extent of, for example,indicating the positions of stitch holes inleather), but can also be easily overlookedif X-raying technique is poor.

Particularly fragile, complex finds orclosely-associated groups of objects areoften lifted in a block of the surroundingsoil to enable careful excavation back atthe laboratory. X-radiography is invaluablein clarifying and locating the contents of asoil block when laboratory excavation isundertaken (eg Watson and Edwards1990, 98, pl 1). Additional X-radiographs,taken after excavation of the soil blocks,are normally required to clarify detail ofthe metal artefacts. Similarly, soilmonoliths can be X-rayed to show wheremetal ions have concentrated or to showthe effects of rootlets or other features

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Fig 3 Medieval iron axe-head.The much accreted blockmasks the identity of the axe-head but is clearly visible inthe X-radiographs taken in side and plan views.There is nometal surviving in the axe-head, which is now largely voidedand is more transparent to X-rays than the surroundingaccretions. Length of axe-head: 168mm. Exposure: 3mA,110kV, 300s, 0.47m FFD, Kodak Industrex MX.

Fig 4 Early medieval knife with horn handle and leather sheath.The junction of the handle with the blade is clearly visible inthe X-radiograph owing to iron mineralization of the different organic layers. Part of the handle is visible in the radiographwhere the horn has been preserved by mineralisation on the tang.The sheath shows as an irregular and discontinuous linearound the blade and also where it extends over part of the handle. Length: 170mm.

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Fig 6 (above) Medieval barrel padlock: the mechanism is revealed in the X-radiograph but is not otherwise visible, even after conservation. Length: 95mm.Exposure: 3mA, 110kV, 240s, 0.47m FFD, Kodak Industrex MX.

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Fig 5 (above) Iron Age iron file, as excavated and X-rayed in side and plan view showing thatthe cuts which form the teeth are on one face only and are raked (length 91mm).

Fig 7 (above & right) Roman dagger sheath platemade of iron and decorated with tin.The plate asexcavated gives no indication of the decorationwhereas the X-radiograph reveals this clearly, as wellas the plated rivet heads.The decoration is revealedowing to different radiopacities of the iron and thetin. Both metals are totally mineralised and so X-radiography provides a simple and non-destructive method of investigation. Length: 105mm.

Fig 8 (below) Early medieval spearhead.The X-radiograph shows the form and levelof deterioration and the detail (left) reveals an inlaid maker’s mark formed of a metalwith greater radiopacity than the ferrous metal.

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Fig 9 (left) Romancross-bow broochmade of leadedbronze anddecorated withopenwork on thefoot. Gilding isvisible as a brighterline to the metalwhere it survivesthrough protectionby the relief. Theradiograph alsoshows that theonion-domedheads are hollow,one of which ismissing and one isbroken.The stemis hollow to takethe pin.

Fig 12 Medieval copper alloy spearhead.The X-radiograph shows complex structure within the blade, which is not visible onthe object after conservation. Exposure: 3mA, 110kV, 210s, 0.47m FFD, Kodak Industrex MX.

Fig 11 (below) Early medieval knife with pattern-welding in the back of the blade. Length: 178mm. Exposure: 3mA, 110kV, 60s,0.45m FFD, Kodak Industrex MX.

(eg Canti 2003, fig 40; English Heritage2004, 20). X-radiography cannot aloneidentify the composition of artefactsalthough it can often provide clues to thenature of the material or materials based onthe micromorphology visible, especially fororganic materials such as bone and wood.

Iron corrodes in a distinctive manner thatis normally recognisable on X-radiographsbut identification of non-ferrous metals inparticular relies on analytical techniquessuch as X-ray fluorescence (eg Bayley et al2001, 25).

Form and structureX-rays will show size, shape and details of construction of the items underexamination that will aid objectidentification as well as contribute to theircharacterisation, technical description,classification and dating. Examples rangefrom details of the cuts and ridges thatform the ‘teeth’ seen on a file (Fig 5), tothe clarification of a complex item such asa barrel padlock mechanism (Fig 6).

Surface featuresX-radiography can elucidate decorativesurface features – such as inlay, a wash

of metal, or fields of enamel or niello –because the different chemicalcomposition of the material comprisingthe object and that forming its surfacedecoration is revealed. Thus non-ferrousmetal can be seen decorating an irondagger sheath (Fig 7) and an inset maker’smark is visible on a spearhead blade (Fig 8). Incised lines, tool marks andmaker’s stamps, show due to differences in metal thickness (see Figs 5, 13 and 16).

Non-ferrous metal coatings, such as gold on copper (Fig 9) or tin on iron (see Figs 2 and 7), will give a distinctivesharpness to the surface of the object in theimage due to the relatively higher densityof the coating material. Similarly, the use oflead-tin solders and copper-based brazingmaterials for joints will be evident.

Fig 10 (below) Early medieval knife with a weld line in the blade where the steel edge was joined to the iron back. Length: 100mm.

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TechnologyX-radiography can provide a range oftechnological information about themanufacture of an object, from details ofthe microstructure of the metals and alloysemployed, whether it was made from sheetmetal, wrought or cast, through to details ofthe construction of complex artefacts.

X-radiography shows the structure of ablade, for example, that might vary from a simple weld line joining the back andthe cutting edge (Fig 10), to complexpattern welding (Fig 11) or otherstructural detail (Fig 12) seen inprestigious edged blades. This informationnot only aids description, identificationand dating but also assists subsequentexamination, such as microstructuralanalysis, informing the choice of samplearea through the consideration ofcondition and structure.

X-radiography may reveal tool marks thatcan indicate if a non-ferrous metal vesselwas made by hammering (Fig 13) orturned or spun on a lathe. It can alsoprovide details of manufacture of a castmetal object and its quality, as revealed bythe extent of porosity visible (Fig 14). Thepositions of ingates, for filling the mould,may be seen. Chaplets that held the corein place may survive or, if destroyed, maybe detectable as voids caused bydifferential corrosion. Evidence for therepair of items or the recycling ofmaterials, seen when features from anearlier object is revealed, may also be

Fig 13 Hammer marks visible in the X-radiograph of a medieval copper alloy bowl show that the vessel was raised from sheetmetal. Dimension across: 108mm. Exposure: 3mA, 110kV, 120s, 0.47m FFD, Kodak Industrex MX.

Fig 14 Late Bronze Age spearhead. Porosity in the tip ofthe blade, visible as dark areas due to voids in the metal,indicates that it was made by filling the mould with hotmetal from the blade tip. Exposure: 10mA, 230kV, 30s.

Fig 15 Late 16th-century ‘Jack of Plates’ made from recycled iron sheet.The detail shows the plateswith distinctive rivet holes of the late 15th-century brigandine. Exposure: 5mA, 130kV, 30s.

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and non-ferrous metal casting waste(Fig 18), is sometimes found withinthe soil or corrosion layers associatedwith artefacts. Lead casting waste, ifoxidised, can be mistaken for mortar inthe soil unless it is X-rayed. Otherevidence can be found by X-raying soilsamples selected for specific

Fig 16 Early medieval shield boss as excavated (upper).The X-radiograph reveals hammer marks aligned around the apex fromforging the iron boss to shape (middle).The studs are made fromsilver alloy and thus show in the radiograph due to differences inmetal composition.These were revealed during conservation (lower).

Fig 17 (bottom) Soil samples containing ferrous hammerscale from a blacksmith’s hearth, comprising flakes andspheres visible in the scanning electron micrograph. (top and middle) X-radiographs: top shows flake hammerscale,comprising individual flakes that show as bright lines if they are vertical to the X-ray beam on exposure; middleshows spherical scale, comprising hollow and part hollow spheres. Hammerscale can also be found within theaccretions surrounding an artefact.

visible in the X-radiograph (Fig 15).Complex artefacts such as shield bossesmay reveal details of manufacture andconstruction, including the use ofdifferent metals (Fig 16).

Evidence of manufacturing processes,such as ferrous hammerscale (Fig 17)

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investigation, such as those from around a smithing hearth (eg Bayley et al. 2001,14; Starley 1995), or by X-raying certaintypes of manufacturing implements,such as ceramic crucibles for evidence ofnon-ferrous waste metal.

ConditionThe archaeological interpretation of findscan depend on features of condition,such as completeness before burial, orsubsequent damage. Knowledge of thecondition of an artefact, in terms of thepresence of fissures (Fig 19), fractures or the extent of mineralisation (Fig 20),can inform decisions on subsequentexamination and the conservationprogramme, particularly when finds arevery fragile.

6. How to make informative X-radiographs

Film or digital?At the time of writing (2005), thestandard image capture method inarchaeology is the production ofconventional film radiographs. In part this is because of the much lower costassociated with setting up such a system. At present, the image quality ofmedium-priced digital systems is notsufficiently good for archaeologicalapplications other than for the basicscanning of soil blocks. Digital systems are improving in quality and may in time become the standard technique forimage capture in relation to archaeologicalarchives (see Richards and Robinson2000). The guidance offered in sections1–5 will still apply.

Digitisation of film radiographs issometimes employed as a means of studyand dissemination between finds workers,although the initial X-radiographs willremain the prime source of data andarchive, and the quality of these originalX-radiographs is paramount. Imageenhancement is also employed, but its useshould be made known to the findsresearcher. These topics are covered inmore detail elsewhere (Lang andMiddleton 2005; O’Connor and Maher2001; O’Connor et al 2002).

Film radiographsFilm radiography is essentially the same as the process used for conventionalmedical X-radiography in which a two-dimensional negative image is captured on photosensitive film (Fig 21).

The image size is approximately 1:1 if the exposure is made with the artefact inclose contact with the film, althoughdimensional distortions can arise for anumber of reasons (see below).

To achieve the maximum information andquality in X-radiographs requires someknowledge of the nature of the assemblageand a rigorous methodology.The first willenable the best orientations and exposures to be selected, while the latter will facilitategood quality images and a clear under-standing of the relationship of the image tothe original artefacts. Operator skill andexperience plays a crucial part in the process.

For the basic principles and methodologyof X-radiography, the reader should refer to standard texts, in particular Lang andMiddleton (2005). Detailed technicalinformation on equipment, materials andmethodology is available in publications on industrial radiography (eg Kodak 1985;1987; Quinn and Sigl 1980; Halmshaw1986) and in manufacturer’s technical data on specific products such as films,processing equipment and chemicals (oftenavailable through their web pages).

The sections below assume basicknowledge of the X-radiography processand offer guidance on how to achieveclear and unambiguous images, suitablefor the purposes of identification, analysisand illustration, and to provide thearchival record.

Health and safety legislationThere are several statutory (legal) healthand safety requirements for any laboratoryusing ionising radiations including X-rays,and also for using certain related materialsand processes.

● The Ionising Radiations Regulations1999

● The Control of Substances Hazardousto Health Regulations 2002

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Fig 21 Figure showing the basic layout during exposure.

Fig 18 Medieval nail.The X-radiograph shows non-ferrousmetal casting waste trapped within the corrosion layers.Length: 65mm.

Fig 19 Bronze buckle from an early medieval grave.Thebuckle is fissured and very fragile and now comprisesmostly tin oxide and copper carbonate.This is aconsequence of decuprification of the bronze in the acidburial environment. Length: 21mm.

Fig 20 Roman nail, completely corroded.,The X-radiographreveals a void within the accreted block.

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The equipmentThe basic X-ray equipment, whether abench-top cabinet unit or a larger-scaleindustrial unit, will have several variablesin terms of exposure:

● intensity, or quantity of the X-rays –controlled by the current inmilliamperes (mA). This might not be a variable for some cabinet units.

● energy, or quality of the X-rays –controlled by the kilovoltage (kV)

● duration, or length of time of theexposure

Film typeThe preferred type of X-ray film to use for archaeological artefacts is very finegrain industrial film. This has filmemulsion on both sides and has highcontrast and high definition.

Film holdersX-ray film is light sensitive and is placedin suitable light-tight holders (cassettes)while being exposed to X-rays. Otherwiseit is only handled in the darkroom underthe illumination of an appropriatesafelight until fully processed. The film is normally used within rigid cassettes,in which lead intensifying screens are held in close contact with the film. Thesescreens are employed with industrial typefilm in order to minimize the effects of

Fig 22 Roman ironstylus decoratedwith brass andsilver (length:108mm).The fiveexposures of thesame stylus weremade by increasingthe X-ray energyfrom left to right.Left, the outline ofthe corrodedlayers is visible aswell as the tip andthe eraser, butvery little detail ofthe decoration canbe seen. Right, thehigher exposuresreveal detail of theinlay, although thisis to the detrimentof the outline ofthe wholeimplement, whichappears to beshorter as the tipand eraserbecome overexposed andmerge into thebackground.Exposures, left toright: 70kV to110kV in 10kVsteps, all at 3mA,60s.

● The Control of Lead at WorkRegulations 2002

● http://www.hse.gov.uk/

The process of X-raying archaeologicalmaterial is usually completed within aconservation laboratory, a museum oruniversity research facility. Staffundertaking this work must comply withthe legislation.

What affects the image?The image produced through X-radiography depends on the interactionof the X-rays with the materials underexamination, which is a function of theartefacts, the equipment and theexposure. The image quality is alsoaffected by film development conditions.Details in the X-radiographic image might be missed if the viewing conditionsare inadequate.

The artefactsCertain characteristics of the artefacts will affect the image produced during X-radiography. These are:

● thickness● density● chemical nature of the artefact (and

associated accretions)● geometry and orientation in relationship

to the X-ray film

scattered radiation, thus improving imagecontrast and increasing the exposurelatitude. At voltages above 120kV, thescreens will also serve to intensify theimage. At lower voltages there are benefits in terms of reduction in scatteredlonger wavelength radiation and improvedclarity and sharpness of the image.Pre-packed film with integral lead screensis also available, which in roll form isparticularly useful for long artefacts,such as swords.

Occasionally it is useful to employ aflexible cassette, for example to wraparound a bulky object such as a vessel.In these cases, the pre-packed filmmentioned above is useful, or the film canbe sealed within light-tight envelopes.Whichever system is employed, everyeffort should be made to incorporate leadscreens within the cassette or envelope.

The exposureMultiple exposures It is often useful to make severalexposures of each artefact, either to adjustthe orientation of the object in the beam,or to present a suite of images to suitdifferentially degraded components orareas of interest. For example, an ironstylus decorated with non-ferrous metalscan be X-rayed to show different aspectsof its construction (Fig 22).

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Another example, a medieval knife –which will have a blade of wedge section,possibly corroded away at the cutting edge– will benefit from one exposure to matchthe back of the blade and another tomatch the cutting edge. If the handle iscomplex, such as a decorated scale tanghandle, a third exposure at 90 degrees tothe other two exposures but in the planeof the blade might be required, to revealthe lengths of the rivets or otherconstructional detail. There are a numberof potential constructional andtechnological details that may be presenton knives, including makers’ marks,microstructural detail such as welded-onedges or pattern-welding (Fig 11),complex decorated handles (Fig 23), andtraces of organic sheaths and handlespreserved as ferrous pseudomorphs wherethe iron corrosion products haveconcentrated (Fig 4).

Generally, multiple incremental exposures,varying either exposure duration or energy of the X-ray beam, will provide aseries of images of the same object toaccurately record the variation in itsmorphology. Similarly, precise and specific object rotations (usually through90°) are equally important (Figs 3 and 5).Suitable props can be made of radiolucentmaterials (transparent to X-rays) such aspolyethylene foam cut into wedges orother shapes.

Masking offThe easiest way to provide multipleexposures is to mask off the film in itscassette during exposure, allowing all the exposures relating to a single item to

be placed on the same X-radiograph.This is readily achieved with square-cutsheets of roofing lead, placed to almostbutt up to the line of the previous sheet.With care, numerous masking offoperations can be performed on one film without the final developedradiograph showing any white cut-offlines, which occur if the lead sheeting is overlapped (ie where film is notexposed). However, image cut-off canresult from inexact masking, overlappingthe lead sheet on an area of previouslyexposed object. The health risks associated with handling lead dictate that the appropriate protective equipmentbe worn (The Control of Lead at WorkRegulations 2002), or the lead sheet canbe coated or covered.

Other factors to consider1 To assist the division of finds to

different specialists, as well as toproduce a more useful archival record,large assemblages can be X-rayed bymaterial or artefact type – for examplethe coins can be separate from theother copper alloy artefacts.

2 Some groups of finds may comprisenumerous very small components or dislodged fragments. These willprobably all need to be X-rayed,and often at different exposure valuesfor the different components. This can lead to complex imaging but the additional time and patience canbe justified. Similarly, fragmentaryartefacts (with recent fractures) may need to be repaired to enabletheir forms to be properly understood.

3 X-radiographs that are overloadedwith artefacts can be confusing tointerpret. There should also besufficient space between items to allow for labelling (see below).

4 Optimum X-radiography is bestachieved when the surfaces of theartefacts are in close proximity to the film in order to minimizedistortions. This is one reason why it is preferable to remove all or most of the packaging surrounding theartefacts, although very small or fragile items can be exceptions to this.

5 Distortions in the image can also occurowing to the radiation beam angle.

6 The exposure required is also affected by the film-to-focus distance(FFD), which is the distance betweenthe film and the focal spot of the X-ray tube.

7 Dimensional and other distortionsneed to be considered if artefacts areillustrated from the X-radiographs.

Film developmentThe development of the film is animportant part of the process for highquality, archival radiographs. A rigorousdarkroom routine is essential, fromdevelopment through to thorough washing and drying. Manufacturers’processing data sheets and darkroom hints are useful sources of informationand can be downloaded from their webpages. There are health and safety

12

Fig 23 Encrusted iron knife, 12th century.The X-radiograph reveals non-ferrous metal components on the bolster and tang, subsequently shown to comprise a bronze bolster at the junction ofthe blade and tang, and numerous close-set brass plates on the tang.The latter were probably separated by thin horn plates, which no longer survive (Ottaway 2003, 272).

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requirements for working with theprocessing chemicals (The Control ofSubstances Hazardous to HealthRegulations (COSHH) 2002).Information on the safe disposal ofchemicals can be found in their MaterialSafety Data Sheets and must comply with local authority regulations on waste.

A high level of attention to detail is critical when developing X-radiographicfilm to produce useful images. If it islacking, the investment made in theexposure process may be wasted. The X-radiographic image should have auniformly black background and the lackof a black background is diagnostic of abadly processed film. The use ofexhausted processing chemicals producesa streaky background in various shades of brown. X-radiographs with thisappearance have no value in the archiveand should not be accepted. It can beavoided by the rigorous monitoring ofprocessing chemicals.

Film labellingThe processed films require clear labellingin a tidy and small format so as not tointerfere with the recorded image. Whiteink is commonly used to mark the filmdirectly, although it is worth noting thatthis method of marking will not be copiedduring any digitisation process. Certainminimum information, such as thedesignated X-ray film number, theaccession numbers of the artefactsexamined, and diagrammaticrepresentations showing orientations ofthe artefacts plus other relevantinformation, must be recorded with theX-radiograph. This information, togetherwith exposure parameters, will also berecorded on the outer protective sleeve of the X-radiographs (see below), as well as in a log book for the X-rayequipment. The data forms part of the ‘X-ray archive’ and can be useful for the interpretation of the X-radiograph.The designated radiograph numbers foreach item or groups X-rayed will formpart of the site archive and the researcharchive, as separate ‘X-ray catalogues’and through cross-reference to the objectcatalogue (English Heritage 1991, A3.1.1,A6.1.1).

It is also worth adding the X-ray filmnumbers to the finds bags or labels as thisprovides a quick method to retrieve thecorrect radiograph when examiningobjects, and may be a requirement of therecipient museum.

Film protectionThe emulsion layers of the film are veryvulnerable to scratching and to otherdamage through handling, such as fingermarks. For protection, transparent coverssuch as polyester sleeves are invaluable,particularly when handling the finds andthe X-radiographs together duringassessments and many conservationprocedures, for example.

It is advisable to send objects and their X-radiographs separately if they aretransferred between project members viacourier services. Nor should they beboxed together: their storage requirementsdiffer and the potential damage to X-radiographs from contact with dust and other particles should be avoided.

StorageThe recommended environmentalconditions for processed X-radiographsare 20 – 50% relative humidity at atemperature less than 25ºC for medium-term storage and less than 21ºC for long-term (extended) storage (Brownforthcoming; British Standards Institution2000).

The enclosures for the films should beinert, acid-free sleeves or envelopes.Commonly, transparent polyester sleevesare used, together with outer acid-freepaper or card envelopes, but therequirements of the recipient museum or repository should also be considered.The outer paper or card envelope can beprinted or labelled in archival-quality ink.Any plastic sleeves should be inert andfree of plasticiser. Chlorinated nitrated or plasticised sheetings are highlyunsuitable and should not be used (British Standards Institution 2000;2001), nor should Glassine envelopes.

7. How to view the X-radiographs

Viewing radiographs in the correctconditions is important to fully appreciatethe range of information available. Ideally,they are viewed in a room where lightlevels are reduced to a minimum, and the radiograph is ‘back-lit’ on a light-box.The unused areas of the front panel of the light-box should be blanked off.The temptation to view and attempt theinterpretation of carefully producedimages by squinting at them against aninappropriate light source, such as awindow or desk lamp, should be avoided,as it is impossible for the eye to cope

effectively with such wide variations in light levels. A low-power lens such as a photographer’s loupe (X4–X8magnification) can be useful for closelyexamining the detail in an image.

Project managers should ensure that staff experienced in the interpretation of X-radiographs are engaged in the analysisof the assemblage. The radiographs should be consulted when objects areillustrated to ensure that the form of theactual items are depicted, rather than theshape of the covering corrosion products.

X-radiographs might often be the bestway to publish artefacts, particularly when they are very accreted or possessintricate technological features. Complexobjects with internal workings such aslock mechanisms (eg Egan 1998, 109,

fig 83) and other items, including the York Coppergate helmet (Spriggs 1992,901), knives (eg Cowgill et al 1987,pls 2 and 5), and pattern-welded blades(eg Lang and Ager 1989) all benefit from the publication of X-radiographs.Finds catalogues can also be enhanced by judicious use of radiographs (egHaughton and Powlesland 1999).

8. How much should it cost?

It is often necessary to provide an estimate of the cost for X-raying findsfrom an excavation at the project planning stage, before the quantity ofmaterial is known. An estimate willanticipate the volume of finds based onpast experience for the site type andperiod within the region. Of course if the volume of finds is known then amore precise costing can be gained from the organisation that will beundertaking the work.

The cost for X-radiography will includematerials (the film and developing costs),time (labour), plus any overheads such as laboratory and equipment expenses not accounted for in the previous costs (eg X-ray equipment maintenance costsand compliance with health and safetyrequirements). The time element willdepend on factors such as the condition of the material (does it require repairingbefore X-raying?), the packaging methods(which can affect handling time),annotating the radiographs and theirpaper or digital records, the desirability of multiple exposures, the size of objects,and so on.

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Costs can be affected by other, lessobvious, factors. When considering severaltenders one should be aware that thelowest prices might not always provide the best value for money. It is importantthat not only high quality radiography butalso appropriate handling of the materialis specified and undertaken. Adequatepackaging to ensure the material is safelytransported is necessary and will affect the overall cost.

Indicative costs (in 2005) for thecommonly employed 180 × 240mm sizeX-ray film are as follows:

● materials and laboratory expenses – £4 to £5 per film

● time (labour) – typically around eightradiograph films can be completed in a day where small items are X-rayed(inclusive of exposure, development and marking-up). A greater number of radiographs can usually be completed in a day where large items or large groups are X-rayed.

The numbers of objects per film willobviously vary as indicated above, buttypically the following examples may apply for each 180 × 240mm film as arough guide:

● 20 to 40 coins (less than 20 per film if multiple exposures are made,more than 40 coins per film in somecircumstances)

● 3 domestic knives (assuming 2 exposures of each knife)

● 1 complex barrel padlock (at 3 orientations)

● 30 nails (less than 30 per film if theseare large, or if they are bulky orcomplex groups with mineralised woodattached, such as those from coffins.Conversely, perhaps 100 or moreindividual hobnails would fit on an X-ray film, depending on the method of numbering, or a single hobnailedshoe sole).

9.Where to get help

Advice on facilities and laboratoriesavailable for commercial and other workcan be obtained from the followingsources:

1 local archaeological conservationlaboratory services through localauthority and county museumservices, universities and other

institutions, and through discussionwith the other finds specialists involvedin the project ‘core team’

2 English Heritage, Fort Cumberland,Portsmouth (tel: 02392 856704)

3 The Conservation Register of theInstitute of Conservation (formerlyUnited Kingdom Institute forConservation, UKIC). This is aregister of privately practisingconservators: Conservation Register,c/o Institute of Conservation,3rd Floor, Downstream Building,1 London Bridge, London SE1 9BG tel: 0207 785 3804e-mail: info@conservationregister.org.ukwww.conservationregister.com

4 English Heritage Regional ScienceAdvisors, listed below with theirregions:

NNoorrtthh WWeessttSue StallibrassDepartment of Archaeology, Classics andOriental Studies, University of Liverpool,William Hartley Building, BrownlowStreet, Liverpool L69 3GStel: 0151 794 5046e-mail: sue.stallibrass@liv.ac.uk

NNoorrtthh EEaassttJacqui HuntleyDepartment of Archaeology,University of Durham, South Road,Durham DH1 3LEtel/fax: 0191 334 1137e-mail: j.p.huntley@durham.ac.uk

YYoorrkksshhiirreeIan PanterEnglish Heritage, 37 Tanner Row,YorkYO1 6WPtel: 01904 601983e-mail: ian.panter@english-heritage.org.uk

WWeesstt MMiiddllaannddssLisa Moffett English Heritage, 112 Colmore Row,Birmingham B3 3AGtel: 0121 625 6875e-mail: lisa.moffett@english-heritage.org.uk

EEaasstt MMiiddllaannddssJim WilliamsEnglish Heritage, 44 Derngate,Northampton NN1 1UHtel: 01604 735400e-mail: jim.williams@english-heritage.org.uk

EEaasstt ooff EEnnggllaannddJen HeathcoteEnglish Heritage, Brooklands House,24 Brooklands Avenue, Cambridge,CB2 2BUtel: 01223 582759e-mail: jen.heathcote@english-heritage.org.uk

SSoouutthh WWeessttVanessa StrakerEnglish Heritage, 29/30 Queen Square,Bristol BS1 4NDtel: 0117 975 0689e-mail: vanessa.straker@english-heritage.org.uk

LLoonnddoonnJane SidellInstitute of Archaeology, UniversityCollege London, 31–34 Gordon Square,London WC1H 0PYtel: 0207 679 4928e-mail: j.sidell@ucl.ac.uk

SSoouutthh EEaasstt Dominique de MoulinsInstitute of Archaeology, UniversityCollege London, 31–34 Gordon Square,London WC1H 0PYtel: 0207 679 1539e-mail: d.moulins@ucl.ac.uk

Bibliography

Bayley, J, Dungworth, D and Paynter,S 2001 Archaeometallurgy. English HeritageCentre for Archaeology Guidelines,2001/01. English Heritage Publications

Brickstock, R J 2004 The Production,Analysis and Standardisation of Romano-British Coin Reports. London:English Heritage Publications

British Standards Institution 2000Imaging Materials – Processed SafetyPhotographic Films – Storage Practices,BS ISO 18911: 2000. London: BritishStandards Institute

British Standards Institution 2001Imaging Materials – Processed PhotographicFilms, Plates and Papers – Filing Enclosuresand Storage Containers, BS ISO 18902:2001. London: British Standards Institute

Brown, A forthcoming ‘The documentaryarchive’, in UKIC Guidelines for thePreparation of Excavation Archives for Long-Term Storage (2 edn). London:UKIC Archaeology Section

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Canti, M G 2003 ‘X-ray studies of thesediments’, in Brennand, M and Taylor,M ‘The Survey and Excavation of aBronze Age Timber Circle at Holme-next-the-Sea, Norfolk, 1998–9’,Proceedings Prehistoric Society 6699, 42–3

Cowgill, J, de Neergaard, M and Griffiths,N 1987 Knives and Scabbards. MedievalFinds from Excavations in London,1. London: HMSO

Department of the Environment 1990Planning Policy Guidance: Archaeology andPlanning. PPG 16. London: HMSO

Department of the Environment 1994Planning Policy Guidance: Planning and theHistoric Environment. PPG 15. London:HMSO

Egan, G 1998 The Medieval HouseholdDaily Living c 1150–c 1450. MedievalFinds from Excavations in London,6. London: The Stationery Office

English Heritage 1991 Management ofArchaeological Projects (2 edn). London:English Heritage

English Heritage 2002 EnvironmentalArchaeology: A Guide to the Theory andPractice of Methods, from Sampling andRecovery to Post-excavation. EnglishHeritage: Centre for ArchaeologyGuidelines 2002/01

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Halmshaw, R 1986 Industrial Radiography.Agfa-Gevaert

Haughton, C and Powlesland, D 1999West Heslerton,The Anglian Cemetery,Vol ii. Catalogue of the Anglian Graves andAssociated Assemblages. Yedingham:The Landscape Research Centre

Institute of Field Archaeologists 2001Standard and Guidance for the Collection,Documentation, Conservation and Researchof Archaeological Materials. Reading: IFA

Kodak 1985 Fundamentals of RadiographicPhotography,Vol III, Radiographic Quality.London: Kodak Ltd

Kodak 1987 Fundamentals of RadiographicPhotography,Vol II, X-ray RecordingMaterials. London: Kodak Ltd

Lang, J and Ager, B 1989 ‘Swords of theAnglo-Saxon and Viking Periods in theBritish Museum: A Radiographic Study’,in Hawkes, S C (ed), Weapons and Warfarein Anglo-Saxon England, Oxford UniversityCommittee for Archaeology Monograph21, 85–122. Oxford: OUCA

Lang, J and Middleton, A (eds) 2005Radiography of Cultural Material (2nd edn,revised). London: Elsevier

O’Connor, S and Maher, J 2001 ‘Thedigitisation of X-radiographs fordissemination, archiving and improvedimage interpretation’. The Conservator 2255,3–15

O’Connor, S, Maher, J and Janaway, R2002 ‘Towards a replacement forxeroradiography’. The Conservator 2266,100–114

Ottaway, P 2003 ‘Knives’, in Hardy, A,Dodd, A, and Keevill, G D, Ælfric’s Abbey;Excavations at Eynsham Abbey, Oxfordshire,1989–92, Oxford: Oxford Archaeology,Thames Valley Landscapes 16, 271–273

Quinn, R A and Sigl, C C (eds) 1980Radiography in Modern Industry (4 edn).Rochester, New York: Eastman KodakCompany

Richards, J and Robinson, D 2000 DigitalArchives from Excavations and Fieldwork:Guide to Good Practice (2 edn). Oxford:Oxbow, and available at http://ads.ahds.ac.uk/project/goodguides/excavation(accessed 20 July 2005)

Roman Finds Group and Finds ResearchGroup AD 700–1700, 1993 The Guidelines for the Preparation of SiteArchives and Assessments for All Finds Otherthan Fired Clay Vessels. Roman FindsGroup and Finds Research Group AD700–1700

Spriggs, J A 1992 ‘Conservation –approach and methods’, in Tweddle, DThe Anglian Helmet from 16–22 Coppergate.The Archaeology of York. The SmallFinds, 17/8, 894–901. London: CBA

Starley, D 1995 Hammerscale. HistoricalMetallurgy Society Archaeology Datasheet10 (www.hist-met.org)

Watson, J and Edwards, G 1990‘Conservation of Material from Anglo-Saxon Cemeteries’, in Southworth E (ed),Anglo-Saxon Cemeteries: A Reappraisal.Proceedings of a Conference held atLiverpool Museum 1986, 97–106. Stroud:Alan Sutton

Health and safetyThe Ionising Radiations Regulations 1999(Statutory Instrument 1999 No. 3232).HSE. The Stationery Office Ltd

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The Control of Lead at Work Regulations2002 (Statutory Instrument 2002 No.2676). HSE. The Stationery Office Ltd

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Published January 2006

Copyright © English Heritage 2006Edited and brought to press by David M Jones,English Heritage PublishingDesigned by Mark SimmonsProduced by English Heritage PublicationsPrinted by [printer]

Product code 51163

Acknowledgements

These guidelines were written andcompiled by Vanessa Fell, Quita Mouldand Rob White*. They have benefitedconsiderably by comments andsuggestions made by numerous colleaguesincluding conservators, finds researchersand other specialists, and the EnglishHeritage Regional Science Advisors. Weare extremely grateful to the followingpeople who commented on theseguidelines:

Liz Barham, Justine Bayley, Hilary Cool,Jane Cowgill, Nina Crummy, Geoff Egan,Liz Goodman, Karla Graham, JacquiHuntley, Jennifer Jones, Jackie Keily,Janet Lang, Jannicke Langfeldt, Jo Mills,Lisa Moffat, Dominique de Moulins,Sonia O’Connor, Ian Panter, SebastianPayne, Jane Sidell, Jim Spriggs, DavidStarley, Judy Stevenson, Jacqui Watsonand Jim Williams.

We thank Tony Wilmott for identifying acoin and permission to use the imagefrom current excavations. Justine Bayleyand Pete Wilson kindly providedinterpretations on other artefacts. JohnNewman and John Sills agreed to the useof images prior to publication of materialfrom excavations in Suffolk and in NorthEast Lincolnshire, respectively. We are alsograteful for permission to use images ofartefacts from excavations funded byArchaeological Services University of

English Heritage is the Government’sstatutory adviser on the historicenvironment. English Heritage providesexpert advice to the Government about allmatters relating to the historicenvironment and its conservation.

For further information (and copies ofthis leaflet, qquuoottiinngg tthhee pprroodduucctt ccooddee),please contact:

English HeritageCustomer Services DepartmentPO Box 569SwindonSN2 2YP

Telephone: 0870 333 1181Fax: 01793 414926e-mail: customers@english-heritage.org.uk

Durham (Figure 9) and by Chester CityCouncil (Figures 7 and 22).

Illustrations were contributed byLincolnshire County Council, HeritageService, and by David Dungworth,Vanessa Fell, Karla Graham, JenniferJones, Lucy Skinner, David Starley andRoger Wilkes.

Figures 14 and 15 are copyright of theBoard of Trustees of the Armouries.Figure 8 is copyright of CTRL UKLimited 2005. Figures 1(upper), 2a, 3,6, 12, 13, 17 (part), 18, 20 and coverimage are supplied by kind permission of Lincolnshire County Council,Heritage Service. Figures 1 (lower),2d, 4, 11, 16, 17 (part) and 21 arecopyright of English Heritage.Illustrations may not be reproduced byany means without written permission.

These guidelines are endorsed by theArchaeology Group of the Institute ofConservation, the Finds Research Group AD 700–1700, and the RomanFinds Group.

* This document should cited as Fell, V,Mould, Q and White, R 2006 Guidelineson the X-radiography of ArcheologicalMetalwork. Swindon: English Heritage

Cover figure: Post-medieval padlock asexcavated and with the padlock mechanismrevealed through X-radiography.

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