2011. reintegración vidrio arqueológico. e-conservation

8/6/2019 2011. Reintegración vidrio arqueológico. e-conservation

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the online magazine No. 20, July 2011



REVERSIBILITY AND MINIMAL

INTERVENTION IN THE GAP‐FILLINGPROCESS OF ARCHAEOLOGICAL GLASS

By Betlem Martínez, Trinidad Pasíes

and Maria Amparo Peiró



Introduction

Archaeological glass is an extremely delicate ma‐

terial that requires particular ability and care when

treated by the conservator‐restorer. Glass objects

from archaeological origin are fragile, often very

fragmented, and have very thin walls. Besides, they

have undergone singular alteration processes when

preserved in unsuitable environments (figure 1).

In this article we put forward new proposals that

concern one of the most controversial processes

carried out by conservator‐restorers: the treat‐

ment of the missing areas. We have developed analternative that, while being coherent with re‐

versibility and minimal intervention criteria, a

necessary prerequisite to any restoration inter‐

vention, does not prevent a reconstruction pro‐

cess that facilitates form legibility of the object.

Putting the concepts of reversibility and minimal

intervention into practice: a question of criteria

Traditionally, the gap‐filling has been understood

as a process carried out in order to return form

unity to a piece. The ICOM 2008 resolution defines

it as a regular treatment in a restoration process

that includes “all actions directly applied to a single

and stable item aimed at facilitating its appreci‐

ation, understanding and use. These actions are

only carried out when the item has lost part of its

significance or function through past alteration

or deterioration and are based on respect for the

original material”1. But the dangers involved in

that sort of direct action do not only arise fromlack of manual ability and experience of conser‐

vator‐restorers themselves, they can also be

caused by the historical moment when actions

take place: applied criteria, protocols, and ma‐

terials used have varied with time.

If there is one thing we can currently learn from

our recent past, that is the frequent mistakes made

when, without awareness of negative effects, ex‐cessive intervention on cultural heritage objects

is applied without absolute respect for the minimal

intervention criteria. The damage done by profes‐

sional conservator‐restorer, when they justified

excessive intervention to achieve a supposed im‐

Figure 1. Group of archaeological glass objects. Cycladic

Museum (Athens, Greece).

41e‐conservation

REVERSIBILITY AND MINIMAL INTERVENTION IN ARCHAEOLOGICAL GLASS

In recent research at the Museo de Prehistoria of Valencia (Prehistory Museum of Valencia) and the

Institut Valencià de Conservació i Restauració (Valencian Institute for Conservation and Restoration),

we have developed different methods of reversible filling based on the use of synthetic films such as

polyethylene terephthalate and polypropylene, materials commonly used in the field of document

conservation. These methods have now been applied to a variety of archaeological glass collections. The

results of this research project are set out in detail in this article.

1 Terminology to characterize the conservation of tangible

cultural heritage, Resolution adopted by the ICOM‐CC mem‐

bership at the 15th Triennial Conference, New Delhi, 22‐26 September 2008, available at URL (accessed 20th April

2011)

http://www.icom-cc.org/54/document/icom-cc-resolution-terminology-english/?id=744

http://www.e-conservationline.com/



provement in the underst anding of a piece, is

precisely the reason why we should make the cri‐

teria of minimal intervention a priority, and con‐

sider it not only viable but also the alternative

that is most coherent with strict respect for the

original material conserved.

The Ministry of Culture, through the Instituto del

Patrimonio Cultural de España (Cultural Heritage

Institute of Spain) published ten criteria for res‐

toration2. In relation to minimal intervention, the

document says that “the principle of minimal in‐

tervention is crucial. Any manipulation of a piece

involves risks, therefore we should limit ourselves

to that which is strictly necessary and accept natu‐

ral decay caused by time. Over interventionist

treatments that can damage an object integrity

should be rejected”. These recommendations also

refer to the gap‐filling process that according to

the document should only take place “when it is

necessary for the stability of the piece or for some

of the materials that form part of it”. The contro‐versial but indispensable reversibility criterion is

also mentioned. Any report or publication regard‐

ing restoration must include it, even though its

meaning can often create some misunderstand‐

ings [1]. Products used for the f ill‐in process must

be reversible but reversibility should not be a

traumatic moment for the piece nor for the con‐

servator‐restorers themselves [2, pp. 60‐61].

Nowadays, we have sufficient resources to make

reversibility and minimal intervention criteria fit

perfectly into the fill‐in process. Acting with this

in mind does not mean no intervention or that it

is not possible to find alternatives that combine

both respect for and legibility of the piece [3]. We

have tried to make both concepts compatible in

our research.

Applied alternatives to the casting of missing

areas in archaeological glass

Reconstruction of missing areas is often justified

as a consolidation process of the piece. Its purpose

is to improve the reading of the forms and the

understanding of the piece as a historical docu‐

ment, where a gap is considered an interruption

in the continuity of the form. Intervention might

be necessary or advisable in some cases, especially

when the stability of the piece is at stake. But we

know that this is not always the case and that con‐

servator‐restorers are often subject to impositions

or wrong criteria that find justification in consi‐

dering that an incomplete piece cannot be under‐

stood or lacks aesthetic quality.

It is important to define certain areas before a

process of conservation‐restoration is carriedout. We must know what the final destination of

the piece is: storage, research, temporal or per‐

manent exhibition. Once this has been established,

a decision could be made regarding whether treat‐

ment should be preventive or if there is a need for

a remedial approach. Other fundamental factors

are the preservation state of the material and

some of its characteristics, like glass thickness,

and the size, shape and localization of gaps. Inany case, there comes a moment when the profes‐

sional will have to face the problem of a possible

reconstruction of missing areas. What alternatives

are there for that challenge?

Unanimity of criteria is hard to achieve, above all

regarding the process that conditions the piece

appearance when it is eventually presented. When

we make a diagnosis for an object and establish

the percentage that has been lost, we have to act

responsibly and decide among different proposals

42 e‐conservation

2 Free translation from Spanish from Decálogo de la Res‐tauración ‐ Criterios de Intervención en Bienes Muebles,

available at URL [pdf] (accessed on 20 April 2011)

BETLEM MARTÍNEZ, TRINIDAD PASÍES & M. AMPARO PEIRÓ

http://www.mcu.es/patrimonio/docs/MC/IPHE/M0901-02-3-PDF1.pdf




that could be considered. The first thing to be

ascertained is whether a casting of the gaps is

really necessary. No intervention could, in fact,

be a good option, especially in those cases where

the piece can be easily read (figure 2).

We may also decide that only a partial interven‐

tion is necessary, with occasional fillings that strengthen strategic areas to give stability to the

piece. Or we may opt for a no integration proposal

and use other means of supporting the piece in‐

stead (figures 3‐5). There are different types of

supports used for glass made of synthetic resins

[4] or with blown glass [5; 6, p. 160]. In some

cases, instead of completing the object form, sup‐

port is minimized and its presence is reduced to

some elements that not only hold the piece but

in some way help to imagine the area of the ob‐

ject that has been lost (figure 6‐7).

43e‐conservation

Figure 2. A piece without casting of missing areas. Casa Romei (Ferrara, Italy).

Figure 3. New support to reconstruct the missing base of the

glass. Casa Romei (Ferrara, Italy).





This approach requires a radical change of attitude

not only in the case of conservator‐restorers, who

are the first to be convinced of the many advan‐

tages of that decision, but also on the part of ar‐

cheologists, museum directors and the general

public who must learn to really value this alterna‐

tive and be aware that nowadays it is possible to

create 3D digital reconstructions that can be used

as a complement for a better understanding of

pieces, avoiding thus the need of acting directly

on them.

But among traditional options there is also the

total intervention, the complete reconstruction

of gaps in the object for conservation, aesthetic

or exhibition related reasons. Many different pro‐

44 e‐conservation

Figure 7. An internal support with a re‐creation of the basemade in the Institut Valencià de Conservació i Restauració

(Valencia, Spain). Photography Pascual Mercé.

Figure 6. External support to hold a glass artifact. Hadrian'sLibrary (Athens, Greece).

Figure 4 (above). The original artifact rests on an internal

support system. Corinto (Greece).

Figure 5. Support system made of moulding resin which bears

original fragments. Museo de Valladolid (Spain).





45e‐conservation

Figure 8. Casting of the missing glass made of plaster. Archaeological Museum of Haniá (Crete, Greece).





duct s have been used in the fill‐in process. For

instance, glass from other objects reused for this

purpose, materials traditionally used for ceramics,such as plaster (figure 8), acrylic resins (Technovit

4000, Plastogen G), polyester (GTS from Vossche‐

mie, C‐32 from Canuts), polyurethane (Crystal

Clear 200) or epoxy resins (Ablebond 342‐1, Fyne‐

bond, Araldite 2020, Hxtal NYL‐1, Epotek 301) [6,

pp. 153‐159; 7, pp. 286‐304; 8, pp. 76‐95; 9‐11]

(figures 9‐10). In recent years epoxy resins have

been, without a doubt, the products most fre‐

quently used and research has focused on analyz‐

ing their long term aging [12, 13]. This process

involves making models (generally silicones,

modeling clays, dental waxes or clays) and fur‐

ther work on the resin finishing in contact with

the piece (f igures 11‐12). Proposal for recon‐

struction of large gaps with resin by means of

molds made from the piece have been occasion‐

ally put forward. A replica of the lost area is ob‐

tained, worked on and then adhered to the piece

[14]. S. Koob experimented with detachable fills as

well: “[…] the making an intermediate fill or cast‐

ing with plaster of Paris. The plaster will be re‐

moved from the object and then molded in silicone

rubber, from which an epoxy f ill or replacement

fragment will then be made. This can be joined to

the original object with B‐72” [8, pp. 95‐104].

Figure 9 (above). Fill in resin in archaeological glass. Museu

de Conimbriga (Portugal).

Figure 10. Yellowing process of f illing resin. Museo Arqueológico de Santa Pola (Alicante, Spain).


46 e‐conservation




Figure 11. Polishing coloured epoxy resin made at the Institut Valencià de Conservació i Restauració (Valencia, Spain).

Figure 12. A casting with coloured epoxy resin made at the Institut Valencià de Conservació i Restauració (Valencia, Spain).


47e‐conservation




But there are other less technical aspects that,

unfortunately, are sometimes forgotten such as

the high financial cost of many of those products,

their short life, toxicity and questionable rever‐

sibility. In general, these products are not rever‐

sible; therefore we believe that the use of a primer

between the original piece and the new material,

in order to make removal easier, should be not

just necessary but compulsory.

Besides, we should not forget the technical com‐

plexity of the fill‐in process; mistakes can easily

occur. The process requires extreme precision, not

just for the preparation stages (making the mold)

but also when pouring the resin and in the polish‐

ing that later takes place. These are all risky ac‐

tions when they are performed on an archaeo‐

logical object of extreme fragility.

Conscious of the problems involved in the applica‐

tion of these reconstruction methods, which use

traditional materials, we are researching in ourlaboratories, a proposal that might solve the ques‐

tion of reconstruction and conform to reversib‐

ility and minimal technical diff iculty requisites.

A reversible fill‐in method: detachable films

The market offers a great deal of synthetic com‐

pounds made for industries whose activities dif‐

fer a lot from those normally carried out in thearea of cultural heritage. Conservators have gradu‐

ally been getting materials that had originally been

created for other purposes. Therefore, each new

product incorporated to our resources requires a

number of studies that make sure that is not dam‐

aging for the materials it might be in contact with,

and to judge the suitability of new ideas.

The use of detachable films in this proposal is not

new (figure 13), but it has not been sufficiently

studied to be considered a generalized practice.

In some areas of conservation, acrylic resin sheets

have been made using products such as Paraloid

B‐72, Technovit 4004A [15, 16] or slow harden‐

ing epoxies (AY 103, Araldite 2020, Hxtal NYL‐1,

Epotek 301) [17‐ 19]. These can be modeled while

they are becoming hard, in order to give them the

shape of the gap and then stick them to the piece

as if they were fragments [7, pp. 304‐306; 8, pp.

104‐106].

There are publications that have mentioned the

much less researched alternative of making de‐

tachable films with acrylic sheet precast (Perspex,

Plexiglass). Some authors define these materials

as less manageable than others and not very suit‐

able for aesthetic reasons [6, p. 161; 7, p. 304; 8,

p. 104; 20]. Although it is true that the use of

detachable films has its limitations, we have in‐

vestigated it as a proposal in relation to reversi‐

bility and minimal intervention requisites. We

have used particular materials and methods and

we outline the results we have obtained below.

There are many comparative studies for the gap‐

filling resins used for interventions on glass, for

their virtues and qualities. But we cannot f ind

contrasted analyses for different type of sheets,

results, possibilities, or the limitations in their

use, even in the cases when they are presented

as an alternative. Therefore, we have based our

Figure 13. A detachable film solution. British Museum

(London, UK).


48 e‐conservation




selection of the laminated products that we have

used on analyses focused on other applications

and materials within the field of conservation.

Research on the causes for glass deterioration

has found evidence of an acceleration process

related to environment acidity [21, pp 79‐80; 22].

Other objects of a different nature such as paper

and metal, share that sensibility but with some

differences concerning direct effects. We have

found detachable films of various types that have

been used, and analyzed, for application in those

specializations for more than fifty years. In our

proposal, we are putting forward incorporating

some of the results obtained in analyses origin‐

ally focused on, among other things, applications

of lamination treatment for paper or for storage

systems for metals, to the area of glass materials.

Some compounds, for instance, those derived

from polyvinyl acetate (PVA), polyvinyl chloride

(PVC) and cellulose acetate (CA), turned out tobe suitable from the aesthetic and morphologic

point of view, and for their malleability and the

fact that they are easy to handle. But they have

all been rejected because of the damage they can

cause to glass surfaces. Plasticizing elements

used in the manufacturing process are the main

agents for acid vapor emissions that make them

brittle and tacky [23, p. 15].

Eventually we considered two compounds as the

most tested and verified as harmless for our work:

polyethylene terephthalate (PET) and polypropy‐

lene (PP). PET has been used since the mid‐20 th

century and has been the object of several studies

related to the multiple applications it has had,

due to its optimal results. In the conservation

world, the use of PET is fundamentally associated

with archival work and graphic document treat‐

ment. Acceptance of this product in these fields

shows it is the most suitable amongst those we

know. This is, to a great extent, due to the absence

of plasticizing elements in its manufacturing

process which avoids later emissions because

“the semicrystalline nature is the basis for the

excellent resistance to chemicals” [24, p. 68].

We have found that PET made in an uncoated, bi‐

axially oriented, and polished form presents the

best and more secure results within the different

varieties. This is the case of Melinex, the product

we use. Melinex shape and surface does not alter

at least until 120 ºC and does not change its tacki‐

ness for forty days at 37’7º C [25, Tables 1‐2].

PP responded to tasting in a less conclusive way

than PET. Therefore, it is considered an acceptabl e

compound to be used in conservation but that

should be tested further in order to confirm the

results. It is a chemically inert material and it is

not toxic, but it is vulnerable to sun light effects.

PP is considered acceptable, provided that ismanufactured, as in the case of PET, without

plasticizing additives. PP is easy to manipulate

and to work with and can have a hazy and matt

finish, characteristics that in some cases turn

out to be an advantage for gap‐filling in archae‐

ological glass that has partly lost transparency.

We have carried out tests with these two mate‐

rials, both at the Institut Valencià de Conservació i Restauració and the Museo de Prehistoria of

Valencia. The tests concern the application of the

materials as sheets in the fill‐in process of glass

from archaeological origin. We have opted for

PET (Melinex) in the case of an islamic lamp

(figure 14). We have placed the sheet to support

a group of fragments that were poorly supported

by the base. PP (Plakene) was used for the loss

compensation of a medieval chalice that had partly

lost its transparency and that had several small

gaps (figure 15).


49e‐conservation




The sheet is made without having any contact

whatsoever with the piece. This avoids excessive

manipulation. Besides, it means that we do not

have to be over concerned with the possibility of

mistakes and with the possibility of having to

repeat the process. In the first place, we should

choose the material best suited to the piece (fig‐

ure 16), and then decide about the sheet thick‐

ness (figure 17). There are different options for

each of the products: 75 to 175 microns for PET

and 300 to 1,200 microns for PP. We have also

made colour tests for these materials. It might

be interesting to colour them slightly in order to

harmonize the materials with the treated piece.

The application of a mix of pigments and Paraloid

B‐72 in ethyl acetate has been successful and we

can give the material an aesthetic finish closer to

the original if we wish to do so. This might depend

on characteristics of the piece and the differenti‐

ation criterion we choose to apply. But being a

totally reversible system, we can eliminate the

application with no complication if we decide todo so. In fact, the main advantages of these sys‐

tems are the possibility of changing and elimina‐

ting sheets and the reduction to a minimum of

the risks involved in changes.

The first step in the process of making a sheet for

gap‐filling is to obtain the gap profile. We place

a thin acetate sheet on the original piece and

faithfully mark on it the gap contour with a per‐manent marker. This acetate sheet will be used

as a pattern to cut the PET or PP sheet later as

accurately as possible; but mistakes can always

be rectified. In order to adapt the material to the

curve shape of a gap, we can heat the sheet by

Figure 14 (above). PET detachable film as a support in an

islamic lamp. Institut Valencià de Conservació i Restauració

(Valencia, Spain). Photography Pascual Mercé.

Figure 15 (above right). Chalice with PP detachable film. SIP

Archive of the Museo de Prehistoria of Valencia (Spain).

Figure 16 (right). Preparations to make detachable f ilms.

SIP Archive of the Museo de Prehistoria of Valencia (Spain).


50 e‐conservation




means of a hot air blower at low temperature. This

will work provided that the curvature is not toopronounced.

Once we have checked that the fitting of the sheet

is optimal, we only have to fix it in the gap, ad‐

hering the fragment to the glass walls with resin

(figures 18‐19). Following our reversible material

criterion we chose to use as adhesives Paraloid

B‐72 or Mowital B60HH at 20 %; we have obtained

excellent results with both of them.

Conclusion

Nowadays we have sufficient resources to make

reversibility and minimal intervention criteria fit

perfectly into the fill‐in process. This can be done

using materials that are harmless and stable in

the long term. Acting with this in mind does not

mean no intervention or that it is not possible to

find alternatives that combine respect for and

legibility of the piece. In our research into the

Figure 17. Selection of the PP detachable film and its colour in connection with the original. SIP Archive of the Museo de

Prehistoria of Valencia (Spain).

Figures 18 and 19. Above: Adhesion of one PP detachable film

once coloured and cut. Below: The scalpel indicates one of the

missing areas made with PP detachable film. SIP Archive of the

Museo de Prehistoria of Valencia (Spain).


51e‐conservation




ent PVC mould used in the process of replacing

missing pieces on hollow glass objects”, Diana 10,

Department for Preventive Conservation, National

Museum Belgrade, 2004‐2005, pp. 154‐159

[10] L. Fernández, L. Schönherr, M. Pugès, “Pro‐

ductes i tècniques per la reconstrucció de vidre

arqueològic”, Quaderns tècnics de l’MHCB: Con‐

servació i Restauració 2, 2007, pp. 63‐79

[11] B. Martínez Pla, “Restauración de alzata de

vidrio y cobre dorado perteneciente a un juego

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17 th International Meeting on Heritage Conservation,

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Cultura i Esport, 2008, pp. 501‐504

[12] N. Tennent, “Clear and pigmented epoxy res‐

ins for stained glass conservation: light ageing

studies”, Studies in Conservation 24(1), 1979, pp.

153‐164

[13] J. L. Down, “The Yellowing of Epoxy Resin

Adhesives: Report on High‐Intensity Light Aging”,

Studies in Conservation31(4), 1986, pp. 159‐170

[14] E. Risser, “A new technique for the casting

of missing areas in glass restoration”, Journal of

Conservation & Museum Studies 3, 1997, DOI:10.

5334/jcms.3973, URL

[15] R. F. Erret, “The repair and restoration of

glass objects”, IIC Bulletin of the American Group

12, International Institute for Conservation, 1972,

pp. 48‐49

[16] P. Jackson, “Restoration of an Italic glass

oinchoe with Technovit 4004A”, Conservator 7 ,

1983, pp. 44‐47

[17] L. Hogan, “An improved method of making

supportive resin fills for glass”, Conservation

News 50, London, 1993, pp. 29‐30

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35‐47, available at URL

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tion of cultural property”, Museums and Monu‐

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[21] L. Osete, Estudios de procesos de corrosión

de vidrio y vidriados arqueológicos y caracteriza‐

ción de sustancias filmógenas tradicionalmente

utilizadas en su restauración, Facultad de Quím‐icas, Universidad de Valencia, 2005

[22] J. M. Fernández Navarro, El vidrio, Consejo

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[23] B. Cope, “Transparent plastic film materials

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53e‐conservation

http://cool.conservation-us.org/coolaic/sg/bpg/annual/v04/bp04-12.html

http://cool.conservation-us.org/jaic/articles/jaic37-01-004_appx.html

http://dx.doi.org/10.5334/jcms.3973




BETLEM MARTÍNEZConservator‐restorer

Contact: [email protected]

Betlem Martínez graduated from the Department of

Fine Arts, Universidad Politécnica de Valencia, spe‐

cializing in conservation in 1997. She has expanded

her knowledge through a number of courses since

1995; she has taken the Máster Oficial en Conser‐

vación y Restauración de Bienes Culturales ‐ at the

Universidad Politécnica de Valencia specializing

in archaeological materials in 2010. She has been

working in public and private projects related to

her specialization since 1998 both as part of a

private enterprise working in restoration of ar‐chaeological materials and, since 2006, at the

Conservation and Restoration Service of Diputa‐

ción de Castellón and the Institut Valencià de Con‐

servació i Restauració de Bens Culturals.

TRINIDAD PASÍESConservator‐restorer


Trinidad Pasíes (PhD) is a restorer at the Museo de

Prehistoria of Valencia. She graduated in Fine Arts

in 1992, specializing in restoration. Since then she

has been expanding her knowledge of archaeolo‐

gical material treatment doing research and work‐

ing at different international centres such as the

Atelier de restauration de mosaiques (France);

Opificio Delle Pietre Dure and ICCROM (Italy);

Parque de Tikal (Guatemala); Ministry of Culture

(Greece). She has directed a large number of ar‐

chaeological conservation and restoration inter‐

ventions at national level. She has been working

as a teacher since 1996. In 2007 Dr. Pasíes com‐

pleted the Máster Oficial en Conservación y Res‐

tauración de Bienes Culturales at the Universidad

Politécnica de Valencia. She has participated as a

researcher in Research, Development and Innova‐

tion projects and her work has appeared in several

national and international publications.

MARIA AMPARO PEIRÓConservator‐restorer


M. Amparo Peiró graduated from the Department

of Fine Arts, Universidad Politécnica de Valencia

in 1999, specializing in restoration. Since then she

has expanded her knowledge and experience inthe field by means of grants and projects in Italy.

Since 2002 her work is mostly focused on archae‐

ology. She collaborates in projects with Museo de

Prehistoria of Valencia. In 2010 she continued her

professional trainning with the Máster Oficial en

Conservación y Restauración de Bienes Culturales,

with a final project on archaeological lead. She

currently combines her work at Museo Arqueoló‐

gico of Burriana and teaching activity with various

restoration projects of archaeological material for

private and public enterprises.


54 e‐conservation


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2011. reintegración vidrio arqueológico. e-conservation

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