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- Sprache: Englisch
Before the 1970s, most information concerning the conservation and restoration of paintings, wood, and archaeological artefacts were focused on the history of the artefacts, previous attempts of conservation, and the future use of these artefacts. The technical methods of how the restoration and conservation were made were dealt with only very briefly. Today, sophisticated methods of scientific analysis such as DNA are common place, and this encourages conservators and scientists to work together to work out the development of new methods for analysis and conservation of artefacts. This book focuses on the chemicals used for conservation and restoration of various artefacts in artwork and archaeology, as well as special applications of these materials. Also the methods used, both methods for cleaning, conservation and restoration, as well as methods for the analysis of the state of the respective artefacts. Topics include oil paintings, paper conservation, textiles and dyes for them, archaeological wood, fossils, stones, metals and metallic coins, and glasses, including church windows.
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Contents
Cover
Title page
Copyright page
Preface
Chapter 1: Paintings
1.1 Cleaning
1.2 Varnishes
1.3 Methods and Materials for Conservation
1.4 Analysis and Analytical Methods
1.5 Forgeries
References
Chapter 2: Textiles
2.1 Textile Colors
2.2 Textiles from Various Locations
2.3 Processing Methods
References
Chapter 3: Archaeological Wood
3.1 Analysis Methods
3.2 Materials for Conservation
3.3 Degradation
3.4 Special Properties
References
Chapter 4: Fossils
4.1 Monograph
4.2 Paleontological Skill and the Role of the Fossil Preparator
4.3 Analysis Methods
4.4 Conservation Methods
References
Chapter 5: Stones
5.1 Deterioration Processes
5.2 Analytical Methods
5.3 Conservation Methods
References
Chapter 6: Glass
6.1 Analytical Methods
6.2 Cleaning Methods
6.3 Production Practices
6.4 Special Uses of Glass Materials
References
Chapter 7: Archaeological Metals
7.1 Cleaning Methods
7.2 Special Uses of Metals
References
Index
Acronyms
Chemicals
General Index
End User License Agreement
Guide
Cover
Copyright
Contents
Begin Reading
List of Tables
Chapter 2
Table 1.1
Dry cleaning materials (28).
Table 1.2
Values for maximal swelling of burnt umber linseed oil films (36).
Table 1.3
Common mixtures used for restoration (61).
Table 1.4
Surfactants and polymers for foam and gel formulations (63).
Table 1.5
Common chemicals used to remove foxing stains (64).
Table 1.6
Four-component systems to solubilize acrylic and vinyl polymers (73).
Table 1.7
Oil-in-water microemulsions (73).
Chapter 2
Table 2.1
Detected colors (12).
Table 2.2
Dyes used for a data base (24).
Chapter 3
Table 3.1
Analytical instrumental techniques (18).
Chapter 4
Table 4.1
Stable carbon and nitrogen isotope results for domestic animal bone collagen and foodstuffs (40).
Table 4.2
Solution adhesives and reaction adhesives (46).
Chapter 5
Table 5.1
Solubility of stone types (6).
Table 5.2
Cyanobacteria and algae affecting building materials (7).
Table 5.3
Fungi affecting building materials (7).
Table 5.4
Bacteria affecting building materials (7).
Chapter 6
Table 6.1
Chemical composition of the glasses (14).
Table 6.2
Glass samples for evaluation of biocorrosion and biodeterioration (16).
Table 6.3
Pigments (38).
Chapter 7
Table 7.1
Optimized experimental conditions for the analysis different materials (22).
Table 7.2
Composition of gold objects from different tombs (41).
Table 7.3
Effectiveness of cleaning methods (56).
Table 7.4
Variation of the elements in the course of the laser cleaning (58).
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Chemicals and Methods for Conservation and Restoration
Paintings, Textiles, Fossils, Wood, Stones, Metals, and Glass
Johannes Karl Fink
This edition first published 2017 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2017 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.
Limit of Liability/Disclaimer of WarrantyWhile the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make This work is sold with the understanding that the publisher is not engaged in rendering professional services The advice and strategies contained herein may not be suitable for your situation You should consult with a specialist where appropriate Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read.
Library of Congress Cataloging-in-Publication DataISBN 978-1-119-41824-5
Preface
This book focuses on the chemicals used for conservation and restoration of various artefacts in artwork and archaeology, as well as special applications of these materials
Also the methods used, both methods for cleaning, conservation and restoration, as well as methods for the analysis of the state of the respective artifacts
The special issues covered concern:
Oil paintings,
Paper conservation,
Textiles and dyes for them,
Archaeological wood,
Fossiles,
Stones,
Metals and metallic coins, and
Glasses, including church windows.
The text focuses on the basic issues and also the literature of the past decade Beyond education, this book may serve the needs of conservators and specialists who have only a passing knowledge of these issues, but need to know more.
How to Use this Book
Utmost care has been taken to present reliable data Because of the vast variety of material presented here, however, the text cannot be complete in all aspects, and it is recommended that the reader study the original literature for more complete information.
Index
There are three indices: an index of acronyms, an index of chemicals, and a general index In the index of chemicals, compounds that occur extensively, e. g., acetone, are not included at every occurrence, but rather when they appear in an important context When a compound is found in a figure, the entry is marked in boldface letters in the chemical index.
Acknowledgements
I am indebted to our university librarians, Dr Christian Hasenhüttl, Dr. Johann Delanoy, Franz Jurek, Margit Keshmiri, Dolores Knabl, Friedrich Scheer, Christian Slamenik, Renate Tschabuschnig, and Elisabeth Groß for their support in literature acquisition In addition, many thanks to the head of my department, Professor Wolfgang Kern, for his interest and permission to prepare this text
I also want to express my gratitude to all the scientists who have carefully published their results concerning the topics dealt with herein This book could not have been otherwise compiled In particular, I would like to thank Dr Virág M Zsuzsanna for the provision of interesting details, which were very helpful for the preparation of this book
Last, but not least, I want to thank the publisher, Martin Scrivener, for his abiding interest and help in the preparation of the text In addition, my thanks go to Jean Markovic, who made the final copyedit with utmost care.
Johannes FinkLeoben, 14th April 2017
Chapter 1Paintings
1.1 Cleaning
Historically, artists have protected oil painting surfaces with varnish. This is a system that allows the varnish to be brushed clean or even washed relatively frequently to remove accumulated surface dirt without exposing the paint to risk (1).
Unfortunately, mastic or other traditional soft-resin varnishes do not last indefinitely. After a few decades the varnish becomes yellow and brittle, losing transparency, and the cleaning process is transformed into the more challenging problem of removing the degraded varnish directly from the painting surface.
Even when new, a varnish may change the appearance of a painting. The varnish increases the transparency of any partly coated pigments or low refractive index medium, and also it imparts a new surface, which is frequently glossy. Mostly, artists have accepted such immediate changes in appearance for the future benefits of protection from dirt and from the risks of dirt removal.
By the eighteenth and nineteenth centuries, when state academies controlled much professional painting practice, the need for a varnish became important.
The concept of finish embodied many notions and became an unwritten contract of quality and reliability between academician and purchaser of art. It seems likely therefore that professional artists and their clients or patrons have always considered the application of varnish as a necessity of permanence and that artists have chosen to exploit its properties for both visual and practical benefit.
Many artists, through ignorance or untidy practice, continued painting up to exhibition deadlines and then immediately brushed varnish onto undried paint. A soft-resin varnish, such as mastic, was mixed into a paint to improve the short-term handling properties. Painting was even continued after varnishing. Adding a soft natural resin to oil paint remained popular into the middle of the 20th century (2).
Annual spring cleaning can be simply done by brushing or vacuuming dust from a varnish. However, washing with water is more effective and may need to be done only every decade or two decades. This procedure requires a wetting agent to ensure a good contact with the varnish surface and to trap dirt within the surface of the liquid.
Traditional recipes using potatoes and onions are well known (3). Saliva is still considered effective. Many other materials have been recommended, including borax and urine.
Conventional varnishes are most susceptible to UV radiation, air pollution, and moisture, and as the varnish ages, it becomes more polar and brittle and more soluble in aqueous mixtures. Aqueous methods for cleaning have been described in a monograph (4).
The varnish surface and, eventually, the body of the varnish disintegrate under the action of repetitive cleaning. Wax or poppy oil coatings can be applied to impregnate the varnish surface to extend its life, but opacity and yellowing may destroy its optical qualities (3).
Perhaps two generations will have passed since anyone saw the painting through a clear fresh varnish. The removal of a well-oxidized mastic varnish from a thoroughly dried oil film using spirits of wine has been carried out for centuries (5, 6).
Alternatives to solvents have been favored by Wolbers (7). The cleaning of paint surfaces is done by using surface active agents in water-based systems. This can be effective in removing oxidized varnishes and oil varnishes as well as dirt. The formulations proposed by Wolbers have provided new tools to remove stubborn material more controllably (1).
1.1.1 Special Considerations
With the rapid developments in new cleaning techniques and analytical techniques it is important and necessary for the conservation community to constantly remind itself of the debate surrounding cleaning. In modern times, this debate began with the National Gallery of London cleaning controversy of 1947 (8). A scientific examination for art history and conservation has been published (9–11). The (surface) cleaning and the removal of varnishes are arguably the most controversial and invasive restoration interventions that a painting will undergo.
Doerner, already in 1921 published warnings about the damage that could be caused by solvents and cleaning (2, 8): The origins of the profession of painting restoration in France have been reviewed (12).
There are countless cleaning materials, most of which are the secret of a particular conservator. One cannot believe all the possible types of materials which are applied to paintings. The strongest caustics, acids, and solvents are used without a second thought. Solutions with unknown composition, so-called secret solutions, are recommended to the public, as something anybody without any knowledge can use to clean pictures. Such cleaning methods are often too successful, right down to the ground layers. In those cases, the conservator covers up his sins by retouching.
It is not uncommon that such locations appear cleaner to the unknowing public than the older version. Even to this day there are conservators who, in all seriousness, claim that they have cleaning materials which remove new paint but stop at the real, original layers. The only thing missing is that a bell should ring when the original paint layer is reached.
The use of balsams for cleaning paintings, in particular copaiba balsam, was fashionable until the end of the 19th century. However, the effect of this balsam was devastating and catastrophic, especially on oil paintings (13).
Copaiba balsam is a resin now known for its softening properties that remain active over a long period of time. An original paint layer treated with copaiba balsam is thus much more sensitive and subject to future damage than prior to the intervention. It is to be noted that commercial solutions such as Winsor and Newton Artists’ Picture Cleaner still contain copaiba balsam (8).
1.1.2 Oxalate-Rich Surface Layers on Paintings
Oxalate salts have been the subject of extensive research as alteration products on calcareous substrates, e.g., stone and fresco. However, there has been relatively little notice concerning their occurrence on other objects such as easel paintings (14). The conservation of easel paintings has been reviewed (15).
An understanding of these materials is important since they can be responsible for significant changes in the surface appearance of artworks and the solubility of the matrices where the oxalates are formed.
Altered, oxalate-rich surface layers can causes substantial challenges for the visual interpretation of the painted surfaces.
Oxalate-containing layers or deposits have been reported on a variety of noncalcareous substrates, including glass (16, 17), bronze (18–20), human remains such as mummy skin (21), and polychrome wood (22) and easel paintings (23–25).
The oxalate salts of calcium, whewellite (calcium oxalate monohydrate) and weddellite (calcium oxalate dihydrate), are those most commonly encountered on painted surfaces, although copper oxalates have also been identified in paint layers containing copper pigments.
Mostly these compounds have been found in deteriorated organic surface layers. Biological and chemical mechanisms have been proposed for the formation of oxalate films on artworks (26).
In the paintings studied in the Philadelphia Museum of Art, the oxalate minerals may likely derive from a gradual oxidative degradation of organic materials in the surface layers and their reaction with calcium-containing pigments or particulate dirt.
The resistance of the calcium oxalates to organic solvents and other cleaning agents presumably affects their enrichment on the surface (14).
1.1.3 Leaching
The cleaning of unvarnished paintings is one of the most critical issues. Several studies exist regarding different cleaning tools, such as gels, soaps, enzymes, ionic liquids, and foams, as well as various dry methods and lasers, but only a few have been performed on the risk associated with the use of water and organic solvents for the cleaning treatments in relation to the original paint binder (27).
The behavior of water gelling agents during cleaning treatments and the interaction of the following elements have been assessed: Water or organic solvents used for the removal of gel residues with the original lipid paint binder.
The study was conducted on a fragment of canvas painting from the 16th to 17th century of Soprintendenza per i Beni Storici, Artistici ed Etnoantropologici del Friuli Venezia Giulia, Udine, by means of Fourier transform infrared (FTIR) spectroscopy, gas chromatography (GC)/mass spectroscopy (MS), and scanning electron microscope (SEM) (27).
1.1.4 Removal of Dirt
The removal of dirt from an unvarnished paint surface may be very challenging, in particular, when the deposit is patchy and resilient; besides which, fragile unvarnished underbound paint surfaces are sensitive to aqueous solvents.
When the dissolved dirt may have impregnated the paint surface irreversibly, nonsolvent cleaning methods are necessary (28).
Dry surface cleaning uses a large range of specific materials like sponges, erasers, malleable materials, and microfiber cloths. However, these materials have not yet been fully integrated into the current practice of conservators. Only a few studies have focused on the use of dry cleaning materials in conservation. Most of the studies have focused on textile and paper conservation (29–32).
The testing methodology and results of dry cleaning materials on underbound and solvent-sensitive surfaces have been reviewed (28).
More than 20 cleaning materials used in conservation have been evaluated. This was based on preliminary cleaning tests on soiled and artificially aged oil paint surfaces. The materials are summarized in Table 1.1.
Table 1.1 Dry cleaning materials (28).
Type
Product name
Composition
Malleable material
Absorene
Starch, white spirit
Malleable material
Groom/stick
Isoprene, chalk
Eraser
Edding R10
PVC, DOP
Eraser
Pentel ZF 11
PVC, DOP, etc.
Eraser
Bic Galet
Vegetable oil
Cloth
Yellow microfiber
PET, PA
Sponge
Smoke sponge
Isoprene rubber
Sponge
Akapad white
Styrene butadiene rubber
Makeup sponge
Etos
Isoprene rubber
Makeup sponge
Hema
Styrene butadiene rubber
Makeup sponge
QVS
Poly(urethane)
Gum powder
Draft clean powder
Styrene butadiene rubber
Aging procedures were performed for 4–6 weeks at temperatures of 50–60°C with variations of relative humidity from 27% to 80% every 6 h. Light aging was done with fluorescent tubes (10,000 Lux) for approximately 600 h at a temperature of 23°C and a relative humidity of 44%. This is equivalent to 11.5 y of aging under museum conditions.
The first series of tests were performed on a naturally aged 30 y old monochrome oil painting on canvas. The second series of tests were performed on water sensitive cadmium red, cadmium yellow, and ultramarine blue tube oil paints. The third series of tests were performed on Gouache samples.
Dry cleaning tests were performed under ambient temperature and humidity. After each test, the paint samples were brushed and vacuum treated. The test results were observed visually, then using light microscopy, followed by electron microscopy.
The test results indicated that the Akapad white and makeup sponges were the least abrasive polishing materials. Both materials are very efficient for the removal of embedded and resilient dirt. In contrast, eraser-type materials proved to be the most harmful materials. Here, chemical residues, i.e., the plasticizers, were detected in the paints. This is a special issue, since plasticisers can soften the paint surface, leaving it more sensitive to dust and vulnerable to abrasion or polishing. On the other hand, Groom/stick and Absorene left a film deposit or particulate residue on both well-bound and porous paint layers. This deposit may harden and embed into the paint layer in the course of aging. In summary, makeup sponges proved to be the most efficient and the safest materials (28).
1.1.5 Effects of Organic Solvents
Several technical studies of the effects of solvents on oil paints in the context of removal of varnish from paintings have been reviewed (33). Also, the historical background of technical studies of cleaning and the various effects of solvents on oil paints have been discussed. These include (33):
Swelling and softening of the paint binder, which can contribute to the vulnerability of paints to pigment loss during cleaning,
Solvent diffusion and retention, and
Leaching, i.e., the extraction of soluble organic compounds from the paint.
The methodological issues in cleaning studies have been discussed, particularly the relationship between studies on model reference paint films and realistic, clinical studies of actual cleaning operations, also considering the related issue of aging of oil paints (33).
1.1.5.1 Solubility Parameters
A number of systems for the specification of solubility properties have found currency in the field of conservation (34). The theoretical foundations of various extant solubility parameter schemes have been critically reviewed in the context of the cleaning of paintings with organic solvents.
Recent advances in solvency specification are discussed, and comprehensive tables of solubility parameter data have been compiled from various sources. One recently developed scheme is that of Snyder and co-workers. This scheme provided the foundation for the proposal of a new composite solubility parameter scheme with potential applications for aiding solvent selection in cleaning and for describing the swelling response of paints to solvents.
It has been proposed that this scheme provides the foundation for an improved understanding of the internal cohesive chemistry of paint films (34). The nature of solubility parameters have been extensively reviewed (35).
The Teas fd solubility parameter is an indicator for solubility (36). Teas solubility parameters are normalized Hansen solubility parameters. The solubility of coatings has been detailed (37).
Values for maximal swelling of burnt umber linseed oil films, aged 12 days at 80°C for various solvents, are collected in Table 1.2. Some of the compounds are shown in Figure 1.1.
Table 1.2 Values for maximal swelling of burnt umber linseed oil films (36).
Solvent
Teas
f
d
Paint film Thickness/
µm
Average area Swelling
Perfluorodecalin
i
-Octane
White spirits
Tetrachloromethane
Ethylbenzene
Dibutyl ether
Dioxane
Amyl acetate
Cyclohexanone
Dichloromethane
Butanone
IMS/iso-octane
Acetone
N
-Methylpyrrolidone
t
-Butanol
DMSO
Propan-2-ol
Butan-1-ol
Methoxypropanol
Ethanol
IMS
Acetone/water 1:1
Methanol
Trifluoroethanol
Triethanolamine pH 9.7
Ammonium hydroxide pH 11.2
Figure 1.1 Solvents for swelling tests.
Further, solvents used for resin solubility testing and their Teas fractional solubility parameters have been detailed (38).
It has been stated that Teas charts have come under fire for a number of simplifications, shortcomings, and fudge factors. Two of the most cogent attacks have been summarized (39, 40)
In short, the Teas system can be criticized for overemphasizing the dispersion forces, neglecting ionic and acid-base interactions, rejecting the overall differences in the magnitude of cohesive energy densities, and assuming solvent and solute randomness (38).
The swelling responses of two oil paint films as a consequence of immersion in solvents of various kinds have been elucidated (41). Two test paint films with the same original formulation are based on proprietary artists’ oil colors containing yellow ocher and flake white pigment bound in linseed oil. One was aged by exposure to high light dosage, and the other was unexposed. Lateral, inplane swelling of the paint films during immersion in solvent was determined by a simple microscopical method using computer-based image analysis.
Results have been reported for the swelling of both paint films in more than 55 common solvents and 14 binary solvent mixtures containing ethanol. The data have been presented as swelling curves of percentage change in area as a function of time and as plots of the degree of maximal swelling as a function of selected solvency indicators. The results have been discussed in comparison with existing data on the swelling of oil films by organic solvents and in relation to the implications for the cleaning of oil-based paints (41).
In research and in actual conservation practice, the conservators have to choose adequate methodologies for carrying out treatments successfully, while respecting the integrity of artworks (42). In particular, the conservators must be able to choose appropriate conservation materials and methods.
Solvents are widely used in cleaning, but solubility issues are also of high importance in consolidation treatments as well as in protective coating applications.
The potential of Hansen solubility parameters for reliable use in the field of artwork conservation has been checked (42). An effort was made to develop an efficient methodology for critical solvent selection.
For this purpose, two different methods were used for the estimation of various artwork conservation materials. A group-contribution method, based on the chemical composition of materials, was applied for the prediction of Hansen solubility parameters of egg yolk, pine resin and seven red organic colorants (Mexican, Polish and Armenian cochineal, kermes, madder, lac dye and dragon’s blood), traditionally used in paintings, textiles and illuminated manuscripts.
Additionally, an experimental setup was used for testing the solubility of the commercial products of synthetic conservation materials, Primal AC-532K, Beva gel 371 a and b, as well as a commercial matt varnish made of dammar and wax. The direct use of Hansen solubility parameters and the relative energy difference between various materials made it possible to carry out ad hoc virtual solubility tests that may apply to real and complex systems such as cultural heritage artworks (42).
1.1.6 Cavitation Energy for Solvent Mixtures
The use of solvent mixtures for surface cleaning in restoration and conservation is widespread. However, there is a lack of knowledge on the true consequences of such a treatment (43).
Azeotropic solvent mixtures have been proposed. It is well known that binary solvent mixtures behave nonideally. This means that the properties of the mixture are neither proportional nor related to the mixing ratio.
The solubility of a material is controlled by the solubilization of the solute and the molecular stabilization of the solute within the liquid phase. There is a difference in the behavior between a solvent mixture and either of the pure solvents as both their solvation properties and their cavitation energy vary significantly.
Solvation relates to the intermolecular forces between the solvent and the solute. A selective solvation may arise from a greater affinity of one component of the solvent mixture to the macromolecules or other components of the paint film (44).
Of particular interest in practice is the cosolvation effect, where each solvent exhibits a selective affinity to one type of structural element. This may lead to an increased solubility of a bistructural material, such as alkyd paints, which contain a phthalic acid polyester backbone in addition to fatty acid substituents. Often, the energy of cavitation is ignored in the considerations.
The free energy of solubilization ΔGm is (45):
In a dissolution process, the free energy of mixing ΔGm must become lower in the course of solubilization. The enthalpy of mixing ΔHm requires similar intermolecular solvent-solvent and solvent-solute forces for a successful action and is mostly positive and small. Therefore, the entropy of mixing ΔSm at a given temperature T is of relevance.
The change in entropy in the course of mixing is mainly dependent on the strength of the intermolecular interaction within the liquid because the liquid cohesion has to be overcome to form a cavity in the liquid to incorporate the solute (46).
The cavity formation can be energetically described by the cohesive energy of the liquid. This can be qualified by the Hildebrand parameter . This parameter controls the entropy of the dissolution process.
In the process of dissolution both endothermic and exothermic steps occur. The exothermic step is an enthalpic process which can be described by the intermolecular interaction between solute and solvent. These interactions may be dispersive, aprotic, or protic.
In a study, the swelling capacity upon immersion of paint films in organic solvent compositions was used to quantify the solvation effects on the binder matrix.
The experiments were done using six solvents, i.e., n-hexane, toluene, chloroform, diethyl ether, acetone, and ethanol, as well as binary mixtures.
Extracts of 2 g of paint sample in 50 ml of solvent were gravimetrically quantified and also characterized using FTIR, direct temperature resolved mass spectrometry, and GC MS. The FTIR studies suggested that the increasing polarity of the solvent mixture results in increased leaching of polar oily components. At swelling levels where changes in volume exceed 7% by volume a massive increase of triglycerides in the leached materials was found.
The swelling data reveal almost equivalent swelling anomalies within oil and alkyd paints. In extreme cases the swelling volume may reach several times the ideal value. This effect is not influenced by the liquid-solid interactions but is caused by liquid-liquid interactions. It has been found that the larger the difference in polarity is between the mixed solvents, the greater the observed deviation is from the ideal behavior.
On the other hand, in apolar mixtures the deviation from the ideal behavior is small. In contrast, mixtures that contain a polar solvent may exhibit strong anomalies in swelling behavior. Thus, ethanol-containing mixtures induce very strong swelling anomalies in oil and alkyd paints, with an increase in volume of up to 200%. This effect is particularly pronounced in ethanol mixtures that form azeotropes.
The swelling anomalies correlate with a change in the boiling point (47). The swelling data have been documented in much detail (43). The properties of solubilization and the swelling capacity of solvent mixtures are directly relevant to the extraction of low molecular compounds in paintings.
1.1.7 Hydrogels Based on Semi-Interpenetrating Networks
Water-based detergent systems offer several advantages over organic solvents for the cleaning of cultural heritage artifacts in terms of selectivity and gentle removal of grime materials or aged varnish, which are known to alter the readability of the painting (48).
Unfortunately, easel paintings show specific characteristics that make the usage of water-based systems invasive. The interaction of water with wood or canvas support favors mechanical stresses between the substrate and the paint layers, leading to the detachment of the pictorial layer.
In order to avoid painting loss and to ensure a layer-by-layer control of grime removal, water-based cleaning systems have been confined to innovative chemical hydrogels, specifically designed for cleaning water-sensitive cultural heritage artifacts.
The hydrogels are based on semi-interpenetrating chemical poly (2-hydroxyethyl methacrylate)/poly(vinylpyrrolidone) networks with a suitable hydrophilicity, water retention properties, and sufficient mechanical strength to avoid residues after the cleaning treatment. The monomeric compounds are shown in Figure 1.2.
Figure 1.2 Hydrogel monomers.
The water retention and release properties have been studied by quantifying the amount of free and bound water using differential scanning calorimetry (DSC). The mesoporosity was obtained from SEM. The microstructure was assessed using small angle X-ray scattering. The efficiency and versatility of the hydrogels in confining and modulating the properties of cleaning systems was shown in a case study (48).
1.1.8 Organogels
Organogels have been described as cleaning tools for painted surfaces (49). These combine the most attractive features of cleaning liquids and normal gels while diminishing the deleterious characteristics of both.
The latent gellant, poly(ether imide) (PEI), reacts with CO2 at room temperature in organic solutions to produce an ammonium carbamate form PEI CO2. Ammonium carbamate is a salt that is formed by the reaction of ammonia with carbon dioxide or carbamic acid. The compound is shown in Figure 1.3.
Figure 1.3 Ammonium carbamate.
The charged moieties turn into three-dimensional polymer networks that immobilize the liquids as gels.
The properties of the initial solution can be reestablished by the addition of a small amount of a weak acid. The acid displaces the CO2 molecules and the PEI chains become positively charged.
Contact angle and FTIR measurements as well as visual comparisons of the surfaces before and after application of the gels have been done. The visual changes are substantiated by rheological measurements. The results indicated that these gels are valuable cleaning tools for painted surfaces of historical and artistic interest.
In particular, the PEI CO2-based organogels are highly effective in removing certain surface patinas from painted supports. A surface layer of dammar was completely removed from a test canvas with oil paint, an aged painting from the 19th century, and a 15th century oil-on-wood panel attributed to Mariotto di Cristoforo. In addition, a surface acrylic polymeric resin which was used in a restoration performed during the 1960s was successfully removed from Renaissance wall paintings decorating the Santa Maria della Scala Sacristy in Siena, Italy.
The isothermally rheoreversible gel represents a new, highly versatile, and very efficient method for removing aged surface patinas from works of art (49).
1.1.9 Microemulsions and Micellar Solutions
Nanoscience and nanotechnology have evolved as revolutionizing materials science. These materials are creating new approaches to conservation science, leading to new methodologies that can revert the degradation processes of works of art. The most advanced current methodologies are (50):
The use of water-based micelles and microemulsions, neat or combined with gels for the removal of accidental contaminants and polymers used in past restorations, and
The application of calcium hydroxide nanoparticles for the consolidation of works of art.
The combination of different nanotechnologies allows a conservator to provide, in each restoration step, interventions respectful of the physicochemical characteristics of the materials used by the previous artists. The methods using nanoscience are continuously growing (50).
The synthesis and preparation of colloidal systems tailored to the consolidation and protection of wall paintings, plasters and stones have been detailed. Also, two case studies, widely representative of typical consolidation problems, have been discussed, namely the preservation of wall paintings belonging to a Mesoamerican archaeological site and the consolidation of two Italian Renaissance buildings (51).
1.1.10 Acrylic Paintings
The Modular Cleaning Program has been developed by the Getty Conservation Institute, California State University, the University of Delaware, and the Conservation Division of the Winterthur Museum.
The target of the project was to systematically investigate the problem of residues left on surfaces after cleaning with a system that contained nonvolatile components.
The Modular Cleaning Program should help the conservator to achieve precision in designing a cleaning system. It does not direct the conservator on how to clean a surface, but rather organizes the cleaning options in a logical and expandable way.
The Modular Cleaning Program models the physical and chemical interaction. It has 19 internal databases, but the conservator interacts directly with only a few, the components database. The components database includes (52):
Components database,
Repository of physical constants,
Chemical names;
Chemical Abstracts Service registry number,
Molecular weight,
Physical form,
Density,
Acid dissociation constants for weak acids and bases,
Formation constants for chelating agents,
Hydrophile-lipophile balance number,
Critical micelle concentration,
Aggregation number,
Cloud point for surfactants,
Boiling point,
Hildebrand, Hansen, and Teas solubility parameters,
Dipole moment,
Index of refraction,
Dielectric constant, and
Molar volume for solvents.
There is also a separate database of binary and ternary azeotropes for solvent mixtures.
Thus, the modular cleaning program accesses the relevant physical constants as it calculates suitable cleaning formulations in the solutions database and another of the databases actively used by the conservator (52).
Aqueous solution calculations are based on the appropriate physical constants from the components database and user-specified data. For pH buffer and chelating agent solutions, acid dissociation constants and user specifications of counterions, cleaning solution concentrations, and pH values are used to calculate the needed concentrations of ionic and molecular species in solution and the amount of the counterion necessary to set the solution to the specified pH.
In addition, solvent mixtures are modelled in the three-dimensional Hansen space, but Teas and Hildebrand parameters are also calculated. The solvent phase of Carbopol®-based solvent gels and Pemulen®-based emulsions are modeled as simple solvent mixtures. The gel phase of solvent gels is based on stoichiometric relationships between the Carbopol, the user-specified organic amine or amines used to neutralize the Carbopol, and the inferred stoichiometric amount of water necessary to form a gel. The aqueous phase of Pemulen-based emulsions is modeled as an aqueous cleaning solution thickened with Pemulen to which a solvent or solvent mixture is added (52). Pemulen polymers are high molecular weight, crosslinked copolymers of acrylic acid and C10–C30 alkyl acrylate. Acrylic acid is shown in Figure 1.4.
Figure 1.4 Acrylic acid.
Information on the background and use of the modular cleaning program has been detailed (53). The modular cleaning program software is freely available to professional conservators.
1.1.11 Acrylic Emulsion Paintings
Cleaning acrylic emulsion paintings is challenging because of the material properties of the paint films, in particular their solubility (54). The effect of aqueous treatments on acrylic paints was investigated. Several paint manufacturers were asked for their recommendations for cleaning these paintings and conservators were also asked to comment on the damage observed in them and on the treatments applied.
Their responses showed that aqueous cleaning treatments are used, despite the associated risks, and that more technical information is needed about the effects of cleaning. The changes in the physical and mechanical properties of an aged cobalt blue paint were evaluated as a result of the exposure to aqueous cleaning solutions. The results suggested that a short immersion in these solutions caused a drop in most of the mechanical properties. Interestingly, longer immersions did not cause a drop in the mechanical properties.
The drop after the short immersions seemed to be mostly due to the great increase in the thickness of the paint films (54).
1.1.12 Complications in the Cleaning of Acrylic Paint Surfaces
Acrylic paints are in many ways remarkably durable. However, the surfaces are rather fragile. This fragility, coupled with the tendency for the paintings to be quite large, leaves them vulnerable to damages from handling. Fingermarks and scuffs can be disfiguring and difficult to ameliorate (52).
In addition, the gradual accumulation of surface grime is of general concern (55). It has been shown that even in a presumably benign museum environment the slow accumulation of surface grime will lead to a discernible visual alteration of a paint surface after approximately 50 years.
Research has been done to identify some of the special properties of acrylic paints (56, 57).
The examination of acrylic paint formulations has revealed the variables that must be considered when devising a cleaning strategy for these paint surfaces (7, 58).
Actually, the base of an acrylic emulsion paint system is the acrylic emulsion itself. It is created from water, the acrylic monomer, or monomers, and some materials to stabilize the monomers into micelles, and also to control the pH, initiate the polymerization reaction, and prevent foaming.
Additional ingredients, such as thickeners and coalescing agents, transform it into a successful paint binder.
Furthermore, pigments and extenders must be mixed with water, dispersing agents, and other components that are then added to the composition of the acrylic emulsion paint.
All of these materials form a very complicated and interdependent system compared to any traditional paint system. So an attempt can be made to predict the potential sensitivity of the paint on the basis of the sensitivities and properties of each of these many components.
These potential sensitivities can be categorized into some broad classes related to the effects of (52):
pH,
Ionic strength, and
Surfactant migration.
Also, it is important to think about the physical structure of the acrylic paint film.
Acrylic emulsion paints dry in a different way than other paint systems. The initial drying occurs by evaporation of much of the water, leaving the pigments and spherical polymer-containing micelles to form a close-packed matrix. In the next stage of drying the water held by capillary forces between the polymer spheres and pigment particles is evaporated.
Coalescing agents and the nature of the polymer itself allow the spheres to diffuse into adjacent spheres, forming a film (59).
The resulting structure of the film will be between the solid membrane of a fully coalesced film and an inhomogeneous structure with pores, irregularities, and imperfections.
So, the physical structure of the acrylic film has particular implications when considering surfactant migration and the ionic strength of the cleaning systems (52).
1.1.13 Poly(vinyl acetate) Paints
Synthetic paints are mostly a complex mixture of additives that are likely to influence the response of the paint to different treatment methods. Furthermore, their high sensitivity to most organic solvents narrows the choice of products that can be used.
The knowledge that has been obtained during the cleaning of traditional oil or egg tempera paintings cannot be correlated to contemporary paintings. Therefore, additional research concerning the chemistry and properties of synthetic paints and their response to the conservation practice is needed.
Preliminary results on the effects on test films of poly(vinyl acetate) (PVAc) paint have been published (60). These studies were done for cleaning paintings by Julião Sarmento. This artist is one of the most prominent contemporary Portuguese artists who has made wide use of PVAc paints. In the 1990s he started a series of paintings, commonly called the White Paintings. These paintings already require a surface cleaning because of a combination of their monochromatic white backgrounds and their affinity for dirt pickup.
Samples of a PVAc emulsion paint containing titanium dioxide in the rutile form were artificially aged using a xenon arc light source for 3250 h. In the course of the irradiation, the mock-ups were exposed to dirt particles, i.e., atmospheric particles. Commonly used cleaning methods were subsequently tested:
Water,
Water and a nonionic surfactant, i.e., Brij700S, and
White spirit.
Brij is a poly(ethylene glycol) octadecyl ether compound. The use of a soft eraser (Akapad white) was also examined to reproduce a real case of cleaning a work by Sarmento. White spirit was included since aliphatic mineral spirits are known to have a minimal effect on acrylic paints (57).
Since it is known that most of the material lixiviated from a paint sample occurs in the first few minutes, the effect of a 5 min immersion was evaluated.
Artificial aging studies indicated that the loss of the paint plasticizer, diisobutyl phthalate, is the major alteration between aged and unaged samples. Diisobutyl phthalate is shown in Figure 1.5.
Figure 1.5 Diisobutyl phthalate.
The FTIR spectra from the artificially aged control sample showed no significant signs of surface enrichment with diisobutyl phthalate. Therefore, it was expected that immersion could only remove negligible quantities of plasticizer still present on the test films. Only minor changes were observed in the FTIR spectra that can be correlated with the extraction of this additive. Moreover, diisobutyl phthalate was not detected among the residues remaining after immersion (60).
Because of the irregular surface topography and the affinity for the dirt that is retained by the paint surface, the immersion of the samples was not efficient for the removal of dirt. This was confirmed by optical microscopy and also by colorimetry.
Pure water did not induce relevant alterations concerning the morphology of the white paint. However, with an aqueous solution of Brij700S, the smoothness of the latex particles is found to be lost. Here, the polymer surface displays a more irregular texture, which could be the result of surfactant residues left on the paint.
The cleaning with an eraser resulted in both particles and/or eraser additives left on the surface.
A physical disruption of the latex paint was only detected at the nanoscale. However, it could result in a flattening of the paint surface that is contrary to the distinctive pigment agglomerates and filler texture typical of these paintings (60).
1.1.14 Surface Cleaning
The developments of cleaning methods for paintings in Italy over the course of the last two decades have been described (61). In Italy the restoration has been mostly practiced by private restorers.
