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In situ dielectric analysis for canvas paintingsT Y Amy Chan a , Marianne Odlyha a & Mikkel Scharff ba Department of Chemistry Birkbeck College , University of London , 29 Gordon Square,London, WC1H 0PPb The School of Conservation , The Royal Danish Academy of Fine Arts , Esplanaden 34,Copenhagen K, DK‐1263, DenmarkPublished online: 17 Sep 2010.

To cite this article: T Y Amy Chan , Marianne Odlyha & Mikkel Scharff (1995) In situ dielectric analysis for canvas paintings,The Conservator, 19:1, 10-19, DOI: 10.1080/01410096.1995.9995089

To link to this article: http://dx.doi.org/10.1080/01410096.1995.9995089

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10

IN SITU DIELECTRIC ANALYSIS FOR CANVAS PAINTINGST Y Amy Chan, Marianne Odlyha and Mikkel Scharff

Abstract

Preliminary measurements have indicated that thecoaxial probe technique in the microwave region mayprove to be useful for monitoring conservation treatmentof paintings. Measurements were performed on anumber of cleaning agents, natural glue samples, paintfilms, primed canvas samples during deacidificationprocesses and localised moisture treatment, andpaintings undergoing humidification using a suctiontable. The effect of localised moisture treatment was alsostudied on samples of both recent and nineteenthcentury oil-based priming, recent priming which hadbeen wax-lined, and then acrylic based priming. Resultsshow that the probe is sensitive to the increasing ordecreasing presence of small polar molecules (eg water,methanol) with time.

Dielectric analysis in conservation science

Recent concern at the Courtauld Institute of Art with theeffect of cleaning agents on paintings prompted twostudies which were carried out as final year studentprojects in collaboration with Birkbeck College12. Theyfocused on the effect of solvents and moisture on themechanical and surface properties of paint films3. In thecourse of these studies a number of techniques wereused for the first time. Amongst these was the techniqueof dielectric analysis in the temperature domain. It wasfound that there were differences in the measuredparameters between untreated lead white paint films andthose which had been immersed in propanone and thendried. The sensitivity of the technique to changes in thechemical composition of the paint film as confirmed bygas chromatography, mass spectrometry4 has led tofurther dielectric studies on paint and primed canvassamples56'7. Recent studies in the low frequency range10"4 to 104 Hz have shown that pigmented paint films dorespond to relative humidity in a manner which dependson pigment type and medium content8. Prepared paintfilms containing flake white, Prussian blue and theirmixtures showed significant differences in response toan increase in relative humidity from 54% to 94%.

The present work also uses the dielectric analysistechnique but in the microwave frequency range. Themeasuring system is also different in that it uses a smallcoaxial probe which can be placed directly on thesample.

Dielectric properties of materials

Dielectric properties of materials are dependent on theirchemical composition and particularly on the permanentdipole moments associated with any molecules whichmake up the samples of interest. Good insulators aretermed non-polar because the constituent molecules do

not have a permanent dipole moment capable of rotatingin an electric field. These materials have a dielectricconstant lying between 2 and 5 which is independent offrequency. The important phenomenon contributing tothe frequency dependence of the dielectric properties isthe polarisation arising from the orientation with theimposed electric field of molecules which havepermanent dipole moments. Hence the dielectricproperties are no longer constant but vary as a functionof frequency.

Fig 1 shows the dielectric spectrum of deionised water at20°C. The relative permittivity in the low frequency region(< 2 GHz, where G is the abbreviation of giga and 1G =109) is constant and is called the static permittivity (ordielectric constant). Water is a strong dipole and has anunusually high static permittivity (80.1) so its presence(or absence) can be detected easily. This particularproperty of water has made it possible to measure themoisture content of a number materials such as grains9,foods10, wood11 and textiles12.

In the study described in this paper, the relativepermittivity of the sample is monitored during treatmentinvolving elevated relative humidity conditions whichoccur in suction tables or localised relative humiditychambers or where moisture is introduced directly ontothe surface of the painting, or in cases where a moisturegradient is applied on account of the differentialresponse to moisture of canvas, glue, and paintlayers13'14. Since the dielectric properties of the samplevary during treatment in the time domain, measurementsof relative. permittivity at a particular frequency arepresented as a function of time. In this study 1 gigahertz(GHz) was chosen. This is where the relative permittivityof water in the static region is 80.1 while thecorresponding values for the paint films before treatmentare comparatively low (Fig 1, Table 1).

Complex permittivity of water at 20°C

oo •

8 0 "

6 0 "

4 0 "

2 0 "

o-

^ 8 0 . 1

1GHz]

Relative permittivity——— Dielectric loss

\

A10 8 10 9 10 10 10 11 10 12

Frequency (Hz)Fig 1 The complex relative permittivity of deionised water

at 20°C as a function of frequency. The staticpermittivity is 80.1 and the relaxation frequency is17 GHz.

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Table 1

Sample

1

2

3

4

Description of paint film samples

Pigmenttype

Prussianblue

basic leadcarbonate

burntsienna

basic leadcarbonate

Date

Jan 1991

Jan 1991

April 1993

April 1993

Thickness(mm)

131

139

322

835

Weight(mg)

3.838

6.197

8.112

50.125

Relativepermittivityat 1 GHz

3.7

9.0

4.6

8.7

The coaxial line technique

The open-ended coaxial line probe with the networkanalyser measuring system is a relatively new technique.It was originally proposed by Tanabe and Joines in1976 and its first use was to measure the electricalproperties (complex permittivity and conductivity) ofsolutions and various tissues17. This work was presentedin 1980. Requirements are that the sample under test ishomogeneous, semi-infinite, non-magnetic, isotropic,and where more than one component is present thateffects are additive.

Recently, the performance of a 3.6 mm OD probe andmeasuring system was investigated, in detail, boththeoretically and experimentally, for polar liquids over afrequency range of 0.13-20 GHz18. This microwavefrequency region has the advantage that measurementsare fast and can follow the changes in sample propertiesin real time.

In this paper a new and small probe (0.86 mm OD) wasdesigned particularly for the purpose of makingmeasurements directly on surfaces and the measuringsystem is described in one of our recent publications19.The small probe was used for the following reasons:i it would provide the required spatial resolution to

distinguish between the differing response in moistureuptake of adjoining regions of a painting with varyingpigmentation and media.

ii the small probe size implies lower penetration power.In terms of measurements directly on the surfaces ofsolids this then compensates for lack of homogeneityof the sample as the probe is only measuring localsurface effects.

Objectives

The aim in this paper is to demonstrate the performanceof this novel small probe with solutions of organicsolvents used for cleaning paintings, and then, and ofparticular value for paintings, that it can be usedeffectively to make measurements on solid surfacestreated with water (liquid and vapour) and solutions

containing small polar molecules. This could thenprovide a totally non-destructive method for monitoringthe progress of moisture treatments or other treatmentsinvolving the use of polar materials on paintings.

Experimental procedure: organic solvents

The performance of the 0.86 mm OD coaxial probe in thefull frequency range was first tested on a number oforganic solvents and their mixtures which are commonlyused to clean paintings. The tested mixtures containedthe following: methanol and oil of turpentine, propanoneand mineral turpentine, cyclohexane and methylbenzene(toluene), propanone and methylbenzene (toluene),propan-2-ol and butan-2-one, propan-2-ol and 2-methylheptane.

Natural glues

The response to moisture of a piece of lining adhesivemixture containing processed sturgeon glue with honeywas monitored with the small coaxial probe. The samplewas placed in a desiccator containing a saturatedsolution of potassium nitrate (about 94% RH) for a periodof three days. The relative permittivity at 1 GHz and theweight uptake of this sample was measured during thehumidification process.

Paint films

Naturally aged oil-based paint samples containing thepigments basic lead carbonate, Prussian blue and burntsienna respectively were used to study the effect ofpigment type on water absorption. A description of thesamples is given in Table 1. These samples wereimmersed in water and their relative permittivity andincrease in weight measured on a daily basis over a totalperiod of four days.

Samples of the original tube paints from C. Roberson &Co. Ltd. were also measured and the values comparedto those obtained from measurements on the solid paintsurfaces. This was carried out to determine whetherthere were any problems in the contact between thesmall probe and the solid paint surfaces.

Non-aqueous deacidification treatment of nineteenthcentury primed canvas

Three samples of nineteenth century primed canvas(1x1 cm2) were treated with a polar solvent, methanol.Samples were provided by the Conservation Departmentof the Tate Gallery and had been taken from the original,primed loose lining of Landseer"s painting "Study of aLion" (No. 1350, c 1862). Methanol is a solvent which isused as a vehicle in the non-aqueous deacidification ofcanvas. The aim in this study was to use the smallcoaxial probe to determine how rapidly the methanolvapour leaves the treated canvas. Several drops ofmethanol were placed on the canvas and measurementswere made over a period of time.

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Non-aqueous deacidification treatment ofseventeenth century manuscripts

Small paper samples (4 mm2) were taken from the edgeof one of the letters in the Fanshawe archive, datedLisbon, 20th August 1665. The effects of conservationtreatment had been assessed previously using severaltechniques20. Of particular relevance to this paper arethe measurements which had been made bythermomechanical analysis (TMA). These showed thatthe degree of swelling of the paper sample after non-aqueous deacidification was of the order of 1.36-1.5times greater than the sample before treatment; thetreated samples had become more sensitive to moistureuptake. It was therefore appropriate to measure thedielectric response of samples before and aftertreatment to determine whether it varied in a similarmanner.

Localised humidification using blotting paper

Samples studied included the following :i APTD: commercially prepared cotton duck, acrylic

priming with titanium dioxide from Winsor & Newton,ii GSP: glue sized linen canvas aged (light 20,000 lux

and uv filtered for 34 days at 70°C and 50% RH) andthen deacidified with Mg (HCO3)2.

iii CD2: cotton duck with two coats of acrylic priming andCD2W: the same sample but wax lined.

i lower the probeto the aperature

'"-' t^iuL-

wet blot tingpaper.

non-PVC film

sample

glass slide

Fig 3 The grey arrows show the measuring procedurefor monitoring dielectric changes in the paintcomposite while using moist blotting papertreatment.

Fig 4 The painting ("Composition" by Pierre Lagarec.1940) undergoing humidity treatment on asuction table.

iv LION: Landseer"s "Study of a Lion" (c 1862). Thepriming contains a mixture of basic lead carbonateand calcium carbonate in linseed oil (Fig 2).

The samples were treated in the following way: a pieceof moist blotting paper was applied to the primed surfaceand the dielectric response was measured with the smallprobe over a period of time. The whole measuringsystem was covered with a piece of commercial non-PVC cling film and particular care was taken duringmeasurements to minimise any moisture loss (Fig 3).

Fig 2 LION: Landseefs "Study of a Lion" (1862) undermagnification (x2).(a) Surface of priming.(b) Canvas sun'ace.

Non-invasive in situ measurements on fabric-supported paintings undergoing humidification andde-humidification on a suction table

The small probe was used, for the first time, to monitorthe dielectric response of a painting ("Composition" byPierre Lagare c 1940) which was undergoing humiditytreatment in the School of Conservation of the RoyalDanish Academy of Fine Arts. The painting washumidified from 40% RH to 78% RH and thendehumidified on a suction table (Fig 4). Spotmeasurements were performed both on the surface ofthe painting and on the canvas side of the painting in aregion about 10 cm from the lower right hand comer ofthe painting. The suction table consists of an air-permeable tabletop, usually a perforated metal plate,onto which the painting is placed. Water vapour isreleased from below during treatment. After a suitable

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period of humidification air is drawn through to drythe painting structure. A complete description of theoperation of the suction table is given elsewhere21.

Results and discussion: organic solvents

Figs 5 and 6 illustrate the dielectric properties of themixtures of methanol and oil of turpentine and that ofpropanone and mineral turpentine respectively. Themolecules of methanol and propanone are polar andrelatively small compared with oil of turpentine andmineral turpentine. Hence they relax at higherfrequencies in the GHz region. Although both oil ofturpentine and mineral turpentine are polar, therelaxation frequency depends very much on the size ofthe molecule and the distribution of charge throughout it.The relaxation of these large molecules occurs in themegahertz region and is well separated from that of thesmaller molecules. Hence their relaxations are notshown in the GHz region. It appears that where thedielectric response of individual components was

Volume content of propanone

20 n

T 15 -

Q .

aDC

10

5 "

Volume content of methanol

10 1010

Frequency (Hz)

(a)

Volume content of methanol

Frequency (Hz)

(b)Fig 5 Dielectric properties of the mixtures of methanol

and oil of turpentine as a function of frequency(a) relative permittivity, and (b) dielectric loss.

>

nK3fftO3xtnttir^

10 10

10 -

~ 8-

Frequency (Hz)

(a)

Volume content of propanone

El•B•BDA

100%80%67%50%33%20%0%

4-»

"ia.

>

a

CC

1 0 1 0

Frequency (Hz)

(b)Fig 6 Dielectric properties of the mixtures of propanone

and mineral turpentine as a function of frequency(a) relative permittivity, and (b) dielectric loss.

significantly different, the response to the mixture wasfound to depend in a quantitative manner on the volumefraction of each component.

Natural glues

Fig 7 illustrates the change in relative permittivity at 1GHz of the sample of sturgeon's glue against itspercentage weight gain. Both parameters increasedduring the humidifying process. The relative permittivityof free water at 1 GHz and 20°C is 80.1 (Fig 1). Theabsorbed and adsorbed water molecules increase therelative permittivity of the sample in a quantitativemanner. Hence the relative permittivity is directly relatedto the moisture content of the sample. It was alsoobserved that the probe could detect differencesbetween the surface of the sturgeon's glue with naturallycrystallised sugar coating and that which was freshly cutwithout the coating (Fig 8).

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NXC3

Sturgeon glue with honey

a 0ID

DC 0 10 20% weight gain

Fig 7 Plot of relative permittivity at 1 GHz againstpercentage weight gain of a sample of sturgeon'sglue with honey.

Fig 8 Sample of sturgeon's glue with honey(1) naturally crystallised coating(2) the freshly cut surface without coating.

The coating thickness was estimated to be of the orderof 0.5 urn. Values for the relative permittivities werefound to be 2.9 and 7.6 respectively. This provides anindication of the capability of the small probe for surfacemeasurements. The ability of the small probe to measurechanges with length of exposure to conditions ofelevated RH could be used to monitor the re-activationof glue layers of canvas paintings where the glue hadlost its adhesive power.

Paint films

Fig 9 illustrates the percentage increase of relativepermittivity at 1 GHz as a function of percentage weightgain for the four samples. It is noted that the Prussianblue film absorbed water most rapidly and hence has thehighest permittivity values. The relative permittivity oftube paint samples of Prussian blue, basic leadcarbonate and burnt sienna were found to be 3.7, 9.0and 3.5 at 1 GHz respectively and are of the same orderas those measured directly on the solid surfaces aslisted in Table 1. It indicates that the contact between

the probe and the paint film surfaces was reasonablygood.

Non-aqueous deacidification treatment of nineteenthcentury primed canvas "Study of a Lion".

Fig 10 (a) shows the variation in relative permittivity at 1GHz of the canvas as a function of time. It appears thatmost of the methanol evaporates in four minutes. Asecond measurement was made with the probe incontact with the surface of the primed layer. This time asseen in Fig 10 (b) the methanol diffused from the canvasto the priming and then evaporated.

A third measurement was made with the probe removedbetween measurements on the surface of the primedlayer to allow the methanol to evaporate more readily asshown in Fig 10 (c). The results are similar to those inFig 10 (b) but the loss occurs over a shorter time andmethanol appears to evaporate after six minutes.

0 2 4 6 8% weight gain

(a)

Sample 3

Sample 4

0 2 4 6 8 10% weight gain

(b)Fig 9 Plots of percentage increase of relative permittivity

at 1 GHz as a function of percentage weight gainduring water immersion over a period of four days.Table 1 describes the paint film samples.

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Measurement 1

N

af 121ta

>. 10 1

Z 8"E

a.>

Time (mins)(a)

Measurement 2

DC

4 "

210 15

Time (mins)(b)

Measurement 3

20

8 10

Time (mins)(c)

Fig 10 Relative permittivity of the "Study of a Lion" at1 GHz after methanol treatment on the canvasside(a) measured on the canvas side(b) measured on the priming side with the

probe in contact with the sample duringmeasurement

(c) measured on the priming side with theprobe removed from the surface betweenmeasurements.

Non-aqueous deacidification treatment ofseventeenth century manuscripts

Figs 11 (a) and (b) show an increase in both thepermittivity and the loss of the samples in the wholefrequency range after rapid water immersion just to wetthe samples. The values of the paper sample aftertreatment are of the order of 1.24-1.42 and 1.3-1.49times greater than those before treatment. The degree ofswelling of the paper sample, as measured bythermomechanical analysis, after treatment is also of theorder of 1.36-1.5 times greater than before treatment*0.The tendency for increase in swelling of the paper inwater is of the same order as the increase in dielectricresponse and confirms the enhanced sensitivity of thetreated sample to moisture.

Localised humidification: cotton duck

Fig 12 (a) shows that the rate of increase in relativepermittivity values, particularly in the first ten minutes ofa localised humidification treatment using damp blottingpaper, is significantly higher for the sample of cottonduck with two coats of acrylic primer than it is for thesame sample when wax lined. The rate of increasecalculated from the slopes of the lines are 0.3 and 0.9respectively. The data show clearly the effect of relativepolarity of the coating : polar liquid (water) diffuses mostrapidly through the more polar coating. It is known thatchemisorption requires similar polarity and so smalleramounts of water will pass through non-polar waximpregnated surfaces22. This concurs with previousobservations on the stress strain response of canvaspaintings where it was .found that .beeswax impregnationof the sample did suppress the stress variations of thepainting in fluctuating values of humidity. The sampletested was freshly impregnated and as pointed out byHedley an aged sample with the presence of fractures inthe wax would behave differently14.

Response of media used for priming : oil, glue andacrylic

Fig 12 (b) shows the difference in response with time forthe three samples which differ in medium type (aged oil,glue, acrylic) and pigmentation (titanium dioxide in APTDand a mixture of basic lead carbonate and calciumcarbonate in LION). The relative permittivity of LION islower than those of APTD and GSP before treatment (atzero minutes). This is due to the difference inpigmentation of the samples. The literature value for thedielectric constant of titanium dioxide is appreciablyhigher (86 and 170) while those of lead carbonate andcalcium carbonate are 18.6 and 8.6 respectively23.

The high dielectric constant of titanium dioxide accountsfor the comparatively high dielectric constant of APTDand GSP. The rate of moisture uptake calculated as theslope in Fig 12 after the first fifteen minutes is greater forthe LION sample than it is for the acrylic and the gluesamples. The calculated slopes are of the order of LION(1.2), APTD (0.6) and GSP (0.3). The aged oil binder ofthe LION sample will contain oxidation decomposition

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25"

•£ 20"

E 15 J

<DQ.

a.

10"

Before treatment (Dry)After treatment (Dry)Before treatment (wet)After treatment (wet)

properties of this sample using thermomechanicalanalysis (TMA) in the compression moder.24

++++l'<

010

10"

8"

6"

4"

5 2 ,

no

ifo

O

^ ^ - +

10 1 0

Frequency (Hz)(a)

Before treatment (Dry)After treatment (Dry)Before treatment (wet)After treatment (wet)

• • • • •

5SL

10 1010Frequency (Hz)

(b)Fig 11 Dielectric properties of seventeenth century

paper sample before and after brief waterimmersion.(a) relative permittivity.(b) dielectric loss.

products which are highly polar and provide locations forthe chemisorption of water. The more recent samplesAPTD (the youngest) and GSP show a slower rate ofmoisture uptake. After about thirty minutes all threesamples reached an equilibrium value. The lower andunexpected value for the glue may be due to the ageingand deacidification treatment of canvas. It is likely thatthe polar decomposition products from canvas duringageing have been affected by the deacidificationtreatment. Further measurements are required toevaluate the effects of ageing and deacidification ondielectric response.

Results in Fig 13 (a) show that the relative permittivity ofthe nineteenth century primed canvas "Study of a Lion"at 1 GHz increased as a function of time during blottingpaper treatment and reached saturation after abouttwenty five to thirty minutes. Though the surface of thesample appears to reach its equilibrium moisturecontent, it appears that moisture continues to penetrate,after the thirty minutes, from the surface into the differentlayers (paint, size and canvas) of the sample. This wasshown by MacBeth in the study of the mechanical

Fig 13 (b) shows the softening of the sample as afunction of temperature after humidification at variouslevels of RH (54%,97%) and localised blotting papertreatment. The percentage compression corresponds tothe changes in thickness of the sample under test. It isclear that the sample treated for ninety minutes is softerthan the one treated for thirty minutes. At 20°C, thedifference in compression is about 2%. This difference,which is not seen in the dielectric measurements, isobserved by TMA as this measures the bulk response ofthe composite sample.

Non-invasive in situ measurements on fabric-supported paintings undergoing humidification andde-humidification on a suction table

Substantial differences were detected in the response ofthe painting "Composition" by Pierre Lagare after onecycle of humidification for periods of thirty to fifty five

Dielectric measurement at 1 GHz15 -i

0 10 20 30Time (mins)

(a)Dielectric measurement at 1 GHz15 T

20 30 40Time (mins)

(b)Fig 12 Relative permittivity values (a) CD2 and CD2W

(b) LION, APTD, and GSP after localisedmoisture treatment as a function of time.

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minutes and then ninety five minutes, when a certainamount of moisture uptake occurs in the painting. This isshown on the lower canvas and upper paint sides duringtreatment Fig 14.

The values of relative permittivity were also observed tofall during the drying process. Results are shown overthe full frequency range since in this case the responsewas not sufficiently large at 1 GHz. This suggests thatthe small probe is not as sensitive to changes inducedby moisture in the vapour state as it is in cases wheremoist blotting paper was actually applied to the surface.This is reasonable since the diffusion of water vapourwill take longer to affect the moisture content of thepainting.

Dielectric measurement at 1 GHz

10

105 n

100 -

2.95co» 90cooQ-85o

°80

75

Time (mins)(a)

TMA

54%RH97%FHB/paper-30mjnsB/paper-90mins

-60 -40 -20 0 20 40 60 80Temperature ( °C)

(b)Fig 13 Nineteenth century primed canvas "Study of a

Lion".(a) Change of relative permittivity at 1 GHz as a

function of time.(b) Softening of the sample as a function of

temperature after various types of moisturetreatment. Sample conditioned at 54% RH,97% RH, 30 minutes, and 90 minutes afterblotting paper treatment.

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>

• • 1

10 1

Frequency (Hz)(a)

Paint side

I

10 1 0

o•a

•

&

0

0

1

0 mln1530

55

95

O•D

•

minmlnmlnmln

0 min15 min30 mln55 mln95 mln

Frequency (Hz)(b)

Fig 14 "Composition" Piene Lagare (c 1940), relativepermittivity during humidification on a suctiontable, measurements made on the (a) canvasside, and (b) paint side.

Conclusions

Preliminary measurements have indicated the potentialof the small coaxial probe to monitor non-destructivelychanges which involve treatment with moisture or polarsolvents. In the case of localised moisture treatment, anddeacidification the response occurs more rapidly sincewater and methanol are present in the liquid state andtheir diffusion into the material and loss from the materialalso occurs rapidly. The rapid change observed is asexpected. In the case of humidification the response ismuch slower and the increase in permittivity occurs overa longer period of time. In this study the quality of thesurfaces undergoing treatment has not been examined.Clearly any micro cracks and imperfections will influencethe moisture sorption properties.

In future studies the surfaces of test samples (used bothfor probe validation and comparison between surfaces ofdifferent polarities) will also be characterised usingscanning electron microscopy. Furthermore the present

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Fig 15 Measurement using the open-ended probe on apainting.

measurements have been made with a hand held probe(Fig 15). This aspect also requires further developmentin order to fix the probe in an optimum measuringposition.

Our present aim is to correlate dielectric response withthe equilibrium moisture content of the sample beingtested. Isothermal thermogravimetry is being used todetermine the equilibrium moisture content at varyingconditions of relative humidity for differently pigmentedpaint films. The overall objective is to provide a non-destructive non-invasive measuring system which can beused to control the moisture treatment of paintings.

Future plans are to bring the instrumentation toconservation departments and to assist in quantifyingparameters during moisture treatment. For example theprobe could be used during the type of treatmentrecently described on Carel Fabritius' painting "A Viewof Delft" involving the Willard multi-purpose lowpressure table25. It would define operating parameterssuch as the length of time of humidification anddehumidification processes. It could also be used tomonitor the deacidification treatment of canvassupported paintings to establish the time for completeloss of the solvent carrier used. The long term objectiveis to develop this methodology for monitoring non-destructively the moisture content of other historic andcultural materials.

Acknowledgements

This paper is dedicated to the memory of Gerry Hedley.The authors acknowledge the support of the NERCScience Based Archaeology Committee, and would liketo thank Mads. Chr. Christensen of the School ofConservation, Royal Danish Academy of Art, StephenHackney and Dr Joyce Townsend of the ConservationDepartment of the Tate Gallery, London, Alan Phenix ofthe Courtauld Institute of Art, London, and Dr OlivierPages, Birkbeck College, for providing samples for themeasurements. They would also like to thank SusanCurtis, curator of Valence House Museum Dagenham,Professor R Walker and Dr Bill Lidell for access to theFanshawe archive, and Rhona MacBeth of the Museum

of Fine Arts in Boston for the TMA data. We are gratefulto Finn Worsor, Teit Poulsen and Jon Mees of HewlettPackard Test and Measurement for arranging the loan ofthe network analyser both in Denmark and in the U.K.,and to Professor R Hill, Dr L Dissado and Dr J Alison,King's College, University of London, for theirdiscussions and interest.

References

1. Tillinghast, J E, The Effect of Solvent Exposure onthe Mechanical Properties of Oil Paint Films, Finalyear project Courtauld Institute of Art (1990).

2. Husband, C, The Effect of Relative humidity on theMechanical Properties of Oil Paint Films, Finalyear project Courtauld Institute of Art (1990).

3. Hedley, G, Odlyha, M, Burnstock, A, Tillinghast, J,and Husband, C, A Study of the Mechanical andSurface Properties of Oil Paint Films Treated withOrganic Solvents and Water. Cleaning, Retouchingand Coatings, Preprints of IIC Congress, Brussels(1990) pp 98-105.

4. Erhardt, D and Tsang, J S, The ExtractableComponents of Oil Paint Films, Preprints of IICCongress, Brussels (1990) pp 93-98.

5. Odlyha, M, Craig, D Q M and Hill, R M, DielectricAnalysis of Relative Humidity Variations in CanvasPaintings, Journal of Thermal Analysis, 39 (1993)pp 1181-1192.

6. Chan, T Y A and Odlyha, M, Dielectric Analysis forthe study of Historic and Artistic Materials, LifeChemistry Reports, 10 (1994) pp 269-283.

7. Odlyha, M, Chan, T Y A and Pages, O, Evaluation ofRelative Humidity Effects on Fabric-SupportedPaintings by Dynamic Mechanical and DielectricAnalysis, Thermochimica Acta (1995) in press.

8. Chan, T Y A and Odlyha, M, The Effect of RelativeHumidity and Pigment Type on Paint Films,Thermochimica Acta (1995) in press.

9. King, R J, Microwave moisture measurement ofgrains, Institute of Electrical and ElectronicEngineers Transactions on Instrumentation andMeasurement, 41 (1992) pp 111-115.

10. Kent, M, Electrical and Dielectric Properties ofFood Materials, Essex, England: Science andTechnology Publishers (1987).

11. Nelson, S O, Electrical properties of agriculturalproducts: a critical review, Trans. American Societyof Agricultural Engineers, 16 no. 2 (1973) pp 384-400.

12. Tinga W R and Nelson S O, Dielectric properties ofmaterials for microwave processing-tabulated,Journal of Microwave Power, 8, 1 (1973) pp 23-65.

13. Hedley, G, Villers, C, Bruce-Gardner, R andMacBeth, R, A new Method for Treating WaterDamage Flaking, Preprints of ICOM Committee forConservation, Dresden (1990) pp 119-123.

14. Hedley, G, Relative Humidity and Stress StrainResponse of Canvas Paintings: UniaxialMeasurements of Naturally Aged Samples, Studiesin Conservation, 33 (1988) pp 57.

15. Grant, E H, Sheppard, R J and South, G P,Dielectric Behaviour of Biological Molecules inSolution, Clarendon Press, Oxford (1978).

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16. Tanabe, E and Joines, W T, A non-destructivemethod for measuring the complex permittivity ofdielectric materials at microwave frequencies usingan open transmission line resonator, Institute ofElectrical and Electronic Engineers Transactionson Instrumentation and Measurement, 25 (1976)pp 222-226.

17. Burdette, E C, Cain, F L and Seals, J, In vivo probemeasurement technique at VHF through microwavefrequencies, Institute of Electrical and ElectronicEngineers Transactions on Microwave Theoryand Techniques, 28 (1980) pp 414-427.

18. Chan, T Y A, Numerical Analysis of Open-endedCoaxial Line Sensors with Application toDielectric Measurements up to 20 GHz, Ph.DThesis, University of London (1993).

19. Chan, T Y A, Odlyha, M and Scharff, M, In situ non-invasive dielectric spectroscopy for monitoringconservation treatment of canvas paintings,Deutsche Gesellschaft für ZerstîrungsfreieUntersuchungen an Kunst-und Kulturgutem e.V.,Berlin, 4th International Conference on Non-Destructive Testing of Works of Art (1994) pp 510-520.

20. Odlyha, M, Walker, R M and Liddell, W H, A Study ofthe Effects of Conservation Treatment on theFanshawe Archive, Preprints of IIC CongressMadrid (1992).

21. Scharff, M, Suction table techniques, lamination andmodern art, Proceedings from the seminar ModernArt, Copenhagen (July 6-8 1994) in press.

22 Payne, H F, Organic Coating Technology 11, JohnWiley & Sons, London (1961) pp 1310.

23 Lide, D R (ed), Handbook of Chemistry andPhysics, 73rd edition (1992-3) pp 12-39 to 12-47.

24. MacBeth, R, Odlyha, M, Burnstock, A, Villers, C andBruce-Gardner, R, Evaluation of Moisture Treatmentof Fabric-Supported Paintings, Preprints of ICOMCommittee for Conservation, Washington DC(1993) pp 150-156.

25. Keith, L, A View in Delft: Some Observations on itsTreatment and Display, National Gallery TechnicalBulletin, 15 (1994) pp 55-63.

Materials and suppliers

Laboratory chemicalsBDH LtdPO Box 15Freshwater RoadDagenhamEssex RM8 1RF, England

Paint film samples 1 & 2 and APTDWinsor & Newton51 RathboneLondon P1 W1, England

Paint film samples 3 & 4 and tube paintsC. Roberson & Co. Ltd.London, England

Distributor:L. Cornelissen & Son105 Great Russell StreetLondon WC1B 3RY, England

T Y Amy ChanDepartment of ChemistryBirkbeck CollegeUniversity of London29 Gordon SquareLondon WC1H 0PP

PhD in Physics from King's College, University ofLondon, 1993. Research fellow at Birkbeck CollegeChemistry Department, 1993-1995, working on NERC(SBAC) funded project "Application of DynamicMechanical and Dielectric Analysis for Monitoring Effectsof Moisture based Conservation Treatment". Researchfellow at Birkbeck College Chemistry Department, 1995to present, working on EU funded project "EnvironmentalResearch for Art Conservation".

Marianne OdlyhaDepartment of ChemistryBirkbeck CollegeUniversity of London29 Gordon SquareLondon WC1H 0PP

MSc in Chemistry and BA in German and French fromthe University of Adelaide. Senior Tutor / Assistantlecturer in Chemistry, University of South Australia andshort courses in Chemistry in Archaeology (FlindersUniversity). Study leave in Doerner Institute (Munich)(six month periods between 1982-1987). CourtauldInstitute of Art (1988-1989) and Birkbeck College(1988 to present) research associate in charge ofUniversity of London Intercollegiate Research Service inThermal Methods and research in their application toconservation science. Co-ordinator of EU funded project"Environmental Research for Art Conservation".

Mikkel ScharffThe Royal Danish Academy

of Fine ArtsThe School of ConservationEsplanaden 34DK-1263 Copenhagen KDenmark

Studied History of Art at the University of Copenhagen1973-1977. BA equivalent degree in PaintingConservation 1980, MSc equivalent degree in PaintingConservation 1987 from the School of Conservation,Copenhagen. 1980-1987 various lecturing and work asconservator in Denmark and abroad. From 1987 lecturerat the School of Conservation and from 1992 Head ofDepartment of Paintings, specialising in structuraltreatment of paintings, analytical photography andpreventative conservation, other work include earlymedieval painting on wood and history of conservation.At present co-ordinator for the ICOM Committeefor Conservation's Working Group on PaintingConservation.

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