Applications of Infrared Microspectroscopy to Art Historical Questions

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Microspectroscopy to Art Historical Questions about Medieval Manuscripts Mary Virginia Orna , Patricia L. Lang , J. E. Katon , Thomas F. Mathews , and Robert S. Nelson 1

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Department of Chemistry, College of New Rochelle, New Rochelle, N Y 10801 Department of Chemistry, Ball State University, Muncie, I N 47306 Molecular Microspectroscopy Laboratory, M i a m i University, Oxford, OH 45056 Institute of Fine Arts, New York University, New York, N Y 10021 Department of A r t , University of Chicago, Chicago, IL 60637

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Fourier transform infrared microspectroscopy applied to Byzantine manuscripts in the Special Collections Department of the University of Chicago Library revealed the use of numerous additives to the paint mixture including kaolin, hide glue, egg yolk, and other proteinaceous materials. Some evidence suggests that cochineal was used as a red pigment.

Pigment Analysis Aims of Pigment Analysis. F o r the art historian, the c h e m i c a l a n a l ysis of p i g m e n t s serves two m a i n purposes: for authentication, that is, to c o n f i r m or d e n y the alleged a t t r i b u t i o n or d a t i n g of a p a i n t i n g o n the basis of c o m p a r i s o n w i t h the k n o w n p a i n t i n g practices of the artist o r p e r i o d ; a n d as a k e y to u n d e r s t a n d i n g the w o r k of a g i v e n artist or p e r i o d . T h e latter use of p i g m e n t analysis is less glamorous (nothing makes b e t t e r news t h a n to declare an accepted masterpiece a fake), b u t i n the l o n g r u n , it is the m o r e i m p o r t a n t enterprise. A l t h o u g h art historians r e l y p r i m a r i l y o n style to describe the interrelationships of artists a n d schools, this task can also be approached f r o m the p o i n t of v i e w of the artist's materials. M u c h progress has b e e n m a d e i n a n a l y z i n g the paintings of the R e n aissance a n d later periods i n terms of materials. It has b e e n d e m o n s t r a t e d , 0065-2393/89/0220-0265$07.00/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

Allen; Archaeological Chemistry IV Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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ARCHAEOLOGICAL CHEMISTRY

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for example, that Tintoretto's r e p u t a t i o n as an innovative colorist has a v e r y real foundation i n the w a y he u s e d p i g m e n t s (J). N o t o n l y d i d T i n t o r e t t o use the w i d e s t range of p i g m e n t s available i n V e n i c e at that t i m e — f o u r different b l u e s , for e x a m p l e — b u t h e also u s e d p i g m e n t s i n mixtures a n d layers q u i t e u n p a r a l l e l e d i n the w o r k of his contemporaries. T h u s , t h r o u g h the c h e m i c a l analysis of p i g m e n t samples, i t is possible to infer h o w the paints w e r e p r e p a r e d a n d , consequently, gain an insight into the artistic t e c h n i q u e itself. O u r k n o w l e d g e of m e d i e v a l p i g m e n t s , h o w e v e r , is still v e r y slight. Rationale for the Approach. L i k e the m e d i e v a l naturalist w h o p r e f e r r e d to speculate o n the n u m b e r o f t e e t h i n a horse's j a w rather t h a n to inspect the beast, the m o d e r n art h i s t o r i a n has taken a d e c i d e d l y t h e o retical approach to the study of artists' materials i n the M i d d l e Ages. R e l y i n g o n artists' r e c i p e books, several o f w h i c h s u r v i v e f r o m those times, art h i s torians have generally left to speculation the question of w h i c h p a r t i c u l a r p i g m e n t s w e r e u s e d i n a g i v e n w o r k of art. A n approach m o r e e m p i r i c a l than the use of such recipe books was d e v i s e d b y R o o s e n - R u n g e a n d W e r n e r (2, 3) i n t h e i r examination o f the 8 t h c e n t u r y L i n d i s f a r n e gospels. T h e i r m e t h o d i n v o l v e d a visual c o m p a r i s o n of the various p i g m e n t surfaces contained i n the m a n u s c r i p t w i t h p i g m e n t s m a n u f a c t u r e d a c c o r d i n g to the m e d i e v a l painters' manuals. T h e p r e p a r e d p i g m e n t s w e r e p a i n t e d o n small pieces of p a r c h m e n t , a n d the m i c r o s c o p i c structures of the p i g m e n t s i n the m a n u s c r i p t w e r e t h e n c o m p a r e d w i t h the k n o w n samples b y u s i n g p o l a r i z e d light, s u p p l e m e n t e d w i t h visual o b s e r vation i n ultraviolet light. A n o t h e r approach is w h o l e - m a n u s c r i p t n e u t r o n activation analysis ( N A A ) i n a n u c l e a r reactor, f o l l o w e d b y gamma-ray analysis a n d autoradiography (4). G a m m a - r a y analysis p e r m i t s identification of the elements c o n t a i n e d i n the m a n u s c r i p t , a n d autoradiographic analysis indicates the specific locations of these elements. A similar approach a p p l i e d to art objects of historical interest is e n e r g y - d i s p e r s i v e X - r a y fluorescence ( X R F ) analysis, w h i c h p e r m i t s n o n d e s t r u c t i v e semiquantitative e l e m e n t a l analysis (5). A l t h o u g h these methods are nondestructive i n the sense that a m a n u script remains intact after analysis, they have some serious shortcomings. T h e m e t h o d of R o o s e n - R u n g e a n d W e r n e r involves the use of reasonably i n t e l l i g e n t guesses m a d e o n the basis of several assumptions. T h e o t h e r methods i n v o l v e r e m o v i n g the m a n u s c r i p t f r o m its location a n d subjecting it to m u l t i p l e handlings a n d n e u t r o n or X - r a y b o m b a r d m e n t . B u t the most serious drawback is the fact that N A A a n d X R F p r o v i d e i n f o r m a t i o n o n l y r e g a r d i n g the e l e m e n t s c o n t a i n e d i n the p i g m e n t s b u t no i n f o r m a t i o n r e g a r d i n g c h e m i c a l formulas of c o m p o u n d s . G i v e n the availability o f samples from a m a n u s c r i p t , there is no s u b stitute for the standard methods of analysis of microscopic particles. G r a n t e d


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that t h e methods j u s t d e s c r i b e d are n o n d e s t r u c t i v e , the r e m o v a l of p i g m e n t samples from a m a n u s c r i p t c a n b e e q u a l l y nondestructive. T h e use o f p i g m e n t samples f r o m manuscripts for analysis is preferable for various reasons. 1. S a m p l i n g does not r e q u i r e the r e m o v a l of the m a n u s c r i p t from its p e r m a n e n t location. 2. A w e l l - p l a n n e d s a m p l i n g is a o n e - t i m e operation that n e e d n e v e r b e repeated.


3. S a m p l i n g involves m u c h less h a n d l i n g of the m a n u s c r i p t t h a n the other methods. 4. O n l y t h e samples, a n d not the entire m a n u s c r i p t , are s u b j e c t e d to h i g h - e n e r g y irradiation. 5. T h e samples can b e taken i n such a w a y that t h e lacunae left b y excision of the p i g m e n t particles are not d i s c e r n i b l e b y t h e n a k e d eye. H o w e v e r , t h e most c o m p e l l i n g reason for t a k i n g samples is the w e a l t h o f information that c a n b e o b t a i n e d f r o m t h e i r analysis, i n c l u d i n g unambiguous c h e m i c a l identification of p i g m e n t components. O f course, it is also necessary to identify cooperative curators. Analysis of Medieval Manuscripts. A n ongoing project (6-8) d e a l i n g w i t h m e d i e v a l manuscripts has h a d as its objective t h e application of small-particle-analysis techniques to the study o f pigments i n m e d i e v a l A r m e n i a n (Greater A r m e n i a n ) , C i l i c i a n (Lesser A r m e n i a n ) , a n d B y z a n t i n e manuscripts. T h e M i d d l e E a s t e r n materials are a n i d e a l starting p o i n t b e cause m a n y o f t h e manuscripts are dated a n d can b e located b y colophons a n d inscriptions. T h i s w o r k has b e g u n to shed m o r e l i g h t o n several art historical p r o b l e m s , i n c l u d i n g t h e tracing of lines of influence or i n t e r c o n n e c t i o n b e t w e e n m e d i e v a l centers o f m a n u s c r i p t p r o d u c t i o n a n d the c l a r i fication of periods o f k n o w n usage o f several i m p o r t a n t artists' p i g m e n t s . A t present, several d o z e n manuscripts f r o m m u s e u m s a n d centers such as t h e W a l t e r s A r t G a l l e r y ; t h e F r e e r G a l l e r y o f A r t ; t h e P i e r p o n t M o r g a n L i b r a r y ; the S p e c i a l C o l l e c t i o n s of the U n i v e r s i t y of C a l i f o r n i a at L o s A n g e l e s ; the Spencer C o l l e c t i o n of the N e w Y o r k C i t y P u b l i c L i b r a r y ; t h e A r m e n i a n Patriarchate o f St. James, J e r u s a l e m ; a n d the M o n a s t e r y of San L a z z a r o , V e n i c e , have b e e n s a m p l e d a n d analyzed. S o m e o f the data a n d results have b e e n p u b l i s h e d i n representative journals. T h i s chapter is a progress report o n t h e analysis o f particles taken f r o m B y z a n t i n e i l l u m i n a t e d manuscripts from t h e U n i v e r s i t y of C h i c a g o L i b r a r y S p e c i a l C o l l e c t i o n s . Because B y z a n t i n e manuscripts have, heretofore, n e v e r b e e n analyzed c h e m i c a l l y (9), this report is the result of i n i t i a l i n q u i r y i n t o a large a n d c o m p l e x subject.


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A n o t h e r objective of this chapter is to introduce the use of F o u r i e r transform i n f r a r e d ( F T - I R ) microspectroscopy for the analysis of p i g m e n t s a n d to p r o v i d e some b a c k g r o u n d about the t e c h n i q u e .


F T - I R Microspectroscopy Background. I R spectroscopy has l o n g b e e n u s e d as an analytical tool for the identification of u n k n o w n materials because it provides selective, m o l e c u l a r i n f o r m a t i o n . H o w e v e r , I R spectroscopy has not b e e n w i d e l y u s e d to analyze historic artwork i n the past p r i m a r i l y because the m i n i m u m sample size n e e d e d to o b t a i n transmission spectra was i n the o r d e r of m i c r o g r a m quantities (10). T h e d e v e l o p m e n t of F T - I R spectrometers, w i t h t h e i r i n h e r e n t advantages o v e r grating i n s t r u m e n t s (II), has increased the usefulness of this t e c h n i q u e for the analysis of w o r k s of art. T h e e n h a n c e d spectral t h r o u g h p u t of F T - I R i n s t r u m e n t s and t h e i r ability to average m a n y scans have a l l o w e d the analysis o f m u c h smaller samples, i n the o r d e r of n a n o g r a m quantities. I n 1983, Shearer a n d co-workers (10) u s e d F T - I R i n c o n j u n c t i o n w i t h a b e a m - c o n d e n s i n g accessory to analyze the paint chips r e m o v e d f r o m Virgin and Child, an Italian p a i n t i n g i n the C l a r k Institute c o l l e c t i o n . T h e i r spectral data s u p p o r t e d the art c o n s e r v a t o r s o p i n i o n that the p a i n t i n g was e i t h e r an i m i t a t i o n of an e a r l i e r w o r k or a h e a v i l y r e w o r k e d fragment. T h e recent c o u p l i n g of F T - I R spectrometers w i t h microscope accessories has f u r t h e r a l l o w e d the analyst to obtain spectra f r o m picogram quantities of samples a n d , m o r e o v e r , to obtain the spectra q u i c k l y a n d easily (12, 13).

Microscope Accessory for F T - I R Analysis. T h e basic e x p e r i m e n t a l design o f a microscope accessory is shown i n F i g u r e 1. A b e a m of I R radiation is focused b y a m i r r o r to a spot approximately 0.5 m m i n d i a m e t e r . T h e focus is fixed o n top of a 1-mm K C 1 w i n d o w . T h e t r a n s m i t t e d I R radiation is c o l l e c t e d a n d magnified 15 times b y a cassegrainian lens. T h i s lens is made of reflective materials, a n d it is used because traditional m i croscope objectives, w h i c h are made of glass, absorb I R radiation. A n iris located i n the focal plane of the m a g n i f i e d image (called an adjustable field stop) allows the analyst to define the sample area u n d e r observation. T h e t r a n s m i t t e d r a d i a t i o n is finally focused b y another m i r r o r onto a m e r c u r y - c a d m i u m t e l l u r i d e ( M C T ) detector. T h e h i g h sensitivity of the M C T detector increases the o v e r a l l performance of the system. A flip m i r r o r (not shown) allows the sample to be v i e w e d t h r o u g h a 10 X ocular a n d b r o u g h t into focus. T h e optical elements of this accessory have b e e n a l i g n e d so that the v i s i b l e focus is coincident w i t h the focus of I R radiation.



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Figure 1. Optical diagram of a typical FT-IR

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microscope accessory.

Materials and Methods Materials. A t the U n i v e r s i t y of C h i c a g o , 10 decorated manuscripts w e r e s a m p l e d b y processes p r e v i o u s l y d e s c r i b e d (6-8). T h e s e manuscripts represent a b r o a d chronological span r a n g i n g f r o m the 10th c e n t u r y u n t i l the p o s t - B y z a n t i n e e r a (16th c e n t u r y or later). T h e earliest book is the E l f l e d a B o n d G o o d s p e e d Gospels, decorated solely w i t h headpieces a n d initials a n d l i n k e d w i t h the Chrysostomos manuscripts i n Paris a n d R o m e (14). T h e second m a n u s c r i p t s a m p l e d was the celebrated R o c k e f e l l e r - M c C o r m i c k A f e u ? Testament, a book redated to the second h a l f of the 12th c e n t u r y b y C a r r (15) a n d associated w i t h manuscripts from C y p r u s or Palestine.


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T h e t h i r d U n i v e r s i t y of C h i c a g o m a n u s c r i p t was the C h r y s a n t h u s gospels, a potentially i n t e r e s t i n g b o o k because i t may be a t t r i b u t e d to the same p e r i o d as the R o e k e f e l l e r - M e C o r m i e k New Testament a n d shares w i t h it various stylistic a n d iconographie details, thus m a k i n g a comparison of the p i g m e n t s u s e d a most i n t e r e s t i n g endeavor. I n a d d i t i o n , the m a n u s c r i p t is valuable because m i n i a t u r e s w e r e a d d e d to it d u r i n g the p o s t - B y z a n t i n e p e r i o d , possibly because the o r i g i n a l i l l u m i n a t i o n s h a d b e g u n to flake.


T h e fourth U n i v e r s i t y of C h i c a g o m a n u s c r i p t , k n o w n as the " A r c h a i c M a r k " , is a book of d e c i d e d l y modest q u a l i t y b u t of u n d e n i a b l e interest to philologists. A s A l l i s o n n o t e d i n an u n p u b l i s h e d d e s c r i p t i o n of the m a n u s c r i p t (16) , some readings find parallels i n the early Codex Vaticanus, a n d others are u n i q u e . O n e scholar was so d a z z l e d b y the textual e v i d e n c e that h e thought that the U n i v e r s i t y of C h i c a g o M a r k m i g h t contain the text of the G o s p e l of M a r k " i n a m o r e p r i m i t i v e f o r m than any other k n o w n m a n u s c r i p t " (17) . O n the other h a n d , others have suggested that its rare text may have b e e n taken from a 19th-century p r i n t e d v e r s i o n of the G r e e k gospels, a n d some have e v e n q u e s t i o n e d the authenticity of the manuscript. T h e p r o b l e m of the A r c h a i c M a r k is q u i t e complex. Its m i n i a t u r e s are based o n the cycle i n a late 12th-century gospel book i n the N a t i o n a l L i b r a r y i n A t h e n s , codex 93 (18). T h e r e exists also a set of fragmentary gospels i n the H e r m i t a g e M u s e u m i n L e n i n g r a d that must have b e e n made b y the same scribe a n d i l l u m i n a t o r , because the similarities i n script, i n i t i a l ; , a n d p a i n t i n g style can h a r d l y be fortuitous. T h e L e n i n g r a d m a n u s c r i p t is i l l u s trated i n reference 19, a n d its contents a n d u n u s u a l text are d e s c r i b e d i n reference 20. B o t h the U n i v e r s i t y of C h i c a g o a n d the L e n i n g r a d manuscripts are fragments. T h e L e n i n g r a d codex has o n l y eight folios f r o m the e n d of the G o s p e l of M a r k a n d the b e g i n n i n g of the G o s p e l of L u k e ; the U n i v e r s i t y of C h i c a g o m a n u s c r i p t contains the G o s p e l of M a r k only. B o t h have the last few chapters of the G o s p e l of M a r k , a n d consequently, these passages c o u l d be profitably s t u d i e d to d e t e r m i n e i f they are i d e n t i c a l . Because of the shared texts, the two manuscripts, m a d e b y the same scribe a n d i l l u m i n a t o r , cannot be fragments f r o m a single gospel book. F u r t h e r m o r e , the page sizes do not agree. T h e L e n i n g r a d v o l u m e measures 170 b y 126 m m , whereas the A r c h a i c M a r k measures 119 b y 84 m m . These n u m b e r s suggest that the smaller U n i v e r s i t y of C h i c a g o m a n u s c r i p t is to the larger L e n i n g r a d m a n u s c r i p t as an octavo is to a quarto. T h u s the two manuscripts m a y have b e e n made out of the same batch of p a r c h m e n t sheets, a n d the folios w e r e f o r m e d b y f o l d i n g a n d r e folding. T h e six r e m a i n i n g U n i v e r s i t y of C h i c a g o manuscripts s a m p l e d w e r e the H a s k e l l gospels (attributed to T h e o d o r e Hagiopetrites) a n d the G e o r g i u s gospels, b o t h of the late 13th c e n t u r y ; the lectionary of C o n s t a n t i n e the reader a n d the lectionary of St. M e n a s the w o n d e r w o r k e r , b o t h of w h i c h are late-12th-century works i n the C y p r i o t style; a 12th-century G r e e k gospel book; a n d the e a r l y - 1 2 t h - c e n t u r y N i c o l a u s gospels, dated A . D . 1133


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T h e p e r t i n e n t i n f o r m a t i o n about the manuscripts r e p o r t e d i n this c h a p ter are s u m m a r i z e d i n Table I.


Sampling Procedure. T h e manuscripts just d e s c r i b e d w e r e s a m p l e d b y abstraction o f p i g m e n t samples w i t h a fine surgical scalpel a n d several s a m p l i n g needles o f various thicknesses u n d e r a 40 X b i n o c u l a r microscope. I n some cases, samples w e r e taken f r o m offsets of p i g m e n t that h a d trans f e r r e d onto the opposite page; thus a r e l a t i v e l y large a m o u n t o f sample c o u l d b e r e m o v e d w i t h o u t d i s t u r b i n g the c o r r e s p o n d i n g m i n i a t u r e . I n several other cases, p i g m e n t flakes that h a d fallen into the gutter of the m a n u s c r i p t w e r e taken. I n those cases w h e n samples w e r e excised d i r e c t l y f r o m the p a i n t i n g s , the lacunae w e r e h a r d l y d i s c e r n i b l e e v e n at 40 X magnification. E a c h sample was c o d e d a c c o r d i n g to h u e , folio n u m b e r , a n d x-y co ordinates i n m i l l i m e t e r s , w i t h the b o t t o m outside c o r n e r of the m a n u s c r i p t page as the o r i g i n . F o r example, a code of R.45v(35,65) identifies a r e d sample taken from folio 45 verso, 35 m m from the left edge a n d 65 m m f r o m the b o t t o m of the page. T h e locations of the samples w e r e also m a r k e d o n photographs m a d e of each page of the manuscripts ( F i g u r e 2). T h e average size of the offset samples was 5 0 - 1 0 0 μπι; the sizes of other samples r a n g e d f r o m 4 to 40 μπι. A n attempt was made to sample the c o m p l e t e range of hues i n each m a n u s c r i p t . H u e s w e r e differentiated b y b o t h v i s u a l assessment a n d b y c o m p a r i s o n w i t h color chips f r o m The Munsell Book of Colors (21). Sample Analysis. T h e samples w e r e i n i t i a l l y a n a l y z e d b y p o l a r i z e d light m i c r o s c o p y a n d X - r a y diffraction b y the methods o u t l i n e d b y M c C r o n e a n d D e l l y (22, 23). T h e microscope was an O l y m p u s P O S - 1 e q u i p p e d for p h o t o m i c r o g r a p h y i n t r a n s m i t t e d light. T h e opaque samples w e r e p h o t o g r a p h e d at 180 X magnification i n reflected l i g h t o n a R e i c h e r t M e F 2 metallograph. T h e X - r a y diffraction patterns w e r e o b t a i n e d b y m o u n t i n g

Table I. University of Chicago Special Collections Manuscripts Analyzed by F T - I R Manuscript 972 1054 965 131 232 46 129 727 879 948

Number

Date

Name Archaic Mark Elfleda Bond Goodspeed Gospels Rockefeller-McCormick New Testament Chrysanthus gospels Greek gospels Haskell gospels Nicolaus gospels Georgius gospels Lectionary of Constantine the reader Lectionary of St. Menas the wonder worker

? 10th century late 12th century late 12th century 12th century late 13th century 1133 late 13th century late 12th century late 12th century




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Figure 2. Moses Receiving the Law, Rockefeller-McCormick New Testament, University of Chicago MS 965, folio 6 verso. The open circle indicates the place where the miniature was sampled. BR6v(127,108) indicates the code for brown sample, page 6v, 127 mm to the right and 108 mm from the bottom left page corner.


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the sample particles o n a glass filament i n a 114.59-m m - d i a m e t e r D e b y e - S c h e r r e r p o w d e r camera a n d irradiating w i t h C u K X - r a y s for 24 h at 30 k V a n d 15 m A . A l t h o u g h these methods e n a b l e d the specific identification of the m i n e r a l p i g m e n t s i n the manuscripts, the organic p i g m e n t s c o u l d b e classified o n l y generally.


a

Samples that w e r e organic i n nature w e r e subjected to the m i c r o c h e m ical tests d e s c r i b e d b y H o f e n k - d e Graaff (24) a n d subsequently analyzed b y F T - I R microspectroscopy. T h e microspectroscopic analysis p r o c e e d e d as follows. S a m p l e particles w e r e transferred f r o m the p r e c l e a n e d glass microscope slides w i t h a fine p o i n t e d m e t a l p r o b e u n d e r a 30 X b i n o c u l a r microscope to a K C 1 w i n d o w m e a s u r i n g 6 m m i n d i a m e t e r a n d 1 m m t h i c k . T h e K C 1 w i n d o w was s u p p o r t e d b y a c e n t e r - h o l e d m e t a l microscope slide that fits r e a d i l y onto the stage of the F T - I R microscope accessory. O n c e on the stage, the sample was visually b r o u g h t into focus, a n d the c i r c u l a r field stop was closed d o w n a r o u n d the sample image. T h e ratio of the sample s p e c t r u m to that of a b a c k g r o u n d s p e c t r u m p r e v i o u s l y o b t a i n e d at a similar aperture setting was obtained. H o w e v e r , s a m p l i n g i n the m a n n e r just d e s c r i b e d sometimes r e s u l t e d i n l o w - q u a l i t y spectra w i t h m a n y a b s o i p t i o n bands that c o u l d not be d i s t i n g u i s h e d from the b a c k g r o u n d noise. T h i s effect is largely caused b y the i n h e r e n t t e n d e n c y of m a n y p i g m e n t samples to scatter m u c h of the t r a n s m i t t e d I R signal away from the optical p a t h l e a d i n g to the detector. I n s u c h cases, spectral q u a l i t y was i m p r o v e d b y m i x i n g a few particles of the p i g m e n t sample w i t h a s m a l l crystal o f K B r a n d flattening the m i x t u r e w i t h a p r o b e tip i n t o a m i c r o s c o p i c K B r plate m e a s u r i n g approximately 3 0 - 5 0 μηι across. T h e quantities of the p i g m e n t samples w e r e insufficient to allow the use of a c o n v e n t i o n a l K B r disk or e v e n a m i c r o d i s k accessory. A n A n a l e c t A Q S - 2 0 M system consisting O f an fx-6260 F T - I R s p e c t r o m eter a n d an fXa-515 microscope m o d u l e was u s e d to obtain 4-ern ^ - r e s o l u t i o n spectra. R e p l i c a t e spectra of each p i g m e n t sample w e r e o b t a i n e d w h e n e v e r sample q u a n t i t y a l l o w e d . T h e n u m b e r of signal-averaged scans was v a r i e d d e p e n d i n g o n the apparent scattering ability of each p i g m e n t . I n g e n e r a l , the b e t t e r the scattering ability, the m o r e scans w e r e n e e d e d to r e d u c e the noise i n the s p e c t r u m . Peaks w e r e located b y u s i n g a cursor a n d t h e n r e corded. Reference spectra w e r e o b t a i n e d from standard organic p i g m e n t s f r o m the F o r b e s c o l l e c t i o n of the C o n s e r v a t i o n C e n t e r , Institute of F i n e A r t s , N e w Y o r k U n i v e r s i t y , a n d f r o m a few organic a n d inorganic p i g m e n t s f r o m various sources. A d d i t i o n a l reference spectra w e r e o b t a i n e d f r o m c o m p o u n d s r e p o r t e d l y u s e d as m e d i e v a l p a i n t additives, such as d r i e d egg y o l k , egg w h i t e , c a l c i u m carbonate, a n d k a o l i n . T h e spectroscopic identification of c o m p o u n d s i n p i g m e n t samples f r o m the manuscripts that w e report h e r e i n was based on a c o m p a r i s o n of sample spectra w i t h these reference spectra.


274


Results and Discussion


T h e p r e l i m i n a r y results o f the X - r a y , microscopic, a n d microspectroscopic analyses are s u m m a r i z e d i n Tables II a n d I I I . I n t e r p r e t a t i o n of I R spectra was c o m p l i c a t e d b y the fact that the p i g m e n t samples often c o n t a i n e d several c o m p o n e n t s i n a d d i t i o n to the p i g m e n t . T h e s e additives often s e r v e d as b i n d i n g m e d i a — t h i c k e n i n g agents or extenders, for example. Because the a m o u n t of p i g m e n t is p r e s u m a b l y s m a l l relative to the amounts of these n o n p i g m e n t components, the spectral bands of the p i g m e n t are often difficult to d i s t i n g u i s h from i n t e r f e r i n g spectral bands of n o n p i g m e n t components. C h r o m a t o g r a p h i c separation p r i o r to spectroscopic analysis was not feasible because of the i n s o l u b i l i t y a n d l i m i t e d quantity of the samples. S p e c t r a l subtraction was u s e d i n m a n y cases as a means to "separate" the components. F o r example, F i g u r e 3 shows a representative s p e c t r u m o b t a i n e d f r o m a y e l l o w p i g m e n t sample from m a n u s c r i p t ( M S ) 46 ( H a s k e l l gospels). S e v e r a l bands can be a t t r i b u t e d to egg y o l k , a t y p i c a l m e d i u m u s e d i n m e d i e v a l times (25), as w e l l as c a l c i u m carbonate a n d kaolin. Subtraction of the spectra of egg y o l k a n d c a l c i u m carbonate from the s p e c t r u m of a replicate y e l l o w p i g m e n t y i e l d e d the s p e c t r u m s h o w n i n F i g u r e 4. ( N o k a o l i n was o b s e r v e d i n the replicate run.) T h e absorption bands at 3382, 3298, a n d 1600 c m " suggest that a p r i m a r y a m i n e is present. P u t r e s c i n e (1,4-diaminobutane), an a m i n e f o u n d i n d e c a y i n g proteinaceous matter, has similar spectral features. Because p a r c h m e n t (being of a n i m a l skin origin) w o u l d be subject to p u t r e faction i f not p r o p e r l y p r e s e r v e d , the a m i n e i n F i g u r e 4 c o u l d be a p r o d u c t of bacteriological degradation. O b v i o u s l y , any such sample degradation w o u l d complicate the i n t e r p r e t a t i o n of spectral data e v e n further. 1

A l t h o u g h the p i g m e n t was not successfully i d e n t i f i e d i n this case, the information that egg t e m p e r a is present as the b i n d i n g m e d i u m is a valuable characterization b y itself. E g g y o l k was also i d e n t i f i e d i n spectra o b t a i n e d from a different-colored p i g m e n t sample f r o m the same m a n u s c r i p t . A different b i n d i n g m e d i u m was i d e n t i f i e d i n M S 965 (Rockefell e r - M c C o r m i c k New Testament), h o w e v e r . F i g u r e 5 shows the s p e c t r u m o b t a i n e d f r o m a sample of the manuscript's s i z i n g material. A l t h o u g h the s p e c t r u m is c l e a r l y not that of egg y o l k , it has the features of a p r e d o m i n a n t l y p r o t e i n c o m p o n e n t . T h e p r o t e i n - c o n t a i n i n g b i n d e r s u s e d i n that p e r i o d are p r e s u m e d to i n c l u d e casein, egg w h i t e , a n d h i d e glue (25). T h e s p e c t r u m closely matches that of h i d e glue (26). T h e s p e c t r u m o b t a i n e d f r o m a magenta sample r e m o v e d from the b i n d i n g of the same m a n u s c r i p t indicates a p r o t e i n substance a n d a d d i t i o n a l u n i d e n t i f i e d component(s). T h e a d d i t i o n a l component(s) h i n d e r i d e n t i f i c a t i o n of the p r o t e i n . A l t h o u g h the p r o t e i n present c o u l d be h i d e glue, it c o u l d also originate f r o m p i g m e n t s extracted f r o m insect sources. P i g m e n t c o m p o n e n t s have also b e e n i d e n t i f i e d f r o m t h e i r I R spectra. F i g u r e 6 shows the s p e c t r u m o b t a i n e d f r o m a p u r p l e p i g m e n t r e m o v e d from


14.

O R N A ET AL.

Infrared

275

Microspectroscopy


M S 965. O n e can r e a d i l y assign bands to u l t r a m a r i n e b l u e a n d w h i t e lead. I n a d d i t i o n , an occasional, isolated b l u e c h i p was f o u n d i n the sample. T h e spectra o f these b l u e chips m a t c h e d reference spectra of u l t r a m a r i n e b l u e o b t a i n e d from the F o r b e s C o l l e c t i o n . T h e r e f o r e , a r e d p i g m e n t m u s t b e present w i t h the b l u e to o b t a i n a p u r p l e color. T h e s p e c t r u m i n F i g u r e 7 is the result of the subtraction of the spectra u l t r a m a r i n e b l u e a n d w h i t e lead. S i m i l a r spectral features are present i n F i g u r e 8, a reference s p e c t r u m of c r u d e cochineal (from P e r u v i a n Coccus cacti). T h i s reference s p e c t r u m was taken f r o m the d a r k e r r e d , almost black, part of the d r i e d insect. A n i m p o r t a n t consideration is that i f the r e g i o n f r o m 1400 to 1800 c m " is e x p a n d e d , a p a i r of doublets b e c o m e e v i d e n t at 1465 a n d 1472 c m a n d at 1708 a n d 1734 c m . T h e s e same bands appear i n the reference s p e c t r u m at 1463 a n d 1472 c m " a n d at 1708 a n d 1734 c m " . T h e fact that the frequencies of these doublets m a t c h suggests that the r e d p i g m e n t m a y v e r y w e l l be cochineal. 1

1

1

1

1

T h e frequencies of all absorption bands o b s e r v e d c o u l d not be m a t c h e d , a n d the spectral differences m a y b e caused b y the presence o f an a d d i t i o n a l component(s), the c h e m i c a l differences b e t w e e n N e w W o r l d (reference) c o c h i n e a l a n d the O l d W o r l d variety (the r e d p i g m e n t o b t a i n e d f r o m Coccus ilicis a n d k n o w n as " k e r m e s " [25]), or differences b e t w e e n c u r r e n t extraction procedures a n d those of m e d i e v a l times. S p e c t r a o b t a i n e d from the b r i g h t e r r e d insect parts a n d those o b t a i n e d f r o m the d r i e d extracted c o c h i n e a l are notably different f r o m each other a n d from that i n F i g u r e 8. T h i s difference is d u e to the i n h o m o g e n e i t y of the reference sample, a fact i n d i c a t i n g that extra care i n s a m p l i n g is r e q u i r e d . T h e d a r k e r r e d part of the insect that gave rise to the s p e c t r u m i n F i g u r e 8 is definitely a m i n o r constituent of the w h o l e insect. F o r the sake of c o m p a r i s o n , the s p e c t r u m of the g r o u n d w h o l e insect is s h o w n i n F i g u r e 9. F l e s h - c o l o r e d a n d r e d p i g m e n t s r e m o v e d f r o m M S 232 ( G r e e k gospel book) have spectral features s i m i l a r to those i n F i g u r e s 7 a n d 8. T h i s s i m i l a r i t y suggests that c o c h i n e a l was also u s e d as the r e d p i g m e n t i n this m a n u s c r i p t . A n o t h e r example of p i g m e n t identification b y I R microspectroscopy is shown i n F i g u r e 10. T h e b o t t o m s p e c t r u m was o b t a i n e d f r o m a b l u e p i g m e n t from M S 972 (Archaic M a r k ) ; the top s p e c t r u m is a reference s p e c t r u m of Prussian b l u e . T h e b a n d c o r r e s p o n d i n g to the feN of ferric ferrocyanide is c o m m o n to b o t h spectra. R e p l i c a t e spectra of b l u e p i g m e n t s r e m o v e d f r o m different locations i n M S 972 indicate that the average frequency of this b a n d is 2083 ± 6 c m " . T h e ubiquitousness of an i r o n b l u e i n this m a n u s c r i p t raises doubts about the a u t h e n t i c i t y of this m a n u s c r i p t . 1

T h e i r o n blues are the first of the artificial p i g m e n t s w i t h a k n o w n h i s t o r y a n d an established date of first p r e p a r a t i o n . T h e color was m a d e b y the B e r l i n color m a k e r D i e s b a c h i n or a r o u n d 1704. M o r e o v e r , a c c o r d i n g to G e t t e n s a n d Stout (25), the m a t e r i a l is so c o m p l e x i n c o m p o s i t i o n a n d m e t h o d of manufacture that there is practically no p o s s i b i l i t y that it was i n v e n t e d i n d e p e n d e n t l y i n other times or places. T h i s fact, i n a d d i t i o n to the e v i d e n c e



Charcoal Carbon black Azurite Ultramarine Verdigris Prussian blue Organic Organic Vermilion White lead, vermilion, and silicates White lead and organic yellow White lead, cochineal, and silicates Organic green Ultramarine, terre verte, and organic yellow

Black

Green

Flesh

Blue green Brown

Blue

Pigment

Color

ο +

+

972

+

1054

+

+

965

+

131

+ ο

+

+

232

Manuscript

Table II. Pigment Components of Manuscripts

+ + +

46

+

129

Number


727

879

948


+

+

+

+

+

+

+

ο

+

+

+

+ + +

+ +

+ ο

+

+

ο

+ +

NOTE:

Colors marked + were detected by microscopic and X-ray analysis, and colors marked spectroscopic analysis.

White Yellow

Red

Orange Pink Purple

Magenta

Ultramarine and organic yellow Ultramarine and gamboge Ultramarine and orpiment Organic magenta White lead (?) Organic orange Organic red Organic purple Ultramarine and organic red Ultramarine, cochineal, and white lead Organic red Vermilion Cochineal White lead Gamboge Organic yellow Orpiment ο

+

were detected

+


278



Table III. Nonpigment Components of Manuscripts Manuscript Number Pigment 972 1054 965 131 232 46 129 727

Color or Component Black Formic acid salts Egg yolk Blue Formic acid salts Calcium Sulfate Brown Formic acid salts Kaolin Calcium carbonate Magenta Protein Formic acid salts Red Formic acid salts Calcium carbonate Yellow Egg yolk Kaolin Calcium carbonate Formic acid salts Ink Protein Size Hide glue ΝΟΤΕ:

879

948

ο ο 0 0

ο ο ο

0

0 0

0 0

ο 0 0

0

ο ο

ο ο ο

The components were analyzed by FT-IR microspectroscopic analysis.

i n d i c a t i n g that b o t h the A r c h a i c M a r k a n d the L e n i n g r a d gospel fragment w e r e copies of the A t h e n s codex 93, suggests that these manuscripts o r i g i nated some t i m e m u c h later than t h e i r p u r p o r t e d 12th-century fabrication. F u r t h e r m o r e , n e i t h e r of these manuscripts has a genealogy that can be traced p r i o r to about 1930, a fact suggesting that t h e i r o r i g i n may v e r y w e l l be d u r i n g the flurry of A t h e n i a n forgeries that came to the m a r k e t i n the 1920s. Bands d u e to c a l c i u m sulfate a n d s o d i u m formate are also e v i d e n t i n F i g u r e 10. T h e s o d i u m formate bands are at 773, 1362, 1591, 2716, 2830, a n d 2954 c m (27). S o d i u m formate bands are e v i d e n t i n various spectra from almost a l l of the manuscripts that w e r e analyzed b y F T - I R m i c r o spectroscopy. I n a d d i t i o n , the presence of s o d i u m formate c o u l d not be associated w i t h any one p a r t i c u l a r color of p i g m e n t . Because f o r m i c a c i d is a c o m p o u n d u s e d i n the m o r e - m o d e r n t a n n i n g processes, w e i n i t i a l l y spec u l a t e d that the formate i n the samples originated from the m e d i e v a l p a r c h m e n t - m a k i n g process as w e l l , v e r y l i k e l y s e r v i n g as a preservative. H o w e v e r , w e recently i d e n t i f i e d s o d i u m formate as a residue on p r e c l e a n e d glass m i c r o s c o p e slides, p r e s u m a b l y left from the m a n u f a c t u r e r s c l e a n i n g process. - 1

Conclusions and Further Work T h e analyses c i t e d p r e v i o u s l y (6-8) indicate that the palette of the A r m e n i a n artist was largely of m i n e r a l o r i g i n a n d contained u l t r a m a r i n e , o r p i m e n t , v e r m i l i o n , w h i t e l e a d , a n d w h i t i n g as the staple pigments. T h e present w o r k on B y z a n t i n e manuscripts, although far from conclusive because of the l i m i t e d n u m b e r of manuscripts e x a m i n e d , suggests that B y z a n t i n e i l l u m i n a t o r s r e l i e d m o r e h e a v i l y o n organic pigments a n d that u l t r a m a r i n e a n d v e r m i l i o n



3400

3100

J

2800

.

1

1

2200

,

1

1900

W A V E N U M B E R S (cm-1)

2500

'

1600

«

1300

1000

700

Figure 3. Spectrum obtained from a yellow pigment sample from MS 46 (300 scans). Abbreviations are as follows: K, kaolin; £Y, egg yolk; and CC, calcium carbonate.

3800


ο

280


I ce

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Applications of Infrared Microspectroscopy to Art Historical Questions

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