Imaging Analysis and Digital Restoration of the Holy Face of Manoppello—Part I

heritage

Article Imaging Analysis and Digital Restoration of the Holy Face of Manoppello—Part I

Liberato De Caro 1,* , Emilio Matricciani 2 and Giulio Fanti 3

1 Istituto di Cristallograﬁa, Consiglio Nazionale delle Ricerche (IC-CNR), via Amendola 122/O, 70126 Bari, Italy 2 Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy; [email protected] 3 Dipartimento di Ingegneria Industriale, Via Gradenigo 6/a, Università di Padova, 35131 Padova, Italy; [email protected] * Correspondence: [email protected]

Received: 28 September 2018; Accepted: 21 October 2018; Published: 25 October 2018

Abstract: The Veil of Manoppello is an icon of the face of Christ (Holy Face). Its particular characteristic is being semitransparent. The face is visible on both sides (front–back) and, depending on the lighting and observation conditions, shows some differences in the anatomical details. An analysis of this icon has allowed us to clarify some aspects of the possible physical mechanism underlying its unusual optical behavior. It is a linen fiber fabric consisting of very thin threads with a thickness of about 0.1 mm, separated by distances even double the thickness of the threads, so that about 42% of the Veil is empty space. The fibers constituting the linen threads may have been cemented by an organic substance of chemical composition similar to cellulose, presumably starch, eliminating the air between them. Such a structure causes the optical behavior of the medium to be intermediate between those of a translucent medium (thin cemented linen threads) and a transparent one (empty space between the threads). The problem of digital image restoration in spatial terms has also been tackled, since the Holy Face is deformed due to distortions of the meshes of the Veil, caused by the yielding of the very fine structure of the fabric.

Keywords: artwork; optical characterization; Veil of Manoppello

1. Introduction The Veil of Manoppello is a small rectangular canvas of dimensions 240 × 175 mm2 characterized by a very thin, semitransparent fabric, on which is depicted the image of a male face with long hair and a divided beard. The represented face is associated in Christian iconography with Jesus Christ and, for this reason, it is called the Holy Face. It is kept in the Sanctuary of Manoppello (PE), Italy. This is a very special icon because the image is visible on both sides, as shown in Figure1 which shows the reliquary containing the Veil of Manoppello photographed front and back. The cheeks are markedly unequal because one of the two is particularly swollen. At ﬁrst glance the chin does not seem to be in line with the rest of the face, and the eyes do not seem to be at the same distance from the nose. All these characteristics give an evident asymmetry to the face. The image has been studied by many scholars because it is not clear how it was created [1–10]. Its peculiarity is due to the fact that, observing the face in reﬂected light, it is visible from both sides. When the observer and the light source are on the same side and look at the front side of the Veil, the image that emerges is that shown in Figure1a; if, instead, the observer looks at the back side, the image represented in Figure1b emerges. The image is as if it were two-faced, although it shows some small differences such as, for example, the direction of the gaze.

Heritage 2018, 1, 289–305; doi:10.3390/heritage1020019 www.mdpi.com/journal/heritage Heritage 2018, 1 290

Heritage 2018, 2, x FOR PEER REVIEW 2 of 17 Moreover, the face is visible even if it is observed by back-illuminating it with grazing light and observingHeritage 2018 the, 2 Holy, x FORFace PEER transmittedREVIEW by the Veil, as schematically shown in Figure2. 2 of 17

Figure 1. Face of the Veil of Manoppello seen from the front (a) and back (b), illuminated frontally and seen in reflection. Figure 1. Face of the Veil of Manoppello seen from the front (a) and back (b), illuminated frontally Figure 1. Face of the Veil of Manoppello seen from the front (a) and back (b), illuminated frontally and Moreover,and seen in reflection.the face is visible even if it is observed by back-illuminating it with grazing light and seen in reﬂection. observing the Holy Face transmitted by the Veil, as schematically shown in Figure 2. Moreover, the face is visible even if it is observed by back-illuminating it with grazing light and observing the Holy Face transmitted by the Veil, as schematically shown in Figure 2.

Figure 2. Images of the Holy Face visible on the Veil of Manoppello depending on the conditions of Figure 2. Images of the Holy Face visible on the Veil of Manoppello depending on the conditions of lighting and observation. lighting and observation. Figure 2. Images of the Holy Face visible on the Veil of Manoppello depending on the conditions of lighting and observation.

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ThatThat is, is, it isit visibleis visible even even when when the the observer observer and an thed the light light source source are are not not on on the the same same side. side. When When the observerthe observer and the and light the source light source are on are the on same theside same and side look and at look the frontat the side front of side the Veil,of the the Veil, reﬂection the imagereflection that canimage be that seen can is representedbe seen is represented by the green by the box green in Figurebox in 2Figure, which 2, which reproduces reproduces the image the ofimage Figure of1a; Figure if, instead, 1a; if, instead, the observer the observer and the and source the source are not are bothnot both on theon the same same side side and and the the VeilVeil is illuminatedis illuminated with with grazing grazing light, light, the imagethe image is transmitted is transmitted by by the the Veil Veil and and is thatis that visible visible in in the the yellow yellow box of Figurebox of 2Figure. Analogous 2. Analogous behavior behavi canor be can obtained be obtained if the iconif the is icon rotated is rotated by 180 by degrees. 180 degrees. In other In other words, twowords, very similartwo very images similar are images visible are and visible are and of the are type of the indicated type indicated in the greenin the boxgreen of box Figure of Figure2—one visible2—one from visible the front from side, the thefront other side, from the theother back from side. the Moreover, back side. it Moreover, is possible it to is view possible another to view couple ofanother images, couple also very of images, similar also to each very other—onesimilar to ea visiblech other—one from the visible front from side, the the front other side, from the the other back sidefrom of thethe icon—ofback side theof the type icon—of indicated the intype the indicate yellowd boxin the of Figureyellow 2box. However, of Figure they 2. However, differ in they some anatomicaldiffer in some details anatomical from the ﬁrstdetails couple from (those the first of thecouple green (those box). of We the pass green from box). the We images pass from of the the type representedimages of the in thetype green represented box to in those the green of the box yellow to those box of by the changing yellow box the by conditions changing the of visualization conditions of visualization and illumination of the icon, i.e., passing from direct to grazing light and from and illumination of the icon, i.e., passing from direct to grazing light and from observer and source observer and source placed on the same side to observer and source placed on opposite sides of the placed on the same side to observer and source placed on opposite sides of the Veil. Veil. Finally, the direct light backlighting does not immediately make the face visible, because the Veil Finally, the direct light backlighting does not immediately make the face visible, because the Veil is so thin that the amount of light that reaches the observer would be excessive. Instead, by placing an is so thin that the amount of light that reaches the observer would be excessive. Instead, by placing obstaclean obstacle between between observer observer and and source source to shield to shield the the excess excess of light,of light, like like the the hand hand shown shown in in Figure Figure3 a, it is3a, possible it is possible to see to the see part the ofpart the of face the face that that falls falls in the in the shielded shielded area. area.

Figure 3. Veil of Manoppello deformation: (a) detail of the Veil of Manoppello visible when light Figure 3. Veil of Manoppello deformation: (a) detail of the Veil of Manoppello visible when light source source and observer are perpendicular to the Veil but are on opposite sides, and the direct beam is and observer are perpendicular to the Veil but are on opposite sides, and the direct beam is partially partially shielded. The thin weave of the fabric is visible. The distortion of the threads which, shielded. The thin weave of the fabric is visible. The distortion of the threads which, obviously, deforms obviously, deforms the image has been highlighted; (b) face always seen in backlighting but with the image has been highlighted; (b) face always seen in backlighting but with grazing light coming grazing light coming from a lateral source. The white box on the right represents approximately the from a lateral source. The white box on the right represents approximately the region of the face on region of the face on the left. the left.

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In the example of Figure3a, the hand allows us to see the right side of the face at the height of the cheekbone and the eye, corresponding to the white box in Figure3b, where the image of the whole face is shown as seen in backlighting with grazing light. Comparing Figure3b with Figure1a,b, it emerges that in this back-illumination mode the teeth are not visible, but they are visible if the lighting is direct, as shown in Figure2 (green box). The possibility of seeing the images of the double-sided face resides in the particular structure of the fabric. These are the characteristics that have attracted the attention of several authors who have, however, studied the Veil of Manoppello from mainly the historical point of view. Our study is divided into several parts. In this paper we ﬁrst discuss the possible nature of the Veil ﬁbers, based on its unusual optical behavior, through an accurate analysis of some experimental data obtained from optical and spectroscopic characterizations already carried out on it. Then, we digitally analyze the image with the aim of correcting the deformations of the face, a consequence of the distortion of the thin structure of the weft and warp threads due to the numerous stresses suffered by the Veil over the centuries.

2. Optical and Spectroscopic Analyses of the Veil of Manoppello Few scientific studies have been conducted on the Veil, and they are reported only in conferences, technical reports, and essays [1–10]. As already highlighted, there seem to be more images of the Holy Face, more or less visible depending on the lighting conditions (Figure2). P. Baraldi [ 10], who performed Raman spectroscopy, was unable to determine the composition of either the colors or the material constituting the Veil because there were no significant spectral peaks in the acquisitions. In the analysis a red laser at 633 nm was used as it passes without a great attenuation through the glass that contained the Veil during the measurements. It is well known that many organic dyes do not give Raman spectra when using this type of wavelength, while other dyes give distinct signals. However, no compound could be identified from the Raman measures [10]. The first investigation conducted on the Veil of Manoppello precedes the analysis of Baraldi and dates back to 1974, when the Veil was observed with ultraviolet light (UV) generated by a Wood lamp [11]. Also in this study, it was found that neither the fabric nor the image show the fluorescence typical of ancient paintings, although it must be kept in mind that the two glass panels that protect the Veil at least partially filter ultraviolet rays. UV analysis was also repeated by the third author [12]; see Figure4a. Although the image has a low resolution, the experimental result confirms what was previously found. Once again, in fact, no fluorescence signals were found that can be traced back to the presence of natural organic substances such as oils, fats, and waxes—traditional pictorial binders used in the past. At the same time, however, comparing Figure4a with Figure3b in some anatomical details such as, for example, the definition of beard and mustache hair, the definition of pupils, and iris or lip contour, in UV these details are almost absent. Moreover, the Holy Face seen under UV radiation can be better correlated to the image obtained in transmission with grazing-incident visible light. Indeed, although the UV image is rather dark and of low definition, the teeth are not visible in UV, as in Figure3b. Moreover, the UV image reveals an upper lip characterized by a greater width than the lower lip, unlike what is visible in Figure3b. This feature may imply that the finer anatomical details, such as beard hairs or the upper lip profile, may have also been retouched or added at a later time on a more ancient icon. Further investigations conducted in 1984 and in 2007 with infrared (IR) light have allowed for verification that in the Veil there are neither traces of preliminary sketches of the face, typical of all paintings, nor a visible signature or brand of the author, as shown in Figure4b [ 12]. Comparing Figure4b with Figures4a and3b, it should be noted how much the IR face is better correlated with the image seen with visible light when observer and light source are on the same side of the icon (Figure1). Heritage 2018, 1 293 Heritage 2018, 2, x FOR PEER REVIEW 5 of 17

Figure 4. Manoppello Veil photographed under ultraviolet and infrared light. (a) Ultraviolet Figure 4. Manoppello Veil photographed under ultraviolet and infrared light. (a) Ultraviolet photograph taken by laying the Veil for 15 seconds and moving the light source; therefore, there are photograph taken by laying the Veil for 15 seconds and moving the light source; therefore, there some reflections due to the glass that were not possible to remove during the measurements; (b) are some reflections due to the glass that were not possible to remove during the measurements; Infrared photograph with source and observer placed on the same side with respect to the icon. (b) Infrared photograph with source and observer placed on the same side with respect to the icon. Infrared photographs generally have the characteristic of highlighting discontinuities caused by pigmentsInfrared or extraneous photographs components generally have added the at characteristic a later stage. of The highlighting Veil was discontinuitieslit with an incandescent caused by pigmentslamp and orphotographed extraneous components with infrared-sensitive added at a later film. stage. It should The be Veil noted, was litin withFigure an 4b, incandescent how some areas lamp andof the photographed hair show a withdifferent infrared-sensitive coloring. In film.this rega It shouldrd, note be noted,that many in Figure materials4b, how reflect, some absorb, areas of and the hairtransmit show infrared a different light coloring. differently In thisthan regard, visible note light. that For many example, materials blue/azure reflect, seen absorb, under and IR transmit usually infraredappears lighter light differently than when than viewed visible with light. visible For ligh example,t; sometimes blue/azure it appears seen underdarker, IR depending usually appears on the lighterchemical than composition when viewed [13,14]. with visible light; sometimes it appears darker, depending on the chemical compositionThe IR characterization [13,14]. indicates that the color of the hair gradually changes from a lower intensityThe IRin characterizationthe upper part of indicates the head that to a the higher color intensity of the hair in graduallythe lower changesarea. These from experimental a lower intensity data incould the be upper compatible part of thewith head a hair to color a higher that intensity from brown in the in lowerthe upper area. part These of the experimental head becomes data brown- could bered compatible in the lower with area. a hairHowever, color thatif you from look brown at the inVeil the with upper direct part visible of the light, head the becomes hair color brown-red is more inuniform, the lower both area. in the However, upper and if youin the look lower at the part. Veil It should with direct also visiblebe noted light, that thethe haircolor color of the is iris more is uniform,clearer than both what in the can upper be observed and in the in lowerthe visible part. lig Itht, should although also becolors noted that that tend the to color brown of the become iris is clearerdarker under than what IR [14]. can Moreover, be observed the incorneal the visible limbus, light, which although is the darker-color colors that circle tend tothat brown characterizes become darkerthe outermost under IR part [14 ].of Moreover,each iris immediately the corneal limbus,before the which white is the sclera darker-color of the eye, circle is clearly that characterizes visible with thevisible outermost light (Figure part of3b), each but iris disappears immediately almost before completely the white and sclera shows of noncircular the eye, is clearly contours visible under with IR visible(in particular, light (Figure see the3b), left but eye). disappears These results almost could completely suggest and the shows possible noncircular presence contoursof some touch-ups under IR (inand particular, the possible see thecontribution left eye). These of another, results coulddiffere suggestnt underlying the possible chromatic presence component, of some touch-ups not directly and thedetectable possible in contribution visible light. of another,Finally, differentnote that underlying the glass chromaticenclosing component, the Veil absorbs not directly part detectableof the IR inradiation. visible light. This Finally,may have note prevented that the glass the detection enclosing and the Veilrecording absorbs of part any of weak the IR signals radiation. generated This may by havethe interaction prevented thewith detection the incident and recordingradiation. of The any same weak signalscould have generated happened by the with interaction the Raman with thespectroscopy, incident radiation. i.e., the Theweaker same signals could coming have happened from the with Veil the could Raman have spectroscopy, been covered i.e., by the the weaker glass signalssignal. To coming confirm from all the these Veil findings, could have more been detailed covered and by accurate the glass UV signal. and To IR confirm characterizations all these findings, would morebe very detailed useful. and To accurateperform UVnew and optical IR characterizations measurements, woulddirect access be very to useful. the Veil To would perform be new necessary; optical measurements,this has to be previously direct access authorized to the Veil by Manoppello’s would be necessary; Sanctuary. this has to be previously authorized by Manoppello’sD. Vittore Sanctuary. [15] analyzed the Veil using a very-high-resolution digital scanner and did not detect any colorD. Vittore residues [15] analyzedin the interspace the Veil usingbetween a very-high-resolution the warp and the weft digital threads. scanner The and presence did not of detect any anypigment, color therefore, residues inis thecircumscribed interspace within between the the regi warpon of and the thethreads, weft which threads. have The dimensions presence ofof anythe order of 100 μm (0.1 mm).

Heritage 2018, 1 294 pigment, therefore, is circumscribed within the region of the threads, which have dimensions of the order of 100 µm (0.1 mm). C. Vigo, an expert in the processing of sea byssus, found some characteristics on the fabric, such as Heritage 2018, 2, x FOR PEER REVIEW 6 of 17 its unusual luster, which could be ascribable to the filaments produced by the Pinna nobilis mollusk, worked sinceC. ancient Vigo, an times expert [in16 the]. Theprocessing third of author sea byss ofus, thisfound paper, some characteristics however, performed on the fabric, asuch microscopic analysis withas its cross-polarizedunusual luster, which light could to be highlight ascribable to any the birefringencefilaments produced useful by the for Pinna characterizing nobilis mollusk, the fibers constitutingworked the threadssince ancient of thetimes fabric. [16]. The He third noted author iridescence of this paper, not however, typical performed of the byssus,a microscopic but rather of analysis with cross-polarized light to highlight any birefringence useful for characterizing the fibers linen [12].constituting It is known the threads that flax of the is birefringentfabric. He noted and, iridescence unlike not sea typical byssus, of the shows byssus, abut marked rather of variability of colors fromlinen [12]. blue-green It is known to that yellow-red flax is birefringent in light and, transmitted unlike sea byssus, to cross shows polarization. a marked variability Themicroscopic of investigationscolors with from crossblue-green polarization to yellow-red highlighted in light transmitted the presence to cross of birefringencepolarization. The on microscopic parts of the fibers, indicatinginvestigations that it may with be linen, cross polarization as shown highlighted in Figure 5the. presence of birefringence on parts of the fibers, indicating that it may be linen, as shown in Figure 5.

Figure 5. Optical characterization of the Manoppello Veil’s fibers with polarized light: (a,b) for Figure 5. comparison,Optical characterization two flax fibers, seen of thein polarize Manoppellod light, Veil’swhich fibersshow the with typical polarized colors due light: to (a,b) for comparison,birefringence; two flax fibers,(c) for comparison, seen in polarized some sea light, byssus which fibers seen show in thepolarized typical light colors that do due not to show birefringence; (c) for comparison,birefringence; some (d–f) seaphotographs byssus fibersof some seen fibers in of polarized the Veil of light Manoppello, that do acquired not show with birefringence; an (d–f) photographsincandescent of lamp; some (g) fibersmagnification of the ofVeil (d), taken of Manoppello, with an incandescent acquired lamp; ( withh,i) same an region incandescent as (g) lamp; but obtained with a neon lamp; all the fibers of the Veil seen in polarized light show birefringence. (g) magnification of (d), taken with an incandescent lamp; (h,i) same region as (g) but obtained with a neon lamp;Figures all the 5a,b, fibers for ofcomparison, the Veil seen show in two polarized linen fibers light as seen show in birefringence.polarized light. In both, the typical color due to birefringence is visible. Figure 5c shows, for comparison, some sea byssus fibers seen in polarized light. They maintain the same coloration that they have under a nonpolarized light beam Figureand5a,b, do fornot comparison,show signs of birefringence. show two linen Figures fibers 5d–has show seen some in polarizedimages of threads light. Inof both,the Veil the typical color due toacquired birefringence in polarized is light visible. at a tear Figure of the5 fabricc shows,. All the for fibers comparison, analyzed show some thesea birefringence byssusfibers that seen in polarizedis light. typical They of linen, maintain jute, and the hemp. same Figures coloration 5d–f are photographs that they have taken underwith an aincandescent nonpolarized source. light beam and do notIn show particular, signs Figure of birefringence. 5f can be compared Figure with5d–h Figure show 5h. Figure some 5g, images which of is threadsa zoom of of Figure the Veil 5d, acquired taken with an incandescent lamp, can be compared with Figures 5h,i, regarding the same region but in polarizedobtained light atwith a teara neon of lamp. the fabric. In these All last the figures fibers the analyzedpresence of showbirefringence the birefringence is also evident. that From is typical of linen, jute,the and analyses hemp. carried Figure out 5ind–f polarized are photographs light, we can exclude taken the with possibility an incandescent that it is sea byssus. source. It seems In particular, Figure5f canconfirmed, be compared instead, that with the FigureVeil consists5h. of Figure thin linen5g, threads. which is a zoom of Figure5d, taken with an incandescent lamp, can be compared with Figure5h,i, regarding the same region but obtained with a neon lamp. In these last figures the presence of birefringence is also evident. From the analyses carried out in polarized light, we can exclude the possibility that it is sea byssus. It seems confirmed, instead, that the Veil consists of thin linen threads. Heritage 2018, 1 295

It should be noted, too, that sea byssus fibers are characterized by slightly higher diameters µ µ (20–30Heritagem) 2018 than, 2, arex FOR those PEER ofREVIEW linen (10–20 m), which are similar in size to those found in the7 of fabric 17 of the Veil (14 ± 6 µm). Moreover, the fibers of the Veil, seen enlarged, show evident traces of cementation, even if theIt should spectroscopic be noted, analyses, too, that sea as byssus already fibers mentioned, are characterized did not by show slightly the higher presence diameters of any (20– specific binder30 usedμm) than in painting are those [ 12of ].linen (10–20 μm), which are similar in size to those found in the fabric of the Veil (14 ± 6 μm). Moreover, the fibers of the Veil, seen enlarged, show evident traces of cementation, 3. Veileven Structure if the spectroscopic and Image analyses, Dependence as already on Lighting mentioned, Conditions did not show the presence of any specific binder used in painting [12]. A very important characteristic of the Veil of Manoppello is its semitransparency, as shown in Figure3. Veil3a. Structure The Veil and is enclosedImage Dependence between on two Lighting glass panels, Conditions which the religious authorities prefer not to be opened; it is framed in a case with a visible space of 240 × 175 mm2, and it exceeds the A very important characteristic of the Veil of Manoppello is its semitransparency, as shown in ± frameFigure dimensions 3a. The Veil by 5 is mm. enclosed In the between horizontal two glass direction panels, there which are the 27 religious2 threads authorities per cm, prefer with not a pitch ∼ of 1/(27to be− opened;1) = 0.39 it is± framed0.03 mm in a. case In the with vertical a visible direction space of there240 × 175 are mm 33 ±2, and2 threads it exceeds per the cm frame of fabric, obtaineddimensions with a by traditional 5 mm. In weavingthe horizontal with direction ratio of 1:1there between are 27 ± the2 threads weft (horizontal)per cm, with anda pitch warp of 1/(27 (vertical) threads.− 1) The≅ 0.39 vertical ± 0.03 mm. step, In therefore the vertical equal direction to 1/(33 there− are1) =∼33 0.31± 2 threads± 0.02 per mm, cm is of on fabric, average obtained narrower with than the horizontala traditional one. weaving Their with ratio ratio is 0.79of 1:1± between0.11. The the threadsweft (horizontal) are on averageand warp 0.12 (vertical) mm thick.threads. Therefore, The betweenvertical the step, threads therefore there equal is a considerableto 1/(33 − 1) ≅ 0.31 gap, ± 0.02 ranging mm, is from on average 0.18 to narrower 0.25 mm, than equal the horizontal to about twice the thicknessone. Their of ratio the thread.is 0.79 ± The 0.11. ratio The betweenthreads are the on empty average region 0.12 left mm by thick. the threadsTherefore, and between the surface the of a threads there is a considerable gap, ranging from 0.18 to 0.25 mm, equal to about twice the thickness rectangle of dimensions 0.31 × 0.39 mm2, equal to the product of the weft and warp steps, is given of the thread. The ratio between the empty region left by the threads and the surface of a rectangle of by (0.31 − 0.12) × (0.39 − 0.12)/(0.31 × 0:39) =∼ 0.42. Therefore, about 42% of the Veil of Manoppello dimensions 0.31 × 0.39 mm2, equal to the product of the weft and warp steps, is given by (0.31 − 0.12) consists× (0.39 of − empty 0.12)/(0.31 space. × 0:39) This ≅ 0.42. explains Therefore, why about it is semitransparent42% of the Veil of Manoppello under direct consists light, of as empty shown in Figurespace.3a. At This the explains same time, why however,it is semitransparent it is not clear under how direct it was light, possible as shown to in obtain Figure the 3a. different At the same images that emergetime, however, depending it is not on clear the how following it was possible lighting to obtain conditions: the different (1) light images and that observer emerge depending perpendicular to theon Veil the onfollowing the same lighting side conditions: (Figure1a,b); (1) light (2) lightand observer and observer perpendicular perpendicular to the Veil toon the the Veilsame but side from opposite(Figures sides 1a,b); (Figure (2) light3a); (3)and observer observer perpendicularperpendicular to to the the Veil Veil but and from grazing opposite backlighting sides (Figure of 3a); the Veil (Figure(3)3 observerb). Further, perpendicular as can be to seen the byVeil comparing and grazing the backlighting images in of Figures the Veil1– (Figure3, it is not3b). clearFurther, how as it is possiblecan tobe highlightseen by comparing different the anatomical images in details, Figures such1–3, it as is the not differently clear how it opened is possible mouth, to highlight to allow the different anatomical details, such as the differently opened mouth, to allow the teeth to be seen or teeth to be seen or not, by changing the lighting conditions. not, by changing the lighting conditions. This singular behavior is summarized in Figure6, in which the details of the mouth and of the This singular behavior is summarized in Figure 6, in which the details of the mouth and of the rightright eye areeye reported.are reported. Figure Figure6a 6a shows shows the the detail detail ofof the mouth and and the the right right eye eye of the of thefront front side sideof of the Veil,the Veil, when when the observerthe observer and and the the source source are are on on thethe same side. side.

Figure 6. Details of the mouth and the right eye of the Manoppello’s Holy Face. (a) Detail of the mouth Figure 6. Details of the mouth and the right eye of the Manoppello’s Holy Face. (a) Detail of the mouth and the right eye of the image reflected from the front side of the Veil when observer and source are and theon the right same eye side. of the The image image reﬂected of the back from side, the ob fronttained side under of thethe Veilsame when lighting observer conditions, and shows source are on theanatomical same side. details The similar image to of those the backof the side,front obtainedside. (b) Detail under of thethe mouth same lightingand the right conditions, eye of the shows anatomicalreflection details image similar transmitted to those from ofthe the back front sideside. of the ( bVeil) Detail when ofit is the illuminated mouth and with the grazing right light. eye of the reﬂection image transmitted from the back side of the Veil when it is illuminated with grazing light.

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Under this lighting condition, the same anatomical details such as, for example, the white teeth and the sclera of the eye, are present when looking at both the front and back sides. However, if the observer and the light source are on opposite sides to the Veil, to see the face it is necessary to use grazing light. In this case, as shown in Figure6b, the white color disappears both in the area of the teeth and in the sclera of the eye. The pupil appears to be reduced in size, since the sclera of the eye of Figure6b appears to be more extended than that of Figure6a, although the dimensions corresponding to the opening of the eye in Figure6a,b are the same. Moreover—and this is the most unusual aspect—not only is the white of the teeth no longer visible, but in grazing light the teeth themselves are no longer visible. It is as if in the image visible in grazing light the upper lip is placed lower and covers the teeth, which are therefore no longer visible. At the same time, however, as evident from the comparison of the red and white arrows shown in Figure6a,b, the highest edge of the upper lip, visible when light source and observer are on the same side, continues to remain visible even when light source and observer are not on the same side. In this regard, note also the change of hue: darker in the part of the upper lip that falls below the profile indicated by the red arrows in Figure6b, with respect to the lighter shade of both the lower lip and the part of the upper lip delimited by the white arrows in Figure6b. Everything should be related to what is visible to UV (Figure4a), and may imply that the lip profile of Figure6b, indicated by the red arrows, is compatible with a retouching performed on an ancient icon. Probably, the profile of the upper lip was that indicated by the white arrows in Figure6b. Finally, as also visible from the anatomical details shown in Figure6, the image obtained with grazing lighting (Figure6b) shows red details much more evident, similar to wounds, excoriations, etc., not so visible in the image obtained with direct illumination (Figure6a). This is an important point we will return to in a later section. Ultimately, as already observed in commenting on Figure2, it is as if several images coexisted on the Veil of Manoppello. Two of them, very similar to each other, almost identical, are visible in front and back if observer and light source are on the same side. Two more images, always visible front–back, emerge from the Veil when source and observer are not on the same side. These images are very similar to one another, but they show some evident differences if compared with the previous couple of images. The color tones of the visible image with grazing lighting tend to yellow-gold in the lighter areas (Figure3b) but, as mentioned above, the semitransparency of the Veil, its natural coloring, and the particular angle of incidence of light grazing, could contribute, not a little, to the final tonality of the image. It should also be noted that with several images on the same support, visible according to lighting conditions (as happens with the Veil of Manoppello), intermediate conditions are often created in which some details of the two images are simultaneously visible. This is the phenomenon of so-called phantom images. In practice, by changing the lighting or visualization modes, neither of the two images can be completely suppressed; they are seen partially overlapped, one more intense than the other. Thus, the profile of the lip indicated by the white arrows in Figure6 is visible both with direct light and with grazing light, but only in one case is it better seen. This also happens for the lip profile indicated by the red arrows in Figure6. This is also present in both images, but is not immediately noticeable because it is superimposed on the profile of the teeth. The phenomenon of phantom images means that while observing the icon, moving sideways with respect to it, the lighting conditions change with respect to the observer. This means that the contribution of the different images is variable and depends on the relative position of the source, Veil, and observer, giving the impression that the face slightly moves the lips and the eyes while the observer changes position. The Veil of Manoppello, therefore, should be considered as the first example in the History of Art of an artwork in which multiple images are visible on the same support, depending on the lighting and/or observation conditions. Indeed, according to the Relatione Historica written by the Capuchin preacher Father Donato da Bomba between the years 1642 and 1645, the Veil could have arrived at the convent, where it is still preserved today, at the beginning of 1500, with more certainty around 1600 [7]. Instead, the first historical example of a painting with the coexistence of Heritage 2018, 1 297

Heritage 2018, 2, x FOR PEER REVIEW 9 of 17 two images, until now considered, is the double portrait of King Frederick IV and Queen Louise GüstowMecklenburg-Güstow of Denmark, painted of Denmark, by Gaspar painted Antoine by Gaspar de Bois-Clair Antoine in de1692. Bois-Clair These are in two 1692. visible These portraits are two independentlyvisible portraits of independently each other, of of the each king other, or queen, of the kingon the or queen,same picture, on the samedepending picture, on depending the angle onof view.the angle But ofthe view. technique But the used technique in this usedcase inclearly this case shows clearly the coexistence shows the coexistence of two paintings of two paintingson the same on painting,the same painting,unlike the unlike Veil of the Manoppello, Veil of Manoppello, for which for the which image the of image the face of the with face the with ajar the mouth ajar mouthis not immediatelyis not immediately visible visible and emerges and emerges only if only it is ifilluminated it is illuminated with grazing with grazing light. light.

4. Hypotheses Hypotheses on on the Optical Mechanism of Image Formation and on the Structure of the Veil The presence presence of of two two images images on on the the same same icon icon is quite is quite unique. unique. It is Itnot is unusual, not unusual, however, however, that dependingthat depending on the on angle the angle of incidence of incidence of light, of light, different different intensities intensities and and spectral spectral components components can can be reflectedbe reflected or transmitted, or transmitted, depending depending on the on thechemic chemicalal composition composition and andmicroscopic microscopic structure structure of the of physicalthe physical medium, medium, thus thus generating generating visible visible color color variations variations depending depending on on the the angle angle lighting lighting and observation. However, However, it it is is unusual unusual that that dependi dependingng on on the the viewing angle, images with different anatomical details can be seen. Figure 77 presentspresents anan enlargementenlargement ofof aa detaildetail ofof thethe VeilVeil ofof Manoppello,Manoppello, mademade byby thethe thirdthird authorauthor of this paper [12]. [12]. A A manual manual joint joint of of two two threads threads and and a a certain certain degree degree of of transparency transparency are are visible, visible, which may have induced C. Vigo to hypothesize marine byssus [16]. [16]. But, But, as as already already summarized, summarized, therethere is experimental evidence that the fibers fibers may be linen. Recall Recall that that the the thickness thickness of of these these fibers fibers is is about 0.12 mm and their section is pseudo-cylindrical.

Figure 7. Zoomed view of a detail of the Veil of Manoppello, showing its fine structure and its Figure 7. Zoomed view of a detail of the Veil of Manoppello, showing its fine structure and its transparency. transparency.

The structure of a linen thread is particularly inhomogeneous inhomogeneous from from the the optical optical point point of of view, view, because regions regions composed composed of of cellulose cellulose (linen (linen micr microfibersofibers of typical of typical size sizeof the of order the order of 10 ofμm), 10 withµm), refractivewith refractive index index values values close closeto 1.5, to alternate 1.5, alternate with air with regions air regions (interspaces (interspaces between between microfibers) microfibers) with awith refractive a refractive index index of 1. of In 1. a In nonhomogeneous a nonhomogeneous micr microstructureostructure the the light light propagates propagates essentially essentially by by diffusion, since the continuous change of the the refractive refractive index index from from 1 1 to to 1.5 1.5 causes causes multiple multiple refractions, refractions, making the medium opaque. The sma smallll thickness allows you to see what is on the other other side, but with with details that that are are not not well well defined. defined. This This is is what what ha happens,ppens, for for example, example, when when looking looking through through a sheet a sheet of photocopierof photocopier paper, paper, essentially essentially made made of ofcellul celluloseose and and of of a athickness thickness of of the the order order of of 100 100 μµm,m, such such as thethe threads threads of the Veil Veil of Manoppello. The sheet is opaque, but given the smal smalll thickness, we see see the shadow of of whatever is is placed beyond beyond it it and in direct contact, without being able to define define well the finerfiner details. The image shown in in Figure Figure 77,, however,however, showsshows thatthat thethe threadsthreads ofof thethe VeilVeil areare characterizedcharacterized byby a certain brightness and transparency. They They do do not not behave, that that is, is, as an opaque medium but as a translucenttranslucent one. Translucency occursoccurs whenwhen thethe transmission transmission of of light light occurs occurs partly partly by by refraction, refraction, as as in in a ahomogeneous homogeneous and and transparent transparent medium, medium, and and partly partly by diffusion, by diffusion, as in aas nonhomogeneous in a nonhomogeneous and opaque and opaquemedium. medium. Even paper Even can paper become can become translucent translucent if, for example, if, for example, a drop of a oildrop soaked of oil the soaked sheet the of papersheet of paper of the previous example. Near the spot, the optical behavior of the medium would radically change, transforming the sheet from opaque to translucent; this would allow us to see even the finer

Heritage 2018, 1 298 of the previous example. Near the spot, the optical behavior of the medium would radically change, transforming the sheet from opaque to translucent; this would allow us to see even the finer details of an object placed beyond it, especially if it is in direct contact with the paper. This is because the air interspaces between the paper fibers would be filled with oil, which has a refractive index closer to that of cellulose. All this would increase the refractive component and decrease the diffusive component, making the optical behavior of the medium translucent. This is the principle that has allowed the development of translucent paper, much used by artists to reproduce faithful copies of the drawings placed in contact with the translucent sheets. The translucent paper was obtained by impregnating common paper with oils, greasy substances, and resins with a refractive index as close as possible to that of cellulose [17]. In this way the opacity of the common paper is considerably reduced. To get an idea of the orders of magnitude, the sheet of a daily newspaper has an opacity of 90% (if it were 100%, the sheet would be totally opaque). For a sheet of ultra-thin white paper, 60 µm thick, it falls to 60%. For a sheet of translucent paper, even thicker at more than 100 µm, opacity drops to 30% [18], demonstrating how important it is to fill (eliminate) the air spaces between cellulose microfibers with a material of refractive index very similar to that of cellulose. One could therefore hypothesize that the fibers of Veil of Manoppello were impregnated with some substance that made them translucent, as seems to emerge from Figure7. However, as mentioned, spectroscopic analyses did not reveal any such compound, and microscopic investigation showed the presence of a cementing substance among the linen fibers. This finding is also deducible from Figure7 showing that the fibers are not well distinguishable due to the lack of empty spaces. They therefore appear to be cemented. But by what? To answer this question, we have to remind ourselves of some historical information concerning the Veil of Manoppello. A report kept in the convent of Manoppello, written in 1618 by Fabritiis, underlines that the image of the Holy Face was in the middle of a square veil with size equal to about four Neapolitan palms, i.e., about 4 × 26.5 cm, so with size equal to about 1 meter [7]. Since the peripheral part of the Veil was all ruined, a certain Father Clemente cut the central rectangle, still kept in the reliquary of Manoppello, leaving only a small border now covered by the reliquary. A veil of linen so big and so thin cannot be placed in relation to a painter’s canvas, but rather to a garment, suitable for a woman of a certain social rank. If, therefore, the linen fibers were impregnated with some substance that had made them even more shiny and translucent, in the hypothesis that it was a garment, it certainly will not have been treated with an oily or greasy substance, or a typical wax of the canvases of painters. Much more likely, the garment would have been impregnated with starch, since the practice of starching clothes and linen is very ancient, dating back to the times of the ancient Egyptians [19,20]. It is beyond the scope of this study to explain why the Holy Face of Manoppello is related, according to our hypothesis, to a garment and not to a painter’s canvas. The sure fact is that the support on which the image is visible is unique in the art world and, by its peculiar characteristic of very thin fabric, the history of the Veil of Manoppello should be placed in relation to that of the Veil of Veronica, the icon/relic of the face of Christ that the Christian tradition binds to his Passion and death. For further information on these interesting historical relationships between the two icons, see the essay by S. Gaeta [7]. Starch is neither an oil nor a fat but a carbohydrate, a polymer having the same chemical composition as cellulose and a very similar structure. There is only one difference between starch and cellulose: in starch, all glucose molecules are oriented in the same way (alpha bonds). In cellulose, on the other hand, each successive glucose unit is rotated 180 degrees around the axis of the main polymer chain with respect to the preceding monomeric unit (beta-glycosidic bonds). The particular orientation of the monomers, compared to the structure of the starch, makes the cellulose polymers much more resistant longitudinally and insoluble in water once combined with hemicellulose and glycoproteins, so as to be suitable for making textile fibers. But the two materials have almost the same index of refraction. Therefore, the starched linen threads would have no gaps between one microfiber and another and, from an optical point of view, the fabric would become particularly Heritage 2018, 1 299 homogeneous; this would allow light to propagate with a dominant refractive component and with a reduced presence of diffusion, making it translucent, even for the small average thicknesses of the threads involved (less than 100 µm). The cellulose structure, in fact, can become so ordered from the atomic point of view as to make it even transparent, and not only translucent, at thicknesses less than 0.1 mm [21]. In the case of the Veil of Manoppello, the linen threads—which, according to our hypothesis, were starched—would behave from the optical point of view as a translucent material. In addition, the linen fibers of the Veil are very thin and are also very distant from each other, so as to preserve the sharpness of the details of the image visible in transmission with backlighting. Indeed, due to the structure of the Veil, diffusion is limited transversally by the dimensions of the order of 0.1 mm; that is, by the dimensions of the linen threads. Because the resolving power of the human eye is of the same order of magnitude, the contours of the image remain clear. For the same reason, given the distance that exists between the fine threads, the Veil also behaves as a semitransparent medium since the propagation between thread and thread, when the light illuminates it with a grazing incidence, takes place in the air. All this implies that the chromatic characteristics of the images of the Holy Face could be explained as a consequence of the combined action of translucency and transparency of the medium, preserving the sharpness of the anatomical details even when the Veil is back-illuminated with grazing light crossing through it to reach the observer. However, the chromatic components are markedly influenced. They can be appropriately compensated in order to reconstruct the original colors of the Holy Face by eliminating the background contribution of the yellow coming from the thin translucent threads (Figure7), consisting of antique linen yellowed over the centuries. This hypothesis could also explain why the spectroscopic analyses carried out on the Veil of Manoppello did not highlight the typical pictorial binders (oils, greases, waxes): not because the binder that cements the fibers does not exist, but because it has the same chemical composition and a polymeric structure very similar to that of cellulose. In this regard, note that in the History of Art, starch has been used as an impregnation of the fibers of canvases, but the use of linen impregnated with starch glue began only in 1850 [22]. The resulting support was resistant, translucent, and suitable for artistic techniques like pencil, ink, and watercolor. Because of the significant production costs, the use of this support began to decline towards 1930 until it disappeared around 1970, thanks to the discovery of transparent plastic materials.

5. The Unusual Optical Behavior of Reddish Details In light of what was discussed in the previous section, given the translucency of the thin starched linen threads, we can understand why the image of the Holy Face is visible both when illuminated in direct light, with observer and light source placed on the same side, and when illuminated with grazing light, with observer and light source placed on opposite sides. In the second case, in fact, the light incident from the back of the Veil is partially diffused, reflected, refracted, and transmitted by the thin threads. Therefore, the more the incidence is grazing, the more the image acquires a yellow-gold hue from the contribution of the yellowish color, typical of the ancient linen, which characterizes the light that passes through the threads. In the first case, instead, when observer and light source are placed on the same side, the reflected component from what is on the surface of the threads dominates, and the colors of the Holy Face are less influenced by the underlying support that is not chromatically neutral. In this regard, it is important to note that the reddish details of the Holy Face are clearly visible only with grazing light, as, for example, can be seen for the lip area (Figure6a,b), i.e., when the component due to transmission through the linen threads becomes important. The fact that reddish details are less visible with direct lighting implies that they are located at greater depths than other colors and, for this reason, become visible only when the light passes through the threads. This would imply that the reddish details colored the linen threads before the addition of the other colors, when there were still no anatomic details of the face, which were added later. All that is singular because the pictorial praxis, and the same practical possibility of realizing the work on such thin threads, would Heritage 2018, 1 300

Heritagerequire 2018 representing, 2, x FOR PEER the REVIEW whole face ﬁrst and only then adding other details, such as, for example,12 of 17 Heritage 2018, 2, x FOR PEER REVIEW 12 of 17 the traces of blood linked to the Passion of Christ represented in the image. Also, this unusual optical ofbehavior the Holy of theFace reddish to a pious details woman of the (“Veronica”) Holy Face of who Manoppellowho wouldwould havehave should wipedwiped be placed thethe sweatsweat in relation andand thethe to onebloodblood of fromthefrom many thethe faceface traditions ofof ChristChrist linked duringduring to the thethe Veil ascentascent of Veronica, toto CalvarCalvar whichy. But, would in addition associate to blood, the image the complete of the Holy image Face of to theathe pious faceface womanwouldwould alsoalso (“Veronica”) appear.appear. WhatWhat who emerges wouldemerges have fromfrom wiped anan accurateaccurate the sweat analysisanalysis and the ofof thethe blood HolyHoly from FaceFace the ofof face ManoppelloManoppello of Christ seemsduring to the represent ascent to this Calvary. tradition. But, It inremains addition unexplainable to blood, the how complete this particular image of sequence the face wouldof coloring also theappear.the icon,icon, What withwith emergesreddishreddish detailsdetails from an placedplaced accurate atat aa analysis greatergreater depth of the and Holy impressed Face of Manoppello before the other seems colors, to represent could havethis tradition. been obtained It remains on a such unexplainable a thin starched how this linen particular veil. A more sequence accurate of coloring Raman thecharacterization icon, with reddish than thosedetailsthose alreadyalready placed done,done, at a greater usingusing modernmodern depth and instrumentation,instrumentation, impressed before oror anan the X-rayX-ray other characterizationcharacterization colors, could have couldcould been provideprovide obtained usefuluseful on experimentala such a thin starcheddata about linen thethe veil. naturenature A moreandand chemicalchemical accurate compositioncomposition Raman characterization ofof thethe reddishreddish than details,details, those Veil,Veil, already andand done,otherother pigments.using modern instrumentation, or an X-ray characterization could provide useful experimental data about the nature and chemical composition of the reddish details, Veil, and other pigments. 6. Deformation of the Veil and Correction of Image Distortions 6. Deformation of the Veil and Correction of Image Distortions The thin structure of the Veil predisposes it to deform easily, as shown in Figure 3a. Moreover, The thin structure of the Veil predisposes it to deform easily, as shown in Figure3a. Moreover, tensionstensions developeddeveloped overover thethe centuriescenturies betweenbetween thethe VeVeilil andand supportingsupporting structurestructure havehave alsoalso generatedgenerated tensions developed over the centuries between the Veil and supporting structure have also generated rips, such as the one shown in Figure 8, the genesis of which is schematically explained in Figure 9 rips, such as the one shown in Figure8, the genesis of which is schematically explained in Figure9[ 12]. [12].[12].

Figure 8. OpticalOptical microscopemicroscope magnificationmagnification magniﬁcation of of a a Veil Veil rip.rip.

Figure 9. Phases of the possible formation of rips in the Veil: (1) Initial state: Veil framed on a rigid Figure 9.9. Phases of the possible formation of rips in the Veil: (1) InitialInitial state:state: Veil framedframed onon aa rigidrigid support. (2) Due to aging, the fabric would shrink if it were not bound to the frame. (3) For the Veil support. ((2)) DueDue toto aging,aging, thethe fabricfabric would would shrink shrink if if it it were were not not bound bound to to the the frame. frame. (3 )(3 For) For the the Veil Veil to toremainto remainremain bound boundbound to the toto frame, thethe frame,frame, internal internalinternal coaction coactioncoaction tensions tensionstensions must be developed;mustmust bebe develodevelo forcesped; have forces been representedhave been representedbyrepresented black arrows. byby blackblack (4) Due arrows.arrows. to the ((4 application)) DueDue toto thethe of applicationapplication these forces, ofof thesethese the fabric forces,forces, gets thethe ripped. fabricfabric getsgets ripped.ripped.

The thin structure of the Veil is certainly at the base of its peculiar characteristics of the Holy The thinthin structurestructure of of the the Veil Veil is is certainly certainly at theat th basee base of itsof peculiarits peculiar characteristics characteristics of the of Holy the Holy Face Face being visible in both sides and of showing different images depending on the angle of beingFace being visible visible in both in sides both and sides of showing and of differentshowing imagesdifferent depending images depending on the angle on of the illumination angle of illumination and observation. At the same time, however, it causes structural failure of the fabric andillumination observation. and observation. At the same At time, the however,same time, it causeshowever, structural it causes failure structural of the failure fabric of (Figures the fabric3a (Figures 3a and 8) and, consequently, non-negligible deformations of the image. In fact, as shown in and(Figures8) and, 3a consequently,and 8) and, consequently, non-negligible non-negligible deformations deformations of the image. of Inthe fact, image. as shown In fact, in as Figure shown3a, in Figure 3a, the red line and the arrows must follow a curvilinear path to remain parallel to the vertical theFigure red 3a, line the and red the line arrows and the must arrows follow must a curvilinear follow a cu pathrvilinear to remain path to parallel remain to parallel the vertical to the threads.vertical threads.threads. ThereThere isis aa consistentconsistent shiftshift withwith respectrespect toto aa straightstraight lineline (vertical(vertical blueblue line).line). QuantitativeQuantitative analysis of Figure 3a shows that the rotation of the vertical threads is equal to ±7°. The white arrow at the bottom indicates the maximum displacement detected in the weaving of the fabric caused by thethe deformation.deformation. ForFor comparison,comparison, anotheranother whitewhite arroarrow is drawn inside the right eye, which allows

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There is a consistent shift with respect to a straight line (vertical blue line). Quantitative analysis of Figure3a shows that the rotation of the vertical threads is equal to ±7◦. The white arrow at the bottomHeritage 2018 indicates, 2, x FOR the PEER maximum REVIEW displacement detected in the weaving of the fabric caused by13 of the 17 deformation. For comparison, another white arrow is drawn inside the right eye, which allows us tous assessto assess the the extent extent of of the the deformation—equal deformation—equal to to about about a a quarter quarter of of the the width width of of the the eye. eye. SinceSince aa human eye, in proportions of 1:1, in correspondencecorrespondence with the anatomical area in which the whitewhite arrow lieslies hashas anan averageaverage openingopening ofof 2.3–2.4 2.3–2.4 cm, cm, it it follows follows that that the the distortion distortion of of the the weaving weaving causes causes a displacementa displacement of of about about 0.5–1.0 0.5–1.0 cm cm in in the the horizontal horizontal dimensions dimensions of of the the rightright cheek.cheek. ThisThis deformationdeformation falls justjust inin correspondence correspondence with with the the swollen swollen cheek cheek and and can, can, for this for reason,this reason, have beenhave unnoticed been unnoticed before. Inbefore. light In of light this analysis,of this analysis, part of part the swelling of the swelling of the rightof the cheek right ischeek due tois due the deformationto the deformation of the fabricof the structure.fabric structure. Also, otherAlso, evidentother evident asymmetries asymmetries of the of face, the likeface, the like different the different distances distances of the of eyes the fromeyes thefrom nose–mouth the nose–mouth axis, axis, could could have have their their origin origin in the in deformationthe deformation of the of the Veil. VeSinceil. Since B.P. B. Schl P. Schlòmeròmer [5] hypothesized[5] hypothesized that that the the face face represented represented on on the the Veil Veil of Manoppelloof Manoppello is superimposable is superimposable to theto the image image of theof the face face of of the the Man Man of of the the Shroud Shroud of of Turin Turin if if it it is is suitably suitably rotated, rotated, in in order order to toverify verify thisthis hypothesishypothesis correctly, itit isis necessarynecessary ﬁrstfirst toto removeremove thethe distortionsdistortions due to thethe yielding of the Veil fabric. To our knowledge, thisthis type of correction toto the image of the Manoppello faceface hashas nevernever beenbeen made.made. But this isis anan importantimportant step,step, sincesince thesethese deformationsdeformations are not negligible,negligible, and exceed the limits of spatialspatial resolution with which the face appearsappears in thethe photographicphotographic negative of the Shroud image, equal to 4.9 ± 0.50.5 mm mm [23]. [23]. The The distortion distortion can can be eliminated digitally by processing the image, imposing that the weaveweave ofof thethe fabricfabric becomesbecomes rectangularlyrectangularly symmetricalsymmetrical asas muchmuch asas possible.possible. It is possiblepossible toto compare aa detaildetail ofof thethe ManoppelloManoppello faceface beforebefore correctioncorrection (Figure(Figure 10 10a)a) andand afterafter (Figure(Figure 10 10b).b).

Figure 10. Detail of the face of Manoppello: (a) before the correction of the distortion; (b) after the Figure 10. Detail of the face of Manoppello: (a) before the correction of the distortion; (b) after the correction. correction.

The verticalvertical dotteddotted whitewhite line, line, together together with with the the arrows, arrows, is usefulis useful to highlightto highlight the the distortion distortion of the of imagethe image (Figure (Figure 10a), 10a), almost almost completely completely absent absent in the in correctthe correct image image (Figure (Figure 10b). 10b). The sameThe same can becan said be forsaid the for horizontal the horizontal dashed dashed line placed line placed near anear defect a defec in thet in weaving the weaving of the of fabric the fabric which which is curved is curved in the distortedin the distorted image image (Figure (Figure 10a) and 10a) becomes and becomes straight straight in the correctin the correct image (Figureimage (Figure 10b). 10b). Figure 11 shows the image as a whole, original (a) and corrected by distortion (b), on a black background to highlight the total deformation of the Veil.

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Figure 11 shows the image as a whole, original (a) and corrected by distortion (b), on a black backgroundHeritage 2018, 2 to, x FOR highlight PEER REVIEW the total deformation of the Veil. 14 of 17

Figure 11. Holy Face of Manoppello: (a) before the correction of the deformation; (b) after the Figure 11. Holy Face of Manoppello: (a) before the correction of the deformation; (b) after the correction. correction.

TheThe dasheddashed white white vertical vertical line line and and the the arrows arrows highlight highlight the presencethe presence of the of distortion the distortion in the fabricin the onfabric the on left the (Figure left (Figure 11a) and 11a) its and absence its absence on the on right the (Figure right (Figure 11b). The11b). horizontal The horizontal dotted dotted blue linesblue intersectlines intersect the pupils the pupils of the face,of the while face, the while vertical the onesvertical are ones on the are axis on of the specular axis of symmetry specular symmetry right–left, takingright–left, the taking mouth the and mouth nose asand a reference.nose as a referenc In Figuree. 11In aFigure the two 11a blue the dottedtwo blue lines dotted do not lines intersect do not perpendicularlyintersect perpendicularly (distorted (distorted image) as image) they do as after they the do corrections after the corrections (Figure 11 (Figureb). 11b). InIn FigureFigures 12 12a,ba,bwe we havehave reportedreported thethe FourierFourier TransformTransform (FT)(FT) module of Figures Figure 1111a,b,a,b, onon aa logarithmiclogarithmic scale,scale, afterafter subtractingsubtracting thethe componentscomponents withwith thethe lowestlowest spatialspatial frequencyfrequency (typical(typical ofof thethe anatomicalanatomical features)features) to to highlight highlight the the higher higher ones, ones, the the latter latter linked linked to theto the smallest smallest details details present present in the in image,the image, such such as, foras, example,for example, the threadsthe threads of the of the fabric. fabric. The The FT ofFT theof the image image in Figurein Figure 11a, 11a, shown shown in Figurein Figure 12a, 12a, is strongly is strongly directional, directional, i.e., it i.e., is not it isotropicis not isotropic from a rotationalfrom a rotational point of view.pointIn of paintings, view. In thepaintings, rotational the anisotropy rotational anisotropy can be a consequence can be a conseq of theuence directionality of the directionality with which with the painter which hasthe painter spread thehas colorspread on the the color canvas, on thatthe canvas, is, how that he has is, movedhow he the has brush, moved or the the brush, pictorial or techniquethe pictorial used. technique In the caseused. of In the the face case of of Manoppello, the face of however,Manoppello, the structurehowever, of the the structure fabric also of contributesthe fabric also to the contributes FT module. to Inthe the FT present module. case, In the it appears present that case, the it peaks appears (white that in the the peaks image), (white are atin multiple the image), spatial are frequenciesat multiple (typicalspatial frequencies of a periodic (typical structure), of a whichperiodic correspond structure), precisely which correspond to the mean precisely distance to between the mean the distance threads ofbetween the Veil. the As threads shown of in the Figure Veil. 12 Asa, sinceshown the in angular Figure 12a, opening since of the the angular visible structuresopening of in the the visible FT is ◦ ◦ aboutstructures 13 in in the the vertical FT is about direction 13° in and the 20 verticalin the direction horizontal and direction, 20° in the and horizontal since the directdirection, measurement and since obtainedthe direct by measurement the local distortion obtained of by the the image local distorti in Figureon3 ofa, the along image the redin Figure line, gives3a, along a total the angularred line, ◦ variationgives a total of 14 angular—of the variation same order of 14°—of of magnitude—it the same order is deduced of magnitude—it that much of is the deduced anisotropy that ofmuch the FT of shouldthe anisotropy be caused of preciselythe FT should by the be distortion caused precisel of they structure by the distortion of the Veil. of the structure of the Veil.

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Figure 12. Fourier Transform module of the Manoppello’s Holy Face: (a) before correction of the Figure 12. Fourier Transform module of the Manoppello’s Holy Face: (a) before correction of the distortion; (b) after correction. distortion; (b) after correction. Furthermore, the anisotropic component of the FT of Figure 12a is rotated about 2° relative to ◦ the verticalFurthermore, and horizontal the anisotropic axes, implying component a rotation of theal FTcomponent of Figure of 12 thea isentire rotated image. about The 2 FTrelative of the tocorrected the vertical image and from horizontal the deformation, axes, implying shown a rotationalin Figure component12b, shows ofa substantial the entire image. change The in the FT ofrotational the corrected anisotropic image component. from the deformation, Indeed, we highligh shownt in (a) Figure the absence 12b, shows of rotations, a substantial present change in Figure in the12a; rotationaland (b) a residual anisotropic rectangular component. symmetry Indeed, that we seem highlights to be directly (a) the absencerelated to of the rotations, FT of the present weaving in Figureof the 12Veil,a; and which (b) aafter residual the rectangularcorrection symmetryof the deformation that seems has to be become directly rectangular, related to the as FT it ofwas, the weavingpresumably, of the originally. Veil, which after the correction of the deformation has become rectangular, as it was, presumably, originally. 7. Conclusions and Future Works 7. Conclusions and Future Works The Veil of Manoppello is a unique artwork in the panorama of the History of Art dedicated to The Veil of Manoppello is a unique artwork in the panorama of the History of Art dedicated to the Passion of Christ. An analysis of the few experimental data available on this icon has clarified the Passion of Christ. An analysis of the few experimental data available on this icon has clarified some aspects of the possible physical mechanism underlying its unusual optical behavior, and the some aspects of the possible physical mechanism underlying its unusual optical behavior, and the probable structure of the fabric that composes it. It should be a linen fiber fabric consisting of very probable structure of the fabric that composes it. It should be a linen fiber fabric consisting of very thin thin threads, 0.1 mm thick, separated by double distances, so that 42% of the Veil consists of empty threads, 0.1 mm thick, separated by double distances, so that 42% of the Veil consists of empty space. space. The linen fibers are probably cemented by starch. This particular structure would make the The linen fibers are probably cemented by starch. This particular structure would make the thin linen thin linen threads translucent to the passage of visible light. This feature allows us to see slightly threads translucent to the passage of visible light. This feature allows us to see slightly different faces different faces depending on the lighting conditions and observer’s viewing angle. In particular, the depending on the lighting conditions and observer’s viewing angle. In particular, the Veil can also be Veil can also be observed with grazing light, which is partially transmitted by the translucent threads. observed with grazing light, which is partially transmitted by the translucent threads. The signs of the The signs of the Passion, the reddish details that should represent the blood, are well visible only Passion, the reddish details that should represent the blood, are well visible only with grazing light. with grazing light. This implies that the reddish details of the image should be found at a greater This implies that the reddish details of the image should be found at a greater depth in the fibers than depth in the fibers than the other colors, as if the Holy Face had been imprinted on the linen threads the other colors, as if the Holy Face had been imprinted on the linen threads after the reddish sign after the reddish sign of the Passion. This is a singular result, as this should have been quite difficult of the Passion. This is a singular result, as this should have been quite difficult to realize, taking into to realize, taking into account how thin the linen threads are. account how thin the linen threads are. The thin structure of the fabric has caused mechanical deformations over the centuries, which The thin structure of the fabric has caused mechanical deformations over the centuries, obviously have also distorted the image of the Holy Face. In this paper, we have digitally restored which obviously have also distorted the image of the Holy Face. In this paper, we have digitally the image by compensating for the distortions in the threads. In a future study it will be possible to restored the image by compensating for the distortions in the threads. In a future study it will be proceed with the chromatic restoration of the undistorted image. In particular, it should be possible possible to proceed with the chromatic restoration of the undistorted image. In particular, it should to correct the Holy Face visible when the icon is illuminated with grazing light, in order to eliminate the yellowish component produced by light crossing the ancient translucent linen fibers. Moreover, it will be possible to investigate the possible technique used to realize the icon by analyzing its Fourier

Heritage 2018, 1 304 be possible to correct the Holy Face visible when the icon is illuminated with grazing light, in order to eliminate the yellowish component produced by light crossing the ancient translucent linen ﬁbers. Moreover, it will be possible to investigate the possible technique used to realize the icon by analyzing its Fourier power spectrum. Of course, other and future experimental measurements (Raman, X-rays, ultraviolet, infrared) could help to clarify the actual chemical composition of the fabric and colors of the Veil.

Author Contributions: Writing of the paper and discussion of the results involved all the authors. G.F. performed the optical characterization of the icon. Funding: This research received no external funding. Acknowledgments: The original photographs of the Holy Face analyzed in this work (Figures2,3,6, 10, and 11) were taken by Paul Badde. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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