Materials and Techniques

The study and identification of the materials in a painting and the manner in which they are employed, in comparison with other securely documented works that have been the subject of similar analyses, help to understand the artist’s process and establish the school and period to which a painting belongs. Technical studies cannot, however, “authenticate” a painting; they cannot prove, in other words, that is by a particular master.

The Artist’s Studio

In Leonardo’s notebooks he describes the ideal studio conditions and light for painting. ¹²

Small rooms or dwellings discipine the mind, large ones distract it . . .The painter who works from nature should have a window which he can raise and lower . . . A broad light high up and not too strong will render the details of objects very agreeable. The light for drawing from nature should come from the north in order that it may not vary. And if you have it from the south, keep the window screened with cloth, so that with the sun shining the whole day the light may not vary . The height of the light should be so arranged that every object shall cast a shadow on the ground of the same length as itself.

An object will display the greatest difference of light and shade when it is seen in the strongest light, as by sunlight, or at night, by the light of a fire. But this should not be much used in painting because the works remain crude and ungraceful.

An object seen in a moderate light displays little difference in the light and shade; and this is the case towards evening or when the day is cloudy, and works then painted are tender and every kind of face becomes graceful. Thus, in everything, extremes are to be avoided: too much light gives crudeness; too little prevents our seeing. The medium is best.

If you should have a courtyard that you can at pleasure cover with a linen awning that light will be good. Or when you want to take a portrait, do it in dull weather, or as evening falls. Note in the streets, as evening falls, the faces of the mean and women, and when the weather is dull, what softness and delicacy you may perceive in them. Hence, O Painter! have a court arranged with the walls tinted black and a narrow roof projecting within the walls. It should be 10 braccia wide and 20 braccia long and 10 braccia high and covered with a linen awning when the sun is shining; or else paint a work towards evening or when it is cloudy or misty, and this is a perfect light.

This is useful advice for lighting paintings in general, not only works by Leonardo. During the hundred of hours I spent with the Salvator Mundi on an easel in my north light studio, I observed that when the light began to fall towards the end of the day, the head assumed a powerful, three-dimensional character, the flesh tones began to glow from within and the figure came to life in an extraordinary way. It was a great privilege to experience this painting as Leonardo himself would have done. I have found this is true of other artists. Titian’s paintings, for example, are transformed in a similarly marvelous and unexpected way when the light is lowered. The high light levels we are accustomed to flatten images and we cannot appreciate them as the artist meant us to do. This also speaks to reproductions and to the distortions produced by photo lamps of different color temperatures referred to in the section of this website with the various images that have been made of the Salvator Mundi.

The Panel

The Salvator Mundi is painted on a walnut panel measuring 65.5 cm. in height and 45.6 cm in width along the top edge and 45.1 cm along the bottom edge. This discrepancy in the dimensions is due to uneven trimming of the reserve along the left side of the panel.

The wood identification was made in 2011 by George Bisacca, Conservator of Panel Paintings, Department of Paintings Conservation, the Metropolitan Museum of Art, based on visual inspection. In 2018 a group of wood scientists from the University of Florence (Professor Luca Uzielli; Dr. Lorenzo Riparbelli; Dr. Giovanni Signorini) confirmed that the support was walnut based on a study of the features of the grain as seen in the x-radiograph.

The panel was thinned when it was marouflaged and cradled and today its thickness ranges from 4.5-5 mm. Other autograph works on walnut panels vary in thickness from 5 to 8mm. Leonardo frequently used single planks of walnut as supports for paintings that he began in Milan. These include: Portrait of a Musician, the Belle Ferronière, Lady with an Ermine and Saint John. The unfinished St. Jerome, although generally considered to have been painted in Florence around 1480, is also on a walnut support.

With one exception, paintings believed to have been painted by Leonardo in Florence are on poplar. These are: The Annunciation, Ginevra de’ Benci, TheAdoration of the Magi, the Virgin and Child with Saint Anne, and the Mona Lisa (a thin single plank). The Virgin of the Rocks in the Louvre was presumably on poplar but was transferred from its original support and is now on canvas. The Virgin of the Rocks in London, although perhaps begun in Milan, is on a panel composed of poplar planks. Walnut, because of its cost and weight, would not have been suitable for such a large work. Dendrochronology, or tree ring dating, is not applicable to either poplar or walnut because the annual growth rings are eratic, unlike oak, for which both the date the tree was felled as well as the geographic region can be identified with accuracy. In any event, dendrochronology provides only a terminus post quem.

The walnut panel adopted for the Salvator Mundi was an inferior plain sawn plank, with both tangential and radial grain. Although the panel specialist who worked on the painting characterized the damage in the lower part of the composition along the central axis as resulting from a knot, this is unlikely since the cut came from close to center of the trunk and may include some heartwood. It is in this, the weakest part of the panel, that the damage occurred. This flaw in the the walnut makes it a substandard cut. Dr. Lorenzo Riparbelli, a Florentine wood scientist, described it is essentially a discard and Leonardo cannot have failed to recognize this. When planning the composition he oriented the panel so that the flaw along the central axis would not coincide with the forehead, as it would had he used it in the other direction. Why and under what circumstances would Leonardo have resorted to such a manifestly defective piece of wood for an important painting? It might be worthwhile to consider this when deciding when and where this painting was begun. Leonardo’s other paintings are on proper wood panels, with the exception of the early Adoration of the Magiwhose support is somewhat clumsily constructed. Although he often experimented with methods to achieve the painterly effects he desired, and sometimes this went drastically wrong, he was careful about choosing his materials, especially for a project on which he was to lavish so much time and care over a period of many years.

DIAGRAM OF A TANGENTIAL CUT

DETAIL OF THE DAMAGE IN THE LOWER PART OF THE PANEL WHERE THE EMBROIDERED STOLE IS SECURED WITH A CABOCHON-SET BROOCH.

The grain of the wood traced in the X-radiograph. The damage occurs in the most tangentially cut part of the plank and may even include some heartwood. It was thus the most vulnerable and least structurally stable area. There is no evidence of a knot. Tracing and observations by Kristin Holder, Fellow in panel conservation, The Metropolitan Museum of Art.

There is a reserve of unpainted wood on three sides of the painting with a slight crest of white ground along the edges of the paint layers. As noted above, the left side has been trimmed slightly but a sliver of the reserve remains along the top left edge. This demonstrates that the composition has not been cut down, although a tiny fraction may be missing along the lower left. The composition is closely cropped: both the blessing hand and the crystal orb were painted right up to the edges of the panel.

The wood reserve and the barbe were created by Leonardo’s practice of inserting the panel into a slotted working frame before the preparatory layers were applied. He did not invent this method. Northern European paintings on oak panel have similar slotted frames. A working frame was described and illustrated by the painter in his notebooks. Although most of Leonardo’s mature works on a smaller scale are painted on a single plank of wood, in the drawing the panel is made up of two planks, glued together, the join reinforced with butterfly inserts.

Page with an illustration of a working frame from Paris MSA

The same page as illustrated and translated by Jean Paul Richter

On the same page the artist describes the following procedure for preparing a panel:

The panel should be pear or service-tree or walnut. You must coat it with mastic and twice-distilled turpentine with some white lead or, if you prefer, chalk, and put it in a frame so it may expand and shrink according to its moisture and dryness.

(Il legno sarà d’arcipresso o pero o sorbo o noce, il quale salderai cō mastico e tremētina secōda destillato e biacca o vuoi calcina, e metti ī telaio ī modo possa cresciere e discresciere secō l’umido o secco)

Detail of the barbe along the lower left edge

Although Leonardo was aware that wood expanded and shrank across the grain with changes in relative humidity, his working frames ultimately did not allow for the convex warp that panels can develop over time. After the recent removal of the cradle, once it was no longer constrained, the exceptionally reactive nature of the walnut support of the Salvator Mundi became apparent: the warp increases and decreases notably and rapidly with even the slightest change in relative humidity. Early Flemish paintings were painted in similar slotted frames, but the panels tend to be quarter sawn Baltic oak, which is stronger and more stable than the tangential cuts of poplar and walnut used by Leonardo. As the warp developed, the central part of the wood plank would be forced against the upper and lower edges of the slots of the working frame, constricting the movement of the wood across the grain and eventually the panel will split. In the case of the Salvator Mundi, a check opened near the center of the panel where the grain of the wood is most distorted. The resulting split eventually traversed the full height of the panel following the curve of the grain.

A hypothesis of this sort has been advanced for the The Mona Lisa, which has been extensively studied by wood scientists. It is painted on a single plank of thin poplar with a 6mm reserve of wood on all sides, demarkated by a crest of white preparation, this time of gesso and an imprimitura layer, created while the panel was in a working frame. ¹³ Any number of Lombard panel paintings of this period have retained a wood reserve and crests of preparatory material, or barbe. The authors of the Mona Lisa study note the the fine quality of the plank, which, although it is a tangential rather than a radial cut, comes from the center of the tree trunk and is characterized by a notably straight grain. The reverse was thinned at some point in the past. According to Élizabeth Ravaud, a split in the panel in the upper left, which extends down to the top of the sitter’s head was provoked by the constriction imposed by the slotted frame. She does not specify whether or not she believes this to be the sort of working frame illustrated in the notebooks. ¹⁴

Early 19th century cradle on the reverse of
the Salvator Mundi

perimeter strainer (since removed.)

Frames

The presumed use of a working frame such as Leonardo describes begs the question of whether its members were composed of decorative moldings, like Flemish frames and when they were replaced by more elaborate frames, suitable for a royal collection. It is documented that Leonardo had three paintings with him in Clos Lucé when he was visited by Cardinal Luigi d’Aragona and his secretary, Antonio de Beatis on October 10, 1517. De Beatis records in his travel diary that they were shown a portrait of a Florentine lady, probably the Mona Lisa, the Virgin and Child with Saint Anne, and a Saint John the Baptist. He does not mention whether or not they were framed. All three paintings were acquired by the French crown in 1518 and were presumably fitted into a traditional frame with a rabbet, rather than a slotted frame. In 1625, when Cassiano dal Pozzo saw the Mona Lisa on a visit to Fontainebleau, he noted that it was in a walnut frame. This would have been appropriate for sixteenth-century taste. The frame historian, Timothy Newbery, informs me that walnut and gold parcel gilt frames derive from Renaissance studioli and were often used for portraits because the warm color of the wood was felt to harmonize with the middle tones and, according to humanist thought, brought the viewer closer to the subject. Walnut frames were used to frame a set of maps in Palazzo Vecchio as late as 1563 but by 1625 would have been out of fashion.

In 1635, a gift from Cardinal Francesco Barberini was presented to Queen Henrietta Maria, a painting, considered to be by Leonardo (now recognized as a portrait, perhaps of Girolamo Casio, by Boltraffio at Chatsworth House). The precious frame of ebony, inlaid with silver and semi-precious stones is described. ¹⁵ Timothy Newbery suggests that this was probably a contemporary frame whose design may have been based on earlier walnut models but enriched.

The question remains: when were the original slotted frames removed and what damage had they already caused? The Louvre believes that the split in the upper left quadrant of the Mona Lisa was caused by the compression exerted by the slotted frame, which it retained for a long time because, it is further suggested, the oblique craquelure patterns located in the four corners and along the perimeter were also provoked by the restrictions imposed by the slotted frame. This is rather difficult to reconcile. As opposed to splits in the wood, which can occur within a few years, cracqulure occurs in the paint and ground layers, which lose flexibility over a longer period of time, perhaps fifty to one hundred years. The rigid layers then shatter, producing a variety of cracks as the support, whether canvas or wood, expands and contracts with changes in relative humity. It is difficult to imagine a scenario in which Leonardo’s paintings retained the utilitarian slotted working frames after they became part of the Royal Collection in 1518. Could they have been incorporated into a decorative frame by applying carved molding over the existing elements? Among the frames in the Kress Collection at the National Gallery in Washington there is a an early 16c Lombard parcel gilt walnut frame that still has the strainer and the edges of the original canvas incorporated into it. There are other examples of sixteenth-century frames applied onto a strainer with a stretched canvas. A panel in a slotted working frame would not have been so easy to adapt in his way but it is theoretically possible. I have never seen an example.

It is is also worth considering what sort of damage to the panel might have been provoked when the original slotted frame was removed.

17th century walnut frame applied over a strainer with the edges of the painting incorporated.

There is another explanation of what provoked the split in the Salvator Mundi. Kristin Holder, a paintings conservator and panel specialist, noticed that there is evidence in the X-radiograph that the panel may have been restrained by pieces of wood nailed into the end grain. Along the top and bottom edges a series of short diagonal splits can be seen at relatively regular intervals and there are some voids where nails would once have been. It is possible that after the check developed, which given the poor quality of the plank might have happened rather quickly, pieces of wood were added to stablize it. This would have had just the opposite effect: by constraining the movement of the wood across the grain the split became worse.

X-ray of Upper edge with nail holes and associated splits

X-ray of lower edge with nail holes and splits

Reverse of the panel during restoration showing extensive worm tunneling.

The split in the Salvator Mundi skirts the proper right side of the face as it traverses the curls. Along this check, the panel divided into two parts, which sprung away from each other so that they were no longer in the same plane. At some point in its history, to realign the two halves, the panel was crudely planed down from the front, removing a wide swath of original preparation and paint from the area of the curls and the proper right cheek along with the wood. As an historical note, Tudor portraits on oak panels from English country house collections were not infrequently subjected to this rough method of obtaining a level surface.

Surface of the Panel. Note the curve of the wood grain.

By 1900 when it was sold to Sir Francis Cook, the panel had been thinned, glued to a pine board, and a cradle had been added. Cradles were invented in France in the late eighteenth century by Hacquin and continued to be used throughout the twentieth century. The cradle on the Salvator Mundi has characteristics of both English and French/low-countries cradles. The members that are glued to the panel in the direction of the grain are a softwood, which are less common in English cradles where they are more often of mahogany or cedar. The sliding members are oak, which would be unusual for a French cradle. The design of the cradle, with the glued members stepped in from the left and right hand side, is also unusual and suggests that it is quite old, perhaps early to mid-nineteenth century. From a structural point of view, the cradle was, in any case, merely window dressing since the panel had first been mounted to a pine board. The two interventions may not have been simultaneous, but there is no way to determine this.

The panel is also extensively worm tunneled by wood boring insects, which further undermines its structural integrity.

Two images with raking light give some sense of the corrugated surface of the panel and the many areas of repair in the background. This three-dimensional aspect of the surface is not reflected in any of the images and contributes to the difficulty of photographing the painting. Originally the surface would have been flat and smooth. As the panel reacted to fluctuations in relative humidity the grain of the wood became more prominent.

The Binder

Analysis of a single sample from the blue robe by Dr. Kenneth Sutherland indicated that the binder was walnut oil for all layers including the preparation. ¹⁶This cannot in itself lead to the conclusion that Leonardo used walnut oil throughout the painting, or in general, although he is reputed to have favored its use and discusses its preparation in his notebooks. It is more fluid than linseed oil and dries more slowly, characteristics favorable for extending the paint mixtures in thin layers. It discolors less than linseed oil.

Media analysis has been undertaken for two other works by Leonardo: the London version of the Virgin of the Rocks¹⁷ and the Madonna of the Carnation in Munich.¹⁸ In both instances the poplar support was prepared with a conventional gesso ground. In the London painting, the two layers of imprimitura consist of lead white mixed with a bit of carbon black bound with a non-heat bodied walnut oil; a second layer, unevenly applied, contained lead white and some lead-tin yellow. Some paint layers were bound with both heat-bodied walnut oil while linseed oil was identified in other mixtures.

For the Munich painting the painter applied only one layer of imprimitura containing lead white, linseed oil, and perhaps some fine particles of glass over a gesso ground. The paint layers were bound with mixtures of walnut oil.

It is notable that the Salvator Mundi does not suffer from the drying defects that afflict many of Leonardo’s works, whether from wrinkling (the Munich painting) or contraction cracks. Saint John the Baptist in the Louvre, a work often considered to have been painted in the same period and stylistically related to he Salvator Mundi, is marred by severely contracted paint in all of the dark passages: the hair, fur and background. Only the head is spared, and perhaps the arm with the pointing hand, although the latter was completely repainted by the seventeenth century, so it is difficult to state that with certainty. The Belle Ferronière and the Paris version of the Virgin of the Rocks are also afflicted by this defect, among other works. There are paintings with fewer film defects such as the Lady with an Ermine and the Mona Lisa, which however exhibits an extensive network of age cracks, perhaps due to the presence of a traditional gesso ground whose aqueous binder would be more reactive to changes in relative humidity.

Detail of the Madonna of the Carnation, AltePinakothek, Munich(Wikimedia Commons)

detail of drying cracks in Ginevra de’ Benci, National gallery of art, Washington, D.C.

detail of drying cracks in Ginevra de’ Benci

In the Salvator Mundi there are some scattered, clearly defined areas of craquelure resembling drying cracks and I described them as such in my 2014 publication. What was perplexing was that they also appear in the X-radiograph, meaning that they affect not only the paint but also the preparatory layers. Usually drying cracks are confined to one or more paint layers. These areas of craquelure are random and do not correspond to particular painted passages. Dr. Lorenzo Riparbelli, who examined the x-radiograph, recognized that they are associated with a certain feature of the grain of the wood support which expands and contracts more than and independently of other areas of wood grain, which explains why they appear in the x-radiograph. ¹⁹

The lack of true drying cracks suggests that Leonardo took particular care with this painting, perhaps choosing a binder that he knew dried properly, using just the right proportion of binder to pigment, and making sure that the under layers were thoroughly dry before applying a successive layer of paint. Could this unusually careful technique be related to his experiments with a “varnish” with which he is said to have been engaged in Rome between 1513-1515, for which he was admonished by Leo X? It is unlikely that the pontiff would have made a distinction between medium and varnish.

The preparatory layers

Leonardo’s instructions regarding the preparation of the panel continue on the same manuscript page as his drawing of the working frame, Paris Ms A:

Then go over it 2 or 3 times with alcohol in which you have dissolved arsenic or sublimate. Then give it a coat of boiled linseed oil that completely penetrates [the wood] and before it cools rub it [the surface] well with a cloth until it appears dry. Over this spread [a layer of] white lead in liquid varnish with a flat stick, then, when it is dry, wash it with urine. Then [transfer] your design with pouncing and outline it finely, and apply an imprimitura over it [composed] of thirty parts of copper green and one [part of] of copper green [mixed with] two parts of yellow. (Author’s notes in brackets)

(dipoi li dà con acquavite, che via sia dentro disoluto arsenico o solimato 2 o 3 volte, di poi dà olio di lino bolito in modo penitri per tutto, e inanzi si freddi fregalo bene con ū panno in modo parrà ascivtto, e dalli di sopra vernice liquida e biacca colla stecca, poi laua con orina, quādo è ascivtta; e poi spoluerezza e proffila il tuo disegnio sottilmēte e dà di sopra l’imprimitura di 30 parti di uerderame e vna di uerderame e 2 di giallo.)

The precise instructions given by Leonardo have not been confirmed by scientific analysis of any autograph works and they are difficult to interpret. Washing down the white lead preparation with corrosive agents might have been a method for driving it deep into the cellular structure of the wood. Something seems to have been left out of this recipe.

Other paintings by Leonardo and his followers on walnut panels that have been analyzed have preparations of lead white bound in oil without an underlying layer of the traditional gesso. ²⁰In the case of the Salvator Mundi, a proteinaceous medium was first applied to the panel, probably an animal glue, which was commonly used for this purpose, rather than the boiled linseed oil described by Leonardo. ²¹ Similar to Leonardo’s recipe, over this is a first ground containing lead white, a drying oil, and particles of glass. Over this a second layer was applied, that can be described as a second ground, or, less precisely, an imprimitura, ²² composed of finer particles of drying oil, white lead and glass, and a bit of lead tin yellow. Both layers are slightly translucent. ²³The particles of glass vary in size from less than one micron up to 30 microns long. Their composition is consistent with manganese-containing soda-lime glass. The lower ground layer contains a higher proportion of glass than the thinner upper layer. (This same soda-lime glass was also identified as an addition to some of the paint mixtures.) The addition of glass is not a singular innovation: glass particles have been noted in the preparatory and paint layers of many other Italian paintings of the fifteenth and sixteenth-centuries. The glass particles functioned as a siccative to aid in drying and would have enhanced the translucency of the paint. ²⁴

In a study of Leonardesque works in the National Gallery London preparations containing lead white applied directly to walnut panels include: Portrait of a Man Aged 20 (the Archinto Portrait) dated 1494, attributed to Marco d’Oggiono, and the Virgin and Child (c. 1500) by a Follower of Giovanni Antonio Boltraffio. ²⁵ A similar preparation has also been verified in the Portrait of Bernardino di Salla (c. 1504 - 1509) by Giovanni Francesco Carota in the Louvre. The Lady with an Ermine also has a preparation of lead white in an oil binder. ²⁷

Preparations containing a small amount of lead tin yellow have been identified in a number of Milanese paintings on walnut from the late fifteenth and early sixteenth centuries as well as in the preparations of the London version of the Virgin of the Rocks and the Adoration of the Magi. In addition to glass particles and lead white, the ground layer contains lead fatty acid soaps (lead carboxylates), formed over time from a reaction between the lead white pigment and the oil medium, contributing to the translucency of the ground layer. ²⁸

Detail of the juniper, Ginevra de’ Benci, National Gallery of Art, Washington, D.C.

Upper left corner, St. Jerome

Detail of Giovanni Bellini, Madonna and Child, National Gallery of Art, Washington, D.C.

Handprints made by the thumb or palm are sometimes visible in Leonardo’s paintings. It is tempting to think of them as Leonardo’s, and they are often described as such, accompanied by the suggestion that the painter used a blotting technique to soften the transitions, but with the exception of one early work, Ginevra de’ Benci, where they are confined to passages where the sky is framed by the spiny leaves of juniper, they have been shown to be actually in the preparatory layers, which were typically smoothed out with the palm of the hand, something that might have been done by a studio assistant.²⁹ This was a common practice, described by Vasari, and can be observed without the aid of scientific imaging in many paintings of the period, for example, on the surface of paintings by Giovanni Bellini and other artists. Another instance are the two fingerprints visible in the upper right corner of St. Jerome, which have also been described as a deliberate softening of the sky. Since this layer is part of the underpainted sketch, and the two prints occur only in that area, it seems more likely that they were caused accidentally when Leonardo, or someone in his studio, carelessly picked up the panel by the corner while the paint was still soft. There is also an adjacent gouge in the fresh paint.

Design

It is possible to see some indication of spolvero in the infrared reflectogram of the Salvator Mundi, indicating that at least parts of the design were transferred to the preparatory layers from a cartoon. Since the 2008 scans were made, instruments capable of producing images of higher resolution and improved clarity have come into use that might reveal clearer images of this pouncing. It also should be noted that the apparent absence in IR images of such pouncing marks need not exclude their presence: Leonardo’s abundant use of black in his paint mixtures, especially in his later paintings, makes the detection of underdrawing difficult. In the 2008 scans there are indistinct impressions of dots of spolvero in the following passages: the eyes, the lips, and, I believe, underlying the knotwork pattern.

In Leonardo’s earlier paintings, the progression from drawing to painting follows more traditional procedures. Pouncing and drawing are evident in IR scans of Ginevra, and some other early works. The evolution of his technique seems to preclude such clear evidence of the transfer or the initial underdrawing. The IR scan of the Salvator Mundi can be compared with that of Saint John in which faint indications of spolvero can be discerned around the eyes.

Underpainted Sketch or Abbozzo

Leonardo describes how to begin painting by laying down the general modelling of the lights and darks:

First, give a general shadow to the whole of that extended part which is away from the light. then put in the half-shadows and the strong shadows, comparing them with each other and, in the same way, give the extended light in half tint, afterwards adding the half-lights and the high lights, likewise comparing them together.

(Dà prima una ōbra vniversale per tutta la parte cōtenēte che nō uede il lume, poi li dà ōbre mezzane e le principali a paragone l’una dell’altra, e così dà il lume cōtēnete di mezzano lume, dādoli poi i mezzi e prīcipali similmēte a paragone.)

There are wash-like black and translucent brown underpainted layers in the Salvator Mundi, analogous to the abbozzo layers visible in his two unfinished paintings of Saint Jerome and the Adoration of the Magi. The black wash is evident in Sample 1, taken from the upper part of the blessing hand. It consists of fine, densely packed black particles of bone black, which do not exhibit any fluorescence in UV light, suggesting an aqueous medium, a carbon based ink, or thin paint with a proteinaceous binder that does not contain oil or resin. In other samples, particularly beneath the shadows of the flesh tone, the initial paint layer is a medium-rich, translucent reddish-brown mixture containing tiny particles of vermilion, black and glass. The layer fluoresces in UV, indicating that the binder is oil or resin.

Similar warm brown underlayers containing red and black pigments have been noted in the Virgin and Child with Saint Anne, Saint Jerome and Saint John the Baptist. ³¹ Brown underlayers, possibly with different pigment compositions, have been observed in the Mona Lisa, Lady with an Ermine, and La Scapigliata. ³²

Section from background (sample 4) showing the finely divided bone black particles of the initial wash

sample from Adoration of the Magi with black wash having characteristics similar to the corresponding layer in the Salvator Mundi. the sample also shows the underawing under the thin lead white imprimitura.

The Adoration of the Magi with details of abbozzo in black wash and translucent brown paint

St. Jerome, Musei Vaticani

IanDagnall Computing / Alamy Stock Photo

Painted Passages

Flesh Tones

The light passages and the mid-tones were underpainted in a brightly saturated, opaque light red color. To create the highlights, this base color was scumbled with progressively cooler and paler layers. In a sample from the blessing hand (see cross-section 1) a mixture containing lead white, vermilion, glass particles, and a small amount of carbon black was followed by a second, slightly cooler pink paint with the same pigments but in different proportions. ³⁰This preparatory modelling was finished with several thin layers of pale pink, each application approximately 4-5 microns thick. These final layers do not contain any glass particles and were thinly and smoothly applied, resulting in an enamel-like final surface.

The wide range of mid-tones were created by interlayering semi-opaque mixtures and dark transparent glazes over the brightly saturated pink base color. The application of these layers was varied to create fine gradations in the shadows. The skillful manipulation of mixtures containing slightly varying proportions of pigment, rendering them more or less opaque, rather than simply blending the transitions between shadow and highlight, creates the sfumato, or smoke, as Leonardo described it:

Detail of check

If as a draughtsman you wish to study well and profitably, accustom yourself when you are drawing to work slowly, and to determine between the various lights, which possess the highest degree and measure of brightness, and similarly as to the shadows, which are those that are darker than the rest, and in what manner they mingle together, and to compare their dimensions one with another; and so with the contours to observe which way they are tending, and as to the lines what part of each is curved in one way or another, and where they are more or less conspicuous and consequently thick or fine; and lastly to see that your shadows and lights may blend without strokes or lines in the manner of smoke. ³³

In fact, in the Salvator Mundi the brush strokes are indistinguishable even under examination with a microscope.

The shadows of the flesh tones are built up with translucent glazes directly over the off-white preparation. The mixtures range in thickness from approximately 3 to 6 microns and vary in tone and in the proportion of pigment to medium. The following sequence is seen in a cross-section of paint from the deep shadow of Christ's raised hand: a lower medium-rich, brown-red layer containing vermilion and a small quantity of carbon black followed by four darker layers containing, in varying proportions, carbon and bone black with a few particles of vermilion. Slight fluorescence under UV could indicate that the final layers might contain a bit of red lake, but this was not confirmed by analysis. However, in the overall image of the painting in UV light, the flesh tones of the face fluoresce a bright pink, suggesting the presence of a madder lake.

Detail of the blessing hand

Detail of wrist and palm

Chin and beard

Etching by Wenceslaus Hollar, 1650

The Thomas Fisher Rare Book Library, University of Toronto

Head of Christ. Drawing for the Last Supper by a contemporary of Leonardo, possible after the original cartoon.

Scala / Art Resource, NY

From medieval times, the appearance of the mature Christ was clearly described:

His hair is wavy and curled with a bluish and bright sheen, flowing over his shoulders. It is parted in two on the top of his head, in the mode of the Nazarenes . . .His beard is abundant, of the colour of his hair, not long but divided at the chin.

In the Salvator Mundi the corkscrew curls of the hair conform with the canonical medieval description. Although Christ has a beard in the Hollar etching, there is no physical evidence of the remains of a beard in the area of the chin. Even though the area has losses and is somewhat abraded, there is enough undamaged original that had there been facial hair, at least some of it would have survived. There is also no moustache, as in the etching. In late antiquity there were variations in how Christ was depicted and there are numerous images of a beardless Christ, including a 4th century sarcophagus that was conserved under the pulpit of Sant’Ambrogio in Milan. (insert footnote: Adam Levine, The Image of Christ in Late Antiquity: A Case Study in Religious Interaction. D. Phil. thesis, Corpus Christi College, 2012.) In the arresting image of Christ in Majesty from the Gospel of Charlemagne there are tiny redish brush strokes on the chin that discreetly suggest some facial hair, like the Naples copy, if not a full beard.

Sarcophagus of Stilicone, 4th c., Sant’Ambrogio, Milan and Christ in Majesty, 8th c., from the Gospel of Charlemagne, BNF. Wikimedia commons
Like the Hollar etching, most of the versions and variants of the Salvator Mundi are bearded, although the copy in Naples, which is probably by one of Leonardo’s close followers, the beard is rather tentative, consisting of tufts of curls on the lower part of the chin so small and pale in color that they only can be discerned by looking closely at the passage in a strong light. In the drawing in the Brera of the head of Christ, once thought to be a cartoon for the Last Supper, is beardless.

Although the areas of the upper lip and lower jaw of the Salvator Mundi are badly damaged, there are no original remants of a moustache or beard. In the area of the chin, where there are several large losses and which has also suffered from abrasion, there are some old, darkened repaints that I did not remove. They may be coeval with the green repainting of the background. Also, the damage around the top of the head where only a few passages of later hair survive has the shape of a halo, perhaps an indication that one may have been removed when the background was scraped. (This halo shaped damage above the head is also present in the Ganay copy.) It is possible that the explanation for the beard in the Hollar etching is that when the original black background was repainted with thick green verdigris, a halo and beard were added to make Leonardo’s disquieting, somewhat androgynous image more conventionally pious and masculine. On an undated sheet, now incorporated in the Codex Atlanticus (680r) the artist wrote an enigmatic note:

When I made a Christ Child you put me in prison, and now if I show him grown up you will do worse to me.

(Quando feci Domene Dio putto, voi mi metteste in prigione; ora, s’io lo fo grande, voi mi farete peggio.)

Leonardo’s interpretation of Christ is still disturbing to some believers. ³⁴

Detail of chin of the Salvator Mundi and the same passage in the copy from San Domenico Maggiore, Naples

Eyes

Detail of eyes of the Salvator Mundi, cleand state

Detail of the eyes after restoration

The eyes are painted with a minimal amount of body color for the cornea, just a thin layer of gray paint, essentially a scumble. The eyes of Saint John and the Mona Lisa are are treated in a similar fashion; they are, however, better preserved. The irises, of a translucent brown, appear to have been painted directly over the preparation. The apparent reflections are not added highlights, but the effect of the nearly white, underlying layer. Both eyes have suffered damage, both mechanical and from abrasion by solvents of the exceptionally thin paint.

Stole

detail of the stole

Design of the knotwork pattern of the stoleCheryl Rossum

The position of the stole was adjusted at a late stage in the evolution of the painting. The initial position, slightly higher and with a minor variation in the angle, can be seen in visible light and is clearer in the infrared reflectogram. Pleats of blue drapery which bordered the earlier design can be seen under the delicate strokes of the embroidered threads, especially on the lower part of the stole on the proper right side. Adjacent to these pleats is a lay-in of thin, pale greenish blue paint, nearly grey, that is quite distinct from the strong blue of the lapis. From evidence elsewhere in the painting, it is clear that the blue drapery was among the last passages to be added. The slight shift in its placement indicates a similarly late stage for the execution of the knotwork design. Fine incisions scored into already dry paint layers record the final adjustment for the lower contour on the proper right side. The band along the top of the bodice was originally a bit lower, at least in the initial underpainted sketch, but the variation is less significant.

An early idea for the design of the knotworkGraphic, Shan Kuang

Mona Lisa, detail of embroidery

Before settling on the final position of the stole and the elaborate knotwork design, Leonardo sketched a simpler knot-work design. It was never fully completed. One element of it can be clearly seen under the final pattern where the paint has become translucent due to natural ageing as well as cleaning. It is similar to examples found in his notebooks and to the knots that decorate the bodice of the Mona Lisa. Using a stiff, narrow brush and pastose pale-yellow paint consisting primarily of lead tin yellow, this initial design is freely sketched, in conception and execution considerably simpler than the final knotwork pattern.

The pattern of groppi, or vinci, that Leonardo devised for the Salvator Mundi is considerably more complex than in any other of his paintings. It is likely that there was a cartoon for the pattern; traces of spolvero can be just barely discerned in the IR scan and the creation of patterns for such elaborate designs conforms with his practice. ³⁵ The threads are not simply a single color, as is, for example, the pattern in the portrait of Cecilia Gallerani, but are described in their three dimensional volume, the effect of the light as it falls over them on the proper right side carefully described. On the proper left, where they are partly in shadow, there a fewer layers, but not just one as in the knotwork of his others works. The entire pattern was first underpainted in grisaille. For the highlighted passages, this was followed by warm red underpaint, a layer of light colored pigment, possibly lead tin yellow, which was then glazed with a warm tone, and a thin dark line of shadow complete.

detail of knotwork pattern of the Lady with an Ermine

Microphotograph of the embroidery

Microphotograph of the embroidery

It has often been observed that the Golden Ratio, the Divina Proportione, can be recognized in every passage of the Salvator Mundi, and in particular in the knotwork pattern. ³⁶

Blue Robe

Detail of degraded blue in the area of the omega fold. Abraded traces of original blue that covered the knotwork pattern in that area are noted.
The blue draperies were underpainted with a translucent warm brown paint containing vermilion, carbon black, and particles of glass. Analysis by MFTIR, Raman and EDS showed the blue pigment it to be natural ultramarine, or lazurite, a mineral derived from lapis lazuli that was imported into Venice from modern-day northeast Afghanistan. It was then, as now, a semi-precious stone used in the manufacture of luxury goods such as jewelry and other decorative arts, and converted into ultramarine pigment of varying qualities. It was often difficult to obtain and by far the most expensive pigment in the Renaissance painter’s palette.

The ultramarine that Leonardo used for the Salvator Mundi contains exceptionally large particles of quartz, an impurity in the mineral. Its presence suggests that the pigment may have been made by simply grinding up the semi-precious stone, or that these impurities failed to be removed during the complex process of extracting the color with lye. Samples of blue from the London Virgin of the Rocks also contain quartz particles but they are considerably smaller.

While the pigment is stable in other media, such as egg tempera, when it is bound in an oil medium it alters. This can be seen in innumerable oil paintings, including many works by Leonardo. The degradation is known as ‘ultramarine sickness,’ and is characterized by severe blanching, rendering the modelling of the lights and darks illegible. ³⁷ In the Salvator Mundi, the dark underpaint has asserted itself in patches throughout the drapery. Details such as the sharp pleats beneath the embroidered band along the top of the bodice and the omega-shaped drapery fold that bunches over the lower left band of the stole, clearly articulated in Leonardo’s preparatory drawing as well as in many copies of the painting, have become difficult to decipher. Recent studies of ‘ultramarine sickness’ have pinpointed its cause and confirmed that saturation of the paint can partially, if only temporarily, restore the earlier appearance of an affected area, a phenomenon already reported in the nineteenth century.

As already noted, the robe is painted up to the contours of the sleeve, the white cuff of the blessing hand, and the hand holding the orb. A small area located below the point at which the diagonal bands intersect, and the lower part of the sleeve was left black and might be considered unfinished. These opaque brownish black passages were not sampled so it is not possible to know if their composition is the same as the underpaint seen in several cross-sections, although additional layers must have been added since they are not translucent. It has been suggested that they are simply the underpainted abbozzo but their opacity makes this unlikely.

Curls

Detail of curls

Details of curls, St. John

Detail of curls, proper right

Microphotograph of curls on proper rightNica Gutman Rieppi

Microphotograph of curls on proper left with grisaille underpainting exposedNica Gutman Rieppi

Elaborate underpainting also underlies the locks of hair. An early indication of tresses was scored into the still wet paint of the adjacent flesh tone and lightly sketched in reddish paint. This initial sketch may have been more extensive but this is not possible to determine. The final curls, like the embroidered threads, were first executed in grisaille over a warm brown underpaint. They closely resemble the unruly locks of Saint John the Baptist in the Louvre, but, as noted above, do not have the pronounced traction craquelure of Saint John’s curls.

Orb

Microphotographs of inclusions
Nica Gutman Rieppi

The orb is one of the most carefully studied passages of the painting. The effect of the quartz crystal and the way in which the hand and drapery seen through it were slightly distorted and magnified must have originally been realistically rendered and appeared breathtaking. Unfortunately much of this illusion is now compromised by the loss of the thin layers of glazes and scumbles with which this illusion was created, removed during harsh cleaning in the past. Of the carefully described inclusions. Martin Kemp noted: “These are not the spherical bubbles found in glass but are the kind of cavity inclusions (small gaps) that appear in some specimens of rock crystal and calcite. Leonardo, we know, was considered an expert in such semi-precious materials.” Kemp further observed a “double refraction produced by calcite. The heel of Christ's hand exhibits two distinct contours, not in this case due to a change of mind.” ³⁸

Although objects seen through a crystal sphere are refracted so that they appear upside down, an effect described by Leonardo in his studies of the mechanics of vision, this only occurs when there is a certain distance between the sphere and the object. In the Salvator Mundi the sphere is held close to the body. A misunderstanding of this phenomenon was reported by the media as a mistake made by the artist, but the reversion of the image is not relevant in this case and was never intended. In its actual position, the heel of the palm part of the hand, seen through the transparent crystal, is magnified and there appears to be the double refraction to which Dr. Kemp refers. Although this passage is abraded, enough remains to understand that this effect is deliberate and not a pentimento. The drapery folds behind the orb may also have originally appeared as slightly magnified, although the effect is no longer apparent due to the abrasion of the final glazes.

It is interesting to compare Leonardo’s approach to this passage with Durer’s nearly contemporary, unfinished painting of the Salvator Mundi where there is a clear reserve for the entire passage and the hand clutches the sphere from the front rather than balancing it so that the double refraction of the palm seen through the sphere did not have to be taken into account because of the different position of the hand. (While the original position of the hand in Leonardo’s painting was originally somewhat higher, it was not as high as it is in Durer’s version.)

The three white highlights that are now isolated may have formed a reflection on the surface, as one sees in some of the copies of the painting but not all. I am not absolutely certain that are original because they do not appear in the x-radiograph, but this is presumably because the white lead preparation overwhelms these three thinly painted specks of paint. Again, the abrasion of the final scumbles and glazes interferes with this reflection. An Austrian scholar, Dr. Stefaan Missinne, has suggested that their orientation of the three dots describes the orthogonal triangle of the Southern Cross, which had only recently been discovered in the Southern hemisphere by Portughese explorers and was described in a 1503 letter from Amerigo Vespucci to Pierfrancesco de’ Medici. ³⁹

Detail of the orb and hand

Albrrecht Durer, Salvator Mundi,
the magnification of the folds seen through the crystal sphere is apparent and must have been a feature of Leonardo’s orb before the passage was damaged

Background

The background was originally a rich, deep black, an effect created by 4 or 5 layers, some of which are mixtures, and others of different black pigments. (See cross-sections 7 and 8.) The use of black backgrounds derives from early fifteenth-century Flemish painters, especially Jan Van Eyck but the immediate source may be Antonello da Messina who is documented in Venice in 1475-76. Antonello’s Salvator Mundi in the National Gallery in London has a cartellino with an inscription that can be variously interpreted as 1460, 1465 or 1475, but is thought to date from the 1470’s. Leonardo’s unfinished 1485 portrait of a musician in the Pinacoteca Ambrosiana has a black background and is much influenced by Antonello.

Original black background around blessing hand

Original black background near curls on right

Photomicrograph of original black background during removal of overpaint

At some point the background was repainted with a thick layer of granular verdigris. This green pigment was often used for backgrounds of portraits during the 16th century, particularly in France, but also by Italian and German painters. It fell out of use after around 1600. At a later date this green color was scraped off, along with much of the original background. Traces of the green, with intact original background color underneath, survived around the contours of the blessing hand and the proper right shoulder. Under the verdigris was intact original background.

Order of Painting

Preparation of panel with a layer of animal glue and two layers of white lead, mixed with glass with a tiny amount of lead tin yellow added to the second layer.
Underdrawing. There are some indications of spolvero.
Abbozzo (underpainted sketch) with a thin wash of bone black in an aqueous medium in some areas and a warm mixture of black, vermilion, and possibly red earth bound in an oil or resin medium in others.
Probably the head and the first position of the blessing hand
The black background
The revision of the blessing hand (this could also have been done at a later stage.)
The stole
The curls
The blue drapery
The orb and hand

Jean Paul Richter, The Literary Works of Leonardo da Vinci. Volume 1. (New York, 1970), pp. 313-317].
Mona Lisa: inside the painting. (New York, 2006), pp. 18-19, 40-42
Ibid., p. 19
Margaret Dalivalle, A Salvator Mundi Attributed to Leonardo da Vinci in the Collections of King Charles I &II. Unpublished paper. That painting, a portrait of a young man, is possibly the portrait of Girolamo Casio, now in the collections of the Duke of Devonshire.
Analysis of the binding medium by Py-GCMS revealed a fatty acid composition characteristic of walnut oil, including a high level of azelaic acid and a ratio of palmitic to stearic acid of 2.7. The ratios of the dicarboxylic acids, which can sometimes provide evidence for heat bodying of a drying oil, did not provide convincing information in this case (ratio of suberic:azelaic acid was 0.23, an intermediate value). Diagnostic values for the fatty acid ratios are drawn from the analysis of reference materials and from published studies. See, for example, John S. Mills and Raymond White, The Organic Chemistry of Museum Objects, (London: Butterworths, London, 1987), pp. 141-143. The single sample analyzed included the blue paint and black wash and warm brown underlayers in addition to a small amount of the white preparatory layers; the layers could not be separated for individual analysis. Further sampling and analysis would be necessary to determine if walnut oil was used in other parts of the composition. Py-GCMS analysis was carried out at the Philadelphia Museum of Art as follows: the sample (c. 10 ug) was placed in a Frontier Lab stainless steel sample cup, and 2uL of a 25% solution of tetramethylammonium hydroxide (TMAH) in methanol added prior to insertion into a Frontier PY-2020iD vertical microfurnace pyrolyzer, attached to an Agilent 6890N GC - 5973N MS. The pyrolyzer furnace temperature was 550°C. The GC was equipped with J&W DB5MS column (30 m, 0.25 mm i.d., 0.25 um film); oven temperature programmed from 40°C, with a 2 minute hold, then increased at 20°C/minute to 140°C, at 15°C/minute to 320°C, and held isothermally for 11 minutes; total run time 30 minutes. The inlet was operated with a split ratio of 1:10, with helium carrier gas at a flow rate of 1 mL/minute. The MS was run in scan mode (m/z 35-600) with the source at 230°C and quad at 150°C. Data were collected and processed using Agilent Chemstation software. [From Nica Gutman Rieppi’s notes.]
Larry Keith, Ashok Roy et.al., “Leonardo da Vinci’s ‘Virgin of the Rocks.’ Treatment, Technique and Display.”National Gallery Technical Bulletin. National Gallery Company (London) 2011
Jan Schmidt et. al., “The Madonna with a Carnation: Technical studies of an early painting by Leonardo da Vinci.” Leonardo da Vinci’s technical practice. paintings, drawings and influence. (Paris, 2014)
Verbal communication from Dr. Lorenzo Riparbelli, University of Florence.
Traditional gesso grounds are thick and, since the binder is aqueous, can be honed to provide a smooth surface. This is necessary for certain relatively soft wood panels, such as poplar, whose surfaces are rough and need to be filled in. The surfaces of hard woods, such as walnut or oak, are already dressed smooth before the ground is applied.
This medium was observed in the cross-sections as a discrete layer below the ground layer. Analysis by ATR-FTIR of this layer in a cross-section of the shadow of the flesh tone of the small finger of the blessing hand, suggested a proteinaceous medium (spectra with an NH stretch at 3295 cm1 and amide bands at 1647 and 1533 cm-1). The data were collected using a Bruker Hyperion 3000 FTIR microscope, equipped with MCT detector and a 20x attenuated total reflectance (ATR) objective with germanium (Ge) crystal. The microscope was attached to a Vertex 70 FTIR spectrometer equipped with a mid-IR Globar source. Data were collected from 4000 to 650 cm1 at 4 cm1 resolution, 512 scans per spectrum using Blackman-Harris apodization and Mertz phase correction. Data were analyzed using Bruker OPUS software, version 6.5. Peng Wang, Applications Scientist, performed the ATR analysis at Bruker Inc., Billerica, MA. [From Nica Gutman Rieppi’s notes.]
An imprimitura,strictly defined, is a priming that isolates a porous underlayer so that it doesn’t absorb medium from the paint. Thus, the layer of white lead in oil on the unfinished Adoration of the Magi in the Uffizi is over a gesso ground, as is a similar layer in the London Virgin of the Rocks.
The presence of both lead white and oil in the preparatory layers was determined by combined MFTIR and Py-GCMS analysis of a microscopic 4taken from the blue robe, which gave results indicative of walnut oil. Lead white, consisting of both neutral and basic lead carbonate, was identified by MFTIR and EDS. Diagnostic IR spectral bands included very strong carbonate stretches centered at 1421 cm1 with additional bands at 1050, 883 and 679 cnr1 for the neutral form and at 1045 and 650 cm1 for the basic form. Lead (Pb) was detected by EDS. Lead tin yellow type I pigment (PbzSnCU) was indicated by characteristic Raman spectral bands at 127,129, 196, 273, 292, 456, and 526 cm1 and the detection of lead (Pb) and tin (Sn) by EDS.IR spectral bands suggesting an oil-based medium were observed at 2929/2851,1739 and 1170 cm1. For MFTIR analyses, the preparatory layers were isolated and representative portions flattened on a Spectra-Tech diamond window with a roller tool for analysis. Analyses were undertaken at the Philadelphia Museum of Art using a Thermo Nicolet Continuum microscope (MCT-A detector) attached to a Nexus 670 spectrometer. The data were collected in transmission mode between 4000 and 600 cm4 at 4 cnr1 resolution and 200 scans per spectrum. Omnic 6.0 software was used; spectra were processed using Happ-Genzel apodization. [From Nica Gutman Rieppi’s notes.]
The siccative properties of this type of glass have been attributed to the presence of manganese, an active drier. IR spectral bands indicating the lead soaps were observed at 1513 and 1401 cm-1. Alterations in translucency brought about by lead soaps in paintings have been discussed widely in the literature.
Marika Spring, Antonio Mazzota, et al., op.cit
Elisabeth Martin in Jean-Pierre Mohen, Michel Menu and Bruno Mottin, "The Preparation of the Panel", Mona Lisa: Inside the Painting. (New York, 2006), p. 58.
Kazimierz Kwiatkowski, "‘La Dame a L'Hermine’ de Leonardo da Vinci”, Etude Technologique, tr. Zygmunt Glinka, Musee National de Varsovie. (Warsaw, 1955) pp. 19-20.
IR spectral bands indicating the lead soaps were observed at 1513 and 1401 cm-1. Alterations in translucency brought about by lead soaps in paintings have been discussed widely described in the literature.
See, for example, Jan Schmidt et. al., op. cit., p. 47
Mercury (Hg) and sulfur (S) indicative of vermilion (HgS) were detected by EDS. Raman analysis produced spectra with characteristic bands for vermilion at 255 and 344 cm1. The red lake pigment particles fluoresced an orange-pink color when excited with UV-violet (355-425 nm) light using FLM. Aluminum (Al), suggesting an alum derived substrate, was detected by EDS. The red colorant was not present in sufficient quantities to enable analysis by such supplemental techniques as high performance liquid chromatography or mass spectrometry.
Bruno Mottin, "Leonardo da Vinci's The Virgin and Child with Saint Anne (Museè du Louvre, Paris): new infrared reflectography," Studying Old Master Paintings- Technology and Practice, The National Gallery Technical Bulletin 30thAnniversary Conference Postprints, ed. Marika Spring, (Lon 2011) p. 69-70.
Elisabeth Martin, Regina da Costa, Pinto Dias Moreira and Jean-Paul Rioux, "The Painter's Palette", in Jean-Pierre Mohen, Michel Menu and Bruno Mottin, Mona Lisa; Inside the Painting, (New York, 2006) p. 70; David Bull, "Two Portraits by Leonardo: Ginevra de’ Benci and the Lady with an Ermine," Artibus et Historiae, vol. 13, No. 25 (1992), p. 78; David Bull "Analisi scientifiche", Leonardo La dama con L’ermellino. (Milano, 1998) p. 83.
Edward MacCurdy, The Notebooks of Leonardo da Vinci, (New York, 1955), p. 887
When I accompanied to painting to the Dallas Museum of Art and it was shown to Ross Perot, he was shocked by it, saying that it looked like a woman and he didn’t know of any Jesus that looked like that. This is one of the reasons, I believe, that the director was unable to persuade his board to buy the painting.
Carmen Bambach, Leonardo, Tagliente, and Dürer: “La scienza del far di groppi.” (Firenze, 1991), p. 79ff.
Gary Meisner has applied a graphic software system of his invention to the Salvator Mundi. His analysis can be viewed at his website: www.goldennumber.net
By Max von Pettenkofer in 1870, as reported in J. Klaas, ‘Die “Ultramarinkrankheit”; Studien der Veränderungen in ultramarinhaltigen Farbschichten an Gemälden,’ unpub. Ph.D diss. (Technical University of Munich 2011), cited by E. René de la Rie, Anne Michelin, Manuela Ngako, Eleonora Del Federico, and Chelsey Del Grosso, “Photo-catalytic degradation of binding media of ultramarine blue containing paint layers: A new perspective on the phenomenon of ‘ultramarine disease’ in paintings,” Polymer Degradation and Stability 144 (2017) 43-52. De la Rie et. al. establishes the primary cause of “ultramarine sickness” as “photo-oxidation of the binding medium of ultramarine blue paint layers” in the presence of ultra-violet radiation, leading to “light scattering and the observed changes in reflectance. This also explains why the color can be restored by the application of oil or resin to the surface.” Such “restoration” of the color, when successful, is temporary. For further information about the degradation of lapis see: For the mechanism of ultramarine discoloration, see E., Del Federico, W., Schoefberger, J., Schelvis, S., Kapetanaki, L., Tyne, A., Jerschow, “Insight into framework destruction in ultramarine pigments.” Inorganic Chemistry, 45, 2006,pp.1270-1276; and Eleonora Del Federico, Jacob Newman, Lindsey Tyne, Cyndi O'Hern, Licio Isolani, and Alexej Jerschow, "Solid-State NMR Studies of Ultramarine Pigments Discoloration", Materials Research Society Symposium proceedings, Vol. 984, 2007. For the fading of red lake, see David Saunders and Jo Kirby, "Light-induced colour changes in red and yellow lake pigments," National Gallery Technical Bulletin 15 (1994) pp. 79-97.
Martin Kemp, “Art History: Sight and Salvation.” Nature, November 10, 2011.
Email communication from Dr. Missinne

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