Does any direct relationship exist between these materials and the essence of the icon? Let us try to answer this question calmly, avoiding the traps of pseudo-mysticism and lyricism. But first, an icon, what exactly is it?
The simple translation of the Greek eikon (= image) is too vague for our purposes. Not all images are icons! We are accustomed to use the word "icon", not for any image, but only for sacred images, those of the Lord Jesus Christ, the Mother of God, angels and other bodiless forces, and saints, as well as episodes of sacred history. In so doing we separate out icons and place them apart from all other artistic work, giving the impression there is art, including painting, as one category, and icon painting as a totally separate category.
But, following this now widespread approach, we must not forget that, originally, this separation between “normal” painting and icon-painting did not exist. But for Saints Basil the Great and Gregory of Nyssa (fourth century), these concepts are synonymous. The founding fathers of the Christian theology constantly used the term ‘artist’ instead of ‘iconographer’, ‘painting’ in lieu of ‘icon painting’ and even – horror horrorum – ‘picture’ instead of ‘icon’ i.e. using for the sacred image a word that has taken on a pejorative meaning in a certain school of the theology of icons. For example, St. John Chrysostom (IVth century) used the terms ‘painting‘, ‘picture’, ‘image’ in the sense in which we use the word ‘Icon’, recognizing as icons the creations of encaustic artists, sculptors, jewellers, chasers, fresco artists, thereby recognizing them all, ipso facto, as icon producers. St. Cyril of Alexandria and St. Sophrony (VIth century) already used the specific word ‘icon’, but not in contrast to, but synonymous with words like as ‘image’, ‘representation’, ‘picture’. In the documents of the Vth and VIth Ecumenical Councils, we again met all these terms as equivalent and interchangeable.
Only in the VIIIth century does St. John Damascene, in his work “Three treatises in defence of the veneration of icons”, start to use the term ‘icon’ as a dominant name for the sacred image. But this classical author does not totally reject from his vocabulary the terms ‘image’, ‘picture’, ‘representation’’ and continues from time to time to use them as synonyms of the word ‘icon’. There is also an antonym – this is not a picture but an idol. It is possible that the very fixing of the term ‘icon’ in the special meaning of ‘ sacred image’ gave rise to the iconoclastic controversy, and the need to express clearly and succinctly the concept of ‘the picture (image, fresco, cameo, mosaic, etc.), representing God and the Saints’.
It is these concepts of picture, icon and idol that are reflected in the decisions of the Seventh Ecumenical Council. Certain pictures are icons, while remaining pictures, and other pictures are idols, while remaining pictures, and certain pictures are simply pictures, neither sacred nor forbidden, created variously for teaching purposes, in memory of people or events, or for the decoration of everyday life, simply out of the abundance of creative energy.
We can therefore, and on a provisional basis, define the icon as a sacred image, in the form of an easel painting (mobile). In so doing we should not, however, lose sight of the fact that sacred monumental painting (frescos), sculpture, illuminated manuscripts, and even certain kinds of decorative art are so close as to interpenetrate at times with the icon proper. In present-day scientific language we also find, for distinguishing the icon from other forms of pictorial expression, the specific term of iconic image, a tautology that perfectly demonstrates the profound unity of Christian sacred art.
Is this definition sufficient to cover fully the icon? We have come all this way without speaking of wooden board, gesso, egg tempera, nor of so-called "Byzantine" style or even of canonicity.
Style and canonicity will be the subject to specific chapters. In this chapter we limit ourselves to the questions of materials and technology that relate to tempera painting and to the board itself, while insisting that these are not exclusive criteria for describing a religious painting as in icon.
In sacred painting, egg tempera is no more than one technique among others, as pigments (fine colored powders) were historically mixed not only with egg yolk, but could also be combined with other binders. The oldest icons that have survived to our day are painted in encaustic, a technique that uses heated beeswax as a binder. This is the main technique used in easel painting up until the iconoclast crisis of the eighth century. Also used as binders, besides wax and egg yolk, were various vegetable glues, and even linseed oil, well before the official onset of oil painting. Certain blue or green pigments, which do not bind in suspension with egg yolk, were crushed with honey, gum arabic or oil. This mixture was then used in the areas in question of icons painted otherwise with egg tempera.
Once oil painting appeared on the scene, it was used not only in the Catholic West but also in the Orthodox East, and not only in sacred painting in the Academic style, but equally much in "Byzantine" painting. There were techniques in which the first layers were placed in oil and finished with egg tempera, and others on the contrary where surfaces painted in tempera were then retouched in oil. Some local iconography schools working with oil paint on glass, with the glass serving both as support and protective layer.
We can also mention here icons in enamel (painted or cloisonné), which were produced in various countries and at different times, as well as the mosaic icons (not to be confused with wall mosaics) which were very popular in the Byzantine Empire until its downfall.
The egg tempera itself was composed to recipes that varied from one school to another. 'Entire' tempera mixes the yolk and the white of the egg. The best known temperas use only the yolk, but this is at times diluted with water, vinegar or beer. These mixtures are used at times fresh, at other times after a period of rest. These different mixtures have varying technical characteristics, calling for specific processes, which can strongly influence the artist's technique and style. It is therefore correct to say that there can exist more technical differences between two schools of tempera painting than between egg tempera painting and oil painting!
Wooden boards are the main, but not the only form of support for icons. Besides natural materials (wood, stone, bone) we also find man-made materials: fabric, glass, metal and, since the eighteenth century, porcelain and papier-mâché.
'Tablets' made of several layers of fabric glued together have been known since the Middle Ages in Greece and Russia. Prototypes of our existing laminated wood, thin, very strong, and lighter than wood, they survive much better than wood: the grain of wood is uneven and capricious, wood can bend, crack along the grain or joins, and even break in two. Tablets icons are well preserved from this type of trouble. Their only disadvantage compared with wood was that, at the time, they were complicated and more expensive to produce, and it was impossible to make them in large dimensions (nowadays it is wood that is the most expensive). Even today, supports in glued fabric or chipboard are used wherever it is useful to have thin, lightweight icons: at the top of an iconostasis, or the series of icons for the daily feasts of the church calendar. .
It is useful to remind ourselves that all the long and costly methods of preparing the wooden board serve to obviate, and ideally to eliminate, the peculiarities of wood as a raw material. The board is gradually dried to remove all moisture from its porous structure. It is soaked with glue to fill the now empty pores. All knots are cut out of the wood, and the holes filled with the same wood; to ensure an even surface porosity. Onto the wood is then glued a linen or cotton fabric, in one or two layers (the first along the grain of the wood and the second diagonally to it) so as to produce a complete barrier against its hydrophilic natural texture, always likely to cause accidents (bubbles, cracking, and loss of gesso). Lastly several layers of gesso (also known as levkas), a mixture of glue and white chalk powder, is spread on top. Each layer is allowed to dry thoroughly, so that the paint has no contact whatsoever with the board, because the support for the tempera is the gesso and not the wood. The sides and the back of the board or panel are imbibed with paint or oil, so as to enclose the wood on all sides, until nothing of the natural aspect of wood is left. In this way the icon board completely loses its own physiognomy, and Fr. Pavel Florensky is telling the truth when he speaks of the icon as a substitute for fresco, an attempt to create a "wall fragment”, regardless moreover of the original appearance of the material (wood, cloth, paper mâché or laminated wood) dissimilated under the gesso layer.
Historically, artists have painted on the supports available to them, supports which have varied from depending on the place and time. St. John Damascene in his Three speeches defending the veneration of icons claimed that any material, in the hands of an artist, can be used to the glory of God. We have no reason to consider egg yolk to be purer or holier than linseed oil, or a board in limewood to be nobler than any other medium. A particular material is selected because it best suits the artist's project, and not because it is "more traditional."
Of all materials used by iconographers, one would expect pigments would to be the least affected by these supposedly tradition-based prejudices. Whether in the form of natural minerals or industrial products, their chemical formula is identical. But, paradoxically, here too fantasy ideas abound, sacralizing unimportant secondary elements to the detriment of the essential.
We know these outlandish and so widespread claims: a true iconographer should himself collect the stones for his colors in Nature and grind them personally into pigments. Leaving aside the curious "chthonic" aftertaste of these claims (Mother Earth, direct contact increasing the mystical power of holy icons and the rest), let us stick to the facts. A village painter was obliged to limit his palette to soot from the oven, local clays and colored particles obtained by scraping oxidized metals. The higher his socio-professional level, the less he participated in person in the preparation of pigments. The market for "ready-made" pigments became more accessible to him, he became more demanding in terms of their quality, and he was less inclined to take the risk of crushing unknown minerals without being sure of their reaction with other pigments, in particular under the action of hot olifa varnishes or the sunlight.
Andrei Rublev certainly did not go picking up himself in the meadows around Moscow the precious cinnabar and shining lapis lazuli of his icons: he imported them from abroad. To prepare a sufficient amount of pigments for the frescoes in the Dormition Cathedral in Vladimir, he would have had to spend all his life grinding them!
The Paterikon tells us that another holy Russian iconographer, St. Alipi of the Monastery of the Caves in Kiev, spent on materials a third of the money he received for his work. Obviously his primary concern was for quality, not for "connecting with Mother Earth."
Today, when ready-made pigments (natural or artificial) are available in powder form and more accessible and cheaper than the natural raw materials, it would be strange to oblige iconographers to set up grinding equipment in their back kitchens. There is no justification - technical or theological - for this whatsoever. Technically, the advantage of industrially produced pigments lies in their compliance with standardized official norms. Their lightfastness, their reactions with other pigments and egg yolk, their grain, their covering power etc. are checked by the manufacturer's specialists, giving a certain degree of security to the artist. By contrast, handmade pigments, even when produced from carefully chosen raw materials and worked with all appropriate care, can always give unpleasant surprises. While for eating or dressing natural products may be the better choice, this does not apply to pigments for painting. It very often happens that artificial pigments are the most resistant to light, have more vivid colors or other equally desirable qualities.
Nor does the very notion of "natural" necessarily means "used by medieval artists." The chemical manufacture of pigments has been known since ancient times (in ancient Egypt, for example), and contemporary recipes differ from medieval recipes mainly by their precision and their concern for the health of those who prepare them. More than this, we can state unhesitantly that chemical science as we know it was born in the laboratories of medieval artists and physicians. Indeed the creation of new colors and new medicaments was the primary purpose of chemical research from the outset. This explains why some famous medieval iconographers were also physicians, two such distant professional areas being chemically linked! And in any case we have no pretext to suppose that Dionysios or any other famous medieval icon painter would have lost their place in heaven if, in addition to only natural pigments, they had had on their palettes red cadmium or blue phthalocyanine or green cobalt. Certainly they would have accepted to use these pigments just as they accepted lead white and yellow arsenic.
As for the theological reasons that supposedly justify the rejection of artificial pigments, we have seen that this runs counter to the opinion of St. John Damascene on the equal dignity of all materials belonging to the created world. The possible contact of the raw material or ready-made pigment with impure substances or impure beings does not make them unfit for icon painting. Let us recall that the recipes given in the Erminia  (the classic manual containing the experience of generations of iconographers on Mount Athos) are very far from any fundamentalist purity. We can cite rotten egg white, the boiling of animal skins to make glue, copper filings covered with a layer of fresh manure, and so on. These artists saw no mystical impurity in these natural substances that seem nevertheless embody the idea of impurity and decomposition.
The rite of the blessing of the icon, celebrated in church, washes it and purifies it of its "earthly” origin, including the will of those who worked on it using earthly and perishable materials, according to the technique specific to a particular school or period.
In summary, although the majority of the icons are painted in egg tempera on wood, this is not an exclusive criterion of authenticity. There is no foundation for describing as "decadent" or "false" icons painted on other media, using a different technique or following other recipes.
THE USE OF GOLD
And gold? Sometimes we hear people saying that the gilding of the background, of the haloes, of the the assistes (gold hatching highlighting the shapes of objects) are neither more nor less than the equivalent of the uncreated light.
The ease with which this expression is used is staggering! Uncreated light is a phenomenon of the spiritual world, invisible by definition except in very rare circumstances. From where does this peremptory and self-confident discourse come which tells us that the uncreated light cannot be transmitted by the pigments themselves, and therefore requires the use of gold? In what proportion (double? triple?) would this noble metal be better able to render uncreated light than the different minerals that are used in composing the colors? No one can answer this question, but the use of gold in the icon is still considered of paramount importance. The popularizers of the icon are unable to avoid such passages as "the flamboyant, golden paintings tell us conclusively that the world opening before us in the icon is the supernatural world, the world of the Absolute, the Eternal."  In its further development this lyricism leads to amusing theological conclusions, as "the supreme beauty of the Theophany is neither in the form nor in the color but in the purifying, form-less, life-giving fire”.
In fact, the habit of using gold in sacred images existed long before Christianity. This is partly explained by the custom of placing on the altar of divinity the most precious objects, and to use for worship works of great value, able to last an eternity. In pagan cults, a mystical meaning was sometimes attributed to the use of gold, the brightness of the metal being directly related to the light of the sun, evoking the life-giving light of a sun god.
Some scholars even tell us that, under the reign of Pharaoh Akhenaten, who had tried to establish the monotheistic cult of the sun god Aten, the concept of this invigorating emanation was very close to the Christian idea of the uncreated light.
We should note, however, that idols in solid gold (like the "golden calf" of the Bible), or plated with it, are characteristic of cults and cultures of a very primitive level. In more developed pre-Christian cultures, there was already a preference to use gold only for the secondary details of the sacred images of the gods, reserving for their faces stone, wood or ivory, all materials that "humanize" more the carved image. The Christian icon also followed this trend: gold is used in the background of the the icon, and sometimes as thin hatching or assiste on certain details of clothing, furniture, architectural constructions or plants. It has become the rule, even though it is not from these objects, but from the unique God, that this uncreated light emanates, that this light should be rendered by a metal alloy containing a predetermined percentage of gold.
Let us quote here some "concrete observations" and the amusing conclusions drawn from these observations, committed to writing a hundred years ago by Prince Yevgueny Trubetskoy, at the very dawn of the rediscovery of the ancient Russian icon: "In this illumination by the divine light, the 'assiste' makes us participate in the glory of everything that surrounds the Godhead, everything that has already entered the divine life or appears as its immediate vicinity. The 'assiste' for example covers the throne and the brilliant mantle of Sophia, Divine Wisdom, as well as the Virgin's mantle when she ascends to heaven. The wings of the angels and the tops of the trees of paradise are often touched in the same way. In several icons, the 'assiste' appears on the onion-shaped domes of the churches.”
Is the Mother of God before her Dormition really more distant from the divine life than treetops or church domes? Little theological training is needed to distance oneself from this preposterous "Marian thesis"!
And what about the Christology in colors? "In general, the colors of the other world (a term Trubetskoy introduces here with neither inverted commas nor definition, leaving us to guess its exact meaning from the context) were used with remarkable tact, especially by the painters of Novgorod. The assiste does not appear on the icons representing the earthly life of the Savior, which focus on the humanity of Christ, and in which his divinity is hidden under his condition of a slave. But as soon as the artist sees Christ appear in glory, or wishes to point forward to his future glorification, the assiste reappears.”
According to this incomparable logic, iconographers did not see the glorified Christ, and did not want to point to his future glory in the icons of the Holy Face. This is because we never see assiste in the icons of the Holy Face, except in very rare instances where it appears on the Savior's hair. In the same way, we suppose, the artist did not see Christ's glory, according Trubetskoy, in the icons of the Crucifixion nor even, despite its title, in the type called King of Glory, depicting Christ after his descent from the cross. The logical conclusion would be that only the Pantocrator type expresses the divine glory, since the clave (the wide, decorated ribbon) on the Savior's shoulder is usually decorated with assiste.
Here is another sample of the same irresponsible lyricism, dating this time from the end of the twentieth century, of "the first and only approach up to now to the old Russian icon, which is at once artistic, historical and theological.": "The strongest artistic effect of this use of ‘assiste’ is obtained in those icons where the artist wants to contrast the two worlds, and to widen the distance between the earthly and the divine, for example in the ancient icons of the Dormition. One look at the best of them tells us that the Virgin, dressed in dark clothing and resting on her deathbed among those familiar to her, remains bodily in this plane of being that we can touch with our physical hands and see with our physical eyes. But Christ, in light-colored clothing standing behind the bed and holding in his arms the Virgin's soul in the guise of a child, is manifestly a supernatural vision. His entire face glows and sparkles, separated from the intentional heaviness of the colors of the earthly plane by the ethereal lightness of the lines highlighted with ‘assiste’."
What then is this "manifestly supernatural" vision? The majority of mortals, including the author of these lines, lack the depth of experience of the afterlife that would allow them to describe it that way. Furthermore, if the figure of Christ is distinguished from "heavy earthly plane" by the "ethereal lightness of its lines highlighted by ‘assiste’", it is useful to note that this these "lines" do not form a complete network which "separates Christ the earthly plane" but that the assiste covers only his clothes, and not either his face or his hands!
So where does this heavy earthly plane, "that we can touch and see" originate in the icon that until now has been considered as being on the celestial plane? These quotes give us an indication of the scientific and literary level of all these theories about gold as a "divine emanation bathing the objects of the terrestrial world."
In the course of this work, we will studiously avoid these quotes of a cheap lyricism that offend the most basic good sense, but that are all too prevalent in the "theology of the icon." This text of Trubetskoy's, a confused torrent of disparate thoughts, the scope of which the author himself has failed to gauge, tells us no more than that the addition of gold in the form of assiste is optional and only applies to inanimate objects, and also that it is in no way a mark of holiness: the clothing of monks is never covered with gold, while assiste often covers that of kings and princes, including the garments of King Herod and Pontius Pilate, two of the most execrable figures in the Gospels!
The essential fact we gain from this is that to depict what is holy (human flesh transfigured in the Spirit) gold is not used. Everyday pigments, both natural and chemical, and lacking any particular luster, are used to paint the One God. In so doing we commit no desecration, no blasphemy, because in front of the glory of God, pure gold or the most common ocher (actually crushed clay) are equal in dignity.
Nothing blasphemous either in sacred images in which gold is completely absent, for economic or any other reasons. Very many images painted in the “profane” Academic manner, but also traditional "Byzantine" icons, are without gold decoration. Even the nimbus, the symbolic expression of the aura of divinity reflected by the face of God or a saint, was not necessarily depicted in gold. It could well be outlined only with white or cinnabar contour line, it could - especially in frescoes - be yellow, blue, greenish, or also represented by several circles of different colors.
In scenes consisting of several characters, haloes may be omitted, but never those of the Savior and the Mother of God. For example, the holy apostles are often presented without haloes in the icons of the Dormition, Pentecost, Ascension, in the subjects of Holy Week and other topics from the Gospels.
The Righteous in the so-called Protection or In You Rejoices icon are sometimes haloed, sometimes not. The icon of the Forty Martyrs of Sebaste sometimes shows these valiant warriors with haloes (as a result of which we see only faces of those in the first row), and sometimes without (in this case forty faces are visible, with forty haloes shown in heaven separated from their "owners").
Is there a rule or theological rational for the absence of the halo here? Yes, an explanation exists, not theological but purely practical. It is a question of artistic composition: haloes are omitted when space is lacking, when the artist prefers painting the protagonists' faces rather than the symbolic attributes of their holiness. Nor is the presence of halos in painting exclusive to Christianity: pagan gods, heroes, and kings were sometimes painted or sculpted with haloes which were round or of another shape. Thin golden crowns surround the heads of mere mortals (neither kings nor heroes) on the famous Fayoum Portraits from Egypt, images for the funerary cult, but that had been painted from nature during the lifetimes of the persons represented. An Egyptian, after his death, changed his name and became Osiris+name (after the name of the resurrected god). A crown of gilded leaves was then added to his portrait. This symbolized that the deceased had passed into the mysterious invisible world of beyond, and had himself become a god. This tradition that preceded — and may have contributed to forming — the Christian tradition is just one of countless proofs that the truth can open itself also to pagans, and that Orthodox spiritual art is connected to the artistic and spiritual traditions of all humanity, much more strongly than is sometimes believed.
These relationships exist, whether one likes it or not. It is probably wisest not to attempt to conceal our "sinister pagan past", but rather to try to recognize in it the presence of elements that will become important for Christians, as well as the presence of other more secondary elements, which we could discard without problems, and finally of traditions from which we need to detach ourselves, as remaining faithful to them could divert us from specifically Christian values.
So let us stop looking for cabalistic explanations of what are simply the technical tricks of the painting trade or the search for decorative effects. These theoretical inventions serve not to make prayer in front of an icon more concentrated, nor to advance us in our understanding of Orthodox holiness.