Cennino Cennini - Il Libro Dell' Arte

This text is the earliest known description of the techniques used for verre églomisé. Written during the 14th Century in Italy, it is contemporary with the early Italian religious pieces. This period was dominated by the use of engraving the design into the gold leaf and is found on reliquaries and small portable alters.

The book describes methods used in many arts and trades of the time, but we are particularly interested in this chapter.

Chapter 172 Translation - 1899

How mosaic work is done for the adornment of reliquaries.

There is another way of working on glass, charming, lovely, and as rare as can be imagined, which is a branch of art in devout use for the adornments of holy reliquaries, and it demands sure and ready design; and this is the manner fof the work, thus: take a piece of white glass, not greenish, very clear, without bubbles, and wash it with lye and charcoal, rub it and rinse it again with clean water, and leave it alone to dry; but before you wash it cut it to whatever shape you want. Then take the white of a fresh egg; and with a very clean whisk, break it upas you do for laying on gold; let it be well beaten, and let it distil for a night. Then take a minever brush, and with the brush and the egg-clear, wet the glass on the back side, and when it is wetted equally take a piece of gold-leaf, which must be thick, that is to say, dead gold (appannato). Put it on the parchment tray (paletta di carta) and gently put it on the wetted glass; and with a piece of very clean cotton-wool press it down gently, not letting the egg-white get over the gold. And in this way gild all the glass.

When it is quite dry, take a very flat tablet of wood, lined with black linen or canvas, and go into your little workroom where no one can disturb you at all, and which should have only one linen-covered window (finestra impannata). Put your table at this window as if for writing, so that window is over your head, and stand with your face turned towards the window; the glass being laid out on the before-mentioned black cloth. Then take a needle bound to a small stick, like a minever brush, which must have a very fine point; and invoking the name of God, begin drawing lightly with this needle the figure which you wish to make; let the first drawing be very faint, for you can efface nothing; so make your drawing light as well as firm; then go on working as if you were drawing with a pen; for the whole work is done with the point; and do you see how you must have a light hand and not tired, for the deepest shade which you can make is only to go with the point of the needle quite down to the glass, and moreover the half shade is just not quite penetrating the gold, which is a delicate matter; and this work must not be done in haste, but with great delight and pleasure. And I give you this advice: that the day before you wish work at such works, you should hold your hand to your neck and breast, to have it well rested from fatigue, and moderate in blood.

Having made your design, you must scrape away certain spaces which are usually painted with ultramarine blue in oil. Do this with a leaden style, rubbing over the gold which comes clean away, and cutting sharp round the outlines of the figure. When this is done take other colours ground in oil, such as ultramarine blue, black, verdigris, and lake: and if you wish any garment or lining to gleam with green,(che risprende in verde) paint in green; or with lake, paint in lake; or with black, put in black. But black has more force and sculptures your figures better than any other colour.

Translation - 1959

How to gild glass for reliqary ornaments.

There is another process for working on glass, indescribably attractive, fine and unusual, and this is a branch of great piety, for the embellishment of holy reliquaries; and it calls for sure and ready draftsmanship. This process is carried out as follows. Take a piece of white glass, with no green cast, very clean, free from bubbles; and wash it, rubbing it down with lye and charcoal. And rinse it with good clear water, and let it dry by itself. But before you wash it, cut it to the size you want. Then take the white of a fresh egg; beat it with a good clean whisk just as you do that for gilding, so that it is thoroughly beaten; and let it distil overnight. Then take a minever brush, and with this brush wet the back of the glass with this glair; and when it is thoroughly wet all over, take a leaf of gold, which should be quite heavy gold, that is, dull; put it on the paper tip, and lay it deftly on the glass where you have wet it; and press it down with a little very clean cotton, gently, so that the glair does not get on top of the gold; and lay the whole glass in this way. Let it dry without sun for the space of some days.

When it is all dry, get a nice flat little panel, covered with black cloth or silk; and have a little study of your own, where no one will cause you any sort of interruption, and which has just one cloth covered window; and you will put your table in this window, as if for writing, so arranged that the window shines over your head when you have your face turned toward this window. With your glass laid out on this black cloth:
Take a needle, fastened in a little stick as if it were a little brush, and have it quite sharp pointed. And, with the name of God, begin to draw lightly with this needle whatever figure you wish to make. And have this first drawing show very little, for it can never be erased; and therefore work lightly until you get your drawing settled; then proceed to work as if you were sketching with a pen, for this work has to be done freehand. And do you want to be convinced that you need to have a light hand, and it should not be tired? - [Know] that the strongest shadow you can make consists in penetrating to the glass with the point of the needle, and no more; that the intermediate shadow consists in not piercing through the gold all over; that it is as delicate as that, and you must not work with haste - rather with great enjoyment and pleasure. And I give you this advice, that the day before the day you want to work at this job, you hold your hand to your neck, or in your bosom, so as to get it all unburdened of blood and weariness.

When you have got your drawing finished, and you want to scrape away certain grounds, which generally want to be put in with ultramarine blue in oil, take a leaden style, and rub the gold, which it takes off for you neatly; and work carefully around the outlines of the figure. When you have done this:
Take various colors ground in oil, such as ultramarine blue, black, verdigris, and lac; and if you want any drapery or lining to glisten [in lines of gold] on green, apply green; if you want it on lac, apply lac; if you want it on black, apply black. But the black is the most striking of all, for it shows up the figures better than any other color.