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- [Voiceover] Gold-ground paintings of the 13 and 1400s were usually made for an altar piece which decorated a public church or as part of a devotional altar commissioned for a private chapel. Creating a gold-ground panel painting was a lengthy process involving many steps and numerous artisans. Before an artist could begin work, a carpenter had to build the panel. First, planks cut from poplar trees were butt joined and held together with a glue made from casein, an adhesive derived from skim milk. Next, the frame was carved and attached. Bare poplar is far too absorbent to paint on. So the panel was coated with a thin layer of warm animal skin glue, an adhesive that was made by boiling animal hides in water. Linen was soaked in warm glue, rid on any excess liquid, then spread across the panel and across the frame sometimes as well. This was done in order to stabilize the seams of the panel and to ensure that the final painting surface would be as smooth as ivory. The linen also provided the foundation for the next step, the application of gesso. The gesso helped give the panel a surface smooth enough for gilding and painting. Gesso, the Italian for gypsum, is calcium sulfate, mined in Medieval times from this quarry at Cavallino in the Tuscan region of northern Italy. Making gesso, a task carried out by workshop assistants, required patience and vigilance. First, the white powdered gypsum was mixed into the animal skin glue. Incorrect proportions of gesso or glue would result in either cracking or a dusty surface. If the mixture overheated, air bubbles ruined the gesso, for when it dried, the surface would be peppered with pinholes. As shown in our demonstration panel, this could be disastrous for an angel's complexion. The gesso was brushed on in thin layers. For good adhesion between layers, six to eight coats of gesso were applied during one day and, if necessary, into the night. After the gesso had dried, artisans used charcoal to help them see where the panel was not uniformly smooth. Black charcoal was rubbed across the now-hard white surface. A steel scraper was used to level the gesso. Hollows would remain black until the whole surface was uniformly worked. At this point the panel was ready for the preliminary drawing. Artists used bits of charcoal to sketch in the design. This design served as a guide for the gilding of the panel. Any mistakes could be easily erased with a feather. The underdrawing could be made permanent by going over the faint lines of charcoal with a pen or brush. The underdrawing often appears somewhat rough. Its main purpose was to fix the boundaries between the painted surfaces and the gilded background. To provide a clearly defined edge to the gold, a needle was used to incise the outlines of the figures. The gold leaf served as the background of the picture so the gilding was done before painting. Once a halo was gilded and decorated, it is too late to alter the position of the saint's head. Over the centuries, gold leaf has retained the traditional measurements of four fingers' width. The main difference between Medieval gold leaf, which was beaten by hand from gold coins, and that produced today, is the degree of thickness. Medieval gold leaf was thicker, but still only semi-opaque. If the leaf was laid down directly onto the hard, white gesso, it appeared rather cold and greenish. So the artist first applied a soft, reddish-brown clay mixed with glue. This material was called bol, a Greek word meaning earth. And it was used to create a rich, warm ground below the gold leaf. The fragile sheet of gold leaf was put on a padded leather surface and cut into small, manageable pieces. The dried bol was then moistened with water, which reactivated the glue. And the gold was dropped immediately into position using a special brush or piece of thin card. This method of applying gold leaf is today called water gilding because the gold will only stick to the moist areas. When first applied, the thin leaf appeared rather matte and wrinkled. But the purpose of gilding was to make the surface look like solid, gleaming gold. To make the gold leaf brilliant, it was burnished with a dog or wolf's tooth. Modern agate burnishers are still made in this traditional shape. The timing of these processes was crucial. If burnishing was attempted too soon, the gold was rubbed away. Too late, and the gesso hardened, making it difficult to achieve a glittering surface. Once burnished, the panel was decorated with ornamental lines and punch marks, a process called tooling. Tooling made the gold sparkle and shimmer and drew a viewer's attention to figures of spiritual importance. A compass was used to incise the halo. But good hand and eye coordination was needed for creating the punch mark pattern. The punch must be held exactly perpendicular to the panel and struck evenly or one side of the punch mark would be more deeply indented than the other. For a more ornate decoration, stippling could be added. The gilded background can be further enhanced by colors painted to simulate expensive brocades. The usual method was to apply paint on top of the gilding and then transfer the design with a pricked cartoon. The dots of white pigment would pass through the prick marks and serve as an outline. Using the dots as a guide, the artisan gently scratched away the paint and revealed the gold underneath. Another method of applying gold leaf is called mordant gilding. Artists would use a small brush to paint thin lines of oil and then apply the gold leaf directly onto these lines as they dried. This technique was used commonly to embellish the edges of garments, as shown here in this altar piece by the Florentine artist Bernardo Daddi. These fine lines of mordant gilding did not require burnishing. After gilding, the panel was ready to be painted. The colored pigments used to make paint were usually obtained from the local apothecary. To make the paint, water was added to the powdered pigment and ground together on a marble slab. Paint is simply coloring matter or pigment mixed with a suitable adhesive, for example, the oil in oil paint. In the case of egg tempera paint, however, egg yolk was the adhesive or medium. The recipe was simple. Egg yolk and water were added in equal quantities and mixed in mussel shells or small pots. The yellowness of the yolk was commented on by the 14th century Florentine painter Cennino Cennini. In his treatise on painting, he explained that for young faces, the paler yolks of town hens should be used. For people of more swarthy complexions, the more orange yolks of country eggs were acceptable. The very fast drawing time of egg tempera did not allow colors to be blended as with oil paint. Three tones of the same color were mixed beforehand. Then the artist hashed fine lines of one tone into the next. These lights and darks could be modified by either applying more layers of color or by allowing the underpainting to show through. This ladder technique is best seen in the painting of flesh tones. The flesh area is underpainted with a layer of green earth as seen in this head of Saint Luke by Simone Martini. The pink flesh tint, a color made by mixing vermilion, ochre, and white has been applied thinly in the shadow areas, allowing the green underpaint to create a cool half-tone. A striking feature of gold-ground painting of the 1300s is the strength and brilliance of the colors. Here in the Daddi altar piece the blue of the Virgin's robe was painted with ultramarine. Imported from Afghanistan, its name derives from the Italian words for beyond the sea. The intensely blue pigment was extracted from crushed lapis lazuli, a long, complicated process that made it a painter's most expensive material. Bright, pure colors were required for a number of reasons. First, only intense, true colors could hold up to the glittering halos and brilliant gold backgrounds. Saints were often identified by their particular color. The Virgin Mary, for example, is shown here wearing her traditional blue robe. Lastly, bright colors were needed because Medieval churches were generally quite dark, and a viewer might see a painting only by a chance ray of sunlight or the glow of a candle. The colors, therefore, had to be brilliant.