This Roman period glass bowl is made of translucent, light green glass. The bowl would have been formed by blowing a bubble of glass into a mold, creating the ribs on the vessel’s exterior and leaving the areas between the ribs extremely thin. Despite its age, this glass object is intact with the exception of one small, oblong loss in the thinnest part of the wall. Conservators do not always fill every loss in an object, especially when an object comes from an ancient and/or archaeological context, or when the loss is small; however, in the case of this bowl, the glass surrounding the area of loss is so thin that filling the loss actually improves the object’s stability by protecting the edges of the loss from further damage.

The loss in the bowl’s wall was filled with a thermoplastic acrylic resin called Paraloid® B-72. Paraloid® B-72 is a favorite material in conservation, used as an adhesive, a consolidant, a fill material, and a coating. B-72 is versatile as it is soluble in a variety of solvents in a range of concentrations, and particularly well suited to conservation as the resin is chemically stable, reversible and manipulable with solvents and heat, structurally strong, optically clear, and bonds well to many materials. Paraloid® B-72 finds use on a wide range of materials, but because the resin sets through solvent evaporation, tiny bubbles visually disrupt the resin film, making B-72 less aesthetically appropriate for glass. To help conservators use this excellent repair material in glass conservation, conservators at the Corning Museum of Glass developed a way to cast Paraloid® B-72 resin films without bubbles.

To make the fill for the bowl, B-72 resin was tinted with dyes and dry pigments, poured into silicone rubber molds, and cast into thin sheets. The molds were placed into polyethylene bags (left) to allow the solvent to evaporate slowly, reducing the formation of bubbles in the resin film. Once set (right), the film was cut to shape and adhered to the loss by applying a tiny bit of solvent to the edges of the fill; because B-72 is itself an adhesive, no additional adhesive is necessary.

Through exposure to its archaeological burial environment, the surface of the bowl has developed a layer of iridescence often referred to as “weathering products.” Unlike accretions that have become adhered to the surface of the glass, weathering products are actually the original surface of the glass that has delaminated, or split, into many layers. As light passes through these extremely thin layers of glass that are separated by small pockets of air, the light bends or refracts, the optical effect creating iridescent colors like oil on water or the colors of some butterfly wings.  

To make the Paraloid® B-72 fill resemble the weathered glass, a layer of goldbeater’s skin painted with acrylic iridescent paints was added. Traditionally used in the beating of gold sheet into gold leaf, goldbeater’s skin is thin, strong, translucent, and has a satiny sheen similar to the glass weathering products. This bowl will be part of the upcoming reinstallation of the Brooklyn Museum’s Asian and Islamic Galleries.

Posted by Victoria Schussler 

Sixteenth Chapter. Here is told how the craftsmen who cast precious metals fashioned their wares

(pages 73-78, Book 9, Florentine Codex, Anderson and Dibble translation with images supplemented from original Codex)

The craftsmen fashioned [and] designed objects by the use of charcoal [and clay molds] and beeswax [models] to cast gold and silver. With this [step] they made a beginning in their craft. To start with, he who presided distributed charcoal among them. First they ground it, they pulverized it, they powdered it. And when they had ground it, then they added it to, they mixed it with, a little potter’s clay; this was the clay which served for ollas. Thus they made the charcoal [and clay mixture] into a paste, kneaded it, worked it with the hands into a cohesive mass, so that it would dry and harden.

And also they prepared it: in just the same manner [as tortillas] they made it into flat cakes, which they arranged in the sun; and others were likewise formed of clay which they set in the sun. In two days [these cakes] dried; they became firm, they hardened. When they had dried well, when they had hardened, then the charcoal [and clay core] was carved, sculptured, with a small metal blade.

[If] a good likeness, an animal, was started, [the core] was carved to correspond to the likeness, the form in nature [that] it imitated, so that from it would issue [in metal] whatsoever it was desired to make - perhaps a Huaxtec, perhaps a stranger, one with a pierced, perforated nose, an arrow across the face, painted [tattooed] upon the body with obsidian serpents. Just so was the charcoal [and clay core] dealt with as it was carved, as it was carefully worked. It was taken from whatsoever thing was intended to be reproduced; howsoever its essence or appearance, so would it become [in metal]. If it were, perchance, a turtle, just so was the charcoal [and clay core] modeled: its shell, in which it can move; its head, which is peering forth from it; its neck, which is moving; and its feet, which are as though extending. Or if a bird were to be fashioned of gold, just so was the charcoal [and clay core] carved, so was it shaped, to give it feathers, wings, tail, feet). Or [if] a fish were to be made, just so was carved the charcoal [and clay core] to give it its scales; and its side fins were formed and its tail stood divided. Or [if] a lizard were to be made, its feet were formed. So was the charcoal [and clay core] carved for whatsoever creature was imitated. Or else a radiating, golden necklace would be completed, with bells about its edge, each designed, decorated, with flowers.

When the charcoal [core of the mold] had been prepared, designed, carved, then the beeswax was melted. It was mixed with white copal, so that it would [become firm and] harden well. Then it was purified, it was strained, so that its foreign matter, its dirt, the impure beeswax, could fall. And when the beeswax had been prepared, it was then flattened, rolled out, upon a flat stone with a round piece of wood. It was a very smooth, flat stone on which [the wax] was flattened-[and] rolled.

When it was well flattened, just like a cobweb, nowhere of uneven thickness, then it was placed over the [carved] charcoal [and clay core]; the surface was covered with it. And carefully it was placed on [the core] ; cautiously little pieces [of wax] were cut off or pared away. By this means a little [wax] entered hollows, covered eminences, filled depressions in places where the charcoal [and clay core] had been carved away. By means of a stick [or sliver of wood] they went making it adhere [to the core].’ And when it was prepared, when everywhere the beeswax was placed, then a paste of powdered charcoal was spread on the surface of the beeswax. Well was the charcoal ground, pulverized; and a rather thick coating [of paste] was spread on the surface of the beeswax.

And when it was so prepared, again a covering was placed over it, to wrap, to envelop completely the [thus far] completed work, in order for the gold to be cast. This covering was also of charcoal, also mixed with clay-not pulverized but relatively coarse. When the mold was thus covered, thus completely enveloped, it dried for another two days, and then to it was affixed what was called the anillotl, likewise of beeswax. This would become the channel for the gold, for it to enter there [into the mold] when it was molten. And once more [the mold] was laid out; it was placed [in] what was called the crucible [a charcoal brazier], also made of charcoal [and clay] hollowed out. Then thus was the melting. The charcoal fire was laid. There the gold was placed in a crucible; it was melted, so that then it entered into the channel [in the mold], there to be led along, flow, spread out into the interior.

And when it was cast, whatsoever kind of necklace it was which had been made- the various things here mentioned - then it was burnished with a pebble. And when it had been burnished, it was in addition treated with alum; the alum with which the gold was washed [and] rubbed was ground. A second time [the piece] entered the fire; it was heated over it. And when it came forth, once more, for the second time, it was at once washed, rubbed, with what was called “gold medicine.” It was just like yellow earth mixed with a little salt; with this the gold was perfected; with this it became very yellow. And later it was polished; it was made like flint, to finish it off, so that at last it glistened, it shone, it sent forth rays.

It is said that in times past only gold [was known to] exist. It was taken advantage of. The goldworkers cast it; they made it into necklaces, and the goldbeaters hammered it, flattened it, into the devices which they required. Silver was not yet in use, though it existed; it appeared here and there. It was highly valued. But today, on the other hand, all is silver; they want gold; it is much treasured.

The goldcasters and beaters who work now also require copper, though only a little, a measured amount. They add it to silver [solder] to give it binding power, to make it adhere. For if only silver were melted [to use as solder], the article joined would only shatter; it would only break [at the seams]. There where the article was soldered, [the seams] would not everywhere bind [and] come together.

And the goldbeaters, in times of old, hammered only gold. They smoothed it, they burnished it, with a stone, and they worked out a design along a black line with a stone. First the feather workers made them a design, and then they chased the design with a flint knife [as a tracer]. They followed the black line to form the design with a flint knife. They embossed it, they went making relief work, copying just as was the [black line] pattern. In the same way they manufacture objects today, wherever their work is needed. Perhaps feather mosaic [or other] feather work is required. [The goldworkers] join with [and] are instructed by the feather workers who cut all manner of feather work which may come their way.

Today the goldworkers work thus. They require sand - fine sand. Then they grind it, they pulverize it well; they also mix it with potter’s clay. Then they set it out [in the sun], in the very same manner as they form the clay so as to bring forth, to cast, whatsoever they would make. And in two days it is dry.

When it is well dried, then with a potsherd the surface is rubbed, smoothed, polished, burnished, shined, so that the surface is smoothed. Then it is carved - sculptured - with a metal knife, as is told elsewhere. In either two or three days [the work] is finished, made good, perfected.

When [the core] is prepared, then powdered charcoal paste is spread on its surface, and the surface is made smooth with a clay paste. Then the beeswax is melted; it is mixed with white copal, as was mentioned. When cooled, when purified, then it is flattened, rolled out on a flat stone with a piece of wood. Forthwith it is placed upon- joined to- the clay object to form the shape of the gold, whatsoever is to be made, perhaps a jar or an incense burner, which they call perfurmador. It is painted; it is designed with a beautiful design.

They especially esteem beeswax; they use it especially to form patterns, to produce works of art. But first, somewhere, a model of beeswax is made. When it has been well prepared, the mold is pressed upon it [to make the wax model]. For there is a model [in wax] of all they make, whether birds’ wings, or flowers, or leaves of plants, or whatsoever beautiful design.

By means of a small wooden stick, called a thorn stick, [the wax] is pressed on; it is made to adhere [to the core of the mold]. In perhaps two days it is perfected; it is made good.

When it has been prepared, when in all places the [modeled] beeswax has been made to adhere [to the core], then on its surface is spread [a thin paste of powdered charcoal]. When it is dried, then in addition a covering is placed upon it, of only coarse charcoal [and clay], in order to envelop the model [of wax with its coating of powdered charcoal paste]. In perhaps two days it dries. Then to it is placed the beeswax channel, called the round anillotl. First it is rounded. This becomes the channel for the gold, for it to enter there.

And when the channel has been set in, once more [the mold] is arranged [in) something like a crucible where the gold is [to be] cast. When they are this far, when all is prepared, then [the mold] is placed on the fire; it is thoroughly heated. Then flows out burning the beeswax [model] which has been placed within it. When the beeswax has come forth, when it has burned, then [the mold] is cooled, for which purpose it is once more set out over sand, quite coarse sand. Then immediately the casting takes place; there [the mold] enters the “fire pot” [a charcoal brazier]“ on a charcoal [fire] ; and the gold, which is to enter there [into the mold], is melted separately in a ladle [and poured].

Here this ends; thus the work is finished. And when the piece has been formed, when it has been cast, when it comes forth, then it is treated with alum; in a copper vessel it is boiled. And if somewhere the piece has cracked, has split, that is the time to mend it. That which is to be joined [soldered] is mended. And then it is rubbed so that like copper it shines. Once more it goes into [and] is treated with alum. So thereafter it is cleaned; it is made like flint, so that it glistens brightly.