Glassworking techniques, the Magic of Glass
There are many different techniques used to shape glass, each of which produce different results as the Glass Master chooses. Below is a description of some of the most important ones.
A translucent glass sprinkled with particles of glittering metal oxides, to imitate aventurine quartz. This Venetian glass is made with particles of copper created by chemical reaction when copper oxide is added to the glass “bolo”.
The glass gets its name from the fact that it is so difficult to make and the result is so uncertain that it is considered an “adventure”. Making avventurina is a long and delicate process, during which tiny brown or greenish copper crystals form inside the mass of glass.
Avventurina is the most precious type of glass in the entire history of Murano glass, dating back to the first half of the seventeenth century. The name itself derives from the word “ventura”, which commemorates how it was initially invented by accident, and highlights the difficulties that even the most skilled masters encounter in working with it. History tells us how a glass master, who was melting the many different components of glass, was called away and forced to leave his fusion, ruining the entire operation. It took a week for the pots of glass and the kiln itself to cool completely; to his great surprise, when he broke the pots, the glass master discovered this wonderful material. The first recipe that taught how to make avventurina dates back to 1644, but it was not until 1860 that the secret to blowing avventurina glass was discovered. Avventurina is a type of glass filled with tiny copper crystals (which precipitate as the glass mixture cools) evenly spread through the glass giving the material a characteristic metal sheen. To ensure that the production of the avventurina is successful, it is important, when the fusion is complete, to repeatedly add a certain quantity of raw materials such as iron shavings, metal silica and coal, until the copper has precipitated. During the cooling process, which lasts several hours, the copper slowly and almost completely separates from the glass base. The quality of this glass depends on how evenly the copper crystals spread through the glass, and how large they are: in some cases they can be as large as one millimeter. The avventurina is then removed from the kiln in blocks, after cooling. Melting it again could compromise its characteristic appearance. It is then cut like a gemstone, but special attention is needed to shape it in hotwork. Making avventurina is a slow and rather delicate process; over the centuries it has been a closely-guarded secret for a small number of glass composition specialists.
This technique consists in polishing the surface by “tapping” it against a grinding wheel during the cold-working process, to obtain a pattern of “bubbles” in various sizes, giving the object the appearance of being hammered like iron.
Glass made by mixing different coloured metals to imitate stones such as agate, calcedonium, onyx, malachite and lapis lazuli. This type of process was created in Venice in the late fifteenth century. The technique was rediscovered by Lorenzo Radi in the late nineteenth century.
Glass made by using round or flat, multi-coloured glass rods laid side by side or one on top of the other, then fused and blown. This gives the object a compelling decorative and chromatic effect.
The word describes a highly transparent and reflective type of glass, made with lead silicate. It is also known as “lead crystal” and was perfected in England, France and Bohemia (made with silica, lime and potassium) towards the end of the seventeenth century. By extension, the word “cristallo” is used to describe any type of pure and transparent glass, the most famous of which is clear Venetian glass.
One of the most ancient techniques, filigree has been in use since the sixteenth century. It is made by laying transparent glass rods with a coloured glass core on a metal plate, and heating them until they reach the fusion point. They are then rolled around a cylinder-shaped object to which they adhere. Almost all Venetian glass furnaces use this technique. In the 1950s and 60s, Archimede Seguso made objects with filigrana, using a particular technique for preparing and polishing the glass rods.
Gold or silver leaf
A very thin leaf of pure gold, usually a 8×8 cm leaf of 24-karat gold, which is “gathered up” by the hot glass in the early phases of the working process. Often the gold is then covered in another layer of transparent glass. If the glass is blown, the gold “leaf” breaks up into a suggestive gold dust. In the nineteenth century, silver leaf was also introduced, but it must be incamiciato, covered in another layer of glass to prevent the silver from oxidising and turning an unattractive brown colour.
A Venetian technique that consists in attaching two or more different-coloured elements that must match perfectly in diameter, and shape them together to achieve the desired form. This procedure requires particular skill on the part of the glass master. The most famous pieces of this type are the double-incalmo vases presented by Venini at the 1962 Biennale, and the “Cappello del Doge” designed by Thomas Stern.
Translucent glass made opaque by adding bone ash and coloured by using metal oxides in various shades of colour.
A technique known as early as the sixteenth century; it is a variation on filigrana, featuring a characteristic criss-cross design of the glass rods used for the decoration.
Glass constituted by an outer layer in varying thicknesses covering a layer of a different colour. It is obtained during hotwork by plunging the glass into pots of different colours. The object is thus made of different layers varying in thickness and colour to create surprising chromatic effects.
A type of glass similar to sommerso, but usually thinner, composed of two layers of glass one over the other.
A technique dating back to the Roman era; later abandoned, it was rediscovered in the mid-nineteenth century, reaching its peak in the early twentieth century in the objects made by Artisti Barovier. The production technique consists in preparing a bunch of multi-coloured glass rods assembled so as to obtain the desired design: it is then fused and later cut into small disks. These disks are laid out on a metal plate to compose the desired design, heated and then rolled over a cylinder-shaped piece of glass attached to the blowpipe.
A technique similar to the murrina technique. It is obtained by assembling a bunch of rods in different colours, and heating it to the fusion point; two blowpipes are then attached to the ends of the glass, which is pulled and rotated to form a spiral. Zanfirico is the modern term used in Venice to describe retorto glass.