Already since childhood, everyone knows how glass looks like and its physical characteristics… but how is it made? Because of the enormous role the famous glass produced on the island of Murano has played and still plays in the history and the wealth of Venice, we decided to collect here a few informations regarding this ancient art.
New techniques have been invented and others have been improved over many centuries, but the basics of Murano glass production are still those of over 600 years ago.
- Sand: silica 70%
- Soda: sodium carbonate
- Chile saltpetre: sodium nitrate
The raw materials are mixed together in the heat-resistant crucible and then fused in a kiln that can reach up to 1.400°C.
The Murano glass in its basic composition is colorless. The colors are obtained by adding small amounts of minerals, oxides, and chemical derivatives to the base composition of the glass powder.
The kilns, “fornaci”, used nowadays on Murano have a large crucible of maximum capacity between 20-25 quintals. There are also kilns made up of a number of smaller crucibles (with capacity varying from 10kg to 2 quintals), the smaller crucibles being used for colored and opalescent glass.
Modern-day kilns on Murano are equipped with heat retainers and with equipment that makes it possible to control temperature and the rate of fuel combustion.
The temperature of the fused viscous paste is reduced from 1400°C to 1100°C. At this temperature, the paste is “solid” enough to be taken from the kiln and worked before the glass sets (at around °500C). If it is to be worked by hand, as in Murano, then this process of solidification must take place slowly. At the end of the working process, the glass is still at a temperature of 500-600°C. It must then be cooled slowly: glass is a poor conductor of heat and therefore the inner and outer surfaces can cool at different rates, creating tensions within the material that might lead it to crack (perhaps some time after it has cooled completely). This slow cooling process takes place in what are known as “annealing ovens”.
Tuning Glass Properties
There are some raw materials, called flux or melting agents, which soften at lower temperatures. The more sodium oxide present in the glass, the slower it solidifies. This is important for hand-working (“mano volante”) because it allows the glassmaker more time to shape the material. Other raw materials that an artisan might add to a glass mixture are sodium (to make the glass surface opaque), nitrate and arsenic (to eliminate bubbles) and, at last, coloring or opacifying substances.
Creation of Complex Pieces
The Master sets up and coordinates his “Piazza”, a team of 2 or 3 people which help him in the preparation of the piece: the “serventino“, handling a blow pipe, takes a mass of incandescent molten glass from the kiln and rolls it against an iron plate (“bronzin”); the “servente” blows in the pipe creating a balloon (“colletto”) which is then rolled again against the iron plate, to ensure symmetry. As soon as this set up is done, the Master has to start working immediately and quickly on the viscous mass whose temperature is rapidly dropping. Not only the drop of temperature below 500° would make the glass totally rigid and fragile, but happening so quickly it would create tensions between the outer colder surface and the inner hotter core and lead the glass to break.
During the whole process, the Master has to work quickly and interrupt its work for his team to heat up the craft back in the oven and immediately continue the creative process as soon as the temperature is at a safer (higher, but not too high else it would lose its shapes) temperature. If the piece requires more material, further masses are added and crafted with the same processes. Once the Master is done with the creation, the piece is put in another kiln (“muffola”) at a lower temperature which is then slowly decreased (“tempera”) until it reaches the room temperature.
Famous Murano Glass Techniques
Murano’s glass history boasts a very long list of techniques, and their improvements, over centuries of production and innovations. Techniques can be divided into two main categories: primary processes and secondary processes. Primary processes are those that occur by changing the initial mix before the melting process in the oven. Secondary processes are those that happen after the initial mix has been created and are used to modify the final result by torching and fusing or when cold (“a freddo”) by carving or sculpting. Amongst these many techniques, the most used and appreciated are:
- Avventurina (colored glass with iridescent particles
- Enameled glass
- Murrina & Millefiori (sliced rods of colored glass fused together)
- Vetro Pezzato (multiple colored glass melt together, giving a patchwork look)
- Gold and silver leaf works
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