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 Glass Dictionary 
Cristallo in artistic glass production is transparent glass, homogeneous, absolutely colorless. The secret of quality in cristallo, which is glass, is the 100% purity of the raw materials, the use of bleaching agents, the preparation of the vitrifiable mixture, the fusion process. In the middle of the XVth century Murano invented a pure and colorless glass, which for the first time in history was called cristallo and was later imitated in other European countries. In contrast with Nordic crystal, which has a concentration of lead oxide and today must be subject to strict controls of the fumes deriving from its fusion, its contact with foods and its waste disposal, Murano cristallo is a sodic-calcic glass whose principal components, aside from silica, are sodium oxide and calcium oxide. Murano has always remained faithful to sodic cristallo because it is most suitable for the production of particularly light blown objects which require long working processes.
Opaque colored glass is obtained on the basis of the same principles but with a base of white opaline glass and a larger amount of raw coloring agents.
Avventurina is a type of glass which was invented in Murano around 1620. It presents within its mass countless particles of gold, which are really minute copper crystals - copper is its basic coloring agent. The secret of preparing avventurina, known over the centuries to very few expert glassmasters, is, when the fusion is finished, to add multiple small and successive doses of reducing raw materials such as iron shavings, metallic silicon, or coal, until the metallic copper begins to precipitate. An adequate very slow cooling cycle for the molten glass determines the separation of the metallic copper from the glass base. The quality of the avventurina depends on how homogeneously the copper crystals are distributed and how large the copper crystals, which at best can reach one millimeter in size, are. The origin of the term avventurina is clearly indicated by the XVll th century glassmaster Giovanni Darduin: "it is called venturina, and with reason, because it comes more out of fortune ("ventura") than science." ( Video Avventurina )
Also called "graffito" during the past century, it is obtained by wrapping a colored vitreous thread in a spiral around the body of the blown piece, then, near the glory-hole, it is combed with a hooked tool to achieve the characteristic festoons which may also be smoothed into the wall by marvering the piece and blowing it again. This decorative technique dates from the XVIl th century.


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