Transformation of Flat Glass

In glassmaking jargon, second processes means all treatments that a sheet can be subjected to and which can be applied to the edges and surface of the sheet.

The various glass cutting techniques are based on two principles: for the glass to break, a thin or deep cut must be made, or a great quantity of heat must be localised on the surface.
The most ancient technique involves running a red-hot metal point on the surface of the object, along a line which the crack should follow. The line is then wet with drops of water: very localised sudden cooling forms great tensions and it is sufficient to lightly and repeatedly hit the opposite surface to trigger and spread the crack.
Modern evolution of this technique is the laser beam which allows cuts to be made that are extremely precise and already finished, without any sharp edges, even along very complex lines. A similar result is obtained with a jet-stream of water mixed with an abrasive powder at a very high pressure; this is a technique that is used industrially.
The most traditional cutting method involves cutting the glass with a steel roller which is harder than the glass (vidiam steel). A few light knocks on the opposite surface is sufficient to spread the crack along the cut-line.
Lastly, the glass can be cut with a metallic wheel, attached to a lathe that spins at high velocity, on the edge of which small diamonds or other materials harder than glass, are attached.

- rough edge : removal of sharp edges of a sheet, with either a grindstone, Carborundum or diamond, of quite thick grain.

- shiny edge : this phase comes after the previous one, involves removal of every irregularity of the edges and shining of them with a diamond grindstone or fine grain one or with powdered pumice.

- shiny industrial edge : involves edge grinding with appropriately grained grindstones, without any successive work. This is the classic shiny edge (semi-opaque) for edges of large-scale production sheets, particularly suitable for thin thicknesses.

- chamfer : working on the edges of a sheet which, having been made with any angle less than 90° compared to the surface of the sheet, involves the surface in addition to the edge. Very delicate technique, still used today for very high quality glass.

Holes can be made on practically any type of sheet with suitable crown diamond tipped drills.
Very big holes can be made with a brass or iron pipe, instead of crown tip, attached to vertical drills, assigning the Carborundum powder - dragged by the tip - the task of making the hole.

Curved glass is made by the progressive heating of the sheet in suitable furnaces up to 600 °C. The sheet is slackened under its own weight and it sags onto the surface of the mould on which it rests, thereby taking its shape. Regular or irregular shapes can be made, which are used in the building and the automobile industries.