Glass of the kind used in buildings is a mixture of pure sand, soda, and chalk, with a proportion of broken glass,1 etc. These are melted together at a very high temperature, and brought by different processes into convenient forms for use.
It is not of importance to the engineer or builder to know the exact nature or proportion of the constituents in different kinds of glass, as he can never be called upon to make these for himself. A knowledge of the processes involved is useful only so far as it enables him to distinguish one kind of glass from the other.
Before considering the various descriptions of glass used by the builder, a few points may be noticed which are common to all kinds of glass.
Large panes are more expensive than small ones, as it is more difficult to preserve the entire sheet of glass in making, whereas the smaller panes can be cut from what is technically called "breakage."
1 The following are the proportions (roughly) for a few different kinds of glass : -
Fine white sand..
Sulphate of soda..
Broken glass ...
A trace. 15
All glass differing from that in ordinary consumption, however trifling the difference, is also charged extra The extra labour and risk in carrying out exceptional work is charged for. Triangular and other irregular shapes are charged as square - i.e. the area measured is that of the circumscribed rectangle.
In bending rolled plate the smooth side is outside unless ordered to the contrary.
In fixing glass those varieties, such as crown glass, that are slightly convex, should have the convexity outwards.
In the case of glass having only one smooth side, it is generally recommended that the smooth side should be placed outwards. It is better, however, to place the rough side outwards, for the rays of light are then retained and the surface appears flat; if the smooth side is outwards, the rays are reflected, and the slightest undulation in the glass is easily perceived.