BUILDING MATERIALS 161 



Glass. Ordinary glass, known as window glass, is manu- 

 factured by a blowing and rolling process. It is blown into 

 the shape of a cylinder, then cut and rolled out on a smooth 

 plate It is naturally possessed of many defects, such as 

 bubbles and waves, as well as being uneven in thickness. 

 This glass is on the market as single-strength (S. S.), supposed 

 to be ^ inch thick, and double-strength (D. S.), supposed 

 to be £ inch in thickness. The quality of glass is indicated 

 as AA, A, and B. The B quality is the poorest that may be 

 marketed. It is used ordinarily in cellar sash. Regular 

 stock is cut to sizes of even inches, and is sold by the box, 

 as nearly 50 square feet to a box as the size of panes will 

 make. Single-strength is limited in size to 34 by 50 inches, 

 double strength to 60X70 inches. 



Plate glass. Plate glass is manufactured by a rolling 

 process and subsequently ground to a perfect surface. It is 

 to be had in thicknesses from ^to^ inch, and in any size 

 desired. 



Special glass. Special glass, such as crown glass, may be 

 had on order. It is made of better materials and more nearly 

 free from defects. Ground, prismatic, rib, and wire glass 

 may be had in any size up to 4 by 10 feet. It should be noted 

 that the price of glass, per square foot, increases very rapidly 

 with the size of the pane. 



Nails and screws. Nails. Nails are made in a variety 

 of materials and shapes and weights. They may be had of 

 steel, wrought iron, brass, or copper, and in finish plain, 

 galvanized, or cement-coated. The old style of cut nails 

 is no longer manufactured; they have been replaced by 

 the wire nail, though the latter has not the same holding 

 power. 



Nails are designated as "penny," which originally indi- 

 cated the number of pennyweights of metal which the nail 

 actually contained. The improved quality of materials has 

 allowed the reduction of weight, but the designation is 

 unchanged. Steel or wire nails run in weights from 2 to 60 



