98 GREENHOUSES 



from 40 to 50 per cent.; and blue, from 50 

 to 80 per cent. 



Glass known as A, or first grade, is blown 

 from the top of the retort and is of bet- 

 ter quality than the B, or second grade,, 

 which may contain some foreign matter or 

 settlings. Some of the less regular panes 

 from the first blowing and those containing 

 small air bubbles are also placed in the B 

 grade. When it is essential that the great- 

 est possible amount of light be had and tight 

 glazing is necessary, A grade is used. 



In most commercial constructions B grade 

 will give satisfactory results. Poorer grades 

 are not satisfactory for greenhouse work. 

 The cost of B grade is about 85 per cent, of 

 the price of A grade. Both A and B grades 

 may be had in two weights or thicknesses, 

 known as single-thick and double-thick. 

 Single-thick runs about 12 panes to the inch 

 and weighs from 19 to 21 ounces per square 

 foot. Double-thick runs about 8 panes to 

 the inch and weighs from 26 to 29 ounces 

 per square foot. Double-thick is almost 

 always used when the panes are more than 

 8 x 10 inches in size. It obstructs but little 

 more light and is much more durable, 

 especially against hail. 



