Building Materials, Their Use and Origin 405 



those used because they are readily accessible ; those because 

 they are easily worked ; those because of durability, strength, 

 fire-resistance ; those brought from other places ; those used 

 for beauty or ornamentation. 



Exercise. Study the different kinds of wood used in the construc- 

 tion of a modern building as a house, school, or church, and give 

 reasons for their selection for each of these uses : structural parts as 

 supporting timbers, rafters, and floor joists; doors, windows, and 

 other parts of interior finish; roof. How much of the work of pre- 

 paring the materials for their present use was done on the ground? 

 How much in a mill or factory? How much was done by hand? 

 by machine? 



190. Lumbering. In olden times buildings were mostly 

 constructed by slaves and with little regard to the labor in- 

 volved. Materials were prepared largely on the ground and 

 by hand. But in our day the economy of materials and con- 

 struction must be considered. Consequently a great deal of 

 the material is prepared in factories or mills by machinery 

 and made practically ready for the builder to put into 

 place. 



The changes in transforming tree trunks into suitable ma- 

 terials for buildings include four groups of operations : first, 

 the logging, which comprises felling the tree, preparing the log, 

 and transporting it to the sawmill ; second, the lumbering, 

 which includes sawing the log into commercial dimensions and 

 generally the drying and surfacing of the sawed lumber ; third, 

 the mill work in which much of the finishing is done of doors, 

 sashes, windows, and the like ; fourth, the assembling and 

 fitting together of the parts by carpenters. 



191. Preserving wood. The chief objections to the use 

 of wood in building are its combustibility and liability to early 

 decay. Neither of these can be wholly prevented by any 

 known means. For the preservation of wood from decay such 

 substances as paint, wood tar, coal tar, and creosote are gen- 

 erally used. Paint is a mixture of oil, such as linseed oil, and 



