BUILDING CONSTRUCTION 129 



gauged stuff only small quantities may be mixed at a time, that is, 

 just as much as the workman can use before setting takes place. 



Plaster of Paris is obtained by the partial calcination of gypsum 

 (hydro-sulphate of lime), so that it parts with its moisture. It is used 

 for gauging plaster, running mouldings, and for forming castings 

 or enrichments to be applied to cornices, ceilings and other plastered 

 surfaces. It is invaluable for running arrises or rounded corners. 



There are a number of prepared plasters on the market, which 

 are usually sold in sacks or barrels ready for use. There is a 

 similarity in the composition of most. These prepared plasters 

 must be kept perfectly dry before use, otherwise they will be useless. 



Keene's Cement is a hardened form of plaster of Paris, which 

 sets rapidly and can be made to take a surface almost like china. 

 It is invaluable for forming angles, skirtings, and other ornamental 

 parts in exposed positions where strength is required. 



Portland Cement. Portland cement has become probably the 

 most important of building materials, being used for foundations, 

 floors, paving, building purposes and an endless variety of structural 

 and engineering works. It derives its name from a supposed 

 similarity in appearance to Portland stone. Chalk and clay are the 

 components of Portland cement. They are mixed together by the 

 " wet " process. The proportions of chalk and clay used depend 

 entirely upon the composition of the chalk before it is burned. It 

 is necessary to obtain a mixture containing from 25 to 30 per cent, 

 of clay. If pure white chalk containing no clay is used, three parts 

 of chalk are mixed with one part of alluvial clay or mud, the 

 measurements being by bulk. If the chalk itself contains a pro- 

 portion of clay, the proportion of clay added will be relatively 

 modified. The method followed is to mix the chalk and clay in 

 water till it attains the consistency of a creamy liquid known as 

 " slurry." This is allowed to settle in large tanks or reservoirs for 

 several weeks. When the deposit becomes nearly solid, the liquid 

 is run off and the solid dug out, dried over coking ovens, burned in 

 kilns at a high temperature, and afterwards ground to a fine powder. 

 London Portland cement, which is in world-wide demand, is made 

 principally on the Thames and Medway. The colour varies from 

 a bluish-grey to a light brown. It weighs from 110 Ibs. to 120 Ibs. 

 per Imperial striked bushel. An average weight for general use 

 may be taken as 1 12 Ibs. per Imperial striked bushel. The variation 

 in weight depends upon the degree of temperature and the duration 

 of burning, as well as upon the fineness of the grinding. 



The strength of the cement depends largely upon the fineness 

 of the grinding. The standard specification for Portland cement 

 9 



