Building Materials, Their Use and Origin 417 



IV. SOME MANUFACTURED MINERAL PRODUCTS 



198. Lime. Lime, whether used for plastering, or with 

 brick and stone, is indispensable in building. Lime manufac- 

 ture is an important industry, the annual production amount- 

 ing in value to more than fifteen million dollars. The lime of 

 commerce is made from limestone, fragments of marble, and 

 sometimes from shells, all of which 

 are grouped in chemistry under the 

 name of calcium carbonate. 



The manufacture of lime is carried 

 on by means of the lime kiln, the 

 essential features of which are illus- 

 trated (Fig. 141). Limestone is quar- 

 ried and broken into pieces of suitable 

 size, and then put into the kiln, where 

 they are heated with free access of 

 air (how does this differ from charcoal 

 making?) so that the carbon dioxid is 

 driven off. The product is in the form 

 of hard, dry lumps, called quicklime 

 or calcium oxid. 



Quicklime is prepared for use in 

 two ways. It may be air-slaked or 

 water-slaked. In air-slaking it absorbs 

 moisture and carbon dioxid from the 

 air, becoming lime carbonate, a fine 

 white powder which is useful for ferti- 

 lizing. In water-slaking, it combines 

 with water more rapidly, forming a product generally used 

 for mortar, plastering, and other building purposes. 



Exercise : Slaking lime. The process will be more clearly under- 



stood by slaking a small quantity of quicklime and observing the 



changes which occur. The following is only suggestive in order to 



enable you to make exact observations. Put 12 ounces by weight of 



2 E 



Gas " 



FIG." 141. Cross section 

 "* 



