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FAEMERS' INSTITUTES. 



extent, and will also tend to hold the ware in shape. Silica in the shape of 

 sand has been found to answer well for such purpose, and this sand does 

 better service if the particles are sharp, so that when the fire contracts it 

 they will uaite together by their sharp sides and not slide or roll one over 

 the other. 



Besides these elements there is usually found with clay many impurities, 

 and also combined with it, a greater or less quantity of other minerals. 

 There is almost always lime and iron oxide, and frequently magnesia. The 

 latter substances act as fluxes and in proportion to their quantity make the 

 clay more easy to be vitrified and fused. When they are absent the clay is 

 so little affected by heat that brick made from it will stand well in the hottest 

 furnaces. In fact such brick will stand more heat than any other material, 

 excepting some rare minerals. 



So that clay, inversely in proportion to its impurities as far as ordinary 

 means of producing heat is concerned, varies from absolute infusibility to 

 fusibility at a low temperature. 



This is about the extent of our scientific knowledge respecting the material 

 of brick making. You see it is exceedingly general and of little practi- 

 cal use in any particular case. We may tell you that a good brick clay may 

 contain one-fifth alumina, three-fifths silica and the remaining one-fifth made 

 up of water, iron, lime, magnesia, etc. Yet it is true that good brick are 

 made when the elements are in quite different proportions, and sometimes 

 even when a large quantity of organic matter, as sawdust or other material 

 is added. This, however, is doubtless removed in the burning. 



The following analysis of several clays gives the amount of the various substances 



entering into the clay : 



There is a great field for investigation by scientific men in this line. What 

 ought to be done is this: Brick or tile should first be made with pure clay, 

 then with successive additions of sand, then with additions of the other im- 

 purities; finding in each case the heat required to burn the brick, the heat 

 the brick will stand without breaking or fusing, the durability as respects 

 water and frost and finally its strength. 



