C E M
CEM
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out fome of our domeftic treafures of tfie famematerial. The compact bafalt abounds in allthe diftri&s where coal is raifed, and may there-fore be procured eafily, and calcined with therefufe coal fo as to be fold at a cheap rate : theCalton hill adjoining to Edinburgh confillsalmofl entirely of cellular bafalt, and being buta fhort diftance from the port of Leith, offersan inexhauftible abundance at fmall coft.
In fome parts of the Low Countries coalafhes are fubftituted for tarras with very goodeffect, of which the valuable cendree de Taurnayis a linking inftance. 1 The deep blue argillo-ferruginous lime Hone of the Schelde is burntin kilns with a llaty kind of pit coal that isfound in the neighbourhood: when the calcina-nation of the lime is compleated, the pieces aretaken out, and a confiderable quantity of duftand fmall fragments remain at the bottom ofthe kiln. This refufe confifts of coal alh mixedwith about ^ of lime dull, from which the cen-dree is thus prepared. About a bulhel of thematerials is put in any fuitable veflel, andfprinkled with water juft fufficient to flack thelime; another bulhel is then treated in thefame way, and fo on till the veflel is filled : inthis Hate it remains fome weeks, and may bekept for a much longer time if covered with moillearth. A ftrong open trough, containing abouttwo cubic feet, is filled about two-thirds fullwith the cement in the above ftate, and bymeans of a heavy iron peftle fufpended at theend of an elaftic pole is well beaten for abouthalf an hour; at the end of this time it becomesof the confluence of foft mortar, and is thenlaid in the fhade from three to fix days accord-ing to the drynefs of the air; when fufficientlydry, it is beaten again for half an hour as before,and the oftener it is beaten the better will bethe cement; three or four times however arefufficient to reduce the cement to the conflu-ence of an uniform fmooth pafte; after thisperiod it is apt to become refraftory on accountof the evaporation of its water, as no more ofthis fluid is allowed to enter the compolition thanwhat was at Hrft employed to flack the lime.The cement thus prepared is found to poflefs thelingular advantage of uniting in a few minuteslo firmly to brick or ftone that flill water maybe immediately let in upon the work withoutany inconvenience; and by keeping it dry for24 hours, it has afterwards nothing to fear fromthe molt rapid current.
Somewhat analogous to the preceding is acement ufed in certain parts of England withadvantage, and called afh-mortar. It is pre-pared by flacking two bufhels of frelh burnt
meagre lime, and mixing it accurately with threebufhels of wood-afhes : the mafs i s to lie till itis cold, and is then to be well beaten: in thisftate it will keep a confiderable time withoutinjury, and even with advantage, provided it isthoroughly beaten twice or thrice before it isufed.*
The fcales of black oxyd which are detachedby hammering red-hot iron, and are thereforeto be procured at the forges and blackfmiths’ftiops, have been long known as an excellentmaterial in water cements, but we believe thatMr. Smeaton was the firft perfon who madeany accurate experiments on their efficacy com-pared with other fubflances. The fcales beingpulverized and fifted, and incorporated with thelime in the fame manner as puzzolana, arefound to produce a cement equally powerfulwith puzzolana mortar, if employed in the famequantity. Induced by the fuccefs of thefe ex-periments, Mr. S. fubftituted roafted iron orefor the fcales, and found that this alfo gave tomortar the property of fetting under water : itrequires however to be ufed in greater propor-tions than either tarras or puzzolana: twobufhels of argillaceous lime, two of iron ore,and one of fand, being mixed with the fame careas we have already mentioned, produce 3.22cubic feet of cement fully equal to tarras mortar.If the common white lime is made ufe of, itwill be advifeable to employ equal quantities ofall the three ingredients.
Nothing more remains to be faid on this partof the fubjecl but a few words concerning thechoice of the water by which the feveral ingre-dients of the cement are to be mingled together.River or pond water, where it can be had eafily,is to be preferred to fpring water, but for worksexpofed to the a£tion of the fea, fuch as piers,light-houfes, &c. it is ufually more convenient)and equally advantageous in other refpe£ls toufe fait water. The great point, and which istoo often negle£ted, is not to put in too muchof either one or the other : it is infinitely betterto employ no more than is requifi'te to flack thelime, and fo incorporate the materials at the ex-pence of a little more labour than by a fuperflu-ity of water to bring the whole to the properconfiftence without beating.
§ 3. Maltha or Majlich.
Under this term we include thofe calcareouscements of a more complicated kind whole hard-nefs appears to depend on the, oily and mucila-ginous fubflances that enter into their compo-fition. The ufe of thefe is at prefent verylimited, at leaft in Europe , but they were highlyefteemed by the ancients, efpecially for ftucco.