Qtri

357

QUI

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8tl ^ retains almost the whole of its fixed air.

And

■Ueti^j, m!ls ons have hardly any other rule for judging" 1estone be sufficiently calcined, except this single"poi, j a . nce of its falling to a powder when water is poured^ay b e ’ 11 may thus easily be perceived, that the same limeto a cjj. rnor e or less fitted for making good mortar, accordingaS Q f Umstan ce that, in a great measure, eludes the obser-ve pj e ° °Perative masons ; for if it should happen that alllime drawn from a kiln at one time were just8r >d „ l v calcined to make it fall to a powder with water,Sin- triore > that powder would be altogether unfit fordJ? mortar of any kind

mortar of any kind. This is a case that can seldomh ' V(i en but as there are a great many intermediate degrees^pp (;ri t , tba t state and perfect calcination, it must oftenV^at tbe stone will approach nearer to one of theseV j) e 8 at one time than at another; so that the mortar° 8 v at mUch more perfect at one time than at another, owingAll t ^ 1011 * n this particular.

h e at k,° Se wbo have written on the subject of lime as aendeavoured to ascertain what is the due pro-

l tle att° r San ^ ^ or ma ki n o the most perfect cement, uui »/Hi be^ 11 ^ 011 *° the matter will show, that all rules which,^1 Uri c .^ r ® Scr ibed as to this particular, must be so vague*' be a jj atn as to be of little utility to the practical mason ;

(] S l ' le variation which may arise from a more or lesst ■ * 8 om e e fr ee calcination as above, it is a certain fact,.'a a ainds of limestone are much more pure, and con-y 8 bein^r ampler proportion of sand, than others do ;

of

;«ri.

■tw m° Un< * almost perfectly pure, while others contain\ Th san< 3 . and all the intermediate proportions

^ ^efore, it would be absurd to say that pure lime^ n , e as small a proportion of sand, when made intolo ~ 8t Pro * . w hich originally contained in itself a muchpHosJf rt * on of sand than any writer has ever ventured..oeaij ° r being put into mortar.

there are variations caused by the different nature

M>cal c '

ln ati°n in the several sorts of limestone ; from‘sou may,

of n u P on the whole, be concluded, that about one-

^ m estone is not enough calcined to admit of

>ii -'hlestntf >nto mortar ; and that of the most impure sorts>t; 0 u ae. - •

^ 80

not above one-fourth part of the lime contained^ i Vat rach calcined as to be in a caustic state.

*l>e Pro0 lat ' On *hat is produced by these means, in regardHe or t°K ^ 0n san d that will be required to the lime int( lly t other case, is found to be so extremely great

ta. h® conceived. It is, however, stated, that the

W i ta r t 1 -

tf liNbee

n - J fta r ' -uu ceiveu. xi is, nowe1' baq i t la t has been seen made was formed of limeI* 118: to ^ oun d to contain eleven parts of sand to one°le bult* 3 tbere was added between twice and thrice,! f(j te 8 ^een a , ^ ®and by measure, which may be allowed tofetrf 8 app 0g - leas t three times its quantity by weight. There-1 8c ^y eii* 11 ® t;ba *' every particle of that lime had been so

^han'fk. b ?^ as h> he in a caustic state, there could not

^ aiay^^y-seven parts of sand to one of lime.\

As

.as ( g however, be allowed for the uncaustic part of theearj 11 >8 harfU S ' re< ^ an< ^ t ^ le calculation made accordingly.tbi a Possible to suppose that above one-hundredth(1 18 ca]„„ ass > independent of the water, consisted of pureS?C e a c reous earth.

\ ^ c i'i otls 'derations it is conceived, that it is impossible'''tii/'Ht determinate proportion of sand to lime, as% 8 Hl C j r ^ according to the nature of the lime, and otherto rCUmstance s. which would form an infinity of!h^he Sa ^any general rule. But it would seem that itVk inferred, that the moderns, in general, rather

y 8 *i b 0 ^ to ° httle sand than in giving too much. It- 1. )j ever > to be noticed, that the sand, when natu-

4 y

rally in the limestone, is more intimately blended with thelime than can possibly be ever effected by any mechanicaloperation ; so that it would be in vain to hope to make goodmortar artificially from pure lime, with such a small propor-tion of caustic calcareous matter as may sometimes be effectedwhen the lime naturally contains a very large proportion ofsand. But there seems to be no doubt, that if a much largerproportion of sand were employed, and if that were morecarefully blended and expeditiously worked than is common,the mortar would be much more perfect than is usual inmodern times, as has been proved by actual trials.

Another circumstance that tends greatly to vary the qua-lity of cement, and to make a greater or smaller proportionof sand necessary, is the mode of preparing lime before it isbeaten up into mortar. When for plaster, it is of greatimportance to have every particle of the limestone slakedbefore it is worked up ; for, as smoothness of the surface isthe most material point, if any particles of lime should bebeaten up in it, and employed in work before sufficientlyfallen, the water still continuing to act on them after it wasworked up, would- infallibly slake such particles, which,forcibly expanding themselves, would produce those excres-cences on the surface of the plaster commonly termedblisters. Consequently, in order to obtain a perfect kind ofplaster, that will remain smooth on the surface, and free ofblisters, there is an absolute necessity to allow the lime to liefor a considerable time macerating or souring in water,before it is worked up. And the same sort of process isnecessary for the lime, when intended for use as mortar,though not so absolutely. Great care is, however, requiredin the management in this respect; the principal thingsbeing the getting of well-burnt lime, and the allowing it tomacerate or sour with the water for only a very short timebefore it is used ; but that which is the best burnt willrequire the maceration of some days in the water before it issufficiently slaked in the whole mass for this purpose.

It has been almost universally admitted, that the hardestlimestone affords a lime that will consolidate into the firmestcement ; hence it is generally concluded, that lime made ofchalk produces a much weaker cement than what is madeof marble or limestone. It would seem, however, that ifever this be the case, it is only incidentally, and not neces-sarily so. As, from the nature of calcareous matter, everykind of lime is equally fit for becoming a firm cement, if itbe first reduced to a proper degree of causticity, and has after-wards a due proportion of sand properly mixed with it, beforeit be employed in work. Different sorts of lime, withoutdoubt, differ much from each other in the proportion of sandthey naturally contain, and, of course, require very differentproportions of sand to be added to them before they can bemade equally perfect as a cement; which is an economicalconsideration of no small moment, in some cases, as it maymake one sort of lime a great deal cheaper than another onsome occasions, and, of course, deserves the attention ofbuilders in general.

The excellencies and defects of other substances, that maybe occasionally mixed with lime, in making cement, may bejust noticed. Those commonly used as an addition to mortar,besides sand of various denominations, are powdered sand-stone, brick-dust, and sea-shells. And, for forming plaster,where closeness rather than hardness is required,, they uselime that has been slaked and kept long in a dry place tillit has become nearly effete, powdered chalk, or whiting, andgypsum, in various proportions ; besides hair and other mate-rials of that nature. But some others have been more latelyadvised, such as earthy balls, slightly burnt and pounded,powdered and sifted old mortar rubbish, and others of a