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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[December, 



ON THE MEANS OF PREVENTING THE APPEARANCE OF 

 SALTPETRE ON WALLS. 



Communication from C. H. Smith, Esq., in reply to Questions proposed to 

 him by the Commissioners on the Fine Arts, respecting Causes of and 

 Means of Preventing appearance of Saltpetre on Surface of Walls. 



The mineral substances chiefly used in building, consist of lime, sand, and 

 different kinds of stone, neither of which contain any saline or deliquescent 

 matter as an integral part of their composition. No trace of salt or alkali is men- 

 tioned in the analyses of various stones that were examined with reference 

 to the selection for building the New Houses of Parliament. Bricks are made of 

 clay, which consists principally of alumina and silica, hut generally containing 

 some portion of lime, in the state of carbonate or sulphate ! carbonate of mag- 

 nesia; iron in the state of oxide, or combined with sulphur; and commou 

 culinary salt : these various materials, when exposed to a red heat, act che- 

 mically on each other ; the magnesia most probably will combine with the 

 sulphuric acid, which it obtains partly from the iron pyrites mixed with the 

 clay, and partly from the fuel, if coal is used. It is this sulphate of mag- 

 nesia (common Epsom salt) which is occasionally found to cover the surface 

 of newly built walls with an efflorescence like hoar frost. 



The presence of saline or deliquescent matter on the surface of a building, 

 either internally or externally, may, to a certain extent, be attributed to 

 carelessness, ignorance, or inattention of those who superintend the con- 

 struction of an edifice. Salts, alkalies, or acids, according to the usual ac- 

 ceptation of such terms, do not necessarily form any part whatever of build- 

 ing materials. Nearly all animal and vegetable substances, when in a state 

 of putrefaction or decay, produce a certain quantity of saline or alkaline mat- 

 ter, which absorbs moisture rapidly; therefore every precaution should be 

 to avoid admitting such substances into a building where damp walls 

 are likely to be of serious importance. It has long been a practice among 

 builders to "parget "all the flues in a building: for this purpose cartloads of 

 excrement, frequently of many kinds, are procured from a cow-shed, and 

 mixed with a little mortar, to put a coating throughout the interior of the 

 chimneys. Another objectionable practice is common during the time 

 that the carcase of a building is progressing, and partially until the " finish- 

 ing" is nearly completed, which is that of allowing workmen to urinate in- 

 discriminately against the angles and recesses of the interior of a new build- 

 ing : no pari is more frequently selected than the fire-places before the stoves 

 are placed therein; anil in an extensive building, where hundreds of work- 

 men are employed during several years, the quantity must be quite sufficient 

 to saturate certain parts of the structure beyond all remedy. Both these 

 causes undoubtedly increase the presence of salts, &c, on such parts of the 

 interior of buildings as are elevated above the influence of the ground. To 

 show that dung and urine have long been considered to yield saltpetre abun- 

 dantly, a proclamation of Charles I., in 1625, ordered all persons to save the 

 urine of their families, and as much as they could of that of their cattle, to 

 supply saltpetre for the manufacture of gunpowder; and in 1627 the salt- 

 petre-makers were authorized to take away the ground of all dove-houses, 

 stables, lairs, or others places where cattle were kept. There are many 

 other sources bj which salts are conveyed into or communicated to the walls 

 of a building, but those already mentioned appear to he the most copious, 

 and which may be considerably decreased. 



Under ordinary circumstances it is scarcely possible to get rid of the vari- 

 ous saline or deliquescent substances that have once been admitted into the 

 walls of a building. The fixed alkalies (potash and soda) may probably be 

 considered imperishable; no length of time can injure them ; they may efflo- 

 resce, or, more properly, the) may crystallize on the surface of a wall, and 

 totally or partially disappear again and again, as often as a change in tempe- 

 rature or of dryness or humidity takes place ; these changes may he daily, or 

 the salts may remain inactive (luring ages, and, from some favourable cause, 

 a crop of crystals may be produced as flourishingly as if the wall had been 

 recently erected. The only way to abate the evil, is to brush off the crys- 

 tals, dry, whenever they appear in the most flourishing condition. If potash 

 has been introduced into the walls, either from the putrefaction of animal or 

 vegetable substances, such as have been named, or from other sources, bow- 

 ever thick the wall may be, it will make its way to the surface, and then 

 absorbing nitrogen from the atmosphere, which contains 7(1 or 80 per cent, 

 of it, nitrate of potash (or saltpetre) is produced. 



If we may imagine the possibility of salts in a crystalline state getting to 

 the interior of a dry wall, beyond the influence either of moisture or consi- 

 derable variation ot temperature, in such case they would unquestionably 

 remain crystallized as they were deposited ; hut such a state of things is never 

 likely to take place : salts are generally communicated to a building in weak 

 solutions; the water very gradually evaporates, carrying with it, from the 

 interior of the wall, the molecules which con-pose salt. The solution having 

 arrived at the surface, so as to be freely in contact with the atmosphere 

 (which is always essential to crystalization), evaporation continues until the 

 solution is sufficiently strong to crystallize, leaving only the mother-water in 

 the wall, which is indicated by a certain dampness. 



Lime, mortar, or some other sort of calcareous earth, seems to act as a 

 vivifying principle to set the molecules of salt and water in action; if no lime 

 were present, crystallization would certainly be much less active. An in- 

 crease of temperature, or a humid atmosphere, will slowly dissolve the salts; 

 if both th se causes occur at the same time, liquefaction will be rapid, and 

 the newly formed fluid will be absorbed into the wall as fast as the salts are 



dissolved. These changes will take place with every variation oj atmosphere: 

 a cool dry air, in a state of absolute rest or stagnation, is favourable to crys- 

 tallization ; a warm one, charged with aqueous vapour, will facilitate solu- 

 tion. It is extremely probable that several kinds of salts may be formed on 

 the same wall, with their crystals intermixed so as to escape the discrimina- 

 tion of a casual observer, and that each will crystallize and liquefy at diffe- 

 rent times, according to the temperature and the quantity of moisture in the 

 atmosphere : should, this be the case, perhaps the wall may never appear per- 

 fectly free from efflorescence, so long as the various stimulants of air, mois- 

 ture, light, heat, and other causes of attraction are in activity; and, since all 

 attraction is mutual, it may readily be understood, that as the particles of 

 water attract those of the alkaline sal£, and retain them in solution, so the 

 particles of alkaline salt will attract those of the water, and hold them in 

 crystallization. 



It is difficult to state with precision the relative power of different bodies 

 to attract moisture from the atmosphere ; that such power exists indepen- 

 dentlv of temperature is scarcely probable, as thermal influence appears ge- 

 nerally diffused over the face of nature. Some substances are more suscep- 

 tible of sudden changes of temperature than others, and thereby may occa- 

 sion a rapid precipitation of vapour, from the serial or invisible state in which 

 it exists in a warm atmosphere, to the fluid form on the surface of cold bo- 

 dies ; this circumstance arises solely from the solid mass of the wall requiring 

 a much longer time to attain the same elevated temperature as the atmos- 

 phere. Bodies in contact with each other in due time arrive at one common 

 temperature, by the hotter communicating the requisite proportion of the 

 excess of its heat to the colder ; the velocity of this communication varies in 

 different bodies, some being quickly heated, and as quickly cooled ; others 

 undergoing these changes much more slowly. It is probable that the atoms 

 or completely solid parts of all simple substances have exactly the same ca- 

 pacities for heat, and that the perfect or imperfect conducting power of sub- 

 si, meet will be proportioned to their porosity, sponginess, or the quantity of 

 vacant space contained in their interstices. Deuce bodies are generally the 

 best conductors of beat; those which are the most porous, conduct it very 

 imperfectly ; the metals, which arc substances of the greatest density, trans- 

 mit beat most rapidly; stones and earthy substances conduct it more slowly; 

 wood is a bad conductor; aud the natural clothing of animals — fur, hair, 

 featners, &c., are inferior to every other material in their power of commu- 

 nicating heat. These remarks are applicable only to the conducting power 

 of solid substances ; liquids are all very bad conductors of heat ; therefore, 

 independently of evaporation, a cold damp wall will continue at a low tem- 

 perature much longer than a cold dry one; and hence it will influence the 

 condensation of vapours during a greater length of time than if it were dry. 



\ si inns circumstances seem to infer the probability that voltaic electricity, 

 considered as a chemical agent, may act some part in conveying moisture 

 from the atmosphere to the walls of a building. All substances naturally 

 possess electrical energies, which are inherent in them; probably there may 

 not be two substances, or even two distinct surfaces of the same substance, 

 that are not in different electrical relations to each other ; and it is a law of 

 electricity that bodies in opposite states attract each other. Lime, sand, bricks, 

 and hair, materials with which walls are usually constructed and plastered, 

 are all, when dry, bad conductors ; whereas water is a good conductor of 

 electricity; and whenever the atmosphere, or water, or any part of the sur- 

 face of a body, gains accumulated electricity of a different kind from the con- 

 tiguous substances, there is an immediate tendency to bring the parts in 

 contact. In this manner, other circumstances being favourable, floating 

 aqueous vapours may perhaps be imparted to a wall, and absorbed into it 

 by capillary attraction. 



Electric influence, as connected with the preceding inquiry, is merely offered 

 as a hint, with the view of inducing scientific men to investigate the sub- 

 ject. Hitherto the public are not in possessiou of any facts which have im- 

 mediate reference to this important object. 



C. II. SMITH. 



2D, Clipstone Street, June 2, 1843. 



. OPEN SEATS IN CHURCHES. 



So great is the present demand for good models of open seats, consequent 

 upon the strong and fast-spreadirrg dislike and condemnation of pews, and 

 so numerous are the applications made to us for advice and assistance in 

 restoring to churches their more Catholic arrangement for the accommoda- 

 tion of worshippers, that we propose to offer a few remarks on the. proper 

 proportions of this kind of seats, and the best method of fixing and disposing 

 them. 



The standards or ends of ancient open seats are generally distributable 

 under three heads : (1) those which are sloped off with a shoulder and ter- 

 minate in a boldly carved finial : (2) those that have an elbow, generally (as at 

 Chesterton and Histon churches,) wrought into the device of a cumbent lion, 

 dog, or griffin, but sometimes (as at Ketton, Rutlandshire) having a plain curva- 

 ture ; (3) those which are simply square or parallelogrammic panels. The last 

 are usually the latest in date, though it may be observed in general of open 

 seats, that they scarcely ever bear the marks of great antiquity. Whether 

 any are to be found as early as the fourteenth century we do not know ; but 

 there is every reason to believe that, till the conclusion of the Edwardian 

 period, the area of churches was quite free and unincumbered, and that if 



