CHAMBERS'S INFORMATION FOR THE PEOPLE. 



mixed. Common mortar, although it stands the 

 effect of water very well when perfectly dry, yet 

 occupies a considerable time in becoming so, and 

 dissolves or crumbles away if laid under water 

 before it has had time to harden. 



The general term Roman has been applied to 

 that class of hydraulic cements made from the 

 natural argillaceous limestones found in Britain, 

 and especially to that formed by burning the 

 nodules of septaria found at Harwich ; these con- 

 tain from 20 to 25 per cent, of clay, without which 

 they would be useless for hydraulic cements, and 

 would not possess the property of hardening in 

 water. 



Several artificial hydraulic cements are now 

 in use, the best known of which we may here 

 shortly describe. Portland Cement : This import- 

 ant cement is 'made from carbonate of lime, 

 mixed in definite proportions with the argillaceous 

 deposit of some rivers running over clay and 

 chalk, together under water, and afterwards dried 

 and burned. The strength of the composition is 

 very remarkable, being nearly four times as great 

 as that of any other kind. Portland cement makes 

 an admirable and most powerful concrete, the 

 proportion of cement required being only a tenth 

 or a twelfth part.' Natural substances are also 

 frequently used for the formation of hydraulic 

 cement. Of these, puzzolana is the most notable. 

 This was used by the Romans in building the 

 foundations of their quays, artificial islands, &c. 

 It is chiefly obtained from Puteoli, or Puzzuoli, 

 near Naples, whence its name. This earth is a 

 light, porous, friable mineral, various in colour, 

 and evidently of volcanic origin ; mineralogically, 

 it may be designated a calcined ferruginous clay. 

 When reduced to powder by beating and sifting, 

 and thoroughly mixed with lime, either with or 

 \vithout sand, it forms a mass of great tenacity, 

 which in a short time cements to a stony hard- 

 ness, not only in the air, but likewise when wholly 

 immersed in water. Dutch trass is a somewhat 

 similar substance, which used formerly to be 

 imported from Holland, where it is extensively 

 used in hydraulic works. It is made from a 

 light vesicular lava found near Andernach, on the 

 Rhine. 



The Selinitic cement of Major-general Scott is 

 the newest introduction, and is rapidly coming into 

 favour. It contains gypsum, but the mode of its 

 manufacture is not made known. 



Plaster, or the material which is used to spread 

 smoothly over walls, is of various kinds. That 

 which is applied to inner walls or partitions, is 

 formed of certain proportions of slaked lime, fine 

 sand, and water, with mixture of cow-hair, to 

 assist in giving cohesion. The best plaster is now 

 prepared by the pug-mill, by which the ingredients 

 are more thoroughly incorporated than by the old 

 process of hand-beating. Spanish white, ochre, 

 and other colouring matters are added when any 

 peculiar tint is wanted ; but we may here remark, 

 that the most durable of all plasters, and that 

 which answers best even for fresco-painting, is 

 composed simply of well-slaked lime and sifted 

 river-sand. The surface of plaster is now seldom 

 finished with a view to permanent exposure 

 whitewashing, sizing in colours, oil-painting, and, 

 above all, papering, being the prevalent fashions 

 of the day. 



Stucco is the name ordinarily given to plaster of 



366 



Paris, which is gypsum reduced to a powder by 

 heating and grinding ; but the term stucco is further 

 extended to embrace all those compositions with 

 which walls are coated or ornamented, in imitation 

 of stone. Gypsum is properly a sulphate of lime ; 

 and, like all other varieties of lime, it has a strong 

 power of absorbing water. The practice is, to put 

 the masses into a heated oven, and when duly 

 baked, to take them out, and grind them to powder 

 in a mill. This powder, when sifted, is a beautiful 

 white substance, resembling flour. A quantity of 

 powder being put in a vessel, water is poured 

 upon it, and immediately the stuff thickens in a 

 surprising manner, and soon becomes a hardened 

 mass. While still thickening or setting, it is poured 

 into a mould for any required shape ; or it may be 

 applied along with a little lime as a fine plaster, 

 which it is desirable should dry speedily. It is 

 used largely for all kinds of casts from pieces of 

 sculpture, mouldings for cornices, &c. There are 

 none of the artificial stuccoes which yield so sharp 

 or delicate a cast ; but most of them excel it in 

 hardness and durability. Gypsum, or sulphate of 

 lime, is the basis of all the cements known as 

 Keene's, Martin's, Parian, &c. In these, the 

 ' plaster in the state of fine powder is thrown into 

 a vessel containing a saturated solution of alum, 

 sulphate of potash, or borax. After soaking for 

 some hours, it is removed, and air-dried, and sub- 

 sequently rebaked at a brownish red heat.' It is 

 finally reduced to a fine powder, which, when 

 required for use, is mixed with a solution of alum. 

 ' When borax is used, the plaster is called Parian, 

 but where sulphate of potash is employed, it forms 

 Keene's Cement. The kind called Martin's Cement 

 is made with pearl-ash as well as alum, and is 

 baked at a much higher heat than the rest.' All 

 these cements are greatly used, and are particu- 

 larly valuable ( when a hard durable substance is 

 required, not affected by damp, and perfectly safe 

 from the attacks of vermin.' They are worked 

 with great facility. 



Scagliola is the Italian term for a composition 

 intended to represent various marbles, porphyries, 

 serpentines, &c. It is composed of fine plaster of 

 Paris, with colouring matters, mixed with glue or 

 isinglass, and is sometimes studded with chips of 

 alabaster, &c. to imitate verd-antique, &c. It is laid 

 on like common stucco, moulded into the desired 

 forms, and allowed to set. When thoroughly set, 

 it is smoothed with pumice-stone, and washed ; 

 then polished with tripoli and charcoal ; next with 

 tripoli and oil ; and finally with pure oil, laid on 

 with cotton wool. The result is a surface of 

 unusual richness ; but from the nature of the 

 ingredients, it is only fitted for internal decoration, 

 and even then requires to be kept dry. 



Bituminous Cement. This is best known by the 

 name of Asphalt. Asphalt, or asphaltum, is a 

 bituminous mineral, allied in its nature to pitch, 

 and is found in the form of rocky masses in differ- 

 ent parts of the world. For pavement, it must be 

 mixed with sifted gravel, pounded iron slag, or 

 river-sand, which gives it more stability, and a 

 degree of roughness that is not unnecessary. The 

 composition is prepared in portable boilers or 

 caldrons, and spread while hot on a properly pre- 

 pared bed ; and being rendered smooth on the 

 surface, it offers an exceedingly agreeable resist- 

 ance to the foot, being not so hard as stone, nor 

 so soft as a mud pathway. 



