1850.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



185 



PREPARATION AND APPLICATION OF GYPSUM. 



On the Peculiar and Distinctive Character of the Gypsum fourul 

 near Paris, and its Preparation and ippliciition as a Plaster. By 

 Geokge R. Bur.vell, C.E. — (Paper read at the Royal Institute 

 of British Architects, April 8th.) 



Amongst tl»e local advantag^es enjoyed hy our professional 

 brethren of the French capital, that afforded hy the unlimited 

 supply of a very superior description of plaster may he ranked 

 as one of the most important. The facilities afforded hy the 

 railway, and steamboat, transit having at length put us (to a 

 certain extent) upon a footing of equality in this matter with 

 them, it becomes important to examine the nature of the ma- 

 terial thus offered for our use. 



Regarding plaster mechanically, it may be considered as a 

 species of lime, which is susceptible of being- employed without 

 admixture with any other ingredient than water; and of attaining 

 with singular rapidity, a moderate degree of hardness. These 

 qualities would render its employment in all cases very desirable, 

 were it of a nature to resist the influences of the atmosphere. 

 But unfortunately it is utterly incapable of resisting the effects 

 of humidity, when used alone. ^ 



Chemically, plaster may be defined, in a manner able to include 

 all its varieties, as being a dehydrised sulphate of lime, or that salt 

 from which the water of crystallisation has been driven off by heat. 

 The sulphate of lime is very plentifully distributed through 

 nature, in numerous positions, and in very large quantities. " It 

 is found either crystallised, fibrous, massive, or earthy: the varie- 

 ties which assume a definite crystallisation are distinguished by the 

 name of selenite; those which are amorphous, or earthy, are 

 known hy that of gypsum : the names are, however, frequently 

 confounded. Wlien crystallised it assumes the form of a straiglit 

 prism of a rhomboidal base, whose angles vary from 113° 5' and 

 60° 3', to 113° 3', and 66° 52', terminated by oblique angled prisms. 

 The natural joints are very visible; the crystals are generally trans- 

 parent with a shining pearly lustre; and are of various shades of 

 white, yellow, grey, brown, red, or violet colour. Sulphate of 

 lime ismuch softer than the carbonate, and it yields easily to 

 the nail. Its specific gravity is about 2'31. When pure it con- 

 tains 32"7 per cent, of lime; 46'3 of sulphuric acid; and 21 of 

 water. 



The crystallised selenite is found at Alston, in Cumberland, and 

 in great al)undance at Shotover-hill, in Oxfordshire. It also occurs 

 in considerable quantities in all argillaceous deposits, in detached 

 crystals, but hardly ever in veins. It is said occasionally to tra- 

 verse fissures in the primary rocks accompanied by mineral veins. 

 In Derbyshire, and some of the mines of the Hartz and Hungary, 

 it is found in remarkably long slender fibres, which are generally 

 associated and curved. At Matlock, a variety with straight fibres 

 is met with, which is of remarkable brilliance and beauty. 



The massive sulphate of lime is termed alabaster, on account of 

 its resemblance to the material properly so called, although this is 

 in fact a stalactical carbonate of lime. The real, or oriential, ala- 

 baster was much used by the ancients for the purpose of statuary, 

 and was extracted in large ijuantities from the mountains of Upper 

 Egypt. The variety of the sulphate of lime, which is at present 

 used under that name, is principally obtained at Mount Cenis. 



Granular massive gypsum is found ovei'lying the most recent of 

 the primitive rocks, and sometimes it is said, enclosed by them. 

 It is found in Siberia mingled with mica, felspar, and serpentine; it 

 occurs between two beds of gneiss near St. Gothard, and also at 

 Bellinziua in the Alps, and also near the Mount Cenis; and at 

 Moutier, near Mont Blanc. It generally accompanies the car- 

 bonate of lime formations; and is largely found in connection 

 with the saliferous system. In Scotland, it covers the transition 

 rocks; in Derbyshire and in the midland counties, the gypsum is 

 also found in connection with, or contiguous to the salt rocks; in 

 the nortli of Spain, and in Tuscany, the same co-relation is to be 

 observed. The gypseous and saliferous formations of the Pyrenees 

 are, equally with the analagous formations in England, of the 

 secondary series; those of Tuscany are of the older pliocene era; 

 whilst tlie most important deposits as the sulphate of lime, namely, 

 those near Paris, are of the eocene formations, according to Sir C. 

 Lyell's classification. It is, however, to he observed that the 

 secondary strata, with which the gypseous rocks are' connected 

 in England, are of the early secondary divisions; whilst in Spain 

 they are only found associated with the chalk, or occasionally with 

 a formation similar to the tertiary sub-Appenine rocks. 



The intimate connection which exists in the great majority of 



cases between the gypseous and the saliferous rocks, is a subject 

 which appears to merit a more elaborate investigation than it has 

 hitherto received. Whenever rock salt is met with, in either the 

 secondary or the tertiary deposits, the gypsum always accompanies 

 it. in an infinite variety of forms. It is true that in some cases the 

 gypsum is met with unaccompanied by the salt; but then the absence 

 of the latter would appear to be accidental, excepting in the Paris 

 basin. The gypsum is found in the same positions; the rocks pre- 

 sent the same appearance; and the strata have the same structure. 

 For instance, at St. Leger sur Dhune, in the department of the 

 Soane and Loire, the gypsum is found alone in the marnes iris'es, 

 or the upper new red sandstone and red marl of our technicology ; 

 they are also found near Aix, in Provence, in tertiary strata. 

 But in both these cases they assume the forms, and all the litho- 

 logical characteristics of the saliferous system; so much so in fact 

 as almost to warrant the term of salt rocks without savour. The 

 structure of the saliferous gypsums is undulated and manimillated ; 

 their texture is fine, compact, often crystalline, and they differ 

 from what are considered as sedimentary deposits of gypsum, such 

 as those of the environs of Paris, by a degree of whiteness, and 

 purity from extraneous ingredients, which fully warrants the sepa- 

 rate classification of the gypsums into those connected with the 

 salt deposits, and those which are purely and simply sedimentary. 

 One very remarkable appearance is presented hy the saliferous 

 variety which never occurs in tlie sedimentary; namely, we often 

 find in it masses, the centre of which is composed of the anhy- 

 drous gypsum, whilst the exterior only has hydratcd; as though 

 the total mass had been formed in the anhydrous state, and the ex- 

 terior had combined with the water at a subsequent period. 



We have an excellent opportunity of tracing on a small scale 

 the class of geological phenomena which accompanied the forma- 

 tion of the saliferous gypsum in the duchy of Tuscany. Near 

 Volterra and Castellana, are found some of the purest of the false 

 alabasters, which are principally worked at the latter district. It 

 is found in glandular masses, enclosed in three beds of a greyish 

 crystalline gypsum, which somewhat resembles the beds of the 

 same nature near Paris. The masses found near Castellana are 

 the purest, and present in the highest degree the whiteness and 

 translucence which are sought for in the modern use of alabaster. 

 At Volterra they are less pure, and are found dispersed in the grey 

 and blueish marls, known under the name of "mattajone." These 

 marls are very much contorted; and at Volterra itself they are in- 

 clined, and upraised. They belong to the sub-Appenine formations 

 of the tertiary period; and occasionally they give rise to brine 

 springs, which have led to the formation of large salt works. 



The most remarkable feature of the saliferous formation of 

 Tuscany is the purity and the mass of the gypsum. The whole 

 formation is evidently stratified; whether we find the gypsum in 

 detached rounded masses, with mamillary faces, enclosed in the beds 

 of marl, and succeeding one another at irregular distances, in the 

 diiection of the stratification, like the nodules of septaria in the 

 London clay; or whether it constitute thick beds intercallated be- 

 tween the marls, and exposed to all the accidents of stratification 

 which affect them. In no cases are the marls affected by purtuiba- 

 tions, or alternations, which might lead us to suppose that the gyj)- 

 sum had been introduced subsequently to their deposition. The 

 gypsum is evidently stratified, and contemporaneous vvith the 

 mattajone; the amygdalordal character of the nodules can then 

 only be attributed to the affinity of the molecules, brought into 

 action by specific causes which affected the waters in which they 

 were in suspension. 



The rock salt appears to be disposed like the gypsum, according 

 to the lines of stratification of the whole formation. It is worked 

 from wells; one of which, executed near the factory called "Move," 

 presents the following beds: — 



ft in, 



1. Blue marl, containing nodules of alabaster, which is about (io thichness) 144 4 



2. Rocltsalt 15 7 



a. Marl, witli gypsum 19 7i 



4. Saliteroui marl, about (in thickness) 14 I 



5. Bine marl „ S7 ^» 



fi. Saliferous marl ,, 29 *> 



7. Gypseous marl „ 26 IJ 



8. Rocksall,(greatcstdeposit) „ 41 



9. Blue marls „ IG6 S 



Now the nature of the causes which led to this intercallation of 

 the salt and gypsum between the strata of this formation (one of 

 the most recent of the saliferous deposits), appears to be inti- 

 mately connected with the existence of tlie lagoni of Tuscany, 

 which are the last traces of a series of phenomena acting, in all 

 probability, with much greater energy at the epoch of the deposi- 

 tion of the tertiary strata. These lagoui are eruptions of aqueous 



26 



