FICTILE MANUFACTURES. 



mosaic-work, so as to bring out the representation 

 desired. The discovery of this art is ascribed to 

 a painter in Marseilles, who went to Rome during 

 the pontificate of Julius II. It was afterwards 

 greatly improved by the celebrated Albeit Diirer 

 and Lucas of Leyden. 



All the pigments used in painting on or staining 

 glass are oxides of metals as gold, silver, cobalt, 

 manganese, &c. which, after being laid on, are 

 subjected to a strong heat, until they penetrate 

 into the body of the glass, or become fixed on its 

 surface, and thus give out their fullest brilliancy 

 and transparency. The colours that are meant 

 to penetrate into the glass for the purpose of 

 staining it, are wholly transparent, while those 

 which are merely fixed upon the surface are only 

 Semi-transparent. Any colour or tint can be com- 

 municated to the glass in this way, and the art 

 is at present practised with great success. The 

 description of glass best adapted for painting 

 upon or staining, is the finest crown or window 

 glass. 



But coloured- glass for windows is of two kinds, 

 called pot-metal and flashed. The first is glass in 

 which the colour is mixed in the melting-pot from 

 which it is taken, blown, and formed into sheets 

 either as crown or sheet glass. Flashed glass is 

 prepared by first dipping the blow-pipe into a pot 

 of ordinary colourless glass metal, and having 

 gathered a proper bulb, it is dipped into a pot 

 of coloured metal, so as to be thoroughly coated 

 with it. It is then blown and formed into sheets 

 as before, and is found to have a pretty equal 

 quantity of each kind equally spread, so that it is 

 in fact a double sheet of glass. This admits of 

 many colours being used which as pot-metal would 

 be too dense, and also of various pretty methods 

 of ornamentation ; for, if any portion of the 

 coloured side be ground or etched off, we get a 

 mixture of plain and coloured material, which, in 

 the hands of skilled workmen, may be rendered 

 very effective. 



' The substances employed ' we quote from 

 ParnelFs Applied 'Chemistry 'for rendering colour- 

 less and some coloured glasses more or less opaque, 

 like enamel, are phosphate of lime, fluor-spar, 

 arsenious acid, peroxide of tin, phosphate of lead, 

 and phosphate of antimony. Phosphate of lime, 

 which is the only one of these materials commonly 

 employed at present, with the exception of fluor- 

 spar, is introduced in the form of finely powdered 

 calcined bones, to the amount of one-twentieth to 

 one-thirtieth of the weight of the glass. A very 

 beautiful opaline crystal is obtained in this way.' 



PASTES ARTIFICIAL GEMS. 



In gem-sculpture, paste is the term for a pre- 

 paration of glass, calcined crystal, oxide of lead, 

 and other ingredients for imitating gems. This 

 art appears to have been well known to the 

 ancients, and after being lost, was restored, at 

 the end of the fifteenth century, by a Milanese 

 painter. The general base of artificial gems is a 

 vitreous compound known as the ' Mayence base' 

 or Strass (from the name of its inventor). It is 

 prepared, according to Fontanieu, in the following 

 manner : 8 ounces of pure rock-crystal, or flint, in 

 powder, mixed with 24 ounces of salt of tartar, 

 are to be baked, and left to cool. The mixture is 

 to be afterwards poured into a basin of hot water, 



and treated with dilute nitric acid till it ceases to 

 effervesce ; and then the frett is to be washed till 

 the water conies off tasteless. This is to be dried, 

 and mixed with 12 ounces of fine white-lead, and 

 the mixture is to be levigated and elutriated with 

 a little distilled water. An ounce of calcined borax 

 being added to about 12 ounces of the preceding 

 mixture in a dry state, the whole is to be rubbed 

 together in a porcelain mortar, melted in a clean 

 crucible, and poured out into cold water. This 

 vitreous matter must be dried, and melted a second 

 and a third time, always in a new crucible, and 

 after each melting, poured into cold water, as at 

 first taking care to separate the lead that may 

 be revived. To the third frett, ground to powder, 

 5 drachms of nitre are to be added ; and the 

 mixture being melted for the last time, a mass of 

 crystal will be found in the crucible of a beautiful 

 lustre. 



A base being thus prepared, the peculiar colours 

 are obtained from the metallic oxides, which, in 

 proper proportions, under the hands of an ex- 

 perienced manipulator, are said to yield imitations 

 so like the natural gems, that none but lapidaries 

 or mineralogists could detect the deception. 



The ancient Greeks were very skilful in making 

 pastes by joining portions of white or tinted opaque 

 glass to glass variously coloured. They imitated 

 the onyx, and used the imitation for cutting 

 cameos, some of which are remarkable for their 

 great beauty. The celebrated Portland Vase was 

 made in a somewhat similar manner. 



CEMENTS ARTIFICIAL STONES. 



Under this section we rank those compositions 

 generally known as cements, mortars, concretes, 

 plasters, and stuccoes. 



The mortar or cement employed to unite stones 

 and bricks into a compact mass in building, is 

 composed of quicklime, sand, and water. Quick- 

 lime is procured by roasting or calcining limestone 

 in kilns, into which moderate-sized fragments of 

 the rock are placed in alternate layers with coal 

 or turf. By this process, water and carbonic acid 

 are expelled, and the limestone converted into 

 what is called shell or unslaked lime. This is 

 then reduced to powdery quicklime by slaking 

 that is, by pouring as much water upon it as will 

 suffice to destroy the cohesion of the particles. 

 When intended for mortar, the quicklime should 

 be immediately incorporated with sand, additional 

 water being mixed in so as to bring it to the con- 

 sistency required when used. 



When common mortar is made so fluid with 

 water as to be poured on a course of brick or 

 stone work, it is known by the name of grout. 

 Where great strength and durability are required, 

 the practice of grouting the hearting or packing of 

 the walls is usually adopted ; as by this means the 

 interstices are filled, and the whole rendered, by 

 the hardening of the lime, a solid compact mass. 

 Foundation concretes are generally formed of small 

 angular stones well packed and grouted. Such 

 concretes are proof against all moisture and decay, 

 and, on indifferent subsoils, form more resistant 

 foundations than isolated blocks of stone, however 

 large and heavy. 



Hydraulic or water cements are those which 

 have the property of hardening under water, and 

 of consolidating almost immediately on being 



