524
ELEMENTARY INGREDIENTS FOR THE MANUFACTURE OF GLASS. [Class XXIV.
he taken lest the remedy, applied for the removal of onedescription of defect, should give rise to others of anopposite character. The presence of too much alkaliattracts humidity, and (to use the glass-makers’ phrase;disposes it to sweat. It also diminishes the refractivepower of the glass, and when used in the state of un-deeomposed sulphate of soda, or potash, renders it opa-lescent. Lead in excess will produce equally had effects:it will soften the substance of the glass, affect its clear-ness, and, in course of time, render the surface liable tohe altered and decomposed. The importance of lead inthis manufacture, whether in the form of litharge or ofminium, which is obtained by oxidizing litharge, is sowell understood in England, that very great attention ispaid to its preparation, as well as to the quality of thelead: it is made an object of exclusive manufacture, andis one cause of the excellence of British flint-glass.
Borax, white arsenic, and nitre, are also useful, andpowerful agents when well applied; but danger may arisefrom the employment of arsenic in vessels intended fordomestic use. Borax, or borate of soda, is too expensiveto admit of its being employed for common purposes. Itis, therefore, far from being generally adopted, and isapplied rarely even to objects of great importance. Itwas formerly imported from India , being a product ofThibet , but in a very impure condition; and a better sortin the form of boracic acid is now supplied from theworks which M. de Larderel has established at MonteCerboli, near Yolterra, in the Tuscan Maremme. Havingascertained the presence of large quantities of boracicacid in the district, he obtained a grant of land for a termof years from the Grand Duke , and immediately com-menced operations. He began by diverting a stream tothe spot where the boracic acid was found to abound;and after the water had become impregnated with theacid, it was allowed to flow on through pipes to largepans, for the purpose of evaporation. Volcanic heat like-wise prevails in the same district, and in order to avoidthe expense of fuel, which M. Larderel found extremelyhigh when he first undertook the works, he has availedhimself of this volcanic agency, and conveyed the heat intubes, so as to make it pass under the pans, by whichmeans the evaporating process is effected, and the boracicacid remains in the pans.
Notwithstanding the facility afforded by this naturalagent, the manufacturers complain of the high price oftins jjpbstance, about 5/. 10s. the metrical quintal (205English lbs.). The price is said to be regulated bv a con-tract, made with some merchant at Leghorn ; but it hasthe effect of excluding the material from the manufactureof glass for domestic purposes; and it is likewise to beobserved that the Tuscan boracic acid, even at this price,contains nearly one-half its weight of water of crystal-lization ; so that the glass manufacturer has to pay at thepresent day nearly Is. per pound for the material in itsavailable form as a flux. There are, also, other objectionsto its employment; for it is so extremely pungent that,unless applied with great caution, it corrodes the pots,introduces alumine into the fused metal, or passes awaythrough the pores of the vessel.
It is said that this material has been found in abundanceon the western coast of America , in combination withlime, as borate of lime; which, if correct, will be of es-sential service in many branches of manufacture. It isdescribed, for the first time, by the American mineralo-gist, Hayes, and has been called Hayessine, from its dis-coverer. It is met with in the Peruvian province ofTarapaca , not far from the port of Iquique , in the samelocality where the nitrate of soda, now so extensively ex-ported to England, has been found. The borate of Timecontains nearly 45 per cent, of boracic acid, combinedwith 19 per cent, of lime, and 35 per cent, of water, sub-stances which cannot deteriorate its quality as a flux;whilst its moderate price, white colour, and absence ofany metallic oxide, render it well suited to the use of theglass crucible.
The first chemists in England, France , and Germany have directed their attention to the fabrication of glass.Experiments have been tried by practical working men,with great liberality and intelligence; but there are cer-
tain principles dependent upon science, according to whichall operations connected with the art must be directed. Itis to the accuracy and judgment exercised in adhering tothese principles, in providing for the selection and appli-cation of the component substances, in determining theirproportions, and securing their purity and quality, in theworking of the metal, the construction of the furnacesand the pots, in the management of the fires, and theannealing process, that we must look for the productionof a good composition, and for improvement in the art ofglass-making.
The silex mostly used in England is sea-sand, consistingchiefly of quartz. The finest qualities are obtained fromAlum Bay, in the Isle of Might, and from near Lynn, onthe coast of Norfolk . Black flint, when raised to a redheat, and plunged in cold water, is frequently used, andprobably gave the name to the species' of glass, flint-glass,or crystal, to which it is most commonly applied.
The alkali used in this manufacture is either soda orpotash, which is preferred for the finest works in the con-dition of the carbonate, and is then called carbonate ofpotash, or pearl-ash, from which the carbonic acid is ex-pelled by the heat of the process in fusing. Soda is usedas dry carbonate of soda, when a more than ordinarydegree of whiteness is required, as in plate-glass. It isalso used in this state in coloured glass. In window-glass(crown and sheet) soda is more generally used as a sul-phate of soda.
Colour is imparted to glass by the application of themetallic oxides, and when it pervades the whole mass istermed pot-metal, as distinguished from that to which thecolour is applied in the form of enamel.—(See ColouredGlass.)
Cobalt produces blue.
Manganese produces violet.
Antimony produces yellow.
Precipitate of cassias, or gold, produces pink.
Uranium produces opaline-greenish colour.
Copper produces ruby, or greenish blue, according toits degree of oxidation.
Copper with iron produces ruby, or green, according tothe degree of oxidation of the copper.
Silver produces a pure and beautiful yellow, but onlyby staining the surface at the fire of a muffle.
These colours, however, will be modified, or even com-pletely altered, by different combinations of the metals,the degree of their oxidation, the greater or less degreeof heat employed, the addition of vegetable carbonaceousmatter, and other circumstances.
The manufacture of glass has been classed by the Com-missioners under the following heads;—
A. Window-glass, including—
1. Crown-glass.
2. Sheet.
3. Brown plate, silvered or unsilvered.
4. Coloured sheet, pot-metal, or flashed.
B. Painted and other kinds of ornamental window-glass.
C. Cast plate-glass.
Rough plate.
Pressed plate.
Rolled plate.
D. Bottle-glass, including—
Ordinary bottles.
Moulded bottles.
Medicinal bottles.
Water-pipes and tubing.
E. Glass for chemical and philosophical purposes.
Matrass-retorts, &c., &c.
Water-pipes and tubing.
F. Flint-glass, or crystal, with or without lead; white,coloured, ornamented, for table-vases, &c., &c.—
1. Blown.
2. Moulded and pressed.
3. Cut and engraved.
4. Reticulated and spun with a variety of colours ;
incrusted, flashed, enamelled of all colours,