A Memoir on British Resources of Sands and Rocks Used in Glass Making by P.G.H Boswell

Author:P.G.H Boswell
Language: eng
Format: epub
Publisher: Longmans Green and Co
Published: 1918-03-25T05:00:00+00:00

CHAPTER VIII.

BBITISH RESOURCES OP GLASS-SANDS, ETC. (continued?). B. DEPOSITS CA&RYIN& ALUMINA AND SILICA.

Alumina commonly occurs in sands and rocks used for glass-making in the form of the mineral felspar or the decomposition product, kaolin (china-clay), which results largely from the .alteration of felspar. Certain rocks—as, for example, those of the Triassic and Carboniferous Systems—are rich in decomposed felspars .and contain a certain amount of clayey material. This renders their •crushed products poorly-graded, but the small proportion of alumina present may actually be an advantage, as in the case of bottle-making, where strength and toughness are required in the glass. Unfortunately, most British sands bearing a high proportion of .alumina contain also much iron, and are useless for the manufacture of glass-ware other than common bottles.

A prejudice against alumina in sands for bottle-making was formerly met with, but it is now slowly dying out. In rare cases Alumina is even added to the batch for bottle-making. Dr. W. E. S. Turner, of the Department of Grlass Technology in the University of Sheffield, is of the opinion that, far from having a tendency to •cause devitrification, alumina if in moderate amount has just the opposite properties and will prevent glass from devitrifying. Experiments have shown that when alumina is added to a window-.glass batch, the glass which is produced does not undergo devitrification when heated in a blow-pipe flame. If the amount of -alumina is high, it is possible for a clouded glass to be produced, -as when felspar itself is used. According to the investigations of Singer *, it is possible to add to a glass-batch containing alumina more sand and more lime, and to reduce in proportion the amount •of alkali, thereby cheapening the cost of production.

Of the sands and rocks described or mentioned in the foregoing pages, those from Sandymount Strand (Dublin), Parsley Hay and Brassington (see below), Charltou, South Cave, Huttons Ambo, and Leyenseat, carry appreciable, and sometimes large, quantities •of alumina f.

The sand from Sandymount Strand is used in the Dubh'n black-bottle industry. To give greater "body" to the glass, broken lumps of Bridgwater or other homogeneous brick are added to the bajxih. The brick is red and is highly ferruginous, as the following analysis indicates:—

* F. Singer, 'Die Keramische Rundachau,' yol. v. 1915, "TJeber den Einfluss von Tonerde anf die Sohmelzbarkeit von GHasern."

t See note on page 34 referring to sand from Martinroda used for ther-mometer-glasB.

Analysis of Bridgwater Brick.

SiO, ..................... 58-20 per oent.

AlA ..................... 16-42

TiO' ..................... 1-29

Fe 3 0 3 ..................... 6-74

FeO ........................ 0-13

CaO ..................... 7-37

MgO ..................... 3-02

K a O ........................ 3-65

Nft.,0 ..................... 0-93

C0 2 ........................ 0-13

P.,0, ..................... 0-07

MiiO ..................... 0-08

LOBS on ignition ......... 2-50

1 [HjO flJid carbonaceous

matter.] ' -

Total ......... 100-53 per oent.

The addition of the brick increases the depth of colour of the "bottles, and undoubtedly strengthens them by toughening the glass — a result of the addition of the 16£ per cent, of alumina. "This is a distinct advantage where the bottles have shoulders and .are required to stand considerable pressure from contained gases.

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