Lustres which are chiefly earth and alkali glazes-such as the common salt-glaze, and the glazes containing metallic oxides used to imitate gold and silver. These glazes and enamels are put on before firing, either by immersing the article in a thin solution of the material, or by watering or pouring it over the article; by dusting the freshly formed and still damp surface of the vessel with a powder of the material; or by placing the material in the oven along with the articles, where it volatilises and combines with the silica of the ware, forming a film of glass as in the case of common salt-glazing.

As with the glazing so with the painting, gilding, silvering, and bronzing of earthenware, all the materials employed are strictly mineral and metallic. The preparation of the colours and the laying of them on are matters of chemistry and technology, but we may here briefly indicate the tints obtained from the oxides, chromates, and chlorides made use of. ThusOxide of iron, for red, brown, violet, yellow, and sepia.

manganese, for violet, brown, and black.

copper, for green and red.

chromium, for green.

cobalt, for blue and black.

In gilding, silvering, and platinising, the metal is laid on in solution, and after being burnt in, is either left flat or made bright by burnishing with agate tools.

As with earthenware, so with glass; the materials for ornamenting it-cutting, colouring, staining, painting, &c.—are all products of the mineral kingdom, and, as such, come within the sphere of geology. The substances for cutting and obscuring will be noticed in the next chapter on "Cutting and Grinding Materials;" the colours employed in staining and painting are much the same as those used for colouring porcelain and earthenware-namely, the oxides, chlorides, sulphides, and chromates of the metals,-copper, cobalt, manganese, iron, uranium, titanium, antimony, silver, and gold, being those most frequently in request.

The preparation and colouring of glass for the production of "artificial gems" might be noticed under this head, but will

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come more appropriately under the Chapter on "Gems and Precious Stones," when the student shall have become acquainted with the nature of the substances intended to be imitated. In the mean time, however, it may be observed that all the factitious gems are merely varieties of glass, rendered denser, clearer, and more brilliant by special admixtures, and coloured by metallic oxides to imitate as nearly as possible the tints of the natural productions.

From what has been sketched in the preceding pages-and it is merely a sketch in outline of a vast and varied subject— the student will perceive that all the varieties of glass and earthenware, their composition, fusing, firing, and ornamentation, are wholly dependent upon the mineral kingdom. The sands and clays, the alkaline earths, the salts, the metallic oxides, as well as the fuel required for their preparation, are all obtained from the rocky crust. Comparing the beauty and elegance of the finished products, tender and fragile though they be, with the raw materials from which they are derived, it is impossible not to accord to the manufacture of glass and porcelain a very high place among the arts and industries of modern civilisation. The raw materials with which we have here mainly to deal occur in widespread abundance; and while it is the business of the chemist and technologist to experiment and apply, it is equally the duty of the geologist to search for and determine the nature (quality, abundance, and accessibility) of new sources of supply. Splendid as are the achievements of the Fictile Arts, it were vanity to suppose that, with all the appliances of modern science, there is nothing higher to be accomplished, nor a wider and cheaper diffusion of their elegancies to be attained. To promote this advancement, the geologist will best contribute his mite by studying the nature of the raw materials employed, and by noting, when in the field, whatever he considers superior in quality, or whatever can be more readily and cheaply obtained.

Works which may be consulted.

Wagner's Handbook of Chemical Technology,' Crooke's edition; Knapp's 'Chemical Technology,' vol. ii. ; Ure's Dictionary of Arts and Manufactures,' Hunt's edition; Watt's 'Dictionary of Chemistry.'

XI.

GRINDING, WHETTING, AND POLISHING MATERIALS.

LITTLE progress can be made in the Arts and Manufactures without appropriate tools and instruments, and especially without what are termed "edge-tools." These, generally fashioned of the finest and best-tempered steel, cannot be furnished with the requisite cutting edge without some process of grinding, whetting, and polishing. Be it axe or adze, sword or spear, plane or chisel, knife or scissors, lancet or needle, each requires to be ground and set for its special work; and the materials for this purpose are, one and all of them, derived from the mineral kingdom. Again, in the Arts and Manufactures, a great deal of crushing, grinding, and pulping of various substances is necessary-food-stuffs, chemicals, paper-pulps, clays, mortars, and cements and hence a great variety of millstones and crushing-stones, according to the difficulty or delicacy of the operation to be accomplished. Further, for utility as well as ornament, much polishing and burnishing is needed, alike of mineral and metal, and the most efficient implements for these purposes are in like manner obtained from the crust of the earth. Grinding, whetting, and polishing materials have been requisite at every stage of human civilisation, and are especially so in an age of mechanical appliances like the present. The rude savage who polished his stone hatchet, the ancient warrior who whetted his bronze spear-head, and the modern cutler who puts the keenest edge on the knife of the surgical operator, have all had recourse to the same mineral source for their materials. The old prehistoric man who crushed his nuts and grain, or pounded the pigment wherewith to bedaub himself, the miller of the nineteenth century who produces the finest flour-meal, and the chemist who reduces his admixtures to the most inpalpable pulp, have all called to their aid much the same rocks and minerals. It is to the lithological nature of these grinders, whetters, polishers, crushers, and pulpers, that we devote the present chapter.

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I. MILLSTONES.

Grits.

In the olden times, the ordinary millstones—that is, stones for the reduction of grains and seeds into meals-were chosen from the harder, tougher, and more silicious sandstones. Wherever a good tough grit with a keen-cutting surface could be obtained, such grits were raised, dressed, and centred for the meal, flour, and barley mill. The grits of the Silurian, Old Red Sandstone, and Carboniferous systems (millstone grit) were those most highly prized, some varieties being brought from great distances, and owing to the then limited means of transport, often at considerable expense. The great object was to obtain a milling surface with a keen "burr," and yet of sufficient hardness and toughness as not to give off any appreciable amount of sandy particles to the meal. In course of time the best of such millstones become smooth in surface, and had again and again to be taken out and re-dressed with delicate pick-points so as to renew their burr. At that timesixty and seventy years ago-wind and water mills, on a small scale, were very numerous; and such millstones as we have spoken of were in great request; but now, like other branches of industry, meal, flour, and barley mills have become gigantic factories, and the heavier work to be done has called into use harder and more durable materials.

Burrs.

One of the keenest and most durable of materials, whether for making meal and flour, or for the trituration of cements, manures, pigments, and other chemicals, is burrstone, a porous silicious rock from the tertiary formations of Europe and America. These burrstones are of various colours, whitish, yellowish, and reddish brown, are almost pure silica, with just sufficient calcareous matter to give them the requisite toughness, are slightly porous or vesicular from the decay of imbedded shells and other minuter organisms, and are of freshwater origin. Those used in Britain are generally obtained from the tertiary deposits of France (Seine et Marne) from whence they are brought in blocks of a foot or more square, to be dressed, built up, and clamped or hooped for millstones of varying dimensions. The dressing, fitting, and building up of these blocks requires much skill and labour, as the jointings of the separate pieces should be scarcely perceptible, and the whole surface rendered true and even. As

burrstone is expensive, the requisite weight and thickness is usually obtained by a backing of concrete, made up of the chippings, which for these purposes answers perfectly well.

These burrstones are now largely employed for all kinds of trituration and milling, not only in meal and flour mills, but in cement-works, potteries, chemical works, and other similar factories. In course of time they wear smooth like other stones, and their "burr" has to be renewed by a tedious process of

As

1. In process of building; 2. Completed; 3. Finished, with grooved milling surface. pick-dressing, though recently diamond points have been applied with great success, and saving of time and money. the majority of burrstones vary from 6 to 6.7 in hardness, while that of the bort or black diamond is about 10, the influence of the latter on the former is readily perceptible, while it is free from the dangerous "fire" which accompanies the use of steel.

Quartzites.

Besides grits and burrstones, quartzites and close-grained lavas have been used for milling-the whiter quartzites of the metamorphic rocks (Norway, Banffshire, and Argyleshire) answering well for certain pottery purposes, and the lavas-the old tertiary lavas of the Rhine, for example-being employed for the trituration and intimate admixture of chemicals which would act upon many other rocks.

II. GRINDSTONES.

Sandstones and Grits.

In an age so eminently mechanical as the present, a great deal depends upon the use of tools of the keenest edge and of the surest adjustments. For the shaping and setting of these tools, grindstones of various sizes and texture are required, and these are obtained from the sandstones and grits of all forma