or may be made in a great number of localities, as reasonably good fire-clay is an abundant substance in this country.

Fire-clay beds are fine sediments which accumulate at the bottoms of shallow pools of water, subsequently filled up with growing vegetation.

The roots of aquatic plants have penetrated this clay-bed and extracted generally its potash, soda, lime, iron, etc., and have removed such a percentage of silica as to leave it with a larger relative quantity of alumina than it had before being subjected to their action. In this manner they have extracted from it its more fusible or fluxing ingredients, and have left it its peculiar property of remaining unchanged at a high temperature.

Many beds of peat have underlying them clays very like our fire-clays, and in such circumstances we may plainly see the formation of fire-clays progressing.

In this country we have two varieties of fire-clay, the one non-plastic and particularly adapted to the manufacture of first quality of fire-bricks, and the other plastic, and used for the production of an inferior quality of fire-bricks, and the production of pottery, terra-cotta, glass pots, etc.

In the first class are those that have been mentioned, to which should be added that at New Lisbon, Ohio. To the second class belong the fire-clays of the coal-measures, which differ greatly among themselves as regards purity and excellence, but which answer for secondary purposes, and are very extensively employed in the manufacture of all classes of terra-cotta and stone-ware.

The analyses given below are of some of the best known. qualities of fire-clays. No. 1 is of the Strourbridge clays of England, to which reference has several times been made in this chapter. No. 2 is from Mt. Savage, Md., No. 3 is from Mineral Point, Ohio; both of the last-named being nonplastic. No. 4 is from Port Washington, Ohio, and No. 5 is from Springfield, Ohio, the two last-mentioned being plastic clays.

For analyses of the English Dorsetshire and North De

vonshire fire-clays, see chapter on Terra-Cotta.

To be acquainted with the chemical qualities of the fireclays is of course useful in their manipulation; but the physical tests of this class of clays are of vastly more importance; analyses answer well for comparisons in theory; but the physical trials and results are the ones which govern in their employment in industry.

The properties of bodies largely depend upon the mode of grouping of the atoms; clays may be of the same chemical constituents, and yet when put to the physical test be of

directly opposite refractory qualities; while clays of greatly different composition may prove to be about equally refractory. The practical manufacturer does not usually care at all about the chemical constituents or composition of fireclays; he seeks, or should seek, for that material which yields the best results in actual use.

If he wants a brick to yield slowly to the corrosive influence, which has been explained in this chapter, a simple test to apply is, to see the number of times in which it can be melted with oxide of lead and not be eaten through. In fire-brick constructions the use of joints of clay containing free silicic acid (quartz) should be avoided, which can be done by previously saturating the material with a basic burt clay.

For practical purposes the relative proportions of alumina and silica, which some manufacturers have laid such undue stress upon as indicating heat-resisting quality, are of but small moment, as both these constituents, whether occurring in combinations or as silicates of alumina, or as free alumina and silica, are essentially the real refractory elements of good fire-bricks; being unvitrifiable of themselves excepting when associated with the alkalies, lime, or oxide of iron.

The plastic character of refractory clays has also but limited influence on their suitability for fire-brick manufacture when the bricks are properly moulded, excepting extreme plasticity, which is usually accompanied by excessive contractibility and vitrifiability, which are of course very prejudicial. Take it as a general rule, but few clays or materials used in the production of fire-clay wares are either

over-plastic or insufficiently plastic to prevent their being moulded by the dry-clay process out of nearly dry pulverized clay. If too plastic, they can be dried by the wind or sun, which greatly lessens their plasticity; if this is not sufficient, the clays can be burned in an oven or kiln, and those which are not plastic can be lightly sprinkled with water before being subjected to pressure. I have in Chapter V. strongly opposed this process of manufacture for all bricks to be used for purposes of engineering or architectural construction, where strength is an essential; but the present is a very different employment, the quality here necessary not being strength, but resistance to heat.

Many clays which have but poor refractory qualities when tempered in a pug-mill can by this process of dry-clay moulding be greatly improved; the clay should be thoroughly dried in thin layers, in the sun if possible, before pulverizing. Refractory clays of still lower grades may be greatly improved, so as to resist a very high temperature, by treating with acids, and then thoroughly drying and afterwards moulding them by the dry-clay process.

There is a preponderating quantity of silica in the English coal-measure fire-clays when compared with the tertiary clays of Devonshire and Dorsetshire, in which a larger proportion of alumina appears, as is shown by the analyses of a great number of these clays.

We find coupled with the latter property, in the tertiary clays, tenacity and plasticity, and necessarily greater contraction both in the processes of drying and burning, and when this is excessive the shrinkage is curtailed by a tho

rough incorporation with clean sand, burned clay which has been pulverized, or sherds. About two per cent. is the average shrinkage during burning of the bricks made from the several coal-measure fire-clays; this percentage of contractibility of course excludes that which occurs during the process of drying. The bricks which formed the basis of the calculation from which this average was computed, were made from nearly dry pulverized clay.

The average contractibility during the process of burning of the fire-bricks produced from the tertiary fire-clays is very much greater than that of the coal-measure clays.

Tenacity of texture in a fire-brick material is a mechanical condition, which, cæteris paribus, assists vitrification, a coarse open body being much more refractory than a close homogeneous brick of similar composition.

A well-manufactured fire-brick should be of a pale cream or clear buff color, uniform throughout its mass, and burnt to the full extent of its contractibility.

The chemical changes which take place in the burning consist, first, of the destruction of the disseminated carbonaceous matter, the dehydration of the silicates of alumina, destroying their plastic character, and the decomposition of the disseminated carbonate or protoxide of iron, converting it into anhydrous sesquioxide, to which the yellow color of the burned bricks is due.

Should the burning be carried to a very high state of vitrification the yellow tint is replaced by a dull gray, due to the partial reduction of the sesquioxide of iron and its conversion into silicate of protoxide or minutely disseminated