TERRA-COTTA CLAYS.
In the manufacture of terra-cotta in England an important clay has long been the potter's clay of North Devon and Dor setshire, the analysis of which, by Weston, is as follows:— North-Devon.
Alumina . 29.38 Silica 52.06 Lime 0.43 Magnesia . 0.02 Iron oxide Potash 2.29 Soda 2.56 Water combined r0.27 Dorsetshire Alumina 32.11 Silica Lime 0.43 Magnesia 0.22 Iron oxide 2.34 Potash 2.31 Soda 2.33 Water combined 9.63 Each of these clays contains a small amount of alkalies. The clays of the coal measures, technically known as the "fine clays," are also much esteemed for this purpose.
In the north of England and in Scotland, the purest lumps of fire-clay, selected by their color and texture, are used by themselves in the production of terra-cotta ; but the concerns of Mr. Blashfield of Stamford, and Doulton and others near London, produce a body of much better texture by a careful and thorough mixture of clays.
It requires greater care, and is slightly more expensive for labor ; but these are small considerations in comparison with the increased compact, homogeneous and better vitrified body which results from using a mixture of clays.
The precise combination of clays varies with the appearance desired for the terra-cotta ; sometimes it is a light cream, or a soft buff color ; at other times it may be a cherry-red, or a hard brownish-red color.
A partial vitrification of the mass is desirable in the produc tion of terra-cotta, as it enhances the durability of the body ; and in order to achieve this, clays like the Dorsetshire are added, the small amount of alkalies which they contain acting as a flux and fusing the body to a harder consistency.
New Jersey produces a great variety of clays, and the belt of country underlaid by them extends entirely across the State, and as described by the State geologist, includes an area of three hundred and twenty square miles ; while the area within which these deposits have been worked to the present time is only about seventy square miles, the actual openings of the clay beds being only a very small fraction of the last-named area.
The average depth of these clay deposits is more than three hundred and fifty feet, and the order of supersession is shown in the following table : Feet.
1. Dark-colored clay (with beds and laminm of lignite) . - 50 2. Sandy clay, with sand in alternate layers 3. Stoneware clay bed 3o 4. Sand and sandy clay (with lignite) 50 5. South Amboy fire-clay bed 20 6. Sandy clay, generally red or yellow 3 7. Sand or kaolin to 8. Feldspar bed 5 9. Micaceous sand bed 20 10. Laminated clay and sand 30 1. Pipe clay (top white) so 12. Sand clay, including leaf bed 5 13. Woodbridge fire-clay 20 54. Fire sand clay 15 Fire-clay 15 15. Raritan clay beds j Sand clay 4 Potter's clay 20 These clays are one of the most important elements of the material wealth possessed by the State.
6 Large quantities of clays are marketed annually for making fire-brick, pottery, terra-cotta ware of all kinds, tiles, retorts, crucibles, facings for wall-papers, etc.
The average price per ton is four dollars, and the average aggregate production of fire-clay alone, in its crude state, ex ceeds one and a half million dollars.
New uses for clay of this character are being developed all the while. The New York Terra-cotta Lumber Company has established large works at Perth Amboy for the manufacture of lumber by mixing resinous sawdust with the wet clay, which is left porous after the burning, by the sawdust being con sumed.
In speaking so highly of the terra-cotta clay of New Jersey, we do not mean to be understood that it is suitable for use with out any mixtures or other special preparation, as no terra cotta clay can be so worked with safety ; neither should the terra-cotta clay be confounded with fire-clay, the requirements for which are different ; but that of New Jersey is also one of the best in this country, or in the world.
The body of the clay which has been described is best de veloped at Woodbridge and Perth Amboy, and is practically inexhaustible ; and although its presence has been known for nearly, if not quite, two centuries, its employment for the pro duction of architectural terra-cotta is of but very recent years.