eldspar is used principally in the manufacture of pottery, china ware,
porcelain, enamel ware, and enamel brick and tile. In the body of these
products it is used to lower the fusing point of the other ingredients
and to form a firm bond between their particles. Its use in forming the
glaze of ceramic products is also due to its low melting point. A less
widespread use of feldspar is as an abrasive (Chapter XIII). One of the
varieties of feldspar carries about 15 per cent of potash, and because
of the abundance of the mineral there has been much experimental work to
ascertain the possibility of separating potash for fertilizer purposes;
but, because of cost, this source of potash is not likely for a long
time to compete with the potash salts already concentrated by nature.

Feldspar is mined in eleven states, but the important production comes
from North Carolina and Maine. The United States also imports some
feldspar from Canada.

HYDRAULIC CEMENT (including Portland, natural, and Puzzolan cements)

Cement is a manufactured product made from limestone (or marl) and clay
(or shale). Sometimes these two kinds of substances are so combined in
nature (as in certain clayey limestones) that they are available for
cement manufacture without artificial mixing. It is not our purpose in
this volume to discuss manufactured products; but the cement industry
involves such a simple transformation of raw materials, and is so
closely localized by the distribution of the raw materials, that a
mention of some of its outstanding features seems desirable.

Hydraulic cement is used almost exclusively as a structural material. It
is an essential ingredient of concrete. Originally used chiefly for the
bonding of brick and stone masonry and for foundation work, its uses
have grown rapidly, especially with the introduction of reinforced
concrete. It is being used in the construction of roads, and its latest
use is in ship construction.

With the exception of satisfactory fuels, the raw materials required for
the manufacture of cement are found quite generally throughout the
world. While practically all countries produce some cement, much of it
of natural grade, only the largest producers make enough for their own
requirements and as a result there is a large world movement of this
commodity. The world trade is chiefly in Portland cement.

Next to the United States, the producing countries having the largest
exportable surplus of cement