The fruit of the olive tree varies just as much in quality as does the
grape, according to the species of the tree itself, the nature of the
soil, exposure, and climate of the locality where it grows. Some
varieties of the olive tree largely grown, because thought to be
better suited to the special conditions of some districts, yield a
fruit which imparts a bitter taste to the oil made from it; such oil,
even when otherwise perfect, ranks as a second rate quality.

The highest quality of oil can only be obtained when the fruit is
perfectly and uniformly sound, well ripened, gathered as soon as it
has dropped from the trees, and crushed immediately with great
attention. Should the fruit remain any time on the ground,
particularly during wet weather, it deteriorates fast and gets an
earthy taste; while if allowed to remain an undue length of time in
the garners it heats, begins to decompose, and will yield only bad
oil.

The process of making oil is as follows: The fruit is crushed in a
stone mill, generally moved by water power; the pulp is then put into
bags made of fiber, and a certain number of these bags, piled one upon
another, are placed in a press, most frequently worked by hand; when
pressure is applied, the oil flows down into a channel by which it is
conveyed to a receptacle or tank.

When oil ceases to flow, tepid water is poured upon the bags to carry
off oil retained by the bags. The pulp is then removed from the bags,
ground again in the mill, then replaced in the bags, and pressed a
second time. The water used in the process of making oil must be quite
pure; the mill, press, bags, and vessels sweet and clean, as the least
taint would ruin the quality of the oil produced.