The only wise preservatives are those long known and employed by our
ancestors; salt, vinegar, and spices are all food preservatives, but
they are at the same time substances which in small amounts are not
injurious to the body. Smoked herring and salted mackerel are
chemically preserved foods, but they are none the less safe and
digestible.
235. The Preservation of Wood and Metal. The decaying of wood and
the rusting of metal are due to the action of air and moisture. When
wood and metal are surrounded with a covering which neither air nor
moisture can penetrate, decay and rust are prevented. Paint affords
such a protective covering. The main constituent of paint is a
compound of white lead or other metallic substance; this is mixed with
linseed oil or its equivalent in order that it may be spread over wood
and metal in a thin, even coating. After the mixture has been applied,
it hardens and forms a tough skin fairly impervious to weathering. For
the sake of ornamentation, various colored pigments are added to the
paint and give variety of effect.
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