To be more lucid, it
need be explained, linseed oil boiled has lost its oleic acid and
glycerine ether, which form with the bases of pigments the insoluble
soap, as well as its albumen, which in boiling is thrown out. It
coagulates at 160 deg. F. heat; each is needed to better withstand the
action of wind and weather, preventing the dust from attaching itself
to a painted surface, a channel for ammonia in damp weather to
dissolve and wash off the paint. In later years linseed oil has been
extracted from linseed meal by the aid of naphtha and percolation, the
product of a very clear, quick drying oil, but lacking in its binding
quality, no doubt caused by the naphtha dissolving the fatty matter
only, leaving the glycerine and albumen in the meal.
All pigments of paint group according to their affinity to raw linseed
oil into three classes. First, those that form chemical combinations,
called soap. This kind is the most durable, is used for priming
purposes, and consists of lead, zinc, and iron bases, of which red
lead takes up the most oil; next, white lead, the pure carbonate Dutch
process made, following with zinc white and iron carbonates, as iron
ore paint, Turkey umber, yellow ocher; also faintly the chromates of
lead--chrome-green and chrome-yellow, finishing with the poorest of
all, modern white lead, made by the wet or vinegar process.
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