326

PAINTING MATERIALS

Atom, the smallest subdivision of a chemical
element which remains unchanged in chem-
ical reactions; composed of electrons, pro-
tons, and other structural units.

Atomic Weight, the number which indicates
the relative weight of an element as com-
pared to hydrogen (1.008) or oxygen
(16.000).

Autoxidation, a complex type of slow oxida-
tion at ordinary temperatures by oxygen of
the atmosphere.

Base, (i) a compound which yields hydroxyl
(OH") ions in aqueous solution; a com-
pound which reacts with an acid to form a
salt and water; an alkali; <?.£., NaOH
(sodium hydroxide) and Ca(OH)a (calcium
hydroxide); (2) an inert material upon
which a dye is struck in the manufacture
of a lake pigment.

Basic Salt, a compound derived from an acid
and a base in which not all of the hydroxyl
radical of the base has been replaced by an
acid radical; *.£., white lead, 2PbCOs-
Pb(OH)2.

Binary Compound, a compound that consists
of only two elements; e.g., KC1 (potassium
chloride) and FeaOs (ferric oxide).

Birefracting, doubly refracting; a character-
istic of transparent anisotropic crystals.

Birefringence, the strength of the double re-
fraction of a material. Birefringent, doubly
refracting.

Blanch, (i) to make white; (2) a pale, misty
cast that comes over a film of varnish or
paint; it can be used, in distinction to blush
and bloom, as a name for the change which
comes in old films when a solvent has been
put on them and has evaporated, leaving a
milKy look, usually irregular in distribution,

Bleeding, the property of a color in a dried
paint film to diffuse and spread away from
the layer in which it is applied to other
paint or varnish layers.

Bloom, the white or cloudy appearance of the
surface of aged varnish films, caused by
the presence of minute cracks or pores
which diffuse light. Such a film has a faint
bluish cast; it is not milky white like a
blanched film.

Blush, the white appearance or turbidity that
sometimes develops in lacquer or varnish
films almost directly after they are applied.
The phenomenon usually occurs only in
humid weather* It is caused by rapid
evaporation of solvents, which produces
cooling at the surface and which, in turn,
causes condensation of moisture within the

film. The effect on light is like that in an
emulsion. Blush is distinct from blanch and
bloom, which affect films previously dried.
Boiling point (b.p.), the temperature at which
a liquid boils; the temperature at which the
vapor pressure of the liquid reaches the
pressure of the surrounding atmosphere.
(The b.p. of water at normal atmospheric
pressure is 212° F., 100° C.)
Calcareous, containing calcium; said of com-
pounds such as calcium carbonate or lime-
stone.
Carbohydrate, a type of organic compound
containing carbon, hydrogen, and oxy-
gen, the latter two in the proportion of
water; e.g.y sucrose, CioHtaOn, or cellulose
(CeHioOiOn.
Catalyst, a substance which increases the rate
of a chemical reaction but is itself un-
changed at the end of the reaction. Some
catalysts are merely contact agents acting
mechanically, but others appear actually to
take a direct part in the chemical reactions
they accelerate, forming unstable inter-
mediate compounds from which the catalyst
is regenerated as they are decomposed.
Caustic Soda, sodium hydroxide (NaOH),
usually an impure form or technical grade.
Cellulose, a carbohydrate, (CeHioOc)^ which
is the main structural ingredient of plant
and woody tissue; hence, also, the principal
constituent of wood, fabric from plant
fibres, and paper. It is almost pure in ab-
sorbent cotton and filter paper. Celiulosic
plant cells in woody tissue are cemented
together with another plant material called
' lignin' and, hence, wood is sometimes
called ' ligno-cellulose/
Chemical Compound, a substance in which
the molecules consist of unlike atoms and
in which constituents can not be separated
by physical or mechanical means. In a
chemical compound the constituent ele-
ments are combined in definite proportions
by weight. In a physical mixture, on the
other hand, there is no fixed relationship
among the components.
Chemical Element, a substance which can not
be decomposed by ordinary types of chem-
ical change or made by chemical union of
other substances; matter which consists of
atoms of one type. There are at least 92
possible elements, from hydrogen, the
lightest, to uranium, the heaviest, but of
these only 90 are known. About a third of
all the elements are comprised in the