210 PAINTING MATERIALS
apparatus are made for testing. The point is usually designated in degrees
Fahrenheit and a liquid is supposed to be flammable when the flash point is below
i5o°F. Those which flash below 4O°F. are exceedingly flammable and dangerous.
When solvents are mixed, the flash point of the mixture is not necessarily the
average of the points of the components. There may be uneven evaporation or
constant boiling mixtures may be formed. A common addition to the flammable
solvents is that of carbon tetrachloride, and this, in an adequate quantity, seems
to be effective. Ethylene dichloride is used in the same way. The explosive power
of solvent vapor depends on temperature and on the proportion of this vapor to
the air. The lower explosive limit is the smallest amount of solvent vapor according
to volume per cent which will allow the propagation of the flame. An upper
explosive limit is reached when the solvent vapor exceeds the quantity which has
oxygen enough to keep up a fire. The danger of flammability can be much reduced
by careful storage. Frequent inspection of containers to guard against leaks, the
use of tin or lead-lined cans for all organic solvents, and metal cases remote from
any excess heat, from sparks, or from flame will go a long way to prevent accidents.
If film materials conveyed by flammable solvents are sprayed mechanically, there
is an attendant fire hazard unless good ventilation is provided. In particular,
sparks from motors and from electrical connections have to be guarded against.
An open flame should not be used near containers of flammable solvents. It is
particularly dangerous to distill or heat organic solvents in a container, especially
a glass container, over a free flame. If the container breaks, there is nothing to
prevent Instantaneous and violent explosion. Such heating, when necessary,
should be done over a steam bath or by properly insulated electrical equipment.
Solvents, miscibility. This is the capacity of a fluid to mix with another.
Interfacial tension between the molecules seems to be the determining factor.
When the difference in tension is small, the two go together easily, but when it is
great, the two are da-awn into separate layers. In general, solvents that have a
similar chemical constitution and properties are miscible with each other. Lower
members of a homologous series are more widely miscible than those of higher
molecular weight. The greater part of organic solvents is immiscible with water,
exceptions being the lower alcohols and ketones. Acetic esters and higher alcohols,
butyl and amyl for example, are limited in their ability to mix with water but go
well with the hydrocarbons. Acetone is the only solvent which will mix with water
in all proportions and with all other organic liquids. Frequently two immiscible
fluids can be made to go together by the addition of a small amount of a third.
Butyl alcohol, for instance, will make a mixture out of petroleum spirit and ethyl
alcohol. Solvents with this capacitv are called ' mutual solvents ' or * coupling
agents/
Solvents, toxicity. Almost any materials used as solvents or diluents in the art
of painting are poisonous if taken into the human body in large amounts. There is,
however, a great variety among their toxic affects, and the risk of their being
drunk as a fluid is slight. The greatest exposure is to their vapors and, in general,