Mortar.]
obtained from the pit, being sharp and angular. It is, however, liable to
be mixed with clay or earth, which must be washed away before the sand is
used. Gravel found mixed with it must be removed by screening or sifting.
River sand is frequently used, but is not so good as pit sand on account of
the particles being rubbed smooth by attrition. Sea sand is objectionable
for two reasons; it cannot be altogether freed from a saline taint, and if
it is used the salt attracts moisture and is liable to keep the brickwork
permanently damp. The particles, moreover, are generally rounded by
attrition, caused by the movement of the sea, which makes it less efficient
for mortar than if they retained their original angular forms. Blue or
black mortar, often used for pointing the joints of external brickwork on
account of its greater durability, is made by using foundry sand or smith's
ashes instead of ordinary sand. There are many other substitutes for the
ordinary sand. As an example, fine stone grit may be used with advantage.
Thoroughly burnt clay or ballast, old bricks, clinkers and cinders, ground
to a uniform size and screened from dust, also make excellent substitutes.

Fat limes (that is, limes which are pure, as opposed to "hydraulic" limes
which are burnt from limestone containing some clay) should not be used for
mortar; they are slow-setting, and there is a liability for some of the
mortar, where there is not a free access of air to assist the setting,
remaining soft for some considerable period, often months, thus causing
unequal settlement and possibly failure. Grey stone lime is feebly
hydraulic, and makes a good mortar for ordinary work. It, however, decays
under the influence of the weather, and it is, therefore, advisable to
point the external face of the work in blue ash or cement mortar, in order
to obtain greater durability. It should never be used in foundation work,
or where exposed to wet. Lias lime is hydraulic, that is, it will set firm
under water. It should be used in all good class work, where Portland
cement is not desired.

Of the various cements used in building, it is necessary only to mention
three as being applicable to use for mortar. The first of these is Portland
cement, which has sprung into very general use, not only for work where
extra strength and durability are required, and for underground work, but
also in general building where a small extra cost is not objected to.
Ordinary l