Masonry Magazine August 2005 Page. 23
There is an old masonry proverb
that says, "Stone is simply a
way-stop for sand in its progress
back to the sea."
While we are inclined to think of stone as a durable,
even impervious, material, the geological forces that
shape it mainly water and pressure ultimately deter-
mine its destination. Also, while you and I may not be
around to see the entire life of a stone unfold, we will cer-
tainly notice the effects of the elements on it in the life-
time of our home or garden. In as little as a year into the
life of a stone structure, you may begin to see any num-
ber of changes in the stone, called weathering.
Weathering refers to a variety of effects, ranging from
destructive to desirable. Some of these effects will
change only the appearance of the stone, while others
may change its structural properties in the worst case
Salt Crystallization
ONE OF THE MOST COMMON corrosive substances
around is perhaps the most common cause of weathering
on stone, especially porous stone. Impermeable stones will
show effects of salt crystallization only under special cir-
cumstances.
In general, salt crystallization is indicated by the forma-
tion of efflorescence, a visible growth or film on the surface
of the stone that is usually light in color. The film later
dries to form a powder, which will flake off or can be
washed off, often taking very small amounts of the stone
with it. In regions close to the sea, crystallization effects can
be dramatic, going far deeper than the surface of the stone
and eventually leading to massive decay of the stonework.
The simple solution is a soft brush, water, and some elbow
grease. Most efflorescence can and should be removed as
part of regular maintenance.
Air Pollutants
MUCH OF THE VISIBLE weathering of stonework in
industrial areas can be attributed to air pollutants and acid
Inevitably, your stone is going to change over time, and
you should understand this as a part of its nature.
causing it to destabilize. The factors that cause many of
these effects are present in any environment and can
never be entirely mitigated. Inevitably, your stone is going
to change over time, and you should understand this as
a part of its nature.
What follows is a rough guide to some of the major
factors in stone weathering, as well as an explanation of
how these factors will affect the appearance of the stone.
rain. In extreme cases, buildings can be blackened by tarry
build-up and the stone can absorb the stain deep into its
pores, making cleaning difficult. In residential locations,
the effects will be less visible. In areas regularly washed by
rain, only a slight darkening over time will be noticed. In
some stones, this washing process will actually deteriorate
the outer layer of the stone, exposing fresh stone and mak-
ing it appear new again, though it is at the same time slow-
ly decaying the stone.
As you might expect, the effects of air pollutants are more
pronounced on lighter than on darker stones. Limestone and
marble can be impacted in areas that have significant acid rain
conditions, evidenced by pitting and degrading of honed and
The Voice of the Mason Contractor
August 2005
Masonry
21