Material: Recycled
Glass
Issue: Crushed, graded recycled glass can be an attractive ground cover or plant
mulch.
Before designing glass
into any landscape project,
however, the designer should
have a thorough understanding
of how to specify the glass. Best Practice: The designer considering using crushed glass for landscaping
should consider all of the issues
below. Gradation: Usually the designer will not want glass to look like broken bottles.
Because of the platy
nature of glass, if the glass
is much larger in maximum dimension
than the thickness of a container
or a window, many of the pieces
will look like broken bottles. For example, containers and windows are usually
1/4-inch or less in thickness.
Pieces larger than 1/4-inch
will have a flat dimension over
twice the thickness. Also, some pieces will show the curvature of
the container.
In addition, larger pieces
will not have been processed
by the type of “tumbling” processor described below, and are more likely to have
sharp edges.
Generally, 3/8-inch and
finer glass should be used in
landscape projects. Dust:
The designer should estimate
the finest aggregate desired
in the landscape project.
Glass pieces smaller
than 100 mesh can become airborne
dust, will slow down the drainage
of water through the glass,
and will wash to the bottom
of the glass layer. The designer should consider whether the installation
is in a location where wind
may carry the glass dust into
a public area.
Although airborne glass
dust has been shown to be no
more dangerous than airborne
sand (see the Analysis of Glass Dusts Best Practice), the public
perception of glass dust may
not be something the site managers
want to deal with.
Pieces of glass finer
than 20 mesh will also make
the glass area less reflective,
if a shimmering look is desired.
A glass processing system
with proper dust control can
remove glass fines without additional
screening (see the Dust Control Strategies for Glass Processing Best
Practice). Shape:
All glass processors do not
produce the same shape of product. In general, “single pass” impact crushers,
where the glass is shattered
with single impact of a hammer,
tend to produce shardy material.
Glass processors that
have either multiple rows of
hammers that beat the glass
into cubical shapes or a centrifugal
action that tumbles the broken
glass on itself to produce an
autogenous shape produce a more
aesthetically appealing crushed
glass. Cleanliness: Glass that has been processed as furnace-ready cullet for container
manufacturing appears to be
dirty.
This is because residual
moisture and sugars in the glass
cause glass dust and ambient
dirt to adhere to the glass
surfaces.
Glass can generally be
rinsed off fairly easily. One method that has worked well is to put graded
glass into a cement tumbler
and flood it with water while
the tumbler turns, overflowing
water until it runs clean. Contamination: Most of the contaminants in glass can be screened off in the sizing
process.
Glass used in landscaping
applications, however, must
be especially contaminant-free. Any loose paper labels or metal caps will stand
out in a landscape environment.
Processing to this quality
may require a system including
pre-crushing, drying, crushing,
screening, and dust control.
For an understanding
of the type of system required
to produce the level of contaminant
removal needed, see the Small-Scale
Glass Processing Costs Best
Practice. Color:
In general, the designer has
three choices: clear, green,
and amber. Blue glass is available, but it may be difficult
to accumulate enough for a substantial
installation. Clear window glass reduced to particles 1/8-inch
and smaller can look white as
snow.
If only the sizes 1/8-inch
to 1/4-inch are used, glass
is more reflective. Glass that is 1/8-inch to 1/4-inch with water
running or spraying over it
shimmers like a million diamonds. The designer should integrate the color with
the environment and think about
the appearance of the site as
the landscape matures.
Glass that looked white
initially may turn gray from
dust in a dry urban environment
or from plants shedding organic
matter.
The green of container
glass may clash with some plant
colors. Over time the native soils may migrate into glass, making it necessary
to either renew the glass layer
or turn the glass into the soil,
using the glass as a natural
aggregate.
The designer needs to
think long-term. Environmental Compatibility: It is common to see weeds growing in stockpiles of
recycled glass.
However, as a more formal
test of whether there are any
obvious compatibility problems
between crushed glass and plant
growth, a study was performed
testing glass as a hydroponic
growth medium (1).
No statistical difference
was seen between basil plants
grown in glass and those grown
in a conventional hydroponic
medium. Glass without fines will drain quickly. However, water retention may be a desirable characteristic. Glass particles will track on shoes in the
same manner as brick pieces
or rock chips. Cost:
Green or mixed-color recycled
glass is available for free
or at minimal cost in most parts
of the United States.
However, processing the
glass to landscaping quality
can cost from $20 to $50 per
ton.
There are some specialty
glass processors who make a
clean enough product to be used
in landscape applications (for
example, call TriVitro
Corp., Kent, Washington).
In some cities there
are specialty aggregate processors
who can process glass to custom
specifications.
It may also be possible
to obtain glass from a container
plant beneficiator and clean
it on-site for acceptability. Implementation: Designers should always see
what they’re buying and, if
possible, build models or small
pilot projects to judge the
appearance of glass in the proper
context. Benefits: Glass can be a beautiful and functional landscaping
material if the issues described
above are considered. Application Sites: Landscape design offices. Contact: For more information about
this Best Practice, contact
CWC, mailto:info@cwc.org. References: Glass as a Hydroponic Rooting Medium,
Clean Washington Center Report
GL-96-2, 1996. Issue Date / Update: November 1996 |
