TECHNICAL PAPER #43
UNDERSTANDING GLASS RECYCLING
By
W. Richard Ott
Technical Reviewers
Dr. Louis Navias
Willam Mahoney
Mir Ali
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
Internet: pr-info@vita.org
Understanding Glass Recycling
ISBN: 0-86619-257-3
[C]1986, Volunteers in Technical
Assistance
PREFACE
This paper is one of a series published by Volunteers in
Technical
Assistance to provide an introduction to specific
state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
situations.
They are not intended to provide construction or
implementation
details. People are urged to contact VITA or a similar
organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis. Some 500 volunteers were involved in the
production
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Betsey
Eisendrath
as editor, Suzanne Brooks handling typesetting and layout,
and Margaret Crouch as project manager.
The author of this paper is a VITA volunteer. VITA Volunteer
W.
Richard Ott is the Dean of the Ceramic College at Alfred
University
in New York. The reviewers are also VITA volunteers. Mr.
Mir Ali is the President of Glass & Ceramics
International, Inc.
of Lomita, California. Dr. Louis Navias is a consultant
specializing
in ceramic engineering. Mr. William Mahoney is the Manager
of Corporate Product Affairs for the Ball Corporation in Muncie,
Indiana.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries. VITA
offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
their
situations. VITA maintains an international Inquiry Service,
a
specialized documentation center, and a computerized roster
of
volunteer technical consultants; manages long-term field
projects;
and publishes a variety of technical manuals and papers.
UNDERSTANDING GLASS RECYCLING
By VITA Volunteer W. Richard Ott
I. INTRODUCTION
The first glass vessels were formed over 3,500 years ago
using
the sand core method. In this method, a clay-sand core of
material
was formed on a metal rod and dipped several times into a
bath of molten glass. The vessel was then formed by digging
out
the core, leaving the hollow glass shell. These containers
were
as valuable as gold and were used until broken. Glass
blowing
techniques, invented about 1,500 years ago, made glass
objects
more widely available, but they were still relatively
precious.
The Industrial Revolution changed all that. Glass, which had
begun as a luxury, became a common commodity. The glass
industry
now produces billions of bottles and millions of tons of
flat
glass each year. With that increase in production has come
the
problem of disposing of or reusing the glass. In industrial
countries, the low cost of glass raw materials has frequently
made it cheaper to produce new glass objects than to reuse
old
glass. Nevertheless, despite the relatively low cost of
glass, it
is still quite expensive in many areas of the Third World.
Recycling
glass refuse may be a way to provide jobs and produce
usable products at lower cost than new manufacture.
This paper briefly describes the production of glass and its
properties, and gives some methods for recycling it. The
uses of
glass are not limited to these examples and there may indeed
be
far more inventive ways to reuse glass than are cited here.
It
is important to remember that any effort to recycle glass
must be
geared to the demand for either the glass itself or objects
made
from it. Because of this, it is critical to work at
identifying
potential markets.
II. OPERATING PRINCIPLES
Glass is a hard, transparent or translucent brittle material
that
is insoluble and nonflammable. It is capable of withstanding
high temperatures and many corrosive substances.
The primary raw material of glass is high silica sand
(silicon
dioxide), which is heated until it melts and then allowed to
cool
in a controlled process. The temperature needed to melt sand
is
very high--about 1,700[degrees] C--so materials are added to
the sand to
reduce the melting point to about 800[degrees] C. The
commonest of these
materials is sodium oxide ([Na.sub.2]O), which is obtained
from sodium
carbonate (soda ash, [Na.sub.2][CO.sub.3]. Potassium oxide
([K.sub.2]O) is also used
frequently. This mixture is unstable, however, so a stabilizer
such as calcium oxide, derived from calcium carbonate
(limestone,
[CaCO.sub.3]), or magnesium oxide (dolomite, MgO) is added
to the mix.
A number of other materials may added, depending on the type
of
glass desired. Metal oxides, for example, such as iron,
manganese,
chromium, or copper, may be used to produce glass in colors
ranging from light green to deep blue to topaz yellow. Lead
oxide
or potassium oxide (obtained from potash,
[K.sub.2][CO.sub.3]) are used to make
very clear glass.
Another important ingredient in glass manufacture is cullet,
which is scrap or recycled glass that is cleaned and crushed
specifically to be remelted and reused. The principal source
of
most cullet is waste or reject glass from the manufacturing
operation. However, glass from other sources can be used.
This
is particularly true in the glass container industry, where
composition does not vary substantially from one
manufacturer to
another. It is evident that when uniform composition must be
maintained, the composition of the cullet must be the same
as the
composition of the glass being produced. Cullet is normally
crushed and stored in much the same way as the other raw
materials
are.
The use of cullet serves two purposes. The first is that
reusing
the scrap glass saves the cost of raw materials. The second
is
that cullet aids in the melting process. Some glass melting
operations use up to 60 percent cullet. For some purposes,
it is
possible to make glass entirely of cullet.
TYPES OF GLASS
While it is possible to form glass out of many materials,
almost
all high volume commercial glass is formed from silica as
the
major constituent. Specialty glasses may be formed from
phosphates,
borates, germanates, etc. Because these glasses have
high performance applications that make recycling difficult,
this
paper is restricted to silica-based glasses.
Commercial silica glass can be classified into several
categories.
The person considering recycling glass must be aware of
these categories, as each has different properties and
applications.
The types of glass include:
1. Fused silica
glass: A pure silica or silicon dioxide ([SiO.sub.2])
glass with
excellent chemical durability and very low thermal
expansion. The
very low thermal expansion results in excellent
thermal shock
resistance. However, high processing
temperatures
limit use of this glass to special applications.
This kind of
glass is not commonly recycled.
2. Soda-lime-silica
glass: The earliest glass made and still
the most common.
It is used for plate glass, window sheet
glass, container
glass, and electric lamp bulb glass. Typical
compositions are
given in Table 1. Soda-lime-silica glass
is, by far, the
most important glass economically and is the
target of most
glass recycling operations. It is relatively
easy to melt and
fabricate.
Table 1. Typical Composition of Window Sheet Glass, Plate
Glass,
Container
Glass, and Electric Lamp Glass
Type of Glass
Oxide(*) (percent by weight)
Alumina(**) Silica
Sodium Oxide/
Calcium Oxide/
Potassium Oxide
Magnesium Oxide
(Al
0 ) (SiO)
(Na O/K CO)
(CaO/MGO)
Window Sheet
0.5-1.5 71-73
12-15
9.5-13.5
Plate Glass
0.5-1.5 71-73
12-14
11-16
Container Glass
1.5-2.5 70-74
13-16
10-13
Electric Lamp
73-74
16-17 8-9
(*) Iron oxide is
normally held in the range of 0.1 percent.
(**) Increased alumina in the container formulation improves
the
chemical
durability.
3. Lead oxide-alkali
silicate glass: The calcium oxide normally
in other glasses
is replaced in these glasses by lead oxide
(PbO). These
glasses can have up to 80 percent lead oxide,
and are popular
for artistic ware because of their brilliance
and ease of
working. The common glass "crystal" is normally
a lead
oxide-alkali silicate glass containing 15 to 30 percent
lead oxide. These
glasses have unique optical and
electrical
properties that compensate for the added cost of
using lead oxide.
Television faceplates, thermometer tubing,
and neon tubing
are commonly made of this type of glass, as
are engraved,
etched, or otherwise decorated glass objects.
4. Borosilicate
glass: Boric oxide is both a glass former and
a glass modifier.
Heat-resistant glasses (such as Pyrex)
are commonly of
this type. These glasses are typically
about 80 percent
silica, 4 percent sodium oxide, 13 percent
boric oxide, and
2 percent alumina.
There are, of course, a large number of other glasses used
in specialized applications. However, these are the four
most
likely to be encountered in a glass recycling project.
MAKING GLASS
There are four basic steps in manufacturing a glass article:
melting, forming, annealing, and finishing.
1. Melting: The
mixed raw materials are placed in a refractory
vessel and heated
to the melting point. The typical operating
temperature is
1,500[degrees]C. Heating may be done in batches
or in a
continuous operation. In batch furnaces, the size
of the melt can
vary from very small to several tons. Continuous
furnaces typically
have capacities ranging from 10
to 1,500 tons.
Since a unit of glass stays in the furnace
approximately 24
hours on the average, these capacities are
the average daily
output. Two small-scale glass furnaces are
shown in the
drawings below.
ugrdr1x4.gif (600x600)
From the
standpoint of reycling, it is important to note
that large-scale
operations require constant attention,
sophisticated
controls, and a steady supply of raw materials
(mined materials
or cullet) of unvarying quality. Suppliers
of cullet must be able to assure plant
operators of the
reliability of
both the quantity and the quality of their
product.
2. Forming: The
liquid glass is next fabricated into a useful
product. There
are a number of ways in which glass can be
formed. The most
straightforward are to press, blow, roll,
or draw the glass
into the desired shape. Glass containers
are commonly
formed in a two-stage operation by blowing into
a mold. Flat
glass is drawn into sheets.
3. Annealing: The
stresses that have been left in the glass
object must be
removed. This process is called annealing and
requires that the
glass be heated to the annealing temperature
(about
600[degrees]C for soda-lime) and slowly cooled. Failure
to anneal a piece
will usually cause failure (cracking)
of the object.
The large amount of stored energy can cause
the piece to
explode, sending sharp fragments of glass
flying over 10
meters.
4. Finishing: Any
sharp edges or stresses developed during
forming are
removed and surface coatings are applied if
needed. Grinding
or fire polishing, in which a flame is
played over the
surface to remove sharp edges, are other
frequently used
finishing processes.
III. RECYCLING
The basic difficulty in making glass recycling profitable is
that
glass itself does not have a high value as a material. The
value
of the materials from which glass is made represents only a
small
fraction of the value of a finished glass product
(approximately
10 to 20 percent, and this percentage falls as the product
becomes
more complex). In other words, the cost of an item made of
glass is determined largely by the complexity of the
processing
that the item requires, and by the volume of production.
APPROACHES
Waste glass can be put to new use in several ways:
o
by reusing it for its original purpose;
o
by altering the original item to make new
products;
o
by using it as cullet in the manufacture of
new glass;
o
by using it as a substitute for a raw
material that
is currently
used in the manufacture of some product;
or
o
by using it as a raw material in a newly
invented
product.
Reuse
The most direct method of making use of waste glass is to
return
it to its original purpose.
Flat glass or tubing can be salvaged, cleaned, perhaps cut,
and
then placed back in service. This process tends to be labor
intensive, but is usually straightforward. In the flat glass
industry it means, for example, cutting window panes from
large
sections of broken glass. (Note that tempered plate glass
cannot
be cut.)
In the case of a glass container, an example of
straightforward
reuse is to again fill the container with the same kind of
material
that it originally held. A soda bottle, for example, would
be
returned to the bottler, cleaned, and refilled with soda. In
many
areas, bottles and jars are reused for everything from
beverages
to medicines because the goods are produced at home or sold
unpackaged. That is, the purchaser must provide the
container.
Local markets throughout Africa and other parts of the Third
World usually contain at least one stall where glass bottles
and
jars are sold to be reused in this way.
Beverage bottles refilled by commercial bottlers must be
designed
for reuse. Even so, such reuse eventually causes even
bottles
designed for it to weaken, and frequently to fail. A bottle
failure while the bottle is in the filling line can result
in an
expensive short-term shutdown of the system, and a failure while
the bottle is in the consumer's possession can result in
personal
injury. Large product liability settlements in the United
States
have reduced the economic feasibility of container reuse.
Bottles
must be checked carefully before reuse to be sure they are
free
of nicks, chips, cracks, or other defects.
Adequate cleaning is another problem. A bottle used to
contain
only the intended product can be cleaned thoroughly enough,
but
consumers sometimes use bottles to store insecticides,
poisons,
etc. In those cases, standard cleaning techniques may not be
suitable or sufficient.
In many areas, beverage bottles are not intended to be
reused. It
is likely that governmental action in such places would be
necessary
to require large-scale introduction (or re-introduction) of
recyclable bottles. If you seek governmental action to
require
container recycling, be sure that it applies to all
materials
(i.e., to plastic, paper, and metal containers as well as to
glass ones). If the law applies only to glass, it will tend
simply to eliminate the glass container industry in your
area.
Alteration
Sometimes glass containers and other objects can be made
into new
useful or novelty items. Bottles can be cut to make drinking
glasses, funnels, candle holders, vases, etc. Preparation of
such
items requires very little capital investment, and they can
be
sold for cash. As with any item to be produced for sale, it
is
necessary to determine first of all if a market exists and
if a
regular supply of raw material can be obtained.
Examples of such uses in developing countries are numerous.
An
organization in Colombia manufactures solar water heaters
that
use recycled fluorescent light bulbs as tubing. A volunteer
in
Papua New Guinea used burned out incandescent light bulbs to
make
beakers, specimen dishes, and other equipment for his
science
classes. A small shopkeeper in Recife, Brazil, makes and
sells
lamps and other items made from bottles and jars that are
cut and
painted with floral designs or traditional motifs.
Glass bottles can be cut with an electric wire, with an
ordinary
glass cutter, or by wrapping them with a string and burning
the
string. Special cutters for use on bottles are also
available in
some areas and may be worth the investment if enough of a
market
for the glass products exists.
The electric wire method requires a small electric
transformer
that reduces the usual 230 or 240 volts to about 15 volts.
The
wire, which can be taken from an electrical appliance such
as an
iron, is wrapped tightly around the bottle or jar at the
exact
place where the cut is desired. The wire is attached to the
transformer and the electric current is turned on. After a
minute
or so, when the wire is red hot, the current is turned off,
the
wire quickly removed, and the bottle plunged into cold
water. The
bottle will crack along the line where it was heated.
Precautions must be taken with this method. The transformer
can
deliver a fatal shock if any of the live metal parts on the
230
volt side are touched. And the red hot wire can cause
serious
burns.
When using an ordinary glass cutter, a groove is made along
the
line where the cut is to be made. The area along the groove
is
heated with an alcohol lamp or similar burner and the bottle
is
immediately dipped in cold water, where it will crack at the
groove (it may have to be helped along with a slight tap).
The "burning string" method uses the same
principle as the electric
wire. A fairly fine string is wrapped tightly around the
bottle and set afire. When the string has burned, the glass
piece, again, is plunged into cold water and will crack
along the
heated area. This method is probably most successful with
thinner
glass, such as the lightbulbs used to make lab equipment for
schools.
With all of these methods, the sharp edges of the cuts must
be
smoothed away. A carborundum stone, used for sharpening
knives
and tools, is probably the best material for working glass.
Drinking glasses and similar containers should have the
entire
edge smoothed and rounded so they will be safer and more
comfortable
to use.
Cullet
Another approach is to sell the glass as cullet to a local
manufacturer
of container or flat glass, assuming that there is one.
Economic Feasibility. The economic feasibility of this
approach
depends on the cost of cleaning and crushing the glass, and
of
transporting the cullet, which is heavy. It also depends on
the
price that the manufacturer is willing to pay; the
manufacturer's
access to alternative raw materials will play a large part
in
determining this. Anyone considering recycling glass as
cullet
needs to prepare a careful cost analysis.
Reliability of Supply. The reliability of the cullet supply
that
you can offer is another important factor. Glass tanks are
difficult
to control; changing a batch by increasing or decreasing the
percentage of cullet may not be economically feasible.
Therefore,
the cullet supply must be dependable in quantity and in
quality.
Container glass is usually the most suitable kind of glass
for
use as cullet, because its composition tends to be
essentially
the same from manufacturer to manufacturer.
Markets. The primary market for cullet is the manufacturer
of
bottles and jars for packaging food, but there may be other
types
of glass objects that can be made with cullet. These include
handicrafts, fiber glass, household goods such as vases,
beads,
etc. Cullet may also be used as a portion of the materials
that
go into concrete or ceramic products, as described below.
Glass as a Substitute for a Raw Material
With this approach too, economic feasibility and reliability
of
supply must be carefully considered. Since glass does not
have
great material value and since raw materials must be
processed to
manufacture it, it will probably not win a place as a
substitute
for plentiful natural minerals like silica or feldspar.
However,
it can be used in several classes of product.
Coarse Aggregate for Concrete. Concrete contains a
substantial
quantity of aggregate, often crushed stone. Low alkali
glasses
(not container glass or flat glass) can be substituted for
much
of the crushed stone. (Only low alkali glasses should be
used for
this purpose, because alkali released from the glass often
causes
the concrete to expand and crack.)
The substitute of glass for crushed rock can help solve a waste
disposal problem, but represents virtually the minimum
product
value possible.
Lightweight Aggregate for Structural Concrete. Since the
late
1960s, the U.S. Bureau of Mines has been doing research on
ways
to put municipal wastes to use. One of their findings has
been
that waste glass can be used as the principal raw material
in
producing lightweight aggregate suitable for use in
structural
concrete.
A mixture of 78 percent waste glass, 20 percent clay, and 2
percent dry sodium silicate fired to 1,550[degrees] F for 15
minutes
produced aggregate with a bulk density of 38 pounds per
cubic
foot. Glass aggregate concrete with an average unit weight
of 104
pounds per cubic foot had an average compressive strength of
2,550 pounds per square inch after steam curing for 28 days.
After one year of exposure to weather conditions, the unit
weight
was 102 pounds per cubic foot, and the compressive strength
was
3,025 pounds per square inch. To meet ASTM (American Society
for
Testing and Materials) standard C-330-69, concrete having a
unit
weight of 105 pounds per cubic foot must have a minimum
compressive
strength of 2,500 pounds per square inch. The study
concluded
that alkali reactivity did not appear to be a problem when
this technology was used.
Recycled Glass in Ceramic Products. Recycled glass is
similar in
composition to feldspar, a naturally occurring material and
a
common ingredient in the clays used to make ceramic
products.
Consequently, glass can be added in varying amounts to most
ceramic products. Studies have shown that face brick can be
produced on a commercial scale without difficulty. Similar
studies
have shown that recycled glass can be substituted for
feldspar
in porcelain bodies, tile bodies, and decorative artificial
stone. Some energy saving is associated with the use of
recycled
glass in these applications.
Most of the ceramic products into which recycled glass can
be
substituted are relatively low performance materials. The
strengths of the bodies are well in excess of those needed
to
perform their functions, and exterior appearance is the
primary
factor. Consequently, the substitution of a similar raw
material
(recycled glass) in small quantities is harmless. However,
the
advantages are at best marginal. With only slight value
added to
the product, it is difficult to overcome the expense of
collecting,
crushing, and cleaning the glass. That, and the possible
uncertainty of supply, has made most manufacturers unwilling
to
switch their processes to utilize waste glass.
Glass as Raw Material in a New Product
The most promising approach is to find a process for which
glass
is the most suitable raw material. To do this, you need to
understand
the basic vitreous, inorganic nature of glass, and take
advantage of it to create a unique product that meets a need
of
the local market.
The creation of inorganic foams is an excellent example of
this
approach. The process requires glass as a starting material;
there are no natural minerals that can be substituted for
the
glass; and the resulting product has distinct advantages
over
competing products in the marketplace.
When glass is mixed with a foaming agent that gives off its
gas
at the same temperature at which the glass softens, that gas
will
cause the glass to foam. The result is a product that, when
properly processed, can be used for thermal and acoustical
insulation.
One version of foamed glass is made by crushing waste
glass to a uniform fine particle size, and then mixing it
with
bentonite, calcium carbonate, and water. The pressed mixture
is
then placed in a furnace where it foams. The finished
product
resembles a slab of polystyrene foam except that it is
rigid. It
is fireproof, impervious to water or acid damage, and can be
easily cut with a saw. Several different foaming agents have
been
used in this process. Cow manure is the most
straightforward.
There may be a number of applications of this type. It takes
imagination and an understanding of glass to invent a
product
that is appropriate for a given place.
OPERATIONS
As the basis of a small business, glass recycling requires a
reliable source of raw materials and a minimum amount of
equipment. It also requires space for sorting, cleaning, and
storing the glass. Before making any kind of investment, it
is
very important to try to determine what the market is for
recycled glass, whether in the form of refillable bottles or
jars, cullet, or glass items to be turned into usable
products.
Raw Materials
Collecting unbroken bottles or jars to be sold for reuse or
alteration will be difficult. In most developing countries
such
containers are used and reused until they break and have to
be
discarded, and finding them depends mostly on chance.
Possible
reliable sources might be breweries or soft drink bottlers
that
throw away containers that are still in basically good shape
but
which could not withstand another round in the factory's
equipment.
Hotels, restaurants, and schools; and trash dumps in
wealthier
neighborhoods are other possibilities.
The most plentiful type of recyclable material is likely to
be
broken glass that can be sold as cullet. Cullet can be
collected
from such places as hotels and restaurants; food processing
plants that package products such as preserves or fruits and
vegetables in glass bottles or jars; dairies and bottlers of
both beer and soda; trash dumps; glass cutting shops and
window
factories; and so on. If enough of these types of
establishments
exist to make the effort worthwhile, the collector might
even
provide containers for workers to use for the broken glass
when
they throw it away.
To bring the best price, cullet usually has to be sorted by
type
and color and given at least a preliminary cleaning. It is a
good
idea to check with potential customers about the types of
glass
they are most interested in (and thus will pay most for). Do
not
include such things as car and truck window glass, light
bulbs,
glass reinforced with wire, or dark colored glass.
Equipment
Collecting glass requires a means of transporting it,
facilities
for sorting and storing, and tools for cleaning and
handling.
Glass is heavy and potentially dangerous, and a large
quantity
will likely bring a better price than a few bags or boxes
full. A
sturdy cart or wagon, preferably one that can be tipped
easily
for dumping, will make quantity hauling easier. Glass should
be
sorted as it is put into storage: different types and colors
should be placed directly into containers. In this way the
glass
will only have to be handled once, reducing risk of injury
and
labor costs, and the containers can be loaded directly into
the
wagon for delivery.
Containers should be large enough to be useful and small
enough
to be manageable; one half of a standard petrol drum is a
good
size. Handlers should wear gloves and footwear (preferably
boots), and goggles and other protective clothing as
necessary,
to minimize the chance of injury. A first aid kit should be
kept
at hand, and any cuts should be treated immediately, no
matter
how small, to avoid potentially deadly infections. A stiff
broom
and a flat shovel or scoop are needed to keep the area
clean--absolutely
essential for the protection of workers and any children
or animals that may come along, as well as to avoid flat
tires on vehicles that may be used.
Customers may insist that the cullet be washed before sale.
In
this case, water must be available, and this can be a
considerable
expense in some areas.
Legal Considerations
Before starting into this or any business, it is a good idea
to
check out any legal restrictions. In some areas glass recycling
may be regulated by law. Health and sanitation laws may
restrict
the activity to certain areas or neighborhoods. Safety
precautions
may be required. This is true even in the big cities in
developing countries, where a large percentage of the people
may
make their living as trash pickers. Cairo is one such city,
attempting to reduce health and sanitation threats by, for
example,
requiring trash collectors to use small tractor-drawn wagons
instead of donkey carts that obstruct traffic and generate
their
own clean up problem. Other regulations may apply in other
areas,
and should be investigated.
OTHER OPTIONS
As noted above, some objects can be made entirely of cullet.
The
final product is not a high quality glass, but it can be
servicable.
If your area is not served by a glass manufacturer, and if
sufficient waste glass is available, it may be possible to
set up
a small furnace to remelt the cullet and produce jars,
bottles,
drinking glasses, or other products. Be advised that even a
simple operation requires a fair amount of capital and a
great
deal of skill, and may require months or years to bring a
return
on the investment. If you have the money and skill, and if
you
think the necessary markets and raw materials are available,
consult a technical assistance organization like VITA or the
Intermediate Technology Development Group for guidance in
setting
up shop.
Bibliography
Augustinik, A.I., Sintsova, I.T., "Manufacture of High
Strength
Porcelain,"
Silikattecknik, 19(4), 111-114, 1968.
Bell, J.M., "The Physical and Chemical Composition of
Municipal
Refuse,"
American Public Works Association Reporter, 29, 1,
11, 1962.
Borax Consolidated Limited, Glasses, London, 1965.
Cutler, Ivan, "Insulation from Recycled Glass,"
Department of
Materials
Science, University of Utah.
Holscher, H.H., "Hollow and Specialty Glass: Background
and
Challenge,"
Vol. 46, June-November, 1965.
Johnston, C.D., "Waste Glass as Coarse Aggregate for
Concrete,"
Testing and
Evaluation, 2, 950, 344-350.
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