TECHNICAL PAPER #43
UNDERSTANDING GLASS RECYCLING
W. Richard Ott
Dr. Louis Navias
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
Understanding Glass Recycling
[C]1986, Volunteers in Technical
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
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
They are not intended to provide construction or
details. People are urged to contact VITA or a similar
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
almost entirely by VITA Volunteer technical experts on a
voluntary basis. Some 500 volunteers were involved in the
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Betsey
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
Richard Ott is the Dean of the Ceramic College at Alfred
in New York. The reviewers are also VITA volunteers. Mr.
Mir Ali is the President of Glass & Ceramics
of Lomita, California. Dr. Louis Navias is a consultant
in ceramic engineering. Mr. William Mahoney is the Manager
of Corporate Product Affairs for the Ball Corporation in Muncie,
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries. VITA
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
situations. VITA maintains an international Inquiry Service,
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING GLASS RECYCLING
By VITA Volunteer W. Richard Ott
The first glass vessels were formed over 3,500 years ago
the sand core method. In this method, a clay-sand core of
was formed on a metal rod and dipped several times into a
bath of molten glass. The vessel was then formed by digging
the core, leaving the hollow glass shell. These containers
as valuable as gold and were used until broken. Glass
techniques, invented about 1,500 years ago, made glass
more widely available, but they were still relatively
The Industrial Revolution changed all that. Glass, which had
begun as a luxury, became a common commodity. The glass
now produces billions of bottles and millions of tons of
glass each year. With that increase in production has come
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
glass. Nevertheless, despite the relatively low cost of
is still quite expensive in many areas of the Third World.
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
glass are not limited to these examples and there may indeed
far more inventive ways to reuse glass than are cited here.
is important to remember that any effort to recycle glass
geared to the demand for either the glass itself or objects
from it. Because of this, it is critical to work at
II. OPERATING PRINCIPLES
Glass is a hard, transparent or translucent brittle material
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
dioxide), which is heated until it melts and then allowed to
in a controlled process. The temperature needed to melt sand
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
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
[CaCO.sub.3]), or magnesium oxide (dolomite, MgO) is added
to the mix.
A number of other materials may added, depending on the type
glass desired. Metal oxides, for example, such as iron,
chromium, or copper, may be used to produce glass in colors
ranging from light green to deep blue to topaz yellow. Lead
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
most cullet is waste or reject glass from the manufacturing
operation. However, glass from other sources can be used.
is particularly true in the glass container industry, where
composition does not vary substantially from one
another. It is evident that when uniform composition must be
maintained, the composition of the cullet must be the same
composition of the glass being produced. Cullet is normally
crushed and stored in much the same way as the other raw
The use of cullet serves two purposes. The first is that
the scrap glass saves the cost of raw materials. The second
that cullet aids in the melting process. Some glass melting
operations use up to 60 percent cullet. For some purposes,
possible to make glass entirely of cullet.
TYPES OF GLASS
While it is possible to form glass out of many materials,
all high volume commercial glass is formed from silica as
major constituent. Specialty glasses may be formed from
borates, germanates, etc. Because these glasses have
high performance applications that make recycling difficult,
paper is restricted to silica-based glasses.
Commercial silica glass can be classified into several
The person considering recycling glass must be aware of
these categories, as each has different properties and
The types of glass include:
1. Fused silica
glass: A pure silica or silicon dioxide ([SiO.sub.2])
excellent chemical durability and very low thermal
very low thermal expansion results in excellent
resistance. However, high processing
limit use of this glass to special applications.
This kind of
glass is not commonly recycled.
glass: The earliest glass made and still
the most common.
It is used for plate glass, window sheet
glass, and electric lamp bulb glass. Typical
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
Table 1. Typical Composition of Window Sheet Glass, Plate
Glass, and Electric Lamp Glass
Type of Glass
Oxide(*) (percent by weight)
0 ) (SiO)
(Na O/K CO)
(*) Iron oxide is
normally held in the range of 0.1 percent.
(**) Increased alumina in the container formulation improves
3. Lead oxide-alkali
silicate glass: The calcium oxide normally
in other glasses
is replaced in these glasses by lead oxide
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
oxide-alkali silicate glass containing 15 to 30 percent
lead oxide. These
glasses have unique optical and
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
etched, or otherwise decorated glass objects.
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
likely to be encountered in a glass recycling project.
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
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
have capacities ranging from 10
to 1,500 tons.
Since a unit of glass stays in the furnace
hours on the average, these capacities are
the average daily
output. Two small-scale glass furnaces are
shown in the
standpoint of reycling, it is important to note
operations require constant attention,
controls, and a steady supply of raw materials
or cullet) of unvarying quality. Suppliers
of cullet must be able to assure plant
operators of the
both the quantity and the quality of their
2. Forming: The
liquid glass is next fabricated into a useful
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
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
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
4. Finishing: Any
sharp edges or stresses developed during
removed and surface coatings are applied if
or fire polishing, in which a flame is
played over the
surface to remove sharp edges, are other
The basic difficulty in making glass recycling profitable is
glass itself does not have a high value as a material. The
of the materials from which glass is made represents only a
fraction of the value of a finished glass product
10 to 20 percent, and this percentage falls as the product
more complex). In other words, the cost of an item made of
glass is determined largely by the complexity of the
that the item requires, and by the volume of production.
Waste glass can be put to new use in several ways:
by reusing it for its original purpose;
by altering the original item to make new
by using it as cullet in the manufacture of
by using it as a substitute for a raw
used in the manufacture of some product;
by using it as a raw material in a newly
The most direct method of making use of waste glass is to
it to its original purpose.
Flat glass or tubing can be salvaged, cleaned, perhaps cut,
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
sections of broken glass. (Note that tempered plate glass
In the case of a glass container, an example of
reuse is to again fill the container with the same kind of
that it originally held. A soda bottle, for example, would
returned to the bottler, cleaned, and refilled with soda. In
areas, bottles and jars are reused for everything from
to medicines because the goods are produced at home or sold
unpackaged. That is, the purchaser must provide the
Local markets throughout Africa and other parts of the Third
World usually contain at least one stall where glass bottles
jars are sold to be reused in this way.
Beverage bottles refilled by commercial bottlers must be
for reuse. Even so, such reuse eventually causes even
designed for it to weaken, and frequently to fail. A bottle
failure while the bottle is in the filling line can result
expensive short-term shutdown of the system, and a failure while
the bottle is in the consumer's possession can result in
injury. Large product liability settlements in the United
have reduced the economic feasibility of container reuse.
must be checked carefully before reuse to be sure they are
of nicks, chips, cracks, or other defects.
Adequate cleaning is another problem. A bottle used to
only the intended product can be cleaned thoroughly enough,
consumers sometimes use bottles to store insecticides,
etc. In those cases, standard cleaning techniques may not be
suitable or sufficient.
In many areas, beverage bottles are not intended to be
is likely that governmental action in such places would be
to require large-scale introduction (or re-introduction) of
recyclable bottles. If you seek governmental action to
container recycling, be sure that it applies to all
(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
Sometimes glass containers and other objects can be made
useful or novelty items. Bottles can be cut to make drinking
glasses, funnels, candle holders, vases, etc. Preparation of
items requires very little capital investment, and they can
sold for cash. As with any item to be produced for sale, it
necessary to determine first of all if a market exists and
regular supply of raw material can be obtained.
Examples of such uses in developing countries are numerous.
organization in Colombia manufactures solar water heaters
use recycled fluorescent light bulbs as tubing. A volunteer
Papua New Guinea used burned out incandescent light bulbs to
beakers, specimen dishes, and other equipment for his
classes. A small shopkeeper in Recife, Brazil, makes and
lamps and other items made from bottles and jars that are
painted with floral designs or traditional motifs.
Glass bottles can be cut with an electric wire, with an
glass cutter, or by wrapping them with a string and burning
string. Special cutters for use on bottles are also
some areas and may be worth the investment if enough of a
for the glass products exists.
The electric wire method requires a small electric
that reduces the usual 230 or 240 volts to about 15 volts.
wire, which can be taken from an electrical appliance such
iron, is wrapped tightly around the bottle or jar at the
place where the cut is desired. The wire is attached to the
transformer and the electric current is turned on. After a
or so, when the wire is red hot, the current is turned off,
wire quickly removed, and the bottle plunged into cold
bottle will crack along the line where it was heated.
Precautions must be taken with this method. The transformer
deliver a fatal shock if any of the live metal parts on the
volt side are touched. And the red hot wire can cause
When using an ordinary glass cutter, a groove is made along
line where the cut is to be made. The area along the groove
heated with an alcohol lamp or similar burner and the bottle
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
heated area. This method is probably most successful with
glass, such as the lightbulbs used to make lab equipment for
With all of these methods, the sharp edges of the cuts must
smoothed away. A carborundum stone, used for sharpening
and tools, is probably the best material for working glass.
Drinking glasses and similar containers should have the
edge smoothed and rounded so they will be safer and more
Another approach is to sell the glass as cullet to a local
of container or flat glass, assuming that there is one.
Economic Feasibility. The economic feasibility of this
depends on the cost of cleaning and crushing the glass, and
transporting the cullet, which is heavy. It also depends on
price that the manufacturer is willing to pay; the
access to alternative raw materials will play a large part
determining this. Anyone considering recycling glass as
needs to prepare a careful cost analysis.
Reliability of Supply. The reliability of the cullet supply
you can offer is another important factor. Glass tanks are
to control; changing a batch by increasing or decreasing the
percentage of cullet may not be economically feasible.
the cullet supply must be dependable in quantity and in
Container glass is usually the most suitable kind of glass
use as cullet, because its composition tends to be
the same from manufacturer to manufacturer.
Markets. The primary market for cullet is the manufacturer
bottles and jars for packaging food, but there may be other
of glass objects that can be made with cullet. These include
handicrafts, fiber glass, household goods such as vases,
etc. Cullet may also be used as a portion of the materials
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
supply must be carefully considered. Since glass does not
great material value and since raw materials must be
manufacture it, it will probably not win a place as a
for plentiful natural minerals like silica or feldspar.
it can be used in several classes of product.
Coarse Aggregate for Concrete. Concrete contains a
quantity of aggregate, often crushed stone. Low alkali
(not container glass or flat glass) can be substituted for
of the crushed stone. (Only low alkali glasses should be
this purpose, because alkali released from the glass often
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
Lightweight Aggregate for Structural Concrete. Since the
1960s, the U.S. Bureau of Mines has been doing research on
to put municipal wastes to use. One of their findings has
that waste glass can be used as the principal raw material
producing lightweight aggregate suitable for use in
A mixture of 78 percent waste glass, 20 percent clay, and 2
percent dry sodium silicate fired to 1,550[degrees] F for 15
produced aggregate with a bulk density of 38 pounds per
foot. Glass aggregate concrete with an average unit weight
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
was 102 pounds per cubic foot, and the compressive strength
3,025 pounds per square inch. To meet ASTM (American Society
Testing and Materials) standard C-330-69, concrete having a
weight of 105 pounds per cubic foot must have a minimum
strength of 2,500 pounds per square inch. The study
that alkali reactivity did not appear to be a problem when
this technology was used.
Recycled Glass in Ceramic Products. Recycled glass is
composition to feldspar, a naturally occurring material and
common ingredient in the clays used to make ceramic
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
have shown that recycled glass can be substituted for
in porcelain bodies, tile bodies, and decorative artificial
stone. Some energy saving is associated with the use of
glass in these applications.
Most of the ceramic products into which recycled glass can
substituted are relatively low performance materials. The
strengths of the bodies are well in excess of those needed
perform their functions, and exterior appearance is the
factor. Consequently, the substitution of a similar raw
(recycled glass) in small quantities is harmless. However,
advantages are at best marginal. With only slight value
the product, it is difficult to overcome the expense of
crushing, and cleaning the glass. That, and the possible
uncertainty of supply, has made most manufacturers unwilling
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
is the most suitable raw material. To do this, you need to
the basic vitreous, inorganic nature of glass, and take
advantage of it to create a unique product that meets a need
the local market.
The creation of inorganic foams is an excellent example of
approach. The process requires glass as a starting material;
there are no natural minerals that can be substituted for
glass; and the resulting product has distinct advantages
competing products in the marketplace.
When glass is mixed with a foaming agent that gives off its
at the same temperature at which the glass softens, that gas
cause the glass to foam. The result is a product that, when
properly processed, can be used for thermal and acoustical
One version of foamed glass is made by crushing waste
glass to a uniform fine particle size, and then mixing it
bentonite, calcium carbonate, and water. The pressed mixture
then placed in a furnace where it foams. The finished
resembles a slab of polystyrene foam except that it is
is fireproof, impervious to water or acid damage, and can be
easily cut with a saw. Several different foaming agents have
used in this process. Cow manure is the most
There may be a number of applications of this type. It takes
imagination and an understanding of glass to invent a
that is appropriate for a given place.
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
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
Collecting unbroken bottles or jars to be sold for reuse or
alteration will be difficult. In most developing countries
containers are used and reused until they break and have to
discarded, and finding them depends mostly on chance.
reliable sources might be breweries or soft drink bottlers
throw away containers that are still in basically good shape
which could not withstand another round in the factory's
Hotels, restaurants, and schools; and trash dumps in
neighborhoods are other possibilities.
The most plentiful type of recyclable material is likely to
broken glass that can be sold as cullet. Cullet can be
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
factories; and so on. If enough of these types of
exist to make the effort worthwhile, the collector might
provide containers for workers to use for the broken glass
they throw it away.
To bring the best price, cullet usually has to be sorted by
and color and given at least a preliminary cleaning. It is a
idea to check with potential customers about the types of
they are most interested in (and thus will pay most for). Do
include such things as car and truck window glass, light
glass reinforced with wire, or dark colored glass.
Collecting glass requires a means of transporting it,
for sorting and storing, and tools for cleaning and
Glass is heavy and potentially dangerous, and a large
will likely bring a better price than a few bags or boxes
sturdy cart or wagon, preferably one that can be tipped
for dumping, will make quantity hauling easier. Glass should
sorted as it is put into storage: different types and colors
should be placed directly into containers. In this way the
will only have to be handled once, reducing risk of injury
labor costs, and the containers can be loaded directly into
wagon for delivery.
Containers should be large enough to be useful and small
to be manageable; one half of a standard petrol drum is a
size. Handlers should wear gloves and footwear (preferably
boots), and goggles and other protective clothing as
to minimize the chance of injury. A first aid kit should be
at hand, and any cuts should be treated immediately, no
how small, to avoid potentially deadly infections. A stiff
and a flat shovel or scoop are needed to keep the area
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.
this case, water must be available, and this can be a
expense in some areas.
Before starting into this or any business, it is a good idea
check out any legal restrictions. In some areas glass recycling
may be regulated by law. Health and sanitation laws may
the activity to certain areas or neighborhoods. Safety
may be required. This is true even in the big cities in
developing countries, where a large percentage of the people
make their living as trash pickers. Cairo is one such city,
attempting to reduce health and sanitation threats by, for
requiring trash collectors to use small tractor-drawn wagons
instead of donkey carts that obstruct traffic and generate
own clean up problem. Other regulations may apply in other
and should be investigated.
As noted above, some objects can be made entirely of cullet.
final product is not a high quality glass, but it can be
If your area is not served by a glass manufacturer, and if
sufficient waste glass is available, it may be possible to
a small furnace to remelt the cullet and produce jars,
drinking glasses, or other products. Be advised that even a
simple operation requires a fair amount of capital and a
deal of skill, and may require months or years to bring a
on the investment. If you have the money and skill, and if
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
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