Recycled Glass and Windows

This article focuses on the most common uses of glass in building construction.^[1] Glass is one of the most durable materials used in construction and requires no maintenance other than cleaning.

Window glass is soda-lime-silica glass, that is, it is made mostly of silica (white quartz sand), sodium oxide (soda ash), lime, and small amounts of magnesia and alumina. Other agents may be added for color, opacity, character, or to eliminate or introduce bubbles.^[2] Glass is transparent, dense, and brittle. For construction purposes, it is formed into flat sheets with a hard smooth level surface. Large pieces can be heavy and awkward to carry. Cut and broken edges are very sharp and can easily slice unprotected skin.

Why recycle glass, windows, and doors?[edit | edit source]

Production vs. Recycling[edit | edit source]

Producing one ton of glass from virgin materials requires:

• 1,300 pounds of sand
• 400 pounds of soda ash
• 400 pounds of limestone
• 151 pounds of feldspar
• 24,000 gallons of water
• 15 million BTUs of energy
• this mix melts at 2,800 degrees Fahrenheit.

Recycling one ton of glass:

• requires 32% less energy
• requires 50% less water
• creates 20% less air pollution
• saves 10 gallons of oil
• saves 1.2 tons of raw materials
• melts at 2,500 degrees Fahrenheit^[3]

There are other methods of recycling glass than just making more glass. Glass fibre (fiberglass) and glass wool mat, blanket, or board. Also, pipe cover, thermal insulation board, ceiling board, and acoustical insulation board. It can be ground fine and used for cement replacement. It can be made into ceramic tiles, paving blocks, added to asphalt and concrete.^[4]

Some glass recycling is one way. Old window glass is recycled in the production of container glass or formed glass, but float glass manufacture requires very pure raw materials and therefore recycled glass cannot be added to the melt.^[5] Although it can be used for low quality glass products such as back encapsulation for solar cells.^[6]

Reusing Glass[edit | edit source]

Reusing glass, doors, and windows requires none of the materials or energy shown above and produces no pollution.

Manufacturing 1000 kg of glass can generate up to 200 kg of mining waste and 14 kg of air pollutants (dust, sulfur dioxide, chlorine, and fluorine.)

Significant amounts of energy are required in glass manufacture. The embedded (embodied) energy of two types of glass commonly found as salvage glass is:

• float glass -- 15.9 MJ/kg
• toughened (tempered) -- 26.4 MJ/kg^[7]

Measuring glass is easier than weighing it. Here are conversions for commonly available glass by thickness.

Architectural Flat Glass - Approximate Weight^[8]

Thickness Inches (mm)	Approximate lb/ft2 (kg/m2)
3/32 (2.5)	1.2 (5.7)
1/8 (3.0)	1.6 (7.6)
5/32 (4.0)	2.0 (9.9)
3/16 (5.0)	2.4 (11.9)
1/4 (6.0)	3.0 (14.6)

When considering construction materials, for embedded energy savings, choose glass. If you can use salvage glass, all the better.

Embedded energy estimates of selected engineering materials (MJ/kg)^[9]

Material	MJ/kg
Aluminum, virgin (recycled)	191 (8.1)
Float glass	15.9
Polypropylene	64.0
Steel, virgin (recycled)	32.0 (10.1)

Types of glass[edit | edit source]

Window and door glass used today is almost entirely float glass which is a processing method developed in the early 1950s. This process greatly improved the surface uniformity and therefore decreased variable surface reflection. Float glass was commonly used in the United States beginning in the late 1950s and early 1960s. If you see wavy reflections in older windows, these were probably installed before float glass came into use and are generally sheet glass, an older processing method. Polished plate glass, a more expensive finishing method, was also used before float glass. If you are recycling glass from older structures, you will find these different types of glass. The other type of glass commonly found is tempered glass.

Where to find used glass and windows[edit | edit source]

• Glass shops
• Used construction materials or salvage businesses, such as Resale Lumber
• Penny-saver newspapers, such as Tri-City
• Remodeling sites and remodeling businesses
• Demolition sites
• Yard sales and giveaways

How to identify[edit | edit source]

In everyday use, ordinary window glass is identified by its thickness:

• 3/32” (2.16-2.57 mm) – Single Strength glass is used in small windows and picture frames
• 1/8” (4.32-5.14 mm) -- Double Strength is used in windows and larger picture frames
• 3/16” and ¼” Plate is used for large windows, shelves, table tops
• 3/8”, ½”, and ¾” is used for larger table tops and shelves

Window and door glass colors[edit | edit source]

• Clear Glass (edges are green)
• Crystal (edges are white)
• Light Grey
• Dark Grey
• Bronze
• Green
• Blue

Specialty glass[edit | edit source]

• Laminated Glass -- Two or more glass layers with one or more interlayers of plastic (PVB) or resin. The interlayer holds the pieces together and provides resistance to objects penetrating the glass.
• Wire Glass or Wired Glass -- Glass with a wire mess inserted into the glass. It provides impact resistance similar to normal glass but in case of breakage the mesh holds the glass pieces together
• Patterned Glass – patterns rolled into glass during manufacture.
• Plexiglass -- Acrylic or polycarbonate material and is available in different colors and thicknesses.
• Frosted Glass or Obscure Glass -- Glass with uneven surface that causes light to be diffused which provides a level of privacy
• Tempered Glass or Toughened Glass -- Stronger than normal glass. It is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.^[10] Suitable for glass façades, sliding doors, building entrances, bath and shower enclosures and other uses requiring superior strength and safety properties. Tempered glass cannot be cut or drilled without shattering.
• Low-Emissivity Glass: Glass coated with a low-emissivity substance can reflect radiant infrared energy, encouraging radiant heat to remain on the same side of the glass from which it originated, while letting visible light pass. This often results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.^[11]

Glazing terms[edit | edit source]

• Beveled Edge - Edge finished at an angle (beveled, angled)
• Glazing - Refers to the use of glass as a material (window, door, etc.)
• Glazing Stop or Bead -- Strip of material (wood, metal, etc.) used to hold the glass
• Glazing Compound or Putty - Soft material used to hold the glass
• Hackout – Glazier’s term for a broken window
• Insulated Unit - Two or more layers of glass enclosing a hermetically-sealed air space with a primary function to reduce heat loss lowering energy consumption and reducing the occurrence of condensation on the warm air side.
• Lights – the individual panes of glass in wood-frame or leaded windows
• Mirror - Smooth, polished glass surface that forms an image when light is reflected. The mirror backing (color) can be silver, gray, bronze, etc. providing different appearances.
• Single Glazing - Window or door with a single pane of glass
• Weather stripping - Strip of material used on windows and doors to reduce leaks and help prevent rain and wind from entering

Edge finishes[edit | edit source]

• Clean Cut - Rough Edge and Usually Sharp
• Swiped or seamed - Rough Edge with Sharp Edge has been lightly Sanded or Ground
• Flat Polished - Smooth with Flat Edge
• Rounded Polished - Smooth with Rounded Edge
• Beveled - Edge is angled. Narrow and wide bevels.

Glazing tools[edit | edit source]

• Glass cutter and oil: My dad used the least expensive cutters and kept them wheel down in a can half filled with kerosene. My son treated himself to a diamond cutter with an oil reservoir. Both scored glass.
• Glass pliers: Glass pliers close flat and parallel, not at an angle. You use them to grasp small pieces of glass or long thin pieces in the breaking process. You can also “chew” off corners or along a side by crushing tiny areas with the pliers. This is a way to salvage a piece of glass cut slightly too large, by nibbling along an edge that is too narrow to cut and break. The old adage, measure twice, cut once, is especially applicable to glass.
• Cutting table: Glass shops use large, sturdy, absolutely flat tables, usually with tightly looped, thin carpeting covering the top and edges. You can cut glass on a study table or on the floor, padded with towels, felt, or a thick blanket. A clean, level wooden work table should also work well.
• Putty knife, points, single-edged razors: Putty knives are usually about an inch in width. Points are small pieces of triangular-shaped metal. Hardware stores sell boxes of single-edged razors.

Additional glazing materials and how they are used[edit | edit source]

• Putty: Used with wood-framed windows. The wood frame is cleaned of any dirt, debris, and old putty. Glass is placed in the wood frame with about an 1/8” ease. Shims can be used to center the glass vertically in the opening (wooden matches without the tip work well in small windows.) Two to eight points are placed flat on the glass and pushed part way into the wood frame to temporarily hold the glass in place. Putty is run on all edges and over the points. Glaziers knead and warm the putty in their hands and then push it into the window edges with the fleshy part of their palms. Use a putty knife to smooth and remove excess putty at a beveled angle even with both edges of the frame, covering glass edges and points. Putty needs to be painted to resist moisture. Paint slightly on the glass to seal the glass, putty, and wood assembly.
• Vinyl: U-shaped vinyl is used with aluminum framed windows. The vinyl is stretched slightly and placed over the cut edges of the glass. The outside of the vinyl is snipped halfway through at the corners to allow a smooth turn. Vinyl ends are butted together. Aluminum frames are screwed together around the glass which is cushioned by the vinyl.
• Silicon: Useful for sealing many types of windows and skylights and sealing aquariums. Use a wet finger to push into place and shape. Trim and clean up excess with razor blades.

How to cut glass[edit | edit source]

You don’t really cut glass, you score it and then break it along the score.

1. Clean the surface of the glass, especially where you plan to score. Grease or grime may cause the glass cutter to skip and not score the glass.
2. Practice on a scrap piece of glass. Start as close to an edge without the carbine wheel of the cutter slipping over the edge. Apply an even pressure as you pull the cutter along the surface. The score should run from one edge of the glass to the other edge. Listen for a smooth sound like ripping silk. A gritty sound means that you are pushing too hard. If you push too hard, your cut gets "hot", meaning that it snaps and pops. If you press too lightly, the cutter will skip and the score will not be continuous. Aim for a uniform score. Try cutting freehand and then using a straightedge. Grasp the cutter in any manner in which you can exert a smooth, even pressure. Most scores for window glass are made by pulling the cutter toward you. If the glass is large, you can move along the table pulling the cutter. You can also push the cutter away from you when cutting small pieces, curves and shapes. This allows you to better see where the cutter is going and to follow a pattern placed under the glass, such as for leaded windows. Keep the cutter vertical.
3. There are several ways to break the glass after scoring.
   • The edge of a glass table can be used to break large pieces of glass by placing the score along the edge and then lifting the glass up and then bouncing it down. The larger piece will remain on the table, the smaller cutoff will remain in your hands. I don’t recommend this for beginners.
   • Medium to small size pieces can be broken by placing the glass cutter or a pencil at the edge of the glass just under the score. Press gently down on both sides gradually increasing pressure until the glass fractures along the score. Sometimes it helps to gently tap the underside of the glass at the edge directly under the score to start the fracture. Some glass cutters have a metal ball for this purpose. The fracture will shine and you will be able to see it “run” along the score.
   • Very small pieces are best broken using pliers on the smallest side and using a pulling and breaking motion, starting gently and increasing the pressure until the glass has broken into two pieces.

Recycled aluminum frame windows and doors[edit | edit source]

These are widely available and are the most common source for double strength and tempered glass. The aluminum can be taken to a recycling center and the glass reused. Windows generally have double-strength glass and sliding glass doors and shower doors should be tempered glass. The double-strength edges will probably be sharp and the glass is thinner. Doors will have thicker glass with dull edges. Check the corners of the glass for a “bug,” a slight etched mark. One corner should have this small mark which identifies it as tempered glass. Also, window glass may have scratches and/or bubbles and tempered will not. Tempered glass comes in standard sizes so it is possible to find the same size from different sources. To reuse it, you must build to the size of the glass since it cannot be cut. Aluminum frame windows in good shape can, of course be reused as is. These windows changed the glazing business as they were complete units that could be installed by carpenters and did not require cutting glass to fit on site.

Vinyl windows[edit | edit source]

The current trend in prefabricated windows, they are generally thermo-paned windows manufactured in standard sizes. They are best used in their framed condition as recycling is not as easy as older types of windows. The frames are usually glued PVC vinyl and are recyclable, but the glass is not easily removed from these frames.

Specialty uses for recycled glass[edit | edit source]

It has been clearly identified that post consumer glass is a major component of solid waste. New York City, and similar cities, have a yearly waste of more than 100,000 tons of mixed colored glass. The amount of glass waste is outrageous. Due to this a new material has been created, ashcrete. This new material consist of activation chemicals, Class-F fly ash, coarse and fine aggregates without any portland cement, and waste glass added as an aggregate. The combination of these materials results in glascrete and is essentially 100% recycled waste. Overall this material is low-cost and is very much an environmentally friendly cementitious material. Many use this material in various applications in construction, such as precast concrete.^[12]

Notes[edit | edit source]