How Glass Works

All of your questions about architectural glass, answered.


What is glass?

The primary component of glass is silica. Silica is made of silicon and oxygen, earths two most abundant elements. Silicon doesn’t occur naturally uncombined. You will find it in 95% of rocks. Quartz is about as close as you can get to finding pure silica. While glass and quartz are both made of silica they are very different structurally. Glass is cooled before it has a chance to form crystals, it is considered an amorphous solid, essentially a liquid in a solid state. Quartz does have a crystalline structure giving it distinct characteristics. It is much stronger. Additionally, manufactured glass will have additives that effect the final product.

How is glass Made?

Glass for architectural applications must start off flat and uniform. Annealed glass is basically how glass is born. Silica is combined with other materials like limestone and soda ash. The raw materials are melted. Then the molten glass is floated on molten metal, typically tin for it’s low melting point. Then the glass is slowly and uniformly cooled in a controlled environment, or annealed. This process ensures a uniform thickness. The width of the ribbon of molten glass spilling onto tin will determine the width of the sheet. The length will be the interval at which the glass is cut. The speed of the float line will determine the glass thickness.

You may also hear this type of glass referred to as sheet glass, flat glass, plate glass or float glass. Float glass is the foundational product for most glass used in construction. Annealed glass can then be cut and fabricated. There are about 40 manufacturers with 230 float glass plants worldwide. Just a couple of plants are in the western United States. Float glass is then shipped to fabrication plants. A lot of what comes into CA is from China.

Where does glass come from? Can I get glass made in the USA?

It’s nearly impossible to trace back the various sources of materials in glass. As is true of most finished manufactured goods. The raw materials are mined from various sources and brought to a batch plant. There materials get weighed and mixed to exacting standards specific to the glass type that will be produced. It then goes to a float plant where flat, sheet glass is created. From there it goes to a fabricator where glass can be cut, tempered or further fabricated to specifications.

It makes sense to do the fabrication nearest to the final market. Most are located in industrial zones just outside of metropolitan areas. This ensures timely delivery of goods. You can probably assume the glass you get for your building project was fabricated in the USA. Those fabricators might source it from a United States float plant or internationally. Their supplier may vary depending on factors in the supply chain, varying energy costs and prices. The National Glass Association has compiled a worldwide map of float plants and fabricators. You can see that a lot of fabricators are concentrated in the US. The glass industry is providing a lot of good manufacturing jobs for Americans.

How does glass Break?

Annealed glass will crack and shatter into sharp, dangerous fragments of varying sizes. Tempered glass “explodes” into many pieces that are small. If the glass is laminated or embedded with wire it will hold the glass in place when broken. This has various implications for safety and security.

Why does it matter how glass breaks?

Broken glass is extremely dangerous. Anything subject to human impacts needs to break in the safest possible manner. This can be achieved by breaking into very small pieces that are less likely to severely cut or stab people, or by staying in place when broken. Any glass that is installed above where people are present, also needs to be safety glass. The abilty of glass to stay in place after breaking is also important for fire safety, as it minimizes air flow and prevents the spread of fire and smoke.

What is tempered glass?

When tempered glass breaks, it explodes into a bunch of tiny pieces that shimmer like ice. If you’ve ever been woken by an explosion in the middle of the night to discover a pile of glass you can’t explain and wondered why did my glass explode, that’s your culprit. The good news is that unless you were standing nearby with your eyes wide open, anything more than a minor cut is unlikely. Tempered glass is four times stronger than annealed glass. So that’s positive. However, it has some weaknesses The edge is one weakness. We have to be very careful transporting tempered glass as one false move with the edge and kablooey. In an architectural situation, much of the time edges aren’t exposed, whew! Metal, however, is the other weakness. (any criminals reading should check out some cute cat videos now.)All it takes is one good pop, and because of the way it breaks, the entire opening is exposed for safe and easy access. Annealed is a much better method for collecting DNA left at the scene. Just kidding, we are not advocating passive violence.

Why did my glass explode?

Have you ever woken in the night to the sound of an explosion to find a pile of broken glass? Tempered glass is the culprit. The process of tempering glass creates a stored tension in the glass. This is due to the rapid cooling that causes the surface and edges to be compressed. Counterintuitively, it makes the glass stronger and safer.

What causes spontaneous glass breakage?

There are several reasons glass might break for no apparent reason. There might be some internal defects that occurred during manufacture or tempering. Several years can pass before the defects cause the glass to break. Chips in the glass edge can also cause a delayed break. Improper installation might be the culprit. Glass edges should not be installed in direct contact with metal as thermal expansion or wind deflection may cause contact and breakage. Those same stresses may cause breakage if proper space for expansion wasn't incorporated. Building settling or shifting as from seismic activity may also put stress on the glass leading to breakage. Glass must also be thick enough to withstand the wind load where it is installed. In any particular instance, it is difficult to definitively determine the exact cause.

Can holes be drilled in tempered glass?

Once glass is tempered it can't be cut or drilled, otherwise it will explode. However, you can have tempered glass with holes as long as they are specified before tempering. It is a common misconception that screw mounted backsplashes can't be tempered. Any design with tempered glass can be accommodated as long as it is planned.

What is laminated glass?

Laminated glass, or lami, is made up of two panes of glass with a plastic interlayer. The glass panes can be tempered or annealed. Visually it looks like a single piece of glass, but the plastic adds an additional layer of protection.

The windshield of your car is laminated glass. If the windshield gets a crack, it stays in place because of the interlayer. The same is true if someone tries to throw a brick through a laminated glass window. It’s not an impenetrable barrier, but it can be enough to deter a would be intruder and it will at the very least slow them down.

What is annealed Glass?

As mentioned above in how is glass made, annealed glass is how glass is born, glass at it’s most basic level. It is the most familiar. Annealed is the glass that shatters when you have to bust through it to escape the bad guys. In fact one of the sharp and very dangerous shards, when carefully placed, may fall with impeccable timing on said bad guys. Spoiler alert, movie glass is made from sugar. Annealed is the weakest type of glass and also very dangerous. Thickness can help a little with strength, but not safety.

What is wire glass?

Wire glass is the glass that has a wire mesh or chicken wire embedded in it. Wire glass is made from annealed glass, it can’t be tempered. Wire glass is half as strong as annealed glass.

The advantage of wire glass is that it will hold the pieces of glass together pretty well when it's broken. That is why you might see it in your old high school gym. Slamming into annealed glass is very dangerous because it will break into large dangerous pieces capable of causing fatal injuries. Wire glass would have been used until the 60’s and 70’s when tempered glass became more widely available. It was formerly considered a safety glass, but does not meet current standards to qualify. There are better, safer options for use in areas that may harm people.

Wired glass is considered a fire-glass not only because it offers some fire-resistance, but also because it remains in place when broken to help mitigate air flow and the spread of fire and smoke. It is still commonly used on commercial doors in hallways or stairwells as a fire-safety measure. The wire mesh layer is superior to laminated glass interlayer in resisting heat. It is recommended for use in small openings, not subject to human impact.

Broken Wire Glass

What is the sheet size & block size of glass?

When glass is made at the float plant, it is cut into uniform sizes, stock sheets, that are then crated and shipped to fabrication facilities.

Fabricators are limited to manufacturing pieces that can be cut from the stock sheet. That means if a stock sheet is 48” x 84” the longest side cannot exceed 84” and the short side must be less than 48”. Inevitably there will be scrap. This waste glass is referred to as offcuts.

Fabricators will typically charge for the block size of glass. So if a notch is cut from the glass, such as to accommodate a pony wall or shower bench. The square footage of the discarded cut-out will be included in the cost. Since each stock sheet, once cut, has scrap pieces and labor cost embedded into each cut, manufacturers will typically have a 2 or 3 square foot minimum charge per piece of glass.

What happens to the waste glass & offcuts?

It usually goes to a landfill. Sometimes architectural glass is crushed into cullet glass for recycling.

What is cullet glass?

Cullet glass is waste glass that is crushed for use in recycling. It can be sourced internally, at the plant where architectural or float glass is manufactured. There is also pre-consumer cullet that comes from the offcuts and waste at the fabricating plant. Lastly, there is end-of-life cullet that would come from construction demolition, any glass that won’t be reused. End-of life glass is rarely recycled.

Can plate glass be recycled?

It can, but it’s not. Architectural glass cannot be recycled curbside mostly due to varying melting points. It would be considered a contaminant. Also anything broken is unsafe and not accepted. Flat glass can’t be processed at standard regional recycling centers. A specialized facility is required and they are not common. Specialized facilities can accept most architectural glass, including insulated glass that must have spacers removed, laminated glass and coated glass as impurities will burn off in the float oven. However fire-rated glass, ceramic glass and anything contaminated with metals can’t be processed with plate glass.

As of this writing there is no accurate resource of where these specialized facilities are located or how contractors and other consumers can participate. We have not been able to identify a local recycler.

As an end consumer, your best bet is to try and reuse it. One of the the best reuses for it is to polish it into decorative sea glass. Crushed glass also has potential as backfill. We are not aware of any local contractors using it or the implications regarding building code. It is most likely to be used as backfill in projects like road construction. We don’t recommend it for residential use. People do it. When digging around an older home if you encounter a bunch of broken glass it is because someone used it as backfill at one time.

Can I buy architectural glass made from recycled glass?

No. Post-consumer glass would make a feed stock of varying compositions. This takes away control over the finished product. Architectural glass has some of the highest standards for glass that cannot be met using recycled materials.

There may be a percentage of internal(manufacturer) or pre-consumer(fabricator) cullet in your flat glass, but it is not something that is necessarily traced you could specifically request. Any cullet glass that isn’t recycled into flat glass can still be used in other applications like container glass, fiber glass or highway beads.

What type of glass is the most eco-friendly?

As mentioned in where glass comes from, it’s impossible to trace back through the supply chain and figure out ethical and sustainable sourcing. Generally speaking, annealed glass will have a lower energy use. Purchasing locally from a supplier who purchases from a local fabricator will reduce fuel use for transportation, but honestly that has very little impact overall. It is a good idea to purchase direct from a contractor rather than a retailer to avoid unnecessary packaging.

As is true of any attempts at conscientious consumption, reduce is the guiding principle. Buy less. That is followed by reuse. Salvaged materials will be significantly greener than new, but probably not cheaper. If you can reduce or reuse that will always be superior to any other choices available to consumers.

If you must purchase new, consider getting products that will last the longest, have a timeless quality and require the least maintenance. Also factor in energy efficiency in the case of windows.

In truth, when it comes to choosing a glazier(glass contractor), eco-friendliness shouldn't carry much weight in deciding who to work with. We are all subject to the same constraints of local suppliers and recycling availability.

Do solar windows exist?

Yes, SolarWindow Technologies has created an invisible photovoltaic coating that produces solar energy. It is currently being used on some innovative buildings as pilot projects. Unfortunately, we can’t get our hands on it yet as it’s not available to the public. It looks like it will be cost effective for new builds. It’s definitely a technology worth keeping an eye on.

How can mirrors rot?

You may have heard of something called mirror rot and thought glass doesn’t rot. Mirror rot is real and it’s not entirely a misnomer. The good news is, it’s not mold. It’s oxidized metal or rust. Mirror rot, or desilvering, happens when moisture is able to get between the mirror silvering and the glass. It will start along the edges as small black tarnished dots that grow.

How can I prevent mirror rot?

Since mirror rot is a result of moisture intrusion causing tarnish, it’s important to keep mirrors dry. Always use a vent fan when showering or bathing. It’s better to use a towel to wipe a mirror than a squeegee as squeegeeing can cause puddling along the bottom edge. If a mirror is installed where it meets a countertop, we recommend sealing the bottom edge with silicone. It’s a good idea to reseal it periodically as you would with any bathroom caulking. Never use ammonia. Ammonia or any other caustic cleaner will desilver the mirror and allow moisture to enter. Popular blue glass cleaners contain ammonia. Spraying cleaner directly on to glass will make the problem worse, especially if it drips down and is allowed to pool at the edge. It’s a common house cleaning practice to do this. Make sure anyone who will be cleaning the mirror knows not to use ammonia or allow cleaners to puddle.

How can I get rid of mirror rot?

Mirror rot can be cut off or covered. It can’t be bleached off. You may have some success with a spray rust remover lubricant and a toothbrush. This will temporarily remove the black tarnish without helping the desilvering.

Most unframed mirrors are installed by gluing directly to the wall. Sometimes they can be removed in one piece and reused, but I wouldn’t count on it. It is very rare to end up with a usable mirror that was glued in. Mirror rot typically occurs along the edge so it can be cut off, but it requires removing it from the wall. Since the mirror will end up smaller, that may not be an acceptable solution.

Maybe you have a framed or vintage mirror with black spots you want to keep. Anywhere there are black spots you can assume the silvering was damaged at some point. You can restore the mirror, but don’t expect it to be perfect.

Start by removing the black tarnish. Make sure any chemicals you use are suitable for mirrors and won’t cause further damage to the silvering. The same goes for what you use to scrub. Don’t use foil, for example. Instead use something with soft bristles. Reseal the area then use a silver paint or tape to cover it. Tape is a better option because it is less toxic and easier to replace if the tarnish returns. If it’s a small area with minor damage, it won’t be very noticeable. Be sure to avoid using adhesives on repaired areas.

Restored mirrors will be particularly susceptible to reoccurrences so keep them out of moist areas like bathrooms.

If you choose to cut the mirror or remove the tarnish, be sure to seal the mirror or it won't be long before it happens again.

Alternatively you can frame it to cover the edges. Beware though as framing may hold more moisture to the edges, exacerbating the problem.

Most people will find that the best solution is just to replace the mirror. No matter what you end up doing, take some steps to prevent mirror rot or all of your efforts will be for nought.

Is there such a thing as a frameless sliding shower door?

The short answer is no. However, there is a give and take as to how frameless you can get. If you want a double slider, where both sides can slide open, you will need both a header and a bottom sill. There is no way around that. However, you may be able to get away with no side jambs or clear side jambs. If you only need one side to slide and the other side can be a fixed panel, you have more options. You won’t need side jambs at all. You will have the option to go headerless or go without a bottom threshold. If you opt to forgo the header there will still be a bottom door guide and vice versa. Using a header in favor of foregoing the bottom sill is generally preferred for tub enclosures. Folding shower doors are a great alternative to consider.