An example of stained glass.

This is a royalty-free photo from Pixabay.


The inspiration for this article started with a short conversation or thread I had with Dr. Randy Holmes-Farley. When I asked about what kinds of things I could put in a saltwater tank that wouldn't hurt the water column and leach bad things into it, he mentioned not to put in objects made of float glass because of how float glass is made on a bed of metal, usually tin.

I started reading about glass and float glass and learned that 90 percent of flat glass made today is float glass. Then I thought, I wonder if the aquariums people buy are made of float glass, and whether the side of the glass that has been against the metal is on the inside or outside of the tank. I also wondered if those that DIY their own tanks are aware of this issue. I couldn't find a lot (or any) mention of this, so I thought "Aha!" I could write about this. It also didn't hurt that my husband had owned an architectural glass company for 30 years, and I had a resident expert.

So, I hope you find this topic as interesting as I do.

History of Glass

The experts don't agree on when man started making glass. I've read papers that say anything from 5000 BCE to 3500 BCE. There is some agreement that glass-making started in or near Mesopotamia, spread to Egypt, and soon all along the eastern side of the Mediterranean was the glass-making center of the universe, starting with glass glazes.

After glass glazes came core-formed glass, in which glass was wrapped around a ceramic core, and after the whole thing cooled, the core was chipped out, leaving something like a bottle. Early on, artisans discovered that they could make glass different colors by adding some different metals to their basic glass recipe, so, we start to see different colors in glass in early core-formed glass.

There were also glass beads being made in India and China by 1200 BCE, but whether it originated there or the glass-making skills were brought there by others, I can't say.

Then came cast glass. Glass blowing followed and probably came from Syria around 1000 BCE. Then came mosaic glass. You started to see stained glass after 1000 CE. By 1200 CE, glass was being made in Venice (Venetian glass and millefiori glass) and by the late 1200's, glass-making had moved from Venice to Murano, Italy.

So, if we fast-forward to relatively modern times, in 1608, glass started being manufactured in the US, and in 1674, an Englishman named Ravenscroft invented leaded glass. All kinds of glass-making boomed in the late 1800s like many other types of manufacturing, and then in 1959, Sir Alastair Pilkington invented float glass.

What is glass?

Glass is fundamentally silica (SiO2--sorry I can't do subscripts or superscripts in this interface) or sand. If you take silica sand and heat it high enough, like to 2000 degrees C (3600F), it melts, and when it cools, you get glass. Well, that's the gist of it.

And that's probably how people in history discovered it by accident. Ancient glass was more than 90 percent silica. The problem with this high percentage of silica was that super high melting point required. Then some clever Sumerian or Akkadian or Babylonian figured out that if some other ingredients were added to the glass recipe, the melting point was lowered, which made it much easier to work with.

This is what's called Melting Point Depression, which is in simple terms that the melting point of many compounds can be lowered if you add some impurities to them. And, by the way, this is different from Boiling Point Elevation, which is that you can raise the boiling point of solvents (like water) if you add impurities (like salt or sugar.)

Today, most flat glass is what's called soda-lime glass. The glass is only about 70-75% silica, about 15% soda (sodium oxide, Na2O), and about 10% lime or calcium oxide, CaO. Roughly. In fact, 90% of all glass produced today is soda-lime glass. Soda comes from naturally occurring soda ash, and lime comes from, for example, limestone.

And naturally occurring glass exists like obsidian, a volcanic glass that is formed when rocks that are primarily (guess what?) silica get heated, launched out of a volcano, and cool.


This is a royalty-free photo from Pixabay.

What's useful to know is that the more silica is in the glass, the higher the melting point and harder the glass is. The less silica is in the glass, the lower the melting point and the softer it is. That's why soda-lime glass lends itself to recycling--because it can be melted down and reformed easily.

One more thing: glass is not a liquid and not a solid. It's called an amorphous solid. It has a regular structure almost like a crystal, but not quite as rigid or orderly.

Remember that story we all used to hear about glass in churches or buildings that's hundreds of years old and heavier on the bottom is because the glass "flows" over time? Well, now the experts say that's not true, and no one knows why that glass is heavier on the bottom. Although I'm reminded of the quote by the English philosopher and mathematician, Bertrand Russell, who said, "When the experts are agreed, the opposite opinion cannot be held to be certain."

How hard is glass?

This is like asking, how long is a piece of string? I had no idea how complex this subject is until I started researching it.

The reason it's so complicated is that in materials science, hardness is a vague term that's measured in countless ways. For example, how resistant is the material to abrasion? How resistant is it to tension, compression or torsion? How resistant is it to impact, and impact from something sharp or something blunt? How hard, how strong, and how tough is it? How resistant is it to deformation? To fracturing? This list goes on and on.

So, in brief, I'll just say, it's pretty hard, which you probably already know. And the thicker it is, the tougher it is. And the higher the silica content, the harder it is, as I've already mentioned.

The one important caveat here is that glass is hard when it's pure, smooth, and blemish-free. Any imperfections like bubbles, scratches, chips, or any blemishes along the edges compromise the integrity of the glass, often severely, which is one reason why if you're making your own tanks, you should ask the glass supplier to polish the edges.

Now I could spend all day writing about any one of these individual topics, but I'm going to move on.

What types of glass are there?

Here are the basic kinds. For flat glass, there's rolled glass, drawn (or sheet) glass, crown glass, and float glass. What's called "annealed" glass is just float glass that has been annealed or cooled in a special way strengthen it, all float glass is annealed glass. Float glass is called float glass because in the manufacture it's "floated" on a bed of tin. And that soda-lime glass we talked about earlier is float glass.

Then there's cast glass, which is molten glass poured into specially shaped molds. There's borosilicate glass, made with boron trioxide (B2O3), which is very hard and very resistant to thermal shock. Today most laboratory glass is made of borosilicate glass. And a friend of mine who is a glass artist who blows glass told me he uses borosilicate glass.

For flat glass today, there's low-e(missivity) glass which reflects radiant infrared energy but allows visible light to pass through it, laminated glass that has a layer of polyvinyl butyral (PVB) resin inside two panels of (float) glass, and 100 other types of architectural glass and others that I won't bore you with here.

But since the glass that interests aquarists is certain types of flat glass, let's concentrate on that.

Architectural glass, tinted and probably "low-e".

This is a royalty-free photo from Pixabay.

Flat glass for aquariums

And now we get to the aquarium part. Finally.

With float glass, which is what most people use, it's annealed, and it can be heat-strengthened, or tempered. And I almost forgot to mention that tempered glass must be drilled *before* it's tempered.

Aquarists also like low-iron glass, which is "extra clear."

So, plain, old, normal soda-lime float glass is the standard. You may notice that float glass has a very slight green tinge to it, which is one reason why people like the low-iron glass--like Starphire--which doesn't. The low-iron glass, however, is very slightly softer, and very slightly more prone to scratches (sorry.) There exists extra-clear float glass, but I don't know how easy it is to get.

Of course, there's the issue of whether the tin side of float glass should be on the inside or outside of the aquarium, which is how this article started. Does tin leach into the aquarium? I don't know. Anybody want to run some tests on that? Tin is not something we typically test for. But since I haven't seen this topic discussed a lot, I doubt this is something to worry about.

I learned reading a lot of not-too-fun-to-read academic papers, many of which are listed below, that the tin side is slightly softer than the not-tin side of the glass because of microscopic imperfections that the metal layer makes on the glass surface.

So, if your kids play soccer in the room where your 200-G sits, then you might want the air side on the outside. If you are housing an extended family of mantis shrimp, then you might want the air side on the inside. You'd also want the air side on the inside if you're worried about metals (tin) leaching into the water.

And how can you tell the difference between sides? Well, there is some very sophisticated laboratory equipment that can do it (see YouTube), and there are also a couple of easy at-home tests you can do.

One is to clean the glass thoroughly and then drop one droplet of water onto the flat (horizontal) glass surface from about an inch away. If the droplet stands up high and tight, it's the air side. If the droplet lays down and spreads, it's the metal side.

The other test is to shine a short-wave ultraviolet light onto the glass at a 45 degree angle. The metal side will glow. And only short-wave UV will work.

As far as heat-strengthened and tempered glass go, heat-strengthened is roughly twice as hard as regular float glass, and tempered glass is about four to five times as hard as regular float glass depending on who you ask. And this information was confirmed to me by Justin Mayfield, in the Kopp Glass marketing department. Kopp Glass manufactures "technical glass for demanding applications" since 1926.

The advantage of using float (annealed) glass (instead of tempered or laminated) is that A) it's cheaper than tempered or laminated glass, and B) if you get a crack, you can sometimes repair it.

The disadvantage of tempered (which is harder) is that A) it's more expensive than plain float glass and B) as many of you know already, if it breaks or is compromised, it breaks all over, and all falls out. In fact, if you break the surface tension of tempered glass, like scratching it with a diamond ring, the glass comes apart.

What broken tempered glass looks like, right before it collapses.

This is a royalty-free photo from Pixabay.

Now if float glass breaks, it breaks into shards of all shapes and sizes. Sharp pieces. Tempered glass breaks into little pebble-sized blunt pieces that aren't (very) sharp. And I once experienced this myself while driving on the highway through the Rocky Mountains one winter when it was minus 25C outside.

I took a big rock to my driver's side window, heard a loud bang, turned my head to the left, saw the impact spot, and within a couple of seconds saw spider web cracks, and after a couple of seconds more, the whole window shattered and fell out onto me. This is why in construction, by code, tempered glass is not to be used in overhead applications or skylights.

We don't hear a lot of stories of people using laminated glass for aquariums, but you certainly could, especially in a big aquarium. The advantage of laminated glass is that when it breaks it holds together and won't fall out of the opening, so you *might* have time to empty your tank and save some livestock. And this is why it's often used as the glass in the windows of buildings that are prone to break-ins because it's hard to get through it even when it's broken.

Laminated glass is made of float glass, so it's not harder than float glass; it just holds together better.

And now for a story. My husband told me that many years ago, his glass company did the glass for one of the exhibits at the Vancouver Aquarium. It was the public viewing glass into a pool for whales. He said for them it was an unusual project because architectural glass companies don't often do the glass for applications where the glass has to withstand so much weight and pressure.

He said the glass was several different thicknesses and types of glass laminated together into a pane of glass (called a "light" in the business) that was at least an inch and a half thick, for them a crazy and amusing thickness and weight of glass. He said the glass was anchored into a channel with rockite, and then some kind of marine-grade silicone sealed everything up. A distant memory of an interesting project.

He also added that this is how bullet-resistant glass is made--with several different types of glass laminated together--and that it's not bullet-proof just bullet-resistant. That this type of glass is made specifically to withstand a certain type of bullet, but that doesn't mean another bigger, heavier, faster bullet couldn't pierce it.

And I did read about some new glass being studied that contains palladium and is harder than steel. But I think that's enough for today.

For the purposes of today's article, I did not research relative costs of different types of glass.


Finding the Strength of Glass

Glassmaking: History and Techniques

The Structural Use of Glass

Special thanks:

Special thanks is due today to my husband, who spent a lot of time talking to me about glass and answering a relentless stream of questions. His comment was, "does anyone care about glass this much?" I hope so.


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Author Profile: Cynthia White

Cynthia received her BA in English from NYU. She has been a freelance writer and editor for over 20 years. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait, the Pacific Ocean between Vancouver Island and mainland British Columbia, where the water temperature ranges from about eight degrees C (46F) in the winter to 15 degrees C (60F) in the summer. Bring your dry suit. And some hot coffee.