When I was a kid, my uncle built a home made rocket. Not an Estes or Century rocket model. A rocket from scratch out of paper towel tubes loaded with a zinc-sulfur fuel. The great day arrived for the maiden voyage and we took the mighty rocket to the Pease River in north central Texas for the launch. He laid a lit cigarette across the fuse and we ran like crazy and hid behind some rocks. The thing lit and burned ferociously but it didn't move so much as an inch off the ground. When the fuel was spent, it just tipped over. In one sense the project was a failure. But it did turn out to be my first exposure to glass-making. Underneath the rocket was a red-hot mass of fused and melted sand. When it cooled we picked it up as a sheet of (ugly) glass about 2 feet across. Not what we were after, but neat nevertheless.
In this class we have already seen and used a form of glass called obsidian. This glass emerges in the molten state from active volcanos and cools underground into the glassy substance we have used for making stone tools. So what is the difference between quartz and obsidian? Both are predominantly silica (silicon dioxide). But quartz cools slowly into hard transparent crystals while obsidian cools quickly into dark, glassy, amorphous masses. To understand this difference lets look at a more familiar example.
When I was a kid we used to make home made ice cream using ice and rock salt to freeze the ice cream. Yes, we had electricity in those days. We did it with ice because it's more fun that way. If you don't know what I'm talking about, you should try it some time. Anyway, when you add salt to ice, the ice melts. Since it take heat to melt ice and the heat has to come from somewhere, the mixture gets colder and colder. Adding salt lowers the freezing point of water. If you add a little salt, the freezing point lowers a little, if you add a lot of salt it lowers a lot. This is generally true: the freezing point of a solution is lower than that of either component of the solution.
Now lets look at the reverse process. When salt water freezes, the ice that forms initially is pure water ice. Icebergs floating around in the salty ocean are always fresh water ice. If the ice is allowed to form slowly, the water molecules pack together into a crystaline arrangement and there is no room in this regular arrangement for the salt, so it is excluded from the ice as it freezes. If the salt water is cooled quickly, on the other hand, the salt is frozen into the ice and the normal crystalline arrangement is disrupted.
There is another important aspect to this freezing process when the salt water cools slowly. Lets say we start with salt water that, because of its concentration, freezes at -3 C (instead of the normal 0 C). As the water freezes out as pure ice, there is less water in the solution, so the salt concentration goes up. As the concentration of salt increases, the freezing point goes down. More ice freezes at this lower temperature, reducing the amount of liquid water, increasing the salt concentration, and further lowering the freezing point. So while the mixture started freezing at -3 C, the freezing poing goes down as the solution freezes. This is a general property of solutions: pure substances freeze at a single, well defined temperature, the freezing point; solutions freeze over a range of temperatures.
Lets look at one more example from the icecream. If you start with ice cream frozen rock-hard solid, how does it melt? As it warms up it gets softer and softer until finally it is liquid. But there is no point where you would say now the ice cream has melted. It just gets softer and softer. Contrast that to ice itself. It starts out rock-hard solid. But as it melts it doesn't get softer and softer: it is either solid or liquid, there is no gray area in between. This is another general property of pure crystaline substances and amorphous mixtures. In the crystaline substance all the molecules are locked into a crystal lattice. When they escape from that lattice, they are immediately liquid. But in the amorphous substance they are not in a crystal lattice to begin with. As the amorphous substance melts, the molecules just get more and more mobile. They don't "suddenly" break free as they do in the crystaline case.
So how does all this apply to glass? When quartz forms, it cools from magma slowly. Magma may contain lots of things in addition to silica, but as the quartz crystals grow, the atoms lock into a regular crystal structure which excludes all the impurities. The result is a crystal, which is hard when it is solid, melts at a distinct temperature of 1580 C at which temperature it is "suddenly" liquid.
By contrast, when magma cools suddenly into obsidian, all the impurities are frozen in. There is no regular crystal lattice and consequently, obsidian gets softer and softer as it melts. And as with all solutions, it does this over a range of temperatures which is lower than the melting point of pure silica.
When we make glass, we intentionally add impurities to the silica to lower its melting temperature. A substance which accomplishes this is called a flux. We may also add other minerals to the glaze to impart colors or opacity.
About 5000 BC, the Egyptians discovered that the addition of soda ash to clay produced pottery with a glossy surface. The addition of minerals, like malachite and azurite contributed intense blue colors to this glaze. Unfortunately, these alkaline glazes are quite soft and tend to be somewhat soluble in water. This combination of properties limits the usefulness of these glazes.
A great advance came with the realization that galena, or lead sulfide (PbS) produced a more durable glaze. The galena was crushed and powdered, mixed with water (it is very insoluble) to form a paste, which was then painted onto the surface of the pottery. When fired, galena decomposes to litharge, or lead oxide (PbO). Litharge is an excellent flux and lead glazes have been the dominant glaze family right up to the present day.
While leaded glazes are durable and beautiful, they have the drawback that lead compounds in the glaze can leach out of the glaze into food and drink, particularly acidic foods such as those containing vinegar, tomatoes, or fruit. Soluble lead compounds are acutely toxic, i.e. if you eat them you get sick right now. Insoluble lead compounds are chronically toxic, i.e. you don't get sick today or tomorrow, but the lead can build up over a lifetime and make you sick some time long after your initial exposure. This was probably not a problem when the average life expectancy was 40 years, but as the life expectancy has increased, we are lucky enough to see more and more effects of chronic exposures.
In the last 25 years we have seen increasing public awareness of both acute and chronic lead poisoning. While paint is not designed to be eaten, we have seen lead oxide removed from paint because small children may eat paint chips. While leaded glazes are still allowed in pottery glazes, those designed to contact food must adhere to standards which minimize the possibility of lead leaching out of the glaze into food.
Given this climate, a variety of non-lead fluxes have become increasingly popular. Boron oxide (B2O2) is an excellent low temperature flux and is the one we will use in this project.
All glazes, then, are based on silica, with a flux added to lower the range of
temperatures over which the glaze melts. Lead has been the basis of the most
popular fluxes of all times.
The only commercially important ore of lead is
galena, PbS. When heated in an oxidizing atmosphere, it is converted
to lead oxide, or litharge:
2 PbS(s) + 3 O2(g) -----> 2 PbO(s) + 2 SO2(g)
Litharge has been widely used in pottery glazes throughout history. If the glaze is properly
formulated, an enormous variety of firing temperatures is possible. But if care
is not taken, the glaze may fuse incompletely, leaving soluble lead compounds
on the surface of the pottery. If used for food, these lead compounds may
dissolve in acidic foods, resulting in lead poisoning. Which is not good.
The safety of lead glazes can be improved by producing a lead frit. Litharge, silica, lime, and clay are carefully weighed and mixed. The mixture is melted in a high-temperature kiln until this frit is completely molten. In essence, we are making leaded glass. The frit is cooled, crushed, and mixed with other glaze ingredients. Because the lead compounds have been completly dissolved in the silica, there is little danger of them leaching out into food. Consequently, leaded crystal and lead-bearing glazes prepared in this manner are safe for food. Notice I said "little danger," not "no danger." Lead-bearing glazes must conform to strict standards to ensure that lead levels are below designated concentrations. Remember, the dose makes the poison. Soluble lead compounds in high concentrations are toxic. Insoluble lead compounds in low concentrations can be considered non-toxic. It all depends on how much lead (or any other toxic substance) makes it into the body.
It is also worth mentioning that lead metal is not particularly toxic. Lead is an ingredient used in solder for plumbing. In fact, the symbol for lead, Pb, comes from the latin word plumbum, from which we get the word plumbing. Someone can live with a lead bullet lodged in his body without suffering from lead poisoning. But if that same person were to eat that bullet, some of the lead could react with stomach acid producing lead compounds that are toxic.
Another mineral used to lower the melting temperature of silica is colemanite, or, gerstley borate, with formula (CaO)2(B2O3)3(H2O)5, where I have put in parentheses subunits of the formula you should recognize. Colemanite is one of the most common fluxes in earthenware non-leaded glazes. A simple earthenware glaze can be made by mixing colemanite with calcined kaolinite. Kaolinite, as you will recall, is our prototypical clay. To calcine it, you just heat the bejeesus out of it in the same way that we calcined limestone to produce lime. Calcining kaolinite, as you recall, produces silica and mullite. Another effective borosilicate glaze can be made from colemanite, kaolinite, and silica. Here are three recipes:
| Glaze | Cone | Colemanite | Calcined Kaolinite | Kaolinite | Silica | Limestone |
|---|---|---|---|---|---|---|
| Earthenware I | 09 | 89% | 0% | 7% | 0% | 4% |
| Earthenware II | 06 | 73% | 27% | 0% | 0% | 0% |
| Earthenware III | 06 | 66% | 0% | 9% | 25% | 0% |
The pigments used to dye cloth are not suitable for coloring glazes as they would simply burn off in the kiln. For glazes, we can use a variety of minerals to add color. Here are some common minerals in our sample set used in glazes:
These materials are classified as minerals (M, homogeneous) or rocks (R, heterogeneous). Rocks are further classified as igneous (IR), sedimentary (SR), or metamorphic (MR).
| Use | Mineral or Rock | Chemical | Formula | Picture |
|---|---|---|---|---|
| silica | Flint(SR) | Silicon Dioxide | SiO2 |
|
| green | Malachite(M) | Hydrous Copper Carbonate | Cu2CO3(OH)2 |
|
| blue | Azurite(M) | Hydrous Copper Carbonate | Cu3(CO3)2(OH)2 |
|
| red, black | Hematite(M) | Iron Oxide | Fe2O3 |
|
| silica | Quartzite(MR) | Silicon Dioxide | SiO2 |
|
| flux | Galena(M) | Lead Sulfide | PbS |
|
| white | Limestone(SR) | Calcium Carbonate | CaCO3 |
|
| silica | Obsidian(IR) | Silicon Dioxide | SiO2 |
|
| white | Cassiterite(M) | Tin Oxide | SnO2 |
|
Glass was almost certainly discovered as a by-product of metal smelting. Lead smelts at a relatively low temperature, lower than glass would form. But iron requires high temperatures for smelting. Iron ores tend to have silicate minerals and limestone included in them and as this material is smelted the molten iron settles to the bottom while the "other stuff" forms a molten layer on the top. This molten layer contains silica, limestone, and anything else from the original ore or the added ballast. When this cools, there is no attempt by the smelter to cool it slowly. After all, it is a waste product as far as the smelter is concerned. As it cools it gets harder and harder until it finally forms an amorphous, glassy material called "slag."
As far as the smelter is concned this slag is a waste product. In ancient times it was simply discarded. You may have heard of a "slag heap." Today it is ground up and mixed with cement and cast into "cinder blocks" used ubiquitously as a relatively inexpensive building material. But somewhere along the line, someone got the idea that if you could "pretty up" this slag it might be useful in its own right. To do this, we will start with relatively pure ingrdients rather than whatever happened to be in the metallic ore. In fact, if our goal is glass, the metal is irrelevant, just an accident in the history of glass.
We start with silica in the form of sand. If this is all we added we could just heat it to high temperatures (1580 C) at which it would melt (suddenly, like ice) to a runny liquid. We could cast it in molds of any concievable shape. If we cool it gradually, it will take the shape of the mold just as ice would. But such a glass, "silica glass," has two distinct disadvantages. It can be cast, but not blown, since it is runny in the liquid state. Second, it requires high temperatures to melt it.
We could solve both problems by adding something to the silica. This would lower the melting point and at the same time cause it to soften as it melts. This "something" should be colorless and soluble in silica. A substance that lowers the melting point is called a flux. We have two such materials in our arsenal so far, and they are the materials which were chosen historically for making glass: lime and soda ash.
Unlike a chemical reaction, in which the relative proportions of the reactants are fixed by the balanced chemical equation, glass is just a solution like salt water. We can add more or less lime and soda ash. There is no "right" answer. A typical proportion is 75% sand, 15% soda ash, and 10% lime and such a "soda-lime" glass begins to soften at about 700 C, far lower than silica alone. But there is nothing chemically special about these proportions. Less soda and lime would produce a higher-melting glass. And we could use other "impurities" than soda and lime. Pyrex glass uses soda and borax to lower the softening point of silica to about 800 C. Magnesia (MgO) and alumina (Al2O3 are also used for specialty glasses. And indeed litharge (PbO), one of the intermediates from lead smelting, makes of 30% of "leaded glass" which is used for fine glassware. All of these materials share two qualities: they are white or transparent, and they dissolve in molten silica.
| Silica | Soda-Lime | Pyrex | Lead Crystal | |
|---|---|---|---|---|
| SiO2 | 100% | 73% | 81% | 56% |
| Na2O | 0% | 17% | 4% | 4% |
| K2O | 0% | 0% | 0% | 9% |
| CaO | 0% | 5% | 8% | 0% |
| MgO | 0% | 4% | 7% | 0% |
| PbO | 0% | 0% | 0% | 29% |
| Softening Point | 1580°C | 695°C | 915°C | 630°C |
The glaze quiz consist of three questions on any of the following topics discussed in this page.
While we will not be using lead glazes, I include information on the toxicity of lead compounds here. The chief concern with our glaze ingredients is dust. Dust is bad for you and silica dust can be harmful if the dust is fine enough and if enough of it is inhaled. We are using small amounts and this should not be a problem. You will know that you inhaled some dust because it will make you cough. If the coughing persists, you should call a doctor.
Information on chemical hazards is summarized in a Material Safety Data Sheet for each compound. These sheets often tell you more than you want to know, but they are worth looking at.
You will need to have a piece of bisqued (already fired) ware for this project. This means you must have already completed the pottery project. You will make up about 5 g of one of the earthenware glazes described above. You may make a clear glaze or you may add 2-3% of hematite (iron oxide) or malachite (copper carbonate) for color. Add enough water to turn the glaze into a thin paste and paint it onto your ware. Important: Do not glaze the bottom of your pot or it will stick to the kiln shelf. Also avoid glazing the bottom 1/4 inch of your pot to avoid dripping glaze onto the shelf. Let your pot dry a couple of days and then place it to be fired.
To pass this project you need to pass the glaze quiz. When you have, bring your passing quiz to me along with your glazed pot. To pass, it must have a waterproof, glassy surface.