The practice of blowing glass may seem hip and modern if you’ve recently hit up an art exhibition by Dale Chihuly or Harvey Littleton, but the technique of glassblowing has actually been around since antiquity. Although not as old as simpler methods like bead making or cutting and carving shapes from larger pieces of glass, glassblowing has been all the rage since the time of the Roman Empire.
First century B.C. Syrians are most widely credited with the serendipitous discovery that glass could be blown from the end of a hollow tube into different shapes or molds — a landmark event in the history of glass manufacturing. Impressively, glassblowing techniques have evolved little since then. While new technologies have introduced more modern equipment, the fundamentals of glassblowing remain the same. But we’ll get into those details in a second — first we need to learn a little more about glass.
Glass is such a ubiquitous product that many of us don’t ever give it much thought. But if you do stop to consider glass, one question that could pop into your head would be — if it’s a solid, how can it be transparent? Well this transparency is due to glass’ interesting molecular structure: It’s a solid, but with a molecular arrangement more akin to a liquid. In most solids, highly organized molecules are rigidly aligned and closely bound. In liquids, the molecules have weaker connections and are positioned more randomly. Known as an amorphous (noncrystalline) solid, glass, while it’s rigid like a solid, has a randomized molecular structure like a liquid. This characteristic (also present in plastics and gels) is why glass is transparent — light waves are better at penetrating through this type of arrangement than through other close-quartered solid molecular arrangements. For more about how this works, read What makes glass transparent?
In the meantime, it’s enough to know that glass can be made out of combinations of many potential ingredients, which create varying levels of traits like transparency, strength and conductivity.
Now that we’ve got a handle on the general properties of glass, on the next page we’ll take a closer look at the kinds of glass used for glassblowing and some other important tools of the trade.
To get things started, let’s go over some of the materials you’ll need. First on the list? Glass, of course. Most of the glass that you run across on a day-to-day basis is a type of oxide glass, and the base component is silica. Silica (or silicon dioxide) is more commonly known as sand. Glassblowers don’t just head to the beach with a bucket though: That sand is too packed with impurities and contaminants. There are certain areas around the world from which glassblowers can get supplies of top-quality sand.
Glassblowers don’t use silicon dioxide alone to make glass — it’s got a really high melting point and it becomes very viscous when it melts. So they add other things into the mix to make the glass easier to blow. These might include different metals and metal oxides, like alumina, magnesia, boron oxide and lead oxide, depending on the properties you desire in the finished product.
Let’s look at one simple recipe for a common glass known as soda lime glass. Soda lime glass can be used in products like light bulbs, bottles, fiberglass, window panes and lots of other applications. It’s generally about 72 percent silica, 15 percent soda (sodium dioxide) and 9 percent lime (calcium oxide). Depending on the product, those amounts — as well as the remaining 4 percent of the ingredients — will vary.
Soda and lime are key additives in glassblowing recipes. They’re examples of what are known as fluxes; they lower the melting point and increase the viscosity (flow rate) of the glass mixture, as well as strengthen it and make it more stable. Other fluxes include alumina, which can make the glass more durable, and zinc oxide, which can promote a brilliant shine while at the same time helping to keep the glass’ molecules from crystallizing (a no-no called devitrification). Barium oxide also helps decrease devitrification and lowers the melting point. Adding lithium will increase the glass’ softness, while lowering its melting point and viscosity.
Although lots of products made out of glass are clear, many others are extremely colorful. Those colors come from adding different metal oxides into the glass during the glassblowing process, and they can appear either transparent or opaque. For example, if you put a little cobalt in your melt, you get deep, rich blue, and a dash of chromium makes an emerald green. A pinch of gold will make a beautiful ruby red, but it’s a tricky one to add and needs to be done in chloride form. Several additives, like silver, copper and manganese vary in the colors they produce. With silver, the color typically depends on how the silver is added to the melt, but with copper you’ve got a grab bag of color possibilities that can easily be altered by other metals in the mix and even factors as unpredictable as the atmospheric conditions in the melting chamber.
Starting to sound a little more challenging, right? Now that we’ve got a better grasp of the materials needed for glassblowing, we’re ready to turn up the heat and see how the process works. Go to the next page to dive in and learn how blowing glass typically goes down.
Glassblowing Techniques: Time to Blow
The batch of glass is mixed and ready to go. Glancing around the glassblower’s workshop (called a hot shop), we see a number of tools and equipment that’ll soon be in use.
First is the initial furnace, inside of which is a pot (sometimes called a crucible). In a process called charging, the furnace is filled with large amounts of batch that melt at temperatures higher than 2,000 degrees Fahrenheit (about 1,100 degrees Celsius).
While glassblowing can be done individually, it’s so challenging that it’s often done by a team. When things start to really get underway, the lead glassblower, called the gaffer, reaches for his blowpipe, which is usually made of iron or steel and measures about 4 feet (1.2 meters) long. The blowpipe is dipped into the furnace and comes out with a gob of molten glass on the end. After the glass is secured, the other end of the pipe is cooled off in a barrel of water.
Once the gaffer is ready, he’ll blow through the tube and start to create a growing bubble in the glass. Whenever not blowing, the end of the tube is capped so the hot air remains in the glass and helps it retain its shape. More layers of glass can be gathered and added with a gathering iron, or by dipping the glass attached to the blowpipe back in the batch.
Glassblowers often make use of a large, flat surface called a marver to roll and shape the glass. Several tools are also used while working the glass. A block is a wetted wooden shaping tool, as is a bladed tool called a jack. Heat shields and paddles are often employed to shield the blower from extreme heat. The paddles can also double as a tools to smooth out hot glass. Tweezers are another tool useful for grabbing and manipulating pieces of hot glass.
During the blowing process, the parison — or partially blown glass — is turned around and around and bits of glass are often added with the use of a smaller metal rod called a punty, as are various colorants. Additional glass can be joined in a number of ways. For example, it can be laminated on with heat or adhesive, threads and wraps can be laid in decorative patterns as the glass is turned, or shards can be melted in. While all this pulling and shaping is going on, a metal rod called a pontil is attached to the base of the blown glass to hold it while the mouth end is being shaped. The pontil mark is usually ground or polished away later.
While the glass is being blown, but before it’s completely finished, it often cools to the point where it’s unworkable, which is where the glory hole comes in. The glory hole is the second furnace in the modern three-furnace setup. It’s commonly a round, insulated cylinder and the partially formed glass can be held suspended from the end of the rotating blowpipe, which rests on metal stands called yokes, until it’s hot enough to continue.
Although a piece of glass may appear done when the last gob has been melted in, there’s still a crucial step that needs to take place.
Glassblowing Furnace: Time for the Cool Down
Once a piece of glass is complete, to help it cool properly, an annealer is used. The third furnace in the series, the annealer has a very important job. This stems from the fact that glass, remember, is an amorphous solid. As the glass cools it may naturally start to crystallize — but that’s not a good thing, because that means the glass has lost the properties that made it so useful and special to begin with. The goal is for the glass to cool and retain its scattered but rigid molecular structure. As this happens, it contracts and loses more and more of its viscosity until it becomes solid glass.
So to ease our newly blown piece of glass through this process, an annealer (sometimes called a lehr) is used to control the rate of cooling. Otherwise, the glass could suffer thermal shock and become unstable. A pyrometer carefully measures the temperature during the annealing process while the glass is brought below the point of softening and is carefully cooled over an extended period of time. This helps get rid of any internal strain or tension in the blown glass, so it’s less likely to break down the line. The size and dimensions of the piece affect the length and rate of cooling.
Last but not least, the nearly finished piece of blown glass is often taken to the cold shop where it can be ground, polished, engraved, enameled and otherwise detailed. Glass etching and relief sculpture are just two of many ways glassblowers can increase the craftsmanship in their blown glass art.
Blowing glass is challenging work, and there can be a lot of risks involved. Glass can crack and explode if it not worked properly — and let’s not forget about burns. Plus, a lot of the ingredients in a glass batch are either toxic or if airborne, can cause respiratory problems over time. To counteract these dangers, glassblowers wear protective clothing, safety glasses, respirators and face shields to defend them against injury and exposure — as well as the ferocious heat pouring out of the furnaces.
Now that we’ve seen what goes in to glassblowing, let’s check out how you can join the ranks of the world’s best.