History of Glassmaking

Little is known about the first attempts to make glass. The Roman historian Pliny attributed it to Phoenician sailors. He recounted how they landed on a beach, propped a cooking pot on some blocks of natron they were carrying as cargo, and made a fire over which to cook a meal. To their surprise, the sand beneath the fire melted and ran in a liquid stream that later cooled and hardened into glass.

That said, no one really knows how glass came to be made. It is thought that the ability to make glass developed over a long period of time from experiments with a mixture of silica-sand (ground quartz pebbles) and an alkali binder fused on the surface. The material called faience had been used for well over a thousand years to make small decorative objects such as beads and amulets.
mesmap_sm.gifAlthough it existed as an ignored, accidental byproduct of copper smelting, true glass probably was first made in western Asia, perhaps Mesopotamia, at least 40 centuries ago.

Perhaps early development began with potters firing their wares. Could the first glass have been colorful, hard, shiny decoration fused to a clay pot's surface in the heat of the furnace? No one knows. It was later discovered that if the material were thick enough, it would stand by itself. Pieces of solid glass could then be ground to shape by grinding it with stones, or sand and water, to produce vessels.


secret_sm.jpgAs early as 3,300 years ago, secret "instructions" for furnace building and glassmaking in Mesopotamia were written on clay tablets in a cuneiform script. These instructions were copied and recopied over the centuries. Furnace-building instructions from that time period have not been discovered. The cuneiform tablet pictured on the left of this page is probably about 2,700 years old. Typical instructions for glassmaking follow:

                 When you set up the foundation of a good furnace to make glass, you first search in a favorable month for a day of good omen, and only then can you set up the foundation of the furnace. As soon as you have finished building the furnace you go and place Kubu-images there. No insider or stranger should enter the building; an unclean person must not even pass in front of the images. You regularly perform libation offerings before them. On the day when you plan to make (glass), you make a sheep sacrifice before the Kubu-images (religions statues); you place juniper incense on the incense burner; you pour out a libation (drink honoring a deity) of honey and liquid butter; only then can you make the fire in the hearth of the furnace and place the glass in the furnace.

              The wood that you burn in the hearth of the furnace should be thick, peeled poplar wood, which has no knots, bound together with leather straps, cut in the month of the Abu (July or August). Only this wood should be in the hearth of the furnace. The persons whom you allow to come near the furnace have to be clean; only then can you allow them to come to the furnace.

            If you want to produce zagindurû-colored (blue) glass, you finely grind separately, ten minas (about one pound) of immanakku-stone (quartz), fifteen minas of naga-plant ashes, and 1 2/3 minas of 'white plant.' You mix these together. You place the mixture into a cold furnace that has four openings, and you arrange the mixture between its openings. You keep a good and smokeless fire burning....As soon as the mixture glows yellow, you pour it on a kiln fired brick and this is called zukû-glass....
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Scientific Glassblowing

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              Glass is widely used in science and industry because of its three main  attributes: it is transparent; it is inert to most chemicals; and it is malleable. In the trained hands of an experienced scientific glassblower, glass can be  manipulated into the numerous shapes required for use in research and  commerce. Not only can scientific glassblowers create items made of glass, their extensive knowledge of glass science can be pooled with the knowledge and experience of a researcher or industry leader.
             Glassblowing is a scientific art. A glassblower of today manipulates hot, malleable glass by using the same skills and techniques similar to those used by Egyptian craftsmen of ancient times. However, unlike earlier craftsmen, whose selection of glass was based on how  it looked (i.e., color), the scientific glassblower of today selects glass on how it  will be used. Today’s glassblower may select glass based on heat requirements,  chemical inertness, or ability to fuse to metals. The average person’s image of glassblowing is that of molten glass drawn on the end of a long steel pipe as it is pulled from a fiery furnace. This is quite different from modern scientific glassblowing with precision equipment,  advanced torch designs, digitally controlled ovens, and often a computer. Although the scientific glassblower of today uses highly sophisticated  equipment, the responsibilities of the    scientific glassblower are just as they were in the past: to assist the researcher. For example, it was a glassblower who coiled a long tube of glass for Galileo’s thermometer, and it was other        glassblowers who helped construct the light bulb for Thomas Edison (as well as the necessary glass vacuum pumps). Similarly, glassblowers played critical roles in assisting Ferdinand Braun as he    developed the cathode-ray tube, and other glassblowers were directly involved in the cathode-ray tube’s evolution to the television.

Types and Chemistry of Glasses
           Glass is a state of matter. Glasses combine some properties of crystals and some of liquids but are distinctly different from both.  Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterizes liquids. Glasses are usually formed by melting crystalline materialswhich are called FORMERS at very high temperatures.  Most commercial glass is made with sand that contains the most common FORMER, Silica. Other FORMERS include: Anhydrous Boric Acid Anhydrous Phosphoric Acid But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F). When the melt cools, the atoms are locked into a random (disordered) state before they can form into a perfect crystal arrangement.
           Traditional glass is made today by melting quartz or sand at a very high temperature and molding it into shape. Glass without any additives are called quatrz glass. They have very high melting points and requires extra hot flames to work with during galssblowing. But melting sand by itself is too expensive  because of the high temperatures required (about 1850°C, or 3360°F). Therefore, FLUXES are added which let the FORMER melt more readily and at lower  temperatures (1300°C, or 2370°). These include:  Soda Ash (Na2O), Potash (K2O), lime (CaO) and Lithium Carbonate(Li2CO3.) However,  FLUXES also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. Therefore, STABILIZERS are added to make the glass uniform and keep its special structure intact.  These include: Limestone, Litharge,     Alumina,Magnesia, Barium Carbonate, Strontium Carbonate,Zinc Oxide, Zirconia
             Glass has no specific melting point and there is a wide temperature range in which the glass can be shaped. This way of making glass is very expensive because of the high temperature needed in melting. Soda ash (Na2CO3) and lime (CaO) can be added to the quartz to lower the melting temperature and creating the most common form of glass called soda-lime glass. Which is what windows and bottles are made of. Making glass this way can result in a very impure glass. Glass consists of covalently bonded Silicon and Oxygen atoms with positively charged metal oxides bonded within the oxygen-silicon matrix. (tetrahedrally bonded)
            Nearly all commercial glasses fall into one of six basic categories or types. These categories are based on chemical composition. Within each type, except for fused silica, there are numerous distinct compositions.

1. Soda-lime glass is the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica, 12-18% soda, 5-12% lime. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair.

2. Lead glass has a high percentage of lead oxide (at least 20% of the batch). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is somewhat more expensive than soda-lime glass and is favored for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature.

3. Borosilicate glass is any silicate glass having at least 5% of boric oxide in its composition. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass, and not as low in cost as lime, borosilicate's cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.

4. Aluminosilicate glass has aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry

5. Pyrax or Borosilicate: Another property of the glasses is varying thermal heat of expansion rates. Knowing these rates can tell you what types of glass will shatter when heated or cooled rapidly. One that you probably all ready know of is Pyrex, which is a borosilicate glass. This glass has the compound B2O3 in the matrix. Ninety-six percent silica glass is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200°C the resulting pores are consolidated. This glass is resistant to heat shock up to 900°C.

6.  Quartz is fused silica glass is pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200°C for short periods.

Properties of Glass

Physical Properties *
Soft Glass
(Soda Lime)
(Duran, Kimax, Pyrex)
(Fused Silica)
Softening Range       696 C    821 C    1580 C
Working Range     1000 C   1200 C    1800 C
Annealing Range       514 C    565 C    1050 C
Max. Service Temp.       450 C    490 C    1100 C

Has great inherent strength. Weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws.
Surface resists scratches and abrasions.
Gives under stress - up to a breaking point - but rebounds exactly to its original shape.
Affected by few chemicals. Resists most industrial and food acids.
Withstands intense heat or cold as well as sudden temperature changes.
Retains heat, rather than conducts it. Absorbs heat better than metal.
Reflects, bends, transmit and abosorb light with great accuracy.
Strongly resists electric current. Stores electricity very efficiently.
Equipment, Material, Tools and Terminology

   Term used to describe the process of removing stresses introduced into the glassware during the glassblowing process.

Annealing Point
   Temperature at which the stress in glass is removed. Annealing point temperatures will be different for each type of glass.

Blowhose Assembly
   The blowhose is what you blow air through to shape the glass. The blowhose will have a mouthpiece at one end and a swivel at the other. The swivel is a device that allows the rotation of glass that will be blown. The swivel is connected to the glass by latex or rubber tubing or a stopper/tubing assembly. The blowhose is usually about six (6) feet long and 3/16" ID. Latex is commonly used because of its light weight and low cost.

Borosilicate Glass
   The type of glass most commonly used in the laboratory today is from the borosilicate glass family. Trade names you may be more familiar with are PYREX (Corning), KIMAX (Kimble) and DURAN (Schott). In scientific glassblowing the glass used comes in tubing, rod or sheet form. It is usually four (4) feet in length. The diameters are expressed in millimeters.

   Burners are usually designed for stationary use at the bench or lathe. The glass being worked is moved into and around the flame. Flame size is determined by valves that adjust the flow and mix of fuel gas and oxygen.

    Tool used to measure the internal diameter and/or the outside diameter of glass tubing or rod, and wall thickness.

   Fixed size stopper for temporarily sealing openings in the glass.

Cutting Tool
   An instrument used to scratch the surface of glass tubing or rod. See Tungsten Carbide Knife.

Diamond Scribe
   Hand held instrument with a diamond point used to make a permanent mark on glass.

Didymium Eyeglasses
   Didymium lens protect your eyes from certain visible and UV light produced in the glassblowing process. They enable the glassblower to see the glass while it is being worked in the flame.

   Files may be used to scratch the glass tube or rod surface. Three corner (triangle) files are the easiest to use.

Fire Polishing
   Term used to describe the process using a flame to smooth the ends of glass tubing or rod.

Flint Lighter
   Hand held tool used to create a spark - igniting your torch or burner.

Fused Silica
   Glasses in the Quartz family.

Glass/Ceramic Tape, Tubing and Pads
   Asbestos substitute - Used as spacers, supports, insulators, etc. while glassblowing

Graphite/Carbon Rods and Shapers
   Used to form or shape hot glass. These hand held tools may be found in rod, flat , taper and custom forms.

Hand Tools

Hard Glass
   Term used by glassblowers to generically describe glass in the borosilicate family.

Hot Glass Rest
   Any object made of non-flammable material used to hold or contain hot glass.

   Fuel gas mixed with oxygen to generate temperatures hot enough to work quartz.

   Inside Diameter

   Wax pencil or inks used in identifying specific points/information on glassware. Marks may or may not be permanent after exposure to heat.

Natural Gas
Fuel gas mixed with oxygen to generate temperatures hot enough to work borosilicate glass.

   Outside Diameter

   Gas mixed with hydrogen, natural gas or propane. Enhances the combustion process.

Pluro Stopper
   An adjustable size rubber stopper used in sealing openings in glass.

   Fuel gas mixed with oxygen to generate temperature hot enough to work borosilicate glass.

   Family of glass that is almost pure silica. Used extensively in the semi-conductor industry and for high temperature applications.

   A single or double stage pressure control device installed in a gas manifold, gas delivery line or on a compressed gas cylinder.

Ring Stand and Clamp Assembly
   A common laboratory stand used with an adjustable clamp to hold glassware stationary. The clamp fingers should be covered or protected from the direct flame of your torch. Soft flexible braided glass/ceramic tubing is available for this purpose.

   The process of continually turning glass tubing or rod when it is in the flame or in a softened state.

Soft Glass
   Term used by glassblowers to describe glasses in the soda-lime family.

Softening Point
   Temperature at which glass will sag under its own weight under certain conditions.

Strain or Stress
   Term used to describe tension or compression in glass developed during the heating and cooling stages of the glassblowing process. Stress points or areas in glass are potential fracture sites.

Thermal Shock
   Sudden, rapid cooling or heating of glass surface that may produce cracks or fractures.

   Torches are usually hand held. Unlike stationary burners (glass is moved into the flame), the torch is moved around the glass, which is stationary.

Tungsten Carbide Knife
   Hand tool used to create a scratch on the glass wall surface.

Tungsten Pick
   Hand held tool used to "sew" small holes and cracks in glass together. A sharpened tungsten rod (1/16 - 1/8"OD) is attached to a handle, preferable one that does not transmit heat. This tool should be about 6 -10 inches in overall length.

Working Range
   Temperature at which glass is hot enough to shape and seal or bond.

Cutting Glass

Glass tubing and rod come in four (4) foot lengths and metric diameters. Four feet of tubing is rather awkward to handle in the flame so we need to cut it down to a manageable size. Select tubing of 10 mm OD and place on a flat surface. Mark the glass into 16 inch lengths.
1. Create a scratch on the glass wall surface by drawing the corner of a file or tungsten carbide knife perpendicularly across the tube.

2. Pick up the glass tube and wet (water, saliva) the scratch with your finger. Turn the scratch away from your body.
3. Place your thumbs on the glass tube. The scratch should be located between the thumbs but on the opposite side of the tube.
4. Push your thumbs away from you. The glass tube should break cleanly at the scratch.

Another Option.

This option is useful on large tubing or when repairing vacuum lines or apparatus. This method should be tried after you have had some experience handling the torch.
1. Scratch tubing with file or carbide knife.
2. Wet scratch.
3. Heat the end of a 6 mm rod red hot - quickly place the end of the hot rod onto the wet scratch - be sure the scratch line extends beyond the contact area of the hot rod.
Tip: If done correctly you will observe a crack forming at and extending from the scratch. If the crack does not travel completely around the circumference: Tap the crack lightly. This action may chase the crack around the tube. Or,  reheat the glass rod and reapply to the crack. Again, be sure the rod does not extend beyond the end of the crack.

Burners and Torches
                Torches and burners will be described as being surface-mix or pre-mix. The difference is where the mixing of the fuel gas and oxygen take place. Remember, your torch will be burning oxygen and propane or natural gas. Do not use torch components designed for use with air.
            Pre-mix torches as a rule are capable of burning a little hotter with a flame that can be very concentrated. Gas and oxygen are mixed in a chamber within the torch body. This style torch is preferred when making seals and/or when  heating small concentrated areas. If you were to be restricted to using only one torch, the pre-mix is recommended. Premix torches have the benefit of having interchangeable tips, allowing the user the option of changing flame characteristics over a broad range.This relatively inexpensive torch, with tip sizes #2,3,4,and 5 will cover most of your needs.
          Surface-mix torches and burners are used most frequently when large areas of glass need to be worked. Oxygen and fuel gases are mixed at the torch tip. The flame characteristics tend to be broader and softer, with less velocity than a comparable sized flame from the pre-mix torch. This is a good torch to use when preheating a large piece of glass apparatus for repair, or when shaping large tubing.
              There are many other makes and models of burners and torches available to the glassblower. Their different flame and body characteristics determine their application in the glassblowing field.
             Torches (handheld) may be used as burners by simply hanging or attaching them to some sort of fixture. This fixture may be something as simple as a ring-stand and clamp assembly or a custom made unit that will hold the torch in a safe and secure manner.

Light Up
1. Confirm gas delivery systems are in safe operating condition. Set regulators at 5 psi gas and 10 psi oxygen.
2. Open gas valve full turn to purge air from the line and then quickly close valve.
3.  Repeat step 2 with the oxygen.
4. Open gas valve approximately 1/4 turn - ignite gas with flint lighter.
5. The torch is lit! Adjust the gas valve to produce a flame about 3/4" long. This is known as a pilot flame. You may leave the torch flame at this position when not in use - but always attended.
Tip: If you experience difficulty in immediately igniting the gas beware of excessive gas buildup. Shut the gas valve off and wait a few minutes to allow the gas to dissipate. Attempt to light the torch again using a different setting on the gas valve.
Adjusting Flame Size
        1. Gas first - open valve to create a flame length about 6" long (about the
        length of your hand). The flame should be still attached to the torch tip. If there
        is a space greater than 1/8" between the flame and torch tip adjust (close) the
        gas valve until the flame "re-attaches" to the tip.
        2. Slowly open the oxygen valve on the regulator. Observe how the flame changes in shape,
        velocity and heating characteristics.
        3. Practice flame control by changing the settings on the gas and oxygen
        valves. Make the smallest flame possible, using an inch long flame as a target.
        Now attempt to create the largest flame possible. Try different tip sizes to
        observe the flame size range each produces.

Shutting Down the Gas Delivery System.
         To shut down and relieve all positive pressure from the gas delivery system:
        1. Close valve at the gas source.
        2. Open valves at the torch and burn off released gas.
        3. Back off pressure adjusting screw at the regulator(s).

Determine the Correct Flame Size

Flame sizes are determined by what you are attempting to do with the glass. Only  glass that has been heated to the working point range will be soft enough for you to form into the shapes or seals desired. Heating an area of glass 10 mm in diameter means only 10 mm of glass will be hot enough to to shape or form. Logically it would follow you do not heat an area 30 mm in diameter to seal on a tube that is only 10 mm OD. Match the area of hot glass to the size of  the seal.

Flame intensity is a factor in determining how quickly the glass reaches working point temperatures. Experience will guide you in choosing the proper flame intensity and flame size to apply to the different types of seals. Each person will develop their own style and technique in using the torch. The information presented here should serve as a starting point.

Blowing Glass BubblesThis demonstration allows students of all ages to actually experience glassblowing . The participants do not handle the hot glass or the torch, and do not come close to the flame. The key is to have a rather long blowhose (with individual throw-away mouthpieces), keeping your students away from the hot glass and flame, but still close enough to observe the glass reacting to the "blowing" into the blowhose.