A Short History of Glass
Obsidian, a black volcanic glass, is probably the best known of the naturally occurring glasses. It was used by early man to form cutting tools, arrowheads and spearheads and is now used by modern man to make the sharpest surgical blades.
Synthetic glass was originally prepared by heating a mixture of sodium oxide (or sodium carbonate), calcium oxide and silicon dioxide (sand). If calcium oxide was not added to the melt, soda glass was obtained. Pure soda glass is not usable because of its high solubility in water. Soda lime glass has a large coefficient of expansion when heated and a low resistance to the effects of acids and bases. It usually has a green color due to the presence of iron oxide in the sand. It was later discovered that this color could be removed by adding manganese oxide to the melt when a colorless glass was desired.
Manufactured glass is presumed to have been first used as a glaze for pottery. The earliest known glaze is on stone beads of the Badarian age of Egypt. These beads ranked in value with precious metals and stones at the time! The Egyptians first made vessels out of glass by the laborious process in which the glass was applied over a wooden or metal rod bit by bit. A cylinder of light blue glass made by this method dates back to the Akkad dynasty in 2600 B.C. Glass was first pressed into open molds in 1200 B.C. There is some evidence that Mesopotamia was the location where glass was first manufactured.
The art of glass blowing was first discovered in the Middle East along the Phoenician coast in 20 B.C. This new technique changed the use of glass from jewelry and ornaments to necessities. Glass containers and other items of high quality (even windowpanes) were found in the ruins of Pompeii.
Glass blowing of vases and art objects is still done in basically the same way as it was originally done. Glass blowers (gaffers) use a hollow iron pipe about four feet long. The gaffer dips the pipe in the melt and rolls a small amount of molten glass (gather) on the end. The gaffer then rolls the gather against a paddle or metal plate to give it an initial shape (marvering). The gaffer then blows into the pipe creating a bubble (parison).
The gaffer controls the shape and thickness by reheating the parison at the furnace and shaping and blowing to create the final form. Wooden paddles with holes and wet newspapers held in the hand are all used to shape the glass. Shears can be used to cut the softened glass. Additional gathers can be applied and shaped into stems, handles, and other decorative artwork. The hot piece of glassware can be dipped into molten glass of a contrasting color (flashed). The gather is attached opposite the blowpipe to a solid iron rod called a pontil. After the blowpipe is broken free, the gaffer can then shape and fire polish the open end. After the pontil is broken off, the rough spot that is left (pontil mark) is removed by grinding and polishing.
Constantinople became the center of glass working after the decline of Rome. The Byzantine glassworkers were skilled in the manufacture of colored glasses and mosaics.
Venice became the center of glass working after the Dark Ages and by the end of the seventeenth century there were over 300 factories located in that city. The Venetians developed a hard soda glass that was ductile, colorless and highly transparent. Venetian glass was known as cristallo because it resembled rock crystal. The growth of glass factories in Europe flourished after this time.
The first true modification of the physical properties of glass was made in 1603 by Ravencroft in England. He added lead oxide to the melt and obtained a new glass that had a higher refractive index than Venetian glass. It thus had better optical characteristics. Lead glass was softer and more durable than cristallo.
English lead glass was considered the finest glass of the 18th century. Lead glass remains today as the glass of choice for artistic objects and crystal. Lead glass is typically cut in order to produce decorative facets in the glass.
Cutting glass is done by grinding and polishing. Glass can be etched by sandblasting or by the application of hydrofluoric acid. Gilding with gold leaf or gold paint involves a low temperature firing to permanently affix the metal to the glass.
There were few other advances in the chemistry of glass until late in the nineteenth century. At that time, German scientists made great strides in changing the composition of glass to improve its properties. Abbe was interested in improving the glass available for optics and, in 1884; he joined with Schott and Zeiss to form the Jena glassworks of Schott and Sons. Their formulas marked the beginning of modern glass making and the new glasses had much lower coefficients of expansion and better optical properties.
In 1903, Owens invented the first functional bottle-making machine. He is known as the father of mechanized glass working in honor of his many inventions.
In 1912, the Corning Glass Works of New York introduced borosilicate glasses. These borosilicate glasses incorporated boron oxide in the melt. They expand, when heated, only one-third as much as soda glasses and thus are more resistant to rapid temperature changes.
Hostetter said, “The imagination is really fired when one considers the many interesting and useful properties of glass. It is as brilliant as a diamond, as fiery as an opal, as colorful as the rainbow, light and delicate as a spider’s web, or as huge and massive as a twenty-ton mirror, fragile as an egg shell or as strong as steel. Truly, it can be said that glass is the unusual material; without it, we would return to the Dark Ages. With it, science and civilization moves on.”
The Structure of Glass
The definition of a glass is actually a matter of considerable debate. In a broad sense, solids can be considered to be either crystalline or amorphous. Crystals have symmetrical and repeating patterns for the constituent atoms, sharp melting points and cleave in preferred directions. Amorphous solids show none of these characteristics. The glass state is a category of the amorphous state and encompasses solids that may be softened by heating to viscous liquids, which revert to non-crystalline solids when cooled. At times, crystallization occurs both in the manufacture and working of glass and this results in a loss of the desirable properties of the glass. Glass may be defined simply as a supercooled liquid with a viscosity that makes it, for all practical purposes, a solid. Glass is rigid at ambient temperatures and soft or fluid-like at elevated temperatures.
Pure silicon dioxide, in the form of quartz, has some of the structural characteristics of diamond structure. Unlike diamond, which has only tetravalent carbon arranged in interconnected six-membered rings, quartz has six-membered rings of alternating silicon and oxygen atoms. The oxygen atoms preclude forming the same structure as found in diamond. The six-membered rings may be arranged in planes held together vertically with twelve-membered rings
There is a helix formed from the layered six-membered rings which can be seen when looking at the structure perpendicular to the rings. This helix explains the optical activity of quartz. This twist also relieves some of the oxygen-oxygen electron pair repulsion’s inherent in the twelve-membered rings. Quartz is characterized by a very long-range crystal order, unlike glasses, which have no regular internal structure.
Pure silicon dioxide has a very low coefficient of expansion. It is difficult to shape into useful objects because of a very high melting temperature (1723oC) and a high viscosity when melted. The low coefficient of expansion is probably best explained by the tetrahedral arrangements of the silicon atoms and the cross-linked structure. When heated, the stretching vibrations do not change the relative positions of the silicon atoms very much since the overall vector of motion of the bridged oxygen atoms will be at mostly at right angles to the two silicon atoms that they connect.
If the cross-linked structure of silicon dioxide is disrupted by the inclusion of sodium or other atoms, the softening temperature and the viscosity will both decrease. This allows the glass to be worked at a much lower temperature.
The most common glass in use today remains the mixture of silicon dioxide, sodium oxide and calcium oxide called soda lime glass or just lime glass.
The Physical Properties of Glass
Soda lime glass is low in cost and can be easily worked at reasonable temperatures. Soda lime glass has the following approximate composition.
|silicon dioxide (silica)
|sodium oxide (soda)
|calcium oxide (lime)
|magnesium oxide (magnesia)
|aluminum oxide (alumina)
Soda lime glass is used for light bulbs, bottles, fiberglass, building blocks, windowpanes and other applications where cost is a factor.
Lead glass has good hot workability, high electrical resistance and a high refractive index. Dense lead glasses can be used as shields for X-rays and gamma radiation. Lead glass has the following approximate composition.
|potassium oxide (potash)
Lead glass is used for stems for the filaments for light bulbs, neon sign tubing, crystal tableware and some optical components.
Borosilicate glasses, such as Pyrex (Corning trademark), have high chemical stability, low coefficients of expansion, high heat shock resistance and excellent electrical resistance. They are the glasses of choice for most industrial and scientific applications. They have the following approximate composition.
High silica content glasses, such as Vycor (Corning trademark), withstand extreme thermal stress and have a very high chemical resistance. The reason for better chemical resistance of the borosilicate glasses is not known. These glasses have the following approximate composition.
Optical fibers are made from pure silicon dioxide and are formed using chemical-vapor deposition. A mixture of silicon tetrachloride and oxygen is burned in a methane-oxygen flame. An amorphous silicon dioxide soot is formed which deposits on a glass rod. The rod is removed and the soot is transformed into a glass by heating at high temperature. This glass is then drawn into a thin, ultrapure fiber.
The following table shows some of the properties of several commercial glasses. The expansion of glass is initially linear to about 300oC and exponential beyond that temperature. The coefficient of expansion is the slope of the initial linear portion of the curve or the average change of length per length per oC between 0oC and 300oC. This figure gives a good indication of the ability of a particular glass to withstand rapid changes in temperature. The working temperature is the temperature needed to soften glass to the viscosity where the glass can be made into usable objects. The initial temperature given in the table is the temperature at which the glass begins to soften. The refractive index gives an indication of the brilliance of the glass. Lead glass has the highest refractive index.
|Type of Glass
|Thermal Expansion 10-7in. /in./oC.
|Working Temperature oC.
|Potash Soda Lead
Compatability of Different Glasses
The coefficient of expansion is so different between soda lime glass and borosilicate glass that the two should never be used in the same working area. If even a trace of soft glass is used to patch a hole in a project, the difference in the thermal expansion will cause the project to crack as it cools.
The identification of the type of glass is crucial when a broken piece of equipment is to be repaired. Visual inspection is not reliable and neither is the hardness of the glass to a file. There are several ways to make better, but not perfect, determinations.
The glass may be heated in a flame and the amount of sodium flare and rate of softening will give some indication of the type of glass. This is not a reliable method. If a difference is observed in these characteristics, the glass should be checked more thoroughly.
A small piece of the glass can be drawn into a short rod and set side by side with a rod of Pyrex. The two rods are heated on the ends and pressed together with a tweezers. The joined section is pulled out to a small filament with a diameter of about 0.5mm. The filament is held taut until it cools and then cut in the middle. If the filament remains straight, the glasses have the same coefficient of expansion. If the filament curves, the glasses are significantly different and should not be used together.
A solution of 16 parts of methanol and 84 parts of benzene has the same refractive index as Pyrex glass. If a piece of unknown glass is placed in the solution and becomes essentially invisible, it is probably Pyrex.
Glass of all types can be colored by the addition of metals, metal oxides or other compounds to the melt. The coloring agent will either be suspended or dissolved in the glass. Generally the physical properties of the glass are not changed unless a high concentration of the coloring agent is used.
Colored glass can be divided into two types. The type of color that is best for glass blowing is one that will not be altered when the glass is heated. This type of color is only dependent on chemical composition.
In general, these colors are in the purple-blue-green end of the spectrum. The other type of colored glass is dependent on temperature and in general, these colors are red, some yellows and all opals. Most of these will lose their color after being heated and thus have limited use in glass blowing. Some can be heated enough to work but care must be taking to avoid getting the glass too hot because then the color will be lost. The following table shows some colors that can be created by the addition of various compounds.
|Compound Added to the Melt
|Color of the Glass Produced
|gold or selenium
|uranium, iron, or silver oxides
|copper or chromium oxides
|calcium fluoride or stannic oxide
|white or opal
Glass can be considered to be perfectly elastic up to the point of fracture. It is brittle and does not deform before fracturing. The strength of glass is as high as steel when no imperfections are present. The composition of the glass has little effect on its strength. The hardness of the borosilicate and high silica glasses resist scratching and thus they maintain their inherent strength better than soda lime glass which is sometimes called soft glass. Glasses are harder than mild steel but can be scratched by sand, emery, silicon carbide, hard steel and diamonds.
Microscopic cracks created by abrasion and chemical action ultimately reduce the strength of glass. Water is one of the most potent chemical agents that affect glass and can accelerate the rate of crack growth by more than a million times.
Glass blowers break glass by initially scratching the surface and have long known that water applied to the scratch facilitates the breaking process. Indians in the Catahoula Lake area of Louisiana used this principle when making arrowheads from flint. They performed a ceremony in which they steamed the flint prior to the knapping process. The knapping process involved applying pressure to the sharp edges of the flint with an antler. This fractured off small chips leaving the desired shape and a razor sharp edge.
Cracks in glass grow continuously at controlled rates ranging from less than one-trillionth of an inch per hour to roughly half the speed of sound when glass shatters. The rate of crack propagation depends on the applied stress and the chemical environment.
Cracks develop and grow when the silicon-oxygen bonds are broken by stress. The tip of the crack is roughly the size of the opening created when the ring structure is broken, about 0.5 nanometers. The amount of energy required to cleave the silicon-oxygen bond decreases by a factor of twenty in the presence of water. This indicates that the water molecule fits into the crack tip and converts a silicon-oxygen bond into two silanol (SiOH) groups. Other compounds, such as ammonia, can also facilitate crack rates. Ammonia is actually more reactive than water with strained silicon-oxygen bonds. If the nucleophilic molecule gets larger than 0.5 nanometers, the effect is no longer experienced. Water and ammonia have molecular sizes of about 0.3 nanometers.
Two factors, stress and the chemical environment, affect the durability of glassware. Stress, in glass that is worked, occurs when the glass cools unevenly. Uneven cooling is unavoidable because glass does not conduct heat well and the surface cools more rapidly than the interior. Commercial glass blowers use ovens (lehrs) in which they first heat the object to the annealing temperature (slightly below the softening temperature) and then cool the glassware slowly to remove any internal strains. This process is called annealing.
Glass that is tempered is stressed intentionally in order to impart strength to the article. Glass breaks as a result of stresses that originate across a microscopic surface scratch. Compressing the surface of the glass increases the amount of tensile stress that can be applied before breakage occurs. Thermal tempering introduces this surface compression. In this process the glass is heated almost to the softening point and then cooled rapidly with an air blast or by plunging it into a liquid bath. The surface hardens very rapidly and the subsequent contraction of the slower-cooling interior of the glass pulls the surface into compression. Tempered glass shatters into very small pieces that are not particularly sharp when stressed to the breaking point.
Strains in glass will rotate the plane of polarized light. A simple device can be made that will show strain in glass by arranging two sheets of Polaroid material at right angles so that minimum light is transmitted through the two sheets. If a glass object is placed between the sheets, strains appear as light spots.
The glass object is reheated to a temperature high enough to relieve any internal stresses and then slowly cooled to avoid introducing any new stress. It is always desirable to anneal all glassware, which a glass blower makes.
Annealing in a lehr is always the most desirable method because the entire object is heated and then cooled in a uniform manner.
Annealing depends upon both time and temperature. The ideal temperature is one that will relieve strain rapidly but not be so hot that the glass softens and sags. Generally a Pyrex article is heated gradually to 580oC and held at this temperature for five minutes. The cooling rate should not exceed 9oC per minute when the glass thickness is 5mm. Two-millimeter glass can be cooled at the rate of 56oC per minute.
Flame annealing is the method that most glass blowers must use. This is accomplished by lowering the temperature of the flame by decreasing the flow of oxygen and heating the glass until it is bathed in the yellow flare of sodium. The oxygen is then turned off and the area is heated with the gas flame until the glass is covered with a layer of soot. The glass must then be allowed to cool without coming in contact with any cool surface. This method can produce satisfactory results but glassware that has been flame annealed should always be used with caution. If the glass object does not break in the first week or two, it usually will not break. One of the little known laws of nature is that an unsightly project, regardless of the internal stress, will last forever as a monument to the skill of the beginning glass blower.
Basic Glass Working Equipment
The equipment needed to work glass is quite simple. The torch needed to work borosilicate glass has to be an oxygen-gas torch. An air-gas torch does not develop enough heat for borosilicate glass but can be used for soft glass. Generally several tip sizes are desirable so that flame size can be changed. A hand torch can be used to make impressive pieces of equipment.
Carbon rods and paddles are useful for shaping glass as well as shapers and flaring tools made from brass. Tweezers are essential for grasping small pieces of hot glass. Files or glass knives are needed to score glass in preparation for breaking. Blowing glass is best accomplished using a short length of thin wall rubber tubing with an outside diameter of 3/16 inch. The tubing must be flexible because the glass must be constantly rotated in the flame to prevent sagging.
Eye protection is essential to prevent eye damage from chips of glass and the intense flare. When glass is heated, a bright flare of sodium surrounds the work area. It is nearly impossible to see anything because of this flare and it must be removed by a suitable filter. Glass blowing goggles are made from didymium glass. The glass used in these goggles contains neodymium and praseodymium oxides. These lenses are extremely effective in filtering out light in the region of the sodium D line. The intensity of the light given off is usually not a problem because if glass is heated to the temperature where flare intensity is uncomfortable, the glass is too hot to work effectively. A dark blue, transparent plastic sheet can be used to make an effective filter. However, this should only be used if regular glass blowing goggles prove unsatisfactory.
One of the most basic operations in glass working involves breaking glass to a desired length. Most of the glass used in the manufacture of scientific glassware will be in the form of rods and tubing of various diameters.
Small diameter rod and tubing can be broken easily by hand. The glass is first scored with a file. If at all possible, the scratch should be made with one stroke of the file because sawing with the file tends to widen the scratch and lowers the chances for an even break. The glass should be supported by the bench top because considerable pressure (about 3 to 6 pounds) is needed to make the scratch. Saliva or water is placed on the scratch, which will lower the strength of the glass by about 20%. The rod or tubing is then grasped firmly with the scratch between and opposite the thumbs. The glass is bent at the same time it is pulled apart and a clean break should result. Do not apply a lot of pressure…if the glass doesn’t break easily, scratch the glass again and wrap it in a towel. Very serious cuts can result from forcing the break! If a small piece of glass breaks off the edge of either piece, it means the glass should have been pulled more and bent less. Glass rods tend to leave this sharp point on one side but it can be easily chipped off with a steel screen.
Large diameter tubing offers a greater challenge. An effective way to break any size tubing involves first making a scratch around the entire circumference, wetting the scratch and then touching the scratch with a very hot piece of glass rod. This method rarely yields an even break but the break can be cleaned up by screening and grinding.
Screening glass is similar to knapping arrowheads in that pressure is applied to the sharp edges, which chip off. This method can be used to clean up an uneven break and even make a bevel. The screen is swung down on the edge at about a 45-degree angle. The exact angle and force of the blow are best determined by trial and error. If an edge becomes rounded, this process will not work. Screening leaves a rough edge which must be ground smooth. Grinding the glass can be done with either wet grinding compound on a glass or steel plate or emery paper wetted with water.
The edges of tubing and rod that have been broken are very sharp and must be rounded in the flame (fire-polished). The end of the tubing is held in the hot part of the flame at an upward angle of about 45 degrees. Surface tension will cause the softened glass to make a round contour. The tubing must be rotated while it is in the flame and shortly thereafter or the glass will sag. Fire from the flame can be directed through an open piece of tubing so it is advisable to plug the other end with a cork.
Glass rod is fairly easy to bend without appreciable distortion even with a fairly small flame. On the other hand, glass tubing offers a great challenge. If a large burner such as a Meeker burner is used, the tubing can be bent fairly readily. The secret is to heat the glass over the entire area to be bent to the softening point but not to the temperature where it will sag. If the glass becomes too soft, there is a tendency to have a crimp form at the curve.
Actually it tends to happen regardless of the temperature of the glass. This tendency is somewhat overcome by matching the radius of the bend to the diameter of the tubing. If large diameter tubing must be bent into a short radius, the glass blower will have to incorporate blowing to keep the glass from crimping or collapsing.
Shaping Solid Glass
Many of the operations of glass blowing involve shaping the glass by pushing, pulling and controlled sagging. Learning to shape solid rod into various shapes is an excellent way to gain skill as a glass blower. These manipulations are used in the construction of scientific glassware as well as in the construction of art objects made from solid glass.
A small ball of glass can be formed on the end of a piece of glass rod by holding the rod upward into the flame at about a 45 degree angle and letting gravity pull the glass into a ball. The glass rod must be rotated constantly to prevent sagging.
A flat paddle can be formed from this ball by laying the ball on a carbon paddle and pressing it flat with a carbon rod. If the rod is pressed vertically down onto the paddle, the ball will flatten out into a maria.
If a larger ball is desired, the rod can be heated in the middle and as the glass softens, both ends are slowly pushed together forming a ball. Again the rod must be rotated at all times when it is soft, especially while it is in the flame. The rotation is more difficult in this case since both hands must rotate the glass at the same speed to prevent twisting. A fairly large ball can be made in this way. An oblate ball can be made by pushing the rods and a prolate ball can be made by pulling the rods apart.
A ball in the center of the rod can also be flattened and made into a glass icicle. Heat the flattened portion in the flame on both sides until it is reasonably soft. Pull the ends slowly apart and at the same time rotate the rods. This will create a tapered icicle with a twist that will reflect light in all directions. Heat the glass at the small end to burn it off and round the tip. At the large end, heat the glass and form a small loop for hanging the icicle.
In order to join two pieces of glass together, both pieces must be heated at the same time, touched together and then worked back and forth to create a good weld. This is important in all phases of glass blowing. Small pieces of rod can be joined together by this manner so that glass is not wasted.
A paddle formed at the end of a piece of rod can be used to make leaves, ears, wings and fins on small glass works of art. All that is required is to heat the end of the paddle and push it against the piece of glass that you are working on. Immediately pull and push again to create a good weld between the two pieces. Now heat the paddle and pull and shape into whatever you desire and finally burn off the rod.
A ball in the center of a rod can be used to make bodies for glass animals. Arms, legs and tails are applied by welding on pieces of glass rod.
Regulating the Diameter and Wall Thickness of Glass Tubing
Pulling decreases both the diameter and the wall thickness.
Blowing increases the diameter and decreases the wall thickness.
Heating decreases the diameter and increases the wall thickness.
Pushing an enlarged tube increases both the diameter and the wall thickness.
Pushing a constricted tube decreases the diameter and increases the wall thickness.
Flaring Glass Tubing
Fit a size three tip onto the torch. Adjust the gas until the flame is about 6 inches long without oxygen. The oxygen is turned on until the inner cone is 3/8 inch long.
The end of the tubing is heated uniformly in the flame until soft.
The end is rotated against a flaring tool after it is removed from the flame. The tool may be a tapered carbon rod or a triangular piece of brass mounted on a wooden handle.
If a bulge occurs during this process, the glass was heated too far from the end.
Patching Holes in Glass Tubing
Holes occur at times when two pieces of glass tubing are joined together. These holes can be filled by the following technique.
Fit a size one tip onto the torch. Adjust the gas until the flame is about 3 inches long without oxygen. The oxygen is turned on until the inner cone is 1/4 inch long.
Patch the hole by applying small amounts of 2-mm diameter rod to the hole. The glass rod can be sealed on in any pattern but thick patches should be avoided.
Once the hole is sealed, the patch is collapsed and blown back to the original dimension several times to smooth out the seal.
Lenses are a problem when holes are patched and can be removed by the following technique.
Lenses are potential weak areas in hollow glass objects due to the uneven annealing that usually occurs as a lens is formed. It is always a good idea to remove lenses if possible.
Fit a size one tip onto the torch. Adjust the gas until the flame is about 3 inches long without oxygen. The oxygen is turned on until the inner cone is 1/4 inch long.
The lens is heated in the center with a small flame until soft.
A section of cold glass rod is touched to the soft glass and immediately pulled away. Some of the glass will be peeled off.
The area can then be collapsed in the flame and blown back to the original dimension.