A Short History of Glass
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.
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.
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.
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
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
became the center of glass working after the decline of Rome. The Byzantine
glassworkers were skilled in the manufacture of colored glasses and mosaics.
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.
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.
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
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.
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.
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.
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."
Structure of Glass
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
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
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.
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.
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.
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
lime glass is used for light bulbs, bottles, fiberglass, building blocks,
windowpanes and other applications where cost is a factor.
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.
glass is used for stems for the filaments for light bulbs, neon sign tubing,
crystal tableware and some optical components.
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.
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.
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
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.
Expansion 10-7in. /in./oC.
Compatability of Different Glasses
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.
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,
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.
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.
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.
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.
Added to the Melt
of the Glass Produced
iron, or silver oxides
or chromium oxides
fluoride or stannic oxide
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
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.
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.
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
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.
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
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.
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.
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.
in a lehr is always the most desirable method because the entire object is
heated and then cooled in a uniform manner.
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.
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
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.
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.
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.
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.
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.
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.
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
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.
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.
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
Shaping Solid Glass
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.
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
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.
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.
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
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.
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.
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
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.
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.