Aerospace Glass Applications

 

Aerospace Glass Applications

Posted Krista Grayson on Jun 18, 2020

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Glass and glass-ceramics exhibit a huge range of physical properties which can be easily tuned during manufacturing. This makes them popular research subjects for the development of new composite materials. In the aerospace industry, glasses and glass-ceramics are prized for their high heat resistance compared to polymers and conventional aerospace alloys, high strength-to-weight ratio, corrosion resistance and comparatively low cost of manufacturing.1,2

Passenger aircraft, satellites, and rockets place incredible demand on their components. As a result, the aerospace sector poses some of the toughest challenges for materials science. From window coatings to engine parts and turbine blades; the aerospace industry uses some of the strongest, lightest, and most heat resistant materials in the world.

It should come as no surprise, then, that glass and glass-ceramics have a range of aerospace applications. Glasses, silicate-based solids with no long-range atomic order, and glass-ceramics, which are chemically similar to glasses but have some degree of crystallinity, are hugely versatile. A tremendous range of compositions and processing techniques means that glass and glass-ceramics can be tailored to suit a large range of technological applications.

When most people think of glass, they think of windows – so perhaps the most obvious application of glass in the aerospace industry is in airplane windows. But this example serves to illustrate just how demanding aerospace applications can be. While simple silicate glass is perfect for windows down on the ground, it’s too heavy to use for the construction of cabin windows, which are in fact generally made entirely from a polymer such as stretched acrylic. Glass is used to make flight deck windows, but only in the form of a thin layer of protective tempered glass bonded to the surface of a thick layer of polymer.3

Other applications of “ordinary glass” – silicate glass – in aerospace are high silica glass glaze used to coat ceramic tiles that protect space shuttles from burning up during reentry into earth’s atmosphere along with LED lighting and cabin interior features such as mirrors and paneling.

Making Composite Materials with Glasses and Glass Ceramics

The wide versatility of glass and glass ceramics materials are realized when they’re used to create composite materials. Composites are combinations of two or more materials which exhibit properties that differ from the individual components.

Composites are increasing in popularity over conventionally used metals for a number of reasons including their lower weight, better fatigue performance, corrosion resistance, and decreased manufacturing costs. For example, composite materials make up more than 20% of the airframe of the Airbus A380 (first flight in 2005); while the Boeing 787 Dreamliner is 80% composite by volume (first flight in 2009.)4,5

One of the advantages of glass (and glass-ceramics) is the ability to vary its structure and properties through composition and processing – this makes these materials a prime candidate for the development of high-performance composite materials. In aerospace applications, glasses and glass-ceramics can play the part of either filler (e.g. fiberglass-reinforced plastics) or matrix within a composite.6

Aerospace Composites with Glass and Glass Ceramic Fillers

One of the most popular ways of harnessing the properties of glass and glass-ceramics in aerospace is in the form of polymers reinforced with glass or glass-ceramic.6,7 Combining the stiffness and low density of glasses and glass-ceramics with the shear properties of a polymeric matrix can produce a number of high-performance materials with aerospace applications.

One such material in widespread use is Glass and Aluminum Reinforced Epoxy (GLARE). This material not only exhibits excellent fatigue resistance, reducing the frequency of needed inspections, but is both lighter and more corrosion resistant than the aluminum alloys conventionally used in aviation. For these reasons it is used in the Airbus A380, both in the upper fuselage and the leading edges of the stabilizers.8

Glass Fiber-Reinforced Plastic (GFRP) is another example of a polymer-matrix composite material with a glass filler. This material exhibits a particularly high strength-to-weight ratio and is used to make Airbus A320 floor panels among other applications. GFRP exhibits similar properties to Carbon Fiber-Reinforced Plastics but can be produced at a fraction of the cost due to the relatively low cost of glass.9

Thermoplastic Composites (TPCs) offer the ability to produce high-performance components via straightforward and versatile thermoforming. Glass-reinforced TPCs with are used to produce a wide range of aerospace materials.10

Glass and Glass-Ceramic Matrix Composites

Many useful composites can be obtained by employing the “opposite” approach: impregnating an inflexible and low-strength glass or glass ceramic matrix with high-strength and/or high-ductility particulates or fibers.

For example, dispersing aluminosilicate reinforcing fillers throughout a glass-ceramic matrix has produced highly refractive, temperature-resistant materials with low thermal conductivity suitable for use as heat shielding for jet engines.1 Mechanically strong and highly refractive composites can be formed by the dispersal of continuous carbon fibers throughout borosilicate, high-silica, and quartz glasses along with a range of glass-ceramic matrices.

Aerospace Glass from Mo-Sci

Mo-Sci produces several high-specification glasses for aerospace applications. Our engineers can work with you to research and develop custom glasses for aerospace and other demanding environments. Contact us for more information.

References and Further Reading

1. Solntsev, S. S. High-temperature composite materials and coatings on the basis of glass and ceramics for aerospace technics. Russ. J. Gen. Chem. 81, 992–1000 (2011).
2. Nurhaniza, M., Ariffin, M. K. A., Ali, A., Mustapha, F. & Noraini, A. W. Finite element analysis of composites materials for aerospace applications Related content Finite element analysis of composites materials for aerospace applications. doi:10.1088/1757-899X/11/1/012010
3. What Are Airplane Windows Made of? Available at: https://thepointsguy.co.uk/news/what-are-airplane-windows-made-of/. (Accessed: 18th May 2020)
4. Aviation – The shape of wings to come | New Scientist. Available at: https://www.newscientist.com/article/dn7552-aviation-the-shape-of-wings-to-come/?ignored=irrelevant. (Accessed: 18th May 2020)
5. Composites flying high (Part 1) – Materials Today. Available at: https://www.materialstoday.com/composite-applications/features/composites-flying-high-part-1/. (Accessed: 18th May 2020)
6. Boccaccini, A. Glass and glass-ceramic matrix composite materials. J. Ceram. Soc. Japan 109, (2001).
7. Dinca, I., Ban, C., Stefan, A. & Pelin, G. Nanocomposites as Advanced Materials for Aerospace Industry. INCAS Bull. 4, 73–83 (2012).
8. Quilter, A. Composites in Aerospace Applications.
9. Dong Goh, G., Dikshit, V., Arun Prasanth, N. & Guo Liang, G. Characterization of mechanical properties and fracture mode of additively manufactured carbon fiber and glass fiber reinforced thermoplastics. Mater. Des. (2017). doi:10.1016/j.matdes.2017.10.021
10. Marsh, G. Reinforced thermoplastics, the next wave? Reinf. Plast. 58, 24–28 (2014).

Glass 101: An Introduction to Glass

 

Glass 101: An Introduction to Glass

Posted Krista Grayson on Jul 17, 2019

Everyone knows what glass is…or do they? We are all surrounded by glass in a myriad of forms and serving a diverse array of functions, from jars and glasses to windows and TV screens, but do we really know much about it? Glass has become so ubiquitous that it is widely accepted as an everyday commodity. Consequently, most of us take it for granted without considering what it is or how it shows such amazing properties and versatility.

Through the ages, glass has provided answers to many technological challenges and enabled unbelievable advances across most areas of our lives.1,2 However, despite being at the center of an ongoing success story, glass receives little attention from the majority.

What is glass?

Glass is commonly categorized as a type of ceramic, but it is not like any other ceramics. Ceramics generally have a crystalline structure and are opaque, whereas glass has a non-crystalline atomic structure and is transparent. Furthermore, glass exhibits a range of remarkable properties that set it apart from other ceramics. In a perfect state, glass is mechanically very strong, even when subjected to extreme changes in temperature, and has a hard surface that is resistant to abrasion and corrosion. Paradoxically, it is also elastic, being able to give under stress (up to a breaking point) and then rebound to its original shape. Glass also has extensive optical properties, is heat-absorbent and an electrical insulator.3

Glass is so unique that it cannot be simply defined. It is neither a crystalline solid nor a liquid; it is a disordered, amorphous solid. It is this amorphous structure that gives glass its unique properties. Neither can the composition of glass be described since there are infinite varieties of glass. A current database lists over 350,000 types of known glass and new workable glass compositions are being developed every day.

The production of glass

Glass is formed when the constituent parts are combined by intense heating and then rapid cooling. The rapid cooling immobilizes the atoms of the glass before they have a chance to assume a regular crystalline structure. This can occur naturally, as in the case of fulgurite that is formed by lightning striking sand, and obsidian that arises from the rapid cooling of volcanic lava.

Man-made glasses are produced from varying mixtures of oxides. Although the precise chemical composition varies widely between different types of glass, it typically includes three components: a former, a flux and a stabilizer. Glass formers, such as silicon dioxide (silica), make up the largest proportion of the mixture and provide the transparency. Fluxes, such as sodium carbonate (soda) lower the temperature at which the formers will melt. Stabilizers, such as calcium carbonate (lime) provide the strength and make the glass water resistant. Without the inclusion of a stabilizer, water and humidity will attack and dissolve the glass.4

Immediately after glasses are batched and melted, they are slowly and evenly cooled. This process is known as annealing. This is an important step that enhances the strength of the glass by reducing internal stresses. It ensures that sections of varying thickness cool at the same rate. This avoids the development of steep temperature gradients that could cause the glass to crack.

Types of glass

The precise chemical composition of the mixture melted to produce glass determines the mechanical, electrical, chemical, optical, and thermal properties of the final product. Glass can thus be manufactured with broad-ranging characteristics. Through careful selection of the basic initial mixture and additives used in production, glass is produced with properties and structures to meet the requirements of specific applications.3

Although there are many thousands of different glass compositions, glass can be categorized as belonging to one of the six basic types, based on the chemical composition that endows it with specific properties.5

Soda-lime glass

Soda-lime glass is the most common, and least expensive, type of glass, accounting for 90% of all glass made. It usually contains 60–75% silica, 12–18% soda, and 5–12% lime. This is the type of glass used to make bottles and windows. It is mechanically strong but does not have good resistance to high temperatures, sudden changes in temperature, and corrosive chemicals.

Lead Glass

Lead glass (more commonly known as crystal) contains at least 20% lead oxide, which makes the glass brilliant, resonant, and heavy. Although lead glass, like soda-lime glass, will not withstand high temperatures or sudden changes in temperature, it exhibits excellent electrical insulating properties. Consequently, it is commonly used for electrical applications. It is also used for thermometer tubing and art glass.

Borosilicate glass

The addition of at least 5% of boric oxide to a silicate glass gives it high resistance to temperature change and chemical corrosion. Borosilicate glass is not as convenient to produce as either lime or lead glass, but is useful for pipelines, light bulbs, photochromic glasses, sealed-beam headlights, and vessels for laboratory or kitchen use.

Silica glass

Removal of almost all the non-silicate elements from borosilicate glass after normal melting and forming produces 96% silica glass. The resulting pores are sealed by reheating the glass to 1200° resulting in glass that is resistant to heat shock up to 900°C. 96% silica glass is used for the outer panes of the forward windshields of space shuttles to enable them to withstand the high temperatures reached during atmospheric re-entry.6

Aluminosilicate glass

Similar to borosilicate glass is aluminosilicate glass that includes aluminum oxide in its composition. Aluminosilicates are more difficult to manufacture than borosilicate glass, but have even greater chemical durability and can withstand higher operating temperatures. Aluminosilicate glass can also be used as a resistor in electronic circuits.3

Fused silica glass

Fused silica glass is the most difficult type of glass to produce, and so it is the most expensive of all glasses. Fused silica glass is pure silicon dioxide in the non-crystalline state and can withstand temperatures up to 1200°C for short periods. Fused silica is used to create astronomical telescopes, optical waveguides, and crucibles for growing crystals.6

Glass additives

Additives can be used to change the characteristics of glass. This may be done be for aesthetic purposes, for example, heavy metals, such as lead or manganese may be added to give the glass color. It may also be altered for functional purposes, for example, the addition of selenium makes the glass a light-sensitive conductor of electricity; a feature that forms the basis of photocopying.

Versatility of glass

Glass has an extensive range of potential forms and shapes. Its desirable properties can be manipulated during manufacturing, such as mechanical strength and chemical stability. This has led to the development of novel glass formats for use across an entirely new scope of applications.

Controlled-pore glass, which is porous glass with a sharply defined and adjustable pore size, can be used as a support for solid-phase oligonucleotide synthesis7 and as a stationary phase for a variety of chromatography techniques.8,9 Hollow glass biospheres have unique optical properties that have enabled the development of new research techniques, which hold huge potential for analytical devices of the future.10

Glass is also increasingly being adopted for a range of applications in medicine and dentistry. Bioactive glass is biocompatible and demonstrates antimicrobial activity. Furthermore, it can bond with both soft tissue and bone to promote healing. Bioactive glass has thus become an invaluable tool in tissue engineering and bone implants11 as well as in dental reconstruction procedures.12 It is also used in toothpaste and dental fillings to strengthen enamel and reduce bacterial colonisation.13

The future of glass

Glass has become the material of choice for solving a range of technological challenges. It is lightweight yet has the potential for strength, durability and optical clarity and its precise properties can be fine-tuned to meet a specific need. It can also be produced in a range of very different formats, including flat sheets, fine tubes, beads, and powder.

The versatility of glass has enabled incredible achievements, but the journey has by no means reached its end. With new production techniques and types of glass being continually developed, potential applications of glass products continue to expand and facilitate further remarkable advances. We are already benefitting from great interactive user experiences through the glass screens of mobile phones and tablets, but prototypes are now in development for touch-activated glass surfaces through which a range of digital devices can be accessed. Similarly, glass screens have been developed that provide a medium for virtual and augmented reality experiences.

Scientists continue to take advantage of the unique characteristics of glass, redefining what is possible. The latest projects include cleaning up nuclear waste by vitrification and using glass to develop safer batteries.

With a long and successful history, glass is still an active field of discovery and innovation with a future of exciting and ever-expanding capabilities.

Mo-Sci is a world leader in high-quality precision glass technology and produces a wide range of specialist glass products, the precise composition of which can be tailored to meet specific requirements.14

References

1. Rasmussen SC. Origins of Glass: Myth and Known History. In How Glass Changed the World. Springer 2012. Briefs in History of Chemistry, DOI: 10.1007/978-3-642-28183-9_2
2. Main D. Humankind’s Most Important Material. Object Lessons 2018. Available at https://www.theatlantic.com/technology/archive/2018/04/humankinds-most-important-material/557315/
3. What is Glass | Corning Museum of Glass. All about Glass. https://www.cmog.org/article/what-is-glass
4. Chemisty of Glass | Corning Museum of Glass. All about Glass. https://www.cmog.org/article/chemistry-glass
5. Types of Glass | Corning Museum of Glass. All about Glass.
   https://www.cmog.org/article/types-glass
6. Glass and The Space Orbiter | Corning Museum of Glass. All about Glass.
   https://www.cmog.org/article/glass-and-space-orbiter
7. Grajkowski A, et al. A High-Throughput Process for the Solid-Phase Purification of Synthetic DNA Sequences. Curr Protoc Nucleic Acid Chem. 2017 Jun 19;69:10.17.1-10.
8. Zucca P and Sanjust E. Inorganic Materials as Supports for Covalent Enzyme Immobilization: Methods and Mechanisms. Molecules 2014, 19, 14139—14194.
9. Igata Y, et al. A ‘catch and release’ strategy towards HPLC-free purification of synthetic oligonucleotides by a combination of the strain-promoted alkyne-azide cycloaddition and the photocleavage. Bioorg Med Chem. 2017 Nov 1;25(21):5962—5967.
10. Ward JM, Dhasmana N, Chormaic N. Hollow core, whispering gallery resonator sensors. The European Physical Journal Special Topics 2014;223(10):1917–1935.
11. Rahaman MN, et al. Bioactive glass in tissue engineering. Acta Biomaterialia 2011;7:2355—2373.
12. Sohrabi K, et al. An evaluation of bioactive glass in the treatment of periodontal defects: a meta-analysis of randomized controlled clinical trials. J Periodontol 2012; 83: 453—464.
13. Chatzistavrou X, et al. Fabrication and characterization of bioactive and antibacterial composites for dental applications. Acta Biomater. 2014;10:3723–3732. Available at https://www.ncbi.nlm.nih.gov/pubmed/24050766
14. Mo Sci Corporation website. http://www.mo-sci.com/en/products

Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler – nominal CoE

Annealing point - 470C/879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493 – 498C / 920 - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

• 
  

Iridescence

What is it?       How permanent is it?

“Many special effects can be applied to glass to affect its colour and overall appearance. Iridescent glass … is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere.” 

Older glass can appear iridised because of the light reflection through the layers of weathering.

“Dichroic glass is an iridescent effect in which the glass appears to be different colours, depending on the angle from which it is viewed. This effect is caused by applying very thin layers of colloidal metals (e.g., gold or silver) to the glass.”

Source: https://www.thoughtco.com/

A rainbow iridescent appearance caused by an oil film on water is seen by light being reflected from both the top oil surface and the underlying water surface.  The light reflected from these two surfaces or boundaries have slightly different wave times and so interfere with each other to create this colourful pattern.

This is also observed in soap bubbles.  Here the light is reflecting from both the inner and outer surfaces of the film.

This iridescent appearance is termed thin-film interference.  It is an occurrence in nature where there is a thin film through which light can penetrate and so reflect off the surfaces of the film.  These surfaces are termed boundaries where the light can reflect. 

The thickness of the film can enhance or reduce the iridised effect. 

At a certain thickness the light waves reflected can cancel each other out.  This is described as a destructive interference pattern as it reduces the reflection.  The phenomenon can be used to provide non-reflective surfaces.

At other thicknesses there is an iridised effect.  This is caused by the reinforcement of the recombination of the two light waves reflecting in phase or nearly so.

Control of the thickness can give the silver or the gold iridised appearance, as in the Bullseye iridised glasses, in addition to the rainbow and other effects.

The nature of the light affects the colours of the iridescence.  If the light is daylight or similar it is a combination of many wavelengths.  The different wavelengths reflecting from the “boundaries” or surfaces provide the multiplicity of colour.  If the film has variations in thickness, there will be variations in the colours created.

A diagram from Wikipedia shows how the reflections work at the microscopic level.

https://en.wikipedia.org/wiki/Thin-film_interference

The permanence of the film causing the iridisation appears to be dependent on the metals used and the way in which they are deposited.

Float Glass

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

More information on float glass in the kiln is here.

Figure Rolled Glass

The elaborate patterns found on figure rolled glass are produced by in a similar fashion to the rolled plate glass process except that the plate is cast between two moving rollers. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.

Rolled Plate Glass

The glass is taken from the furnace in large iron ladles and poured on the cast-iron bed of a rolling-table. It is rolled into sheet by an iron roller. The rolled sheet is roughly trimmed while hot and soft and is pushed into the open mouth of a lehr, down which it is carried by a system of rollers.  The method is similar to table glass, except in size and thickness.

Table Glass

This glass was produced by pouring the molten glass onto a metal table and sometimes rolling it. The glass thus produced was heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century.

Modern example of rolling glass. The operator is waiting to take the rolled sheet off the table

Broad Sheet Glass

Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. (This is the forerunner of the cylinder process). The quality of broad sheet glass is not good, with many imperfections. Due to the relatively small sizes blown, broad sheet was typically made into leadlights.

According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.

Drawn Sheet Glass

Drawn sheet glass -sometimes called window glass or drawn glass – is made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardens just out of the vat. This film or ribbon is pulled up continuously and held by tractors on both edges while it cools. After 12 meters (40 feet) or so it is cut off the vertical ribbon and tipped down to be further cut.

This glass has thickness variations due to small temperature variations as it hardens. These variations cause slight distortions. You may still see this glass in older houses.

In more recent times, float glass replaced this process.