Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx

Glass 101: Glass Formers – The Backbone of Glass

 

Glass 101: Glass Formers – The Backbone of Glass

Posted Krista Grayson on Oct 15, 2019

Glass is a state of matter that exists separately from the conventional forms of metal, polymers, and ceramics. There is a wide and varied number of applications for glass that each require a different set of properties. Glass properties are controlled by the composition. One of the most fundamental changes that can be made to the composition is the basic unit of the glass: the network former. The former can be thought of as the backbone of the glass, and changing this element or compound will fundamentally change the properties of the final material.

How are formers used in glass production?

Glass formers are added to the bulk material to facilitate the formation of a glass and form the interconnected backbone of the glass network. The first paper to discuss these components was written in 1932 by Zachariasen, which is considered a landmark in glass research.1 In this paper, he outlines his theories on glass and classifies the three types of cation that glass networks are composed of: network formers, network modifiers, and intermediates. 

Some common cations that are in network formers are boron, silicon, germanium, and phosphorus. They have a high valence state (meaning they have a surplus or deficit of electrons allowing them to bond easily with other atoms) and will covalently bond with oxygen. Network ions that alter the glass network and intermediates are added to gain special properties in the glass.

Applications of glass formers

Various glass formers are used in varying ratios with modifiers and intermediates to produce a glass that can withstand the rigors of a specific application. For example, a glass suitable for handling high-temperature liquids will be made of a different composition than one that is being used for decorative purposes. 

Silicate glass

Silicate glass is one of the earliest types of glasses and is still produced worldwide today. It is formed of Si4+ ions that are covalently bonded to four oxygen atoms, to form SiO4 in tetrahedral shapes.2 They then connect to other tetrahedra by bridging oxygen ions. If each of the tetrahedra is joined corner to corner, it forms the SiO2 polymorph cristobalite that has significant long-range order. This long-range order produces a highly connected network which leads to a high softening point, a low diffusion coefficient, and a small coefficient of thermal expansion (CTE).

If the silica is worked at lower temperatures, it possesses only short-range order. Intermediates, such as sodium ions, can then be introduced to the silica glass to form alkali silicate glass. This is done through the addition of Na+ ions, each one of which creates one non-bridging oxygen. This reduces the network connectivity, resulting in decreased viscosity, and an increased diffusion coefficient and CTE. There is also increased ionic conductivity and reduced chemical resistance.

Boron oxide glasses

When boron oxide is used as the former, the glass is known as borate glass. In this case, the structure is composed of corner-sharing BO3 triangles connected by bridging oxygen. Borate glass can also be made into an alkali form through the addition of Na+ ions. 

However, in contrast to silicate alkali glasses, the initial addition of these alkali ions has the inverse effect on borate glasses. It causes an increase in network connectivity, a reduction in the CTE and an enhancement in thermal and chemical resistance.3 Alkali borosilicate glass is also known as Pyrex® glass; the improved physical properties make it useful for lab equipment and piping, as well as the tiled coating on the space shuttle.

Phosphate glass

Phosphate glasses are based on P2O5, with CaO and Na2O as modifiers.4 Initially these glasses were used for industrial applications such as clay processing and pigment manufacturing, but more recently they have been put to more specialised uses. Research has found that the constituent ions of phosphate glass are similar to those present in the organic mineral phase of bone.5 This chemical affinity to bone sees these glasses being developed for biomedical use, as the inert nature of the glass is ideal for deployment within the body.

Mo-Sci glasses

Mo-Sci has extensive experience in developing and manufacturing glass produced from various types of formers.6 We offer a number of standard glass compositions such as soda-lime silicate, barium titanate, and type 1A borosilicate, as well as being able to develop custom glass compositions for specific applications. Contact us for more information.

References

1. Zachariasen, W. H. The atomic arrangement in glass. J. Am. Chem. Soc. 54, 3841–3851 (1932). https://pubs.acs.org/doi/pdf/10.1021/ja01349a006
2. Hosford, W. F. & Hosford, W. F. Amorphous Materials. Mater. Sci. 153–167 (2009). https://ocw.mit.edu/courses/materials-science-and-engineering/3-071-amorphous-materials-fall-2015/lecture-notes/MIT3_071F15_Lecture2.pdf
3. Yuntian Zhu. MSE200 Lecture19(CH.11.6,11.8)Ceramics. 19, 1–21 https://people.engr.ncsu.edu/ytzhu/Class-Teaching/MSE200/Lecture19-Nov23.pdf
4. Richard K. Brow. Review: the structure of simple phosphate glasses. J. Non. Cryst. Solids 263–264, 1–28 (2000). https://www.sciencedirect.com/science/article/pii/S0022309399006201
5. Rahaman, M. N. Bioactive ceramics and glasses for tissue engineering. Tissue Engineering Using Ceramics and Polymers: Second Edition (2014). https://www.sciencedirect.com/science/article/pii/B978085709712550003X
6. Mo-Sci Glass Products https://mo-sci.com/en/products

Glass 101: Using Glass Modifiers to Change Glass Characteristics

 

Glass 101: Using Glass Modifiers to Change Glass Characteristics

Posted Krista Grayson on Sep 16, 2019

Glass modifiers such as lithium oxide, calcium oxide, and zinc oxide can be used to fine-tune the properties of silicate and borate glass to suit a number of niche engineering applications. In this article we take a look at the ways in which common glass modifiers are used to create high-specification glasses for various applications.

Ordinary glass is a unique material. It’s heat-resistant, exhibiting low thermal expansion and excellent thermal shock resistance; chemically durable; exhibits high electrical resistivity; and of course, is highly optically transparent. These properties have made glass an indispensable material in architecture, labware, electronics, and engineering.

Glass can be further transformed into a true wonder-material through the use of glass modifiers. Just like other materials such as steel, the properties of glass can be precisely tuned and augmented through the careful addition of chemical modifiers to suit a huge array of demanding applications.

Glass structure and composition

The constituents of glass can be broadly divided into three categories: network formers, modifiers, and intermediates.1 Network formers form a highly cross-linked network of chemical bonds and constitute the bulk of the glass. Silicon oxide is the most common network-forming constituent of glass, but glasses based on other oxides such as boron and germanium are also commonly produced.

Modifiers are chemicals that can be added to glass in small quantities to further alter the properties of a glass. These include lithium, sodium, potassium, and calcium; which exist as charged single atoms (ions) amongst the cross-linked network formers, reducing the relative number of strong bonds in the glass and lowering the melting point and viscosity. 

Intermediates; which include titanium, aluminum, and zinc; are chemicals that can behave as network-formers or modifiers depending on the glass composition.2 Glasses are naturally highly disordered, and require a carefully tuned balance of network formers, intermediates, and modifiers to prevent the formation of ordered crystallites within the material.

Effects of glass modifiers

As glass generally acts like a solution, the properties of modified glass can be approximately described by additivity relationships: that is, each ingredient contributes to the bulk properties of the glass by an amount roughly proportional to its concentration.3

Glass modifiers interrupt the normal bonding between glass-forming elements and oxygen by loosely bonding with the oxygen atoms. This creates “non-bridging oxygens,” and lowers the relative amount of strong bonding within the glass. As a result, glass modifiers generally have significant effects on glass properties. 

These include a reduction in melting point, surface tension, and viscosity due to weaker overall bonding within the material. These are some of the primary reasons for using glass modifiers – they make glass easier to work with at lower temperatures without affecting transparency.4 Glass modifiers affect the coefficient of thermal expansion, chemical durability, and the refractive index.

Glass modifiers for high-specification applications

Despite a number of common properties, the unique chemical properties of different glass modifiers can have varying effects on the properties of the glass produced. 

Chemical Durability

The use of alkali metals such as sodium and potassium as modifiers generally reduce the chemical durability of glass, whereas alkaline earth metals such as calcium can increase the chemical durability of glass.5

Resistivity

In electronics, the high resistivity and permittivity of glass lend it to applications in resistors and capacitors. The addition of tellurium, germanium or titanium oxides to glasses in low concentrations have been shown to drastically increase resistivity, making them popular as glass modifiers for ultra-high resistance applications such as hearing aids and infrared detectors.6

Glass for labware

Glass with strong chemical durability and resistance to thermal shock is highly valued in labware manufacturing. The addition of zinc oxide to silicate glass can reduce thermal expansion effects, making it especially resistant to thermal shock. Borosilicate glasses, which use borate as well as silicate as a network former, are also especially thermally resistant and chemically durable, making them a popular choice of material for reaction vessels, test tubes, and other labware.

Specialty optical properties

Some glasses are prized for unusual optical characteristics: zinc-modified glass is widely used in photochromic lenses, while silver, gold, and copper can produce photosensitive glass which changes color in response to incident light.4,7

Bioactive glass

Of particular interest to the biomedical community, bioactive glass is a form of modified glass that closely emulates the properties of the mineral portion of living bone. Bioactive glass is highly biocompatible and forms strong chemical bonds with bone. 

This material consists of around 45% silicate with calcium and sodium acting as the primary modifiers. This results in a comparatively soft glass which can be readily machined into implants for use in the repair of bone injuries.8

Mo-Sci leading precision glass technology

Mo-Sci is a world-leader in precision glass technology and produces a range of high-specification glasses for application in healthcare, electronics, and engineering. With expertise including bioactive glass, high refractive index glass, and fluorescent glass; Mo-Sci is able to produce custom solutions for virtually any glass application. Contact us today with your specifications!

References

1. Karmakar, B., Rademann, K. & Stepanov, A. L. Glass nanocomposites: synthesis, properties, and applications.
2. Kienzler, B. Radionuclide source term for HLW glass, spent nuclear fuel, and compacted hulls and end pieces (CSD-C waste). (KIT Scientific Publishing, 2012).
3. Industrial glass | Britannica.com. Available at: https://www.britannica.com/topic/glass-properties-composition-and-industrial-production-234890#ref608298. (Accessed: 17th May 2019)
4. Phillips, G. C. A Concise Introduction to Ceramics. (Springer Netherlands, 1991). doi:10.1007/978-94-011-6973-8
5. Hu, J. MIT 3.071 Amorphous Materials 2: Classes of Amorphous Materials. https://ocw.mit.edu/courses/materials-science-and-engineering/3-071-amorphous-materials-fall-2015/lecture-notes/MIT3_071F15_Lecture2.pdf
6. Weißmann, R. & Chong, W. Glasses for High-Resistivity Thick-Film Resistors. Adv. Eng. Mater. 2, 359–362 (2000).
7. Photosensitive glass. (1948). https://patents.google.com/patent/US2515275
8. Rahaman, M. N. et al. Bioactive glass in tissue engineering. Acta Biomater. 7, 2355–2373 (2011).

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+

• 
  

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point

Viscosity Changes with Temperature

This image is taken from Pate de Verre and Kiln Casting of Glass, by Jim Kervin and Dan Fenton, Glass Wear Studios, 2002, p.27.

It shows in graphic form how the viscosity of glass decreases with increases in temperature. The temperatures are given in Fahrenheit.  

The coefficient of expansion also changes with temperature. 

This graph is also from Kervin and Fenton

 It is these two forces of viscosity and expansion that must be balanced around the annealing point to give a stable and compatible range of fusing glass.

Specific Gravity

This is an important concept in calculating the amount of glass needed to fill a pot melt, and in glass casting.  This will also help in the calculation of the amount of glass required to fill a given area to a defined thickness.

Specific gravity is the relative weight of a substance compared to water. For example, a cubic centimetre of water weighs 1 gram. A cubic centimetre of soda lime glass (includes most window and art glass) weighs approximately 2.5 grams. Therefore, the specific gravity of these types of glass is 2.5.  

If you use the imperial system of measurement the calculations are more difficult, so converting to cubic centimetres and grams makes the calculations easier. You can convert the results back to imperial weights at the end of the process if that is easier for you to deal with.

Irregular shapes

Water fill method
Specific gravity is a very useful concept for glass casting to determine how much glass is needed to fill an irregularly shaped mould. If the mould holds 100 grams of water then it will require 100 grams times the specific gravity of glass which equals 250 grams of glass to fill the mould.

Dry fill method
If filling the mould with water isn't practical (many moulds will absorb the water) then any material for which the specific gravity is known can be used. It should not contain a lot of air, meaning fine grains are required. You weigh the result and divide that by the difference of the specific gravity of the material divided by 2.5 (the specific gravity of soda lime glass). 

This means that if the s.g. of the mould filling material is 3.5, you divide that by 2.5 resulting in a relation of 1.4   Use this number to divide the weight of the fill to get the amount of glass required to fill the mould.   If the specific gravity of the filler is less than water, then the same process is applied.  if the specific gravity of the filler is 2, divide that by 2.5 and use the resulting 0.8 to divide the weight of the filler.  This only works in metric measurements.

Alternatively, when using the dry fill method, you can carefully measure the volume of the material.  Be careful to avoid compacting the dry material as that will reduce the volume.  Measure the volume in cubic centimetres.  Multiply the cc by the specific gravity of 2.5 for fusing glasses.  This will give the weight in grams required to fill the mould.  If you compact the measured material, you will underfill the mould. The smaller volume gives a calculation for less weight.

Regular shapes

If you want to determine how much glass is required for a circle or rectangle, use measurements in centimetres.  

Rectangles
An example is a square of 20cm.  Find the area (20*20 =) 400 square cm. If you want the final piece to be 6mm thick, multiply 400 by 0.6cm to get 240 cubic centimetres, which is the same as 240 grams. Multiply this weight by 2.5 to get 600gms required to fill the area to a depth of 6mm.

Circles
For circles you find the area by multiplying the radius times itself, giving you the radius squared.  You multiply this by the constant 3.14 to give you the area.  The depth in centimetres times the area times the specific gravity gives you the weight of glass needed.

The formula is radius squared times 3.14 times depth times specific gravity.   R*R*3.14*Depth*2.5
E.g. 25cm diameter circle:
Radius: 12.5, radius squared = 156.25 
Area: 156.25 * 3.14 = 490.625 square cm.
Volume: 490.625 * 0.6 cm deep =294.375 cubic cm.
Weight: 294.375* 2.5 (s.g.) = 735.9375 gms of glass required.  
You can round this up to 740 gms for ease of weighing the glass.