Glass in Electronics

"Glass, a highly functional material integral to modern technology, plays an active role in the world of electronics and energy. From the screens of our smartphones to the fibers that carry internet signals across oceans, glass is at the heart of many technologies that rely on its unique electrical properties. " 

Image generated with the assistance of AI

Glass, a highly functional material integral to modern technology, plays an active role in the world of electronics and energy. From the screens of our smartphones to the fibers that carry internet signals across oceans, glass is at the heart of many technologies that rely on its unique electrical properties.

This article aims to shed light on these properties—from dielectric constants to electrical resistivity—exploring how they are influenced by glass composition. We will delve into the science behind these properties and their diverse applications in today’s technology-driven world. This journey will reveal not just a material but a versatile and essential component of countless electrical and electronic innovations.

The Relationship Between Composition and Electrical Properties

Dating back to 1845, glass was initially chosen for its insulating capabilities in telegraph communications. Fast forward to today, over 360,000 different compositions of glass have been documented, each engineered for specific electrical, mechanical, and thermal properties to meet diverse technological needs. This broad spectrum of compositions is a testament to the material’s adaptability in electrical applications.

The following discussion will detail some of the commonly measured electrical properties of glass,² examine the effects of compositional variations on these properties, and provide insight into the role of different components in enhancing or moderating these properties for advanced technological applications.

The Dielectric Constant

The dielectric constant is a critical factor in a material’s ability to store electrical charge under an electric field. As a dielectric material, glass acts as an electrical insulator, meaning that electric charges do not flow through it as they do in conductors like metals. Instead, when exposed to an electric field, the positive and negative charges within the glass are displaced—positive charges move towards the field, while negative charges move away. This charge separation, or polarization, reduces the internal electric field in the glass, enabling it to store charge.³

This dielectric behavior can be observed when glass is inserted between the plates of a parallel-plate capacitor. This augments the capacitance value, enhancing its ability to store opposing charges on each plate.³

The relationship between the composition of glass and its dielectric constant is an active research area. This relationship is complex, requiring an understanding of how the various elemental components within glass contribute to polarization.

Generally, glasses with higher dielectric constants tend to contain alkalis, alkaline earth, and heavy-metal cations like lead, bismuth, and tungsten,⁴ and research has revealed that altering the composition of glass can impact its dielectric constant. For instance, replacing silicon with aluminum in sodium aluminosilicate glasses, increasing tungsten oxide in sodium germanium borosilicate glasses, and adjusting copper oxide levels in borate zinc-fluoride glasses can raise the dielectric constant. On the other hand, adding rare earth oxides to alkali-free aluminoborosilicate glass can lower its dielectric constant.^4,5,6,7

By manipulating the elemental makeup of glass, researchers and engineers can tailor its electrical characteristics to meet specific technological needs, paving the way for advanced and specialized uses in various industries.

Loss Tangent

When glass is exposed to an alternating electric field, a portion of the electrical energy is stored in line with its dielectric constant. However, not all energy is efficiently stored; some is inevitably lost, predominantly as heat, a phenomenon known as dielectric loss. The dielectric loss tangent of glass relates to this dissipation of electrical energy and can result from various physical processes, including electrical conduction, dielectric relaxation, and dielectric resonance.^8,9

For applications like high-frequency electronic circuits, where minimizing energy dissipation is crucial, the composition of the glass must be meticulously tailored to achieve a low loss tangent. This careful adjustment ensures that the glass performs optimally, avoiding unnecessary energy waste.

While it has been observed that the levels of impurities in glass can impact dielectric loss, in the case of borosilicate (BRS) glass-reinforced PTFE composites used for microwave substrate applications, researchers found that decreasing BRS filler size can reduce the loss tangent.^10,11 This finding underscores the importance of material composition and structure in achieving desired electrical properties for specific technological applications.

Conductivity and Resistivity

Conductivity and resistivity are also fundamental properties, especially with regard to electronic applications. Put simply, electronic conductivity involves the transmission of electrical charge by electrons, which are negatively or positively charged electron holes. Glass, characterized by its covalent and ionic bonding structure, typically shows conductivity levels far lower than those in metals, making it an ideal insulator.

Notably, two glass families exhibit significant electronic conduction:

1. Oxide glasses containing high amounts of transition metal oxides exhibit enhanced conductivity. These oxides introduce unpaired electrons, facilitating electron flow.
2. Chalcogenide and tetrahedral glasses, where the inherent covalent bonding contributes to their conductive properties.¹²

By adjusting specific elements within the glass, such as incorporating elements with free electrons or modifying the bonding structures, manufacturers can tailor glass to have specific conductivity and resistivity characteristics. This ability to customize glass properties makes it versatile for various electronic applications, from insulators to conductive components.

Applications

Glasses with low dielectric constants are crucial in applications where minimizing electrical losses, signal distortion, or interference is essential. For this reason, they are used in high-performance microelectronic systems, such as substrates in semiconductor packaging and in thick-film resistors. On the other hand, glasses with high dielectric constants are important for high-magnitude capacitors and multilayer dielectrics.^4,14

The exploitation of electronic conductivity in glass has also opened up a realm of opportunities in modern technology. Notably, the photoconductivity of selenium and arsenic-selenium glasses, driven by electronic conduction, has been pivotal in revolutionizing the photocopying process.

Moreover, certain glasses exhibiting electronic conductivity can dynamically switch between insulator and semiconductor states. This unique property has been applied in the development of computer memory devices, showcasing the versatility of glass in electronic applications. The photovoltaic characteristics of amorphous hydrogenated silicon, a product of its electronic conduction, also form the cornerstone of solar cell technology.¹²  

Overall, the applications of glass with electrical properties are diverse and continually expanding, driven by ongoing research. This progress highlights the pivotal role that the electronic conductivity of glass plays across multiple high-tech industries, showcasing its versatility and importance in shaping modern technology.

How can Mo-Sci help?

Meticulous calibration and consideration of glass composition are crucial for accurate and meaningful measurements of the aforementioned electrical properties. Mo-Sci specializes in creating high-performance sealing glasses suited for a wide range of applications. Our expertise extends to customizing solutions, including the production of specialized glass substrates and ultra-pure glass frit.

For inquiries on sealing solutions or custom development services, contact the Mo-Sci team today.

References and Further Reading

1. Chen, J., et al. (2017) Generation of shock lamellae and melting in rocks by lightning-induced shock waves and electrical heating. Geophysical Research Letters. doi.org/10.1002/2017GL073843
2. Jones, R. (2010) An Incomplete History and Timeline of the Electric Telegraph and the CD 731 Compromise Insulator. Available at: https://www.insulators.info/shows/handouts/cd_731.pdf
3. Al-Amoudi, M.A. (2020) Determining Dielectric Constants of Glass and Thin Film Using a Parallel Plate Capacitor. International Journal of Applied Science and Engineering Review. ISSN: 2582-6271.
4. Hsieh, C., et al. (1996) Correlation between dielectric constant and chemical structure of sodium silicate glasses. Journal of Applied Physics. doi.org/10.1063/1.363824
5. Mundher, M. (2023) Tungsten oxide effects on conductivity, dielectric parameters, and density of sodium germanium borosilicate glass. Journal of Materials Science: Materials in Electronics. doi.org/10.1007/s10854-023-10280-6
6. Shaaban, S.M., et al. (2023) Influence of Copper Ions on the Structural, Mechanical, Radiation Shielding and Dielectric Properties of Borate Zinc-Fluoride Glasses. Journal of Electronic Materials. doi.org/10.1007/s11664-023-10564-x
7. Zhang, L., et al. (2020) Influence of rare earth oxides on structure, dielectric properties and viscosity of alkali-free aluminoborosilicate glasses. Journal of Non-Crystalline Solids. doi.org/10.1016/j.jnoncrysol.2020.119886
8. Poplavko, Y.M. (2019) Chapter 7 – Dielectrics. Electronic Materials: Principles and Applied Science. doi.org/10.1016/B978-0-12-815780-0.00007-4
9. Sebastian, M.T. (2008) Chapter Two – Measurement of Microwave Dielectric Properties and Factors Affecting Them. Dielectric Materials for Wireless Communication. doi.org/10.1016/B978-0-08-045330-9.00002-9
10. Rodriguez-Cano, R., et al. (2023) Broadband Characterization of Silicate Materials for Potential 5G/6G Applications. IEEE Transactions on Instrumentation and Measurement. doi.org/10.1109/TIM.2023.3256463
11. Alhaji, I.A., et al. (2021) Effects of Particle Size on the Dielectric, Mechanical, and Thermal Properties of Recycled Borosilicate Glass-Filled PTFE Microwave Substrates. Polymers. doi.org/10.3390/polym13152449
12. Varshneya, A.K., et al. (2019) Chapter 16 – Electronic Conduction. Fundamentals of Inorganic Glasses (Third Edition). doi.org/10.1016/B978-0-12-816225-5.00016-X
13. Mo Sci. (2022) Applications of Thin and Thick Glass Films. Available at: https://mo-sci.com/applications-of-thin-and-thick-glass-films/ (Accessed on 17 November 2023).
14. Li, J.C., et al. (2012) Characterization of Semiconductor Surface Conductivity by Using Microscopic Four-Point Probe Technique. Physics Procedia. doi.org/10.1016/j.phpro.2012.03.568

Annealing temperatures by colour

 It has been suggested that there are different annealing temperatures for different colour groups. This is not so.

All the colours in a single fusing compatible range are annealed at the same temperature. It is true that there are variations in the viscosities of different colours, but these are designed by the makers to be minor. Also relevant is that annealing can occur over a range, making concern about different viscosities within a fusing compatible line of glass less important. The anneal soak gives time for all the glass to reach the same temperature differential of ∆T = 5°C, where the viscosity differences will be so small as to be insignificant.

There are precautions that should be observed when combining strongly contrasting colours or contrasting styles. In general, hot and dark colours are less viscous than light and cool colours at slumping and above temperatures. There are also contrasting viscosities between opalescent and transparent colours. A cautious approach to these differences in viscosities is to anneal them as for one layer thicker than that for the profile of the finished item.

The annealing temperature remains the same for all the glass in a fusing compatible line, regardless of colour or style. The length of the anneal soak and cool rates may be altered for these contrasts, but not the anneal temperature.

Glass vs Ceramics

 Glass and ceramics have distinct differences, but can be combined into a fine-grained microstructure that uniformly disperses crystalline phases within an amorphous glass matrix.

Image source: CILAS

Glass and ceramics have similar material properties, including high strength and hardness. However, at the microscopic level, there are many differences in the structures of glasses and ceramics, which ultimately influence their suitability for particular applications.

Key Characteristics of a Glass

Glass is a solid characterized by its amorphous or non-crystalline microscopic structure.¹ Typically transparent to visible light, many glasses are valued for their chemical inertness and hardness, allowing them to withstand highly corrosive environments, including extreme pH levels and biological conditions.²

In contrast, crystalline materials possess a high degree of regularity in their atomic structure, featuring a periodic crystal lattice. The planes of atoms present in crystalline materials can easily slip past one another, which helps relieve internal stresses. This regularity is absent in glasses, contributing to their typically brittle nature. An important characteristic of glass is the glass transition temperature, which is the point where glass transitions from a hard, brittle state to a molten state. This temperature significantly influences the thermal properties and behavior of the glass.³

Commonly, glasses are composed of network formers such as SiO₂, B₂O₃, P₂O₅; and network modifiers designed to achieve specific properties. For optical fibers, minimizing unwanted dopants is crucial to prevent the formation of color centers and radiation damage. However, dopants can also enhance the optical and optoelectronic properties of glasses for other applications.⁴

Bioactive glasses form a distinct category, designed for medical devices and technologies. These materials are biologically safe and promote healing or treatment processes, often through ion release.⁵ Typically made from a mixture of SiO₂, calcium oxide, sodium oxide, and phosphate (P₂O₅), bioactive glasses can be engineered with specific degradation kinetics to enable drug release or to create dissolvable scaffolds for wound healing.

Key Characteristics of a Ceramic

Ceramic materials are renowned for their high thermal resistance. They belong to a diverse family that includes inorganic materials, metallic oxides, nitrides, and carbides. The microstructure of ceramics is made up of small crystalline areas called grains, which can vary in size.

The size and composition of grains significantly influence the material properties of ceramics, and the interfaces between these grains are crucial for optimizing hardness and durability.⁷

Ceramics can be very brittle and have poor resistance to shearing and tension forces. However, like many glasses, they exhibit excellent resistance to chemical erosion. With the appropriate chemical composition, ceramics can be engineered into semiconductors and electrical components, with many capacitors being made from ceramic materials due to their superb thermal and electrical resistance.

Ceramics are now extensively used across various industries, and the development of composite ceramics has broadened their applications, including in the medical field for creating devices like dental implants.⁸

Glass-Ceramics

While a vast array of glass and ceramic materials exists, the ideal material properties for a specific application sometimes require merging the best attributes of both. Glass-ceramics are such a hybrid, possessing the chemical compositions of glasses but differing in their microstructure. Unlike purely glassy materials, which are entirely amorphous, glass-ceramics typically exhibit a predominantly crystalline structure interspersed with amorphous characteristics. This is typically achieved through a fine-grained microstructure that uniformly disperses crystalline phases within an amorphous glass matrix.

This hybrid microstructure makes glass-ceramics stronger than their purely glass counterparts and allows them to retain some of the beneficial electrical properties associated with ceramics, while still remaining transparent.⁹

Glass-ceramics are particularly valued as bioactive materials, with variants like Bioglass 4555 receiving FDA approval for medical device applications. The ability to further refine their properties through controlled crystallization processes during manufacturing enhances their adaptability for complex uses.

Non-metallic materials, such as glass, ceramics, and glass-ceramics, exhibit a broad range of properties influenced by the degree of crystallinity in their microstructure. Generally, a higher degree of crystallinity results in harder materials, but it can also increase light scattering, which is why specialized processing is required to render ceramic materials transparent.

Mo-Sci Solutions

At Mo-Sci, we are experts in the development and creation of glass, ceramic, and glass-ceramic materials, no matter what the application. Whether you need very high-purity silicon dioxide or a more complex custom-made bioactive material, contact us today to see how our services and capabilities could benefit you and help you find the perfect material solution to your product needs.

References and Further Reading

1. Doremus, R. H. (1972). Structure of inorganic glasses. Annual Review of Materials Science, 2(1), 93-120. https://doi.org/10.1146/annurev.ms.02.080172.000521
2. Axinte, E. (2011). Glasses as engineering materials: A review. Materials & Design, 32(4), 1717-1732. https://doi.org/10.1016/j.matdes.2010.11.057
3. Tanguy, A. (2021). Elasto-plastic behavior of amorphous materials: a brief review. Comptes Rendus. Physique, 22(S3), 117-133. https://doi.org/10.5802/crphys.49
4. Griscom, D. L. (2013). A Minireview of the Natures of Radiation-Induced Point Defects in Pure and Doped Silica Glasses and Their Visible / Near-IR Absorption Bands , with Emphasis on Self-Trapped Holes and How They Can Be Controlled. Physics Research International, 379041. http://dx.doi.org/10.1155/2013/379041
5. Jo, W., Kim, D., & Hwang, N. (2006). Effect of Interface Structure on the Microstructural Evolution of Ceramics. Journal of the American Ceramic Society, 8, 2369–2380. https://doi.org/10.1111/j.1551-2916.2006.01160.x
6. Cannio, M., Bellucci, D., Roether, J. A., & Cannillo, V. (2021). Bioactive Glass Applications : A Literature Review of Human Clinical Trials. Materials, 14, 5440. https://doi.org/10.3390%2Fma14185440
7. Jo, W., Kim, D., & Hwang, N. (2006). Effect of Interface Structure on the Microstructural Evolution of Ceramics. Journal of the American Ceramic Society, 8, 2369–2380. https://doi.org/10.1111/j.1551-2916.2006.01160.x
8. Vallet-Regí, M. (2001). Ceramics for medical applications. Dalton Perspective, 97–108. https://doi.org/10.1039/b007852m
9. So, M., Górny, A., Pisarska, J., & Pisarski, W. A. (2018). Electrical and optical properties of glasses and glass-ceramics. Journal of Non-Crystalline Solids, 498, 352–363. https://doi.org/10.1016/j.jnoncrysol.2018.03.033

Notes on Kiln wash

 

Composition

Kiln wash generally is made up of a mix of aluminium hydrate and kaolin, although some contain vegetable extracts instead of the kaolin (china clay).

Application

The kiln wash mix should be applied in at least four directions with a soft brush, such as a hake, one immediately after the other. The object is to have all the layers incorporating into one smooth layer. There are fuller descriptions elsewhere in this blog.

Drying of layers

Some people suggest drying after each direction of application. This is not recommended because the following layers of kiln wash drag at the dry layers and create an uneven surface. A full description is given here.  

Air drying the shelf before use reduces the amount of moisture introduced to the kiln and extends the life of the metal structure of the kiln. This can be done by air drying on top of the heated kiln if you have two sets of shelves, or simply by leaving in a ventilated room for 6 – 8 hours.  Drying in the kiln could be done more efficiently with the glass on the prepared shelf – the moisture will be driven out before the glass has reached its strain point., so the glass will not be affected by a damp shelf.

Performance

A Bullseye video suggests the kiln wash should be completely removed and renewed every time it is fired over 730°C/1350°F. The reason for this is that the kaolin in kiln wash changes its form – with added heat – from its slippery platelets to a crystalline structure. It is the crystalline form of kaolin that sticks to the glass on a second fuse.  Removal involves cleaning all the kiln wash off, down to the bare shelf before applying the new layers. Continual painting over old kiln wash builds up the thickness of exhausted kiln wash and risks cracking and flaking which is mirrored on the back of the glass.

Freshly applied kiln wash prevents it sticking to the glass. However even fresh kiln wash is prone to stick to opalescent glass at full fuse temperatures.  It is easy to remove the adhered kiln wash by using a solution of citric acid.

The use of kiln wash is cheaper, simpler and easier than many people suggest.

Glass Bonding

 This post may help with choosing high performance fastenings to glass objects.  Glass bonding with silane and polymer coatings.

Although glasses are often valued for their chemical inertness, this property also presents challenges when attempting to form strong chemical bonds with other materials. Silanes and polymer coatings offer effective solutions by enhancing the bond between a glass and other materials in a composite.

The Challenge

The challenge of bonding glass to polymers spans across several industries, including industrial, automotive, and healthcare. In biomaterial applications, polymer carriers are often used to deliver glass or ceramic particles to specific treatment sites. However, bioactive glasses, commonly used in treatment delivery and bone regeneration, face a similar issue: the mismatch in critical surface tension and adhesion properties between the materials in the composite.¹ This mismatch is primarily driven by differences in hydrophobicity and hydrophilicity, making it difficult to create strong, stable bonds.

The key question, then, is how to improve and strengthen the bonds between glass and polymer materials to create stable composites that benefit from the properties of both material types.

The Solution

Fortunately, there are several approaches to improving the bonding characteristics of glass. One such solution is the use of silane or polymer materials as adhesive treatments. These materials help bypass the challenges of forming direct chemical bonds between the surface oxide groups on the glass and the substrate of interest.

Silanes

Silanes are particularly effective in improving the bonding between glass and other materials due to their ability to form highly stable siloxane bonds. These strong covalent bonds enhance the compatibility between glass and various organic or inorganic surfaces, creating a stronger interface than unmodified glass. Silanes are an excellent choice for composites, retaining the properties of glass while significantly improving surface chemistry and wettability.

One challenge in forming glass-bonded materials is the introduction of a new surface type, which can create potential regions of weakness and shear. Silanes act as effective coupling agents by forming strong covalent bonds with both the glass and the substrate, reducing these weak points and enhancing the overall stability and durability of the material.²

While physical abrasion and etching with hydrofluoric acid can improve adhesion by roughening the surface, chemical modification using silanes is often preferable. Chemical bonds offer superior grafting properties compared to physical methods, resulting in stronger, more durable connections between glass and other substrates.

Polymers

Polymer materials are widely used for bonding to glass due to their flexibility, which is highly advantageous in adhesive applications. While silicone-based materials can bond to glass, they are generally not as strong as many polymer adhesive options.

One such polymer adhesive, polyurethane, is a popular choice for bonding glass in various industries. This popularity is due to its flexibility, which helps absorb and mitigate vibrations induced by movement, enhancing the durability and integrity of the bonded structure. Similarly, acrylic adhesives are the ideal choice for oily or corrosive environments or for use in high-temperature applications. Epoxy adhesives also offer similar benefits, with excellent chemical and electrical resistance.

While many polymers show good adhesion to glass surfaces, their bonding interactions tend to be weaker than the covalent bonds formed with silanes, relying instead on intermolecular forces.³

Polymer adhesion is sufficient for many applications, especially where motion or substrate deformation is likely, as flexibility in the bonding is beneficial. However, for applications that demand the highest levels of adhesion, combining silane treatment with polymer bonding provides a superior solution. This approach significantly enhances bond strength, making it ideal for situations requiring both flexibility and durability.

MO SCI Solutions

MO SCI has a long history of developing custom glass solutions for even the most challenging applications. We can help you find innovative and effective ways to overcome the challenges of glass bonding and adherence to create devices that not only have the properties for peak application performance but are also stable and resistant to environmental degradation.

Contact us today to discuss your application.

References and Further Reading

1. Brauer, D. S. (2015). Bioactive Glasses — Structure and Properties Angewandte. Angewandte Chemie – International Edition, 54, 4160–4181. https://doi.org/10.1002/anie.201405310
2. Yavuz, T., & Eraslan, O. (2016). The effect of silane applied to glass ceramics on surface structure and bonding strength at different temperatures. Journal of Advced Prosthodontics, 75–84. https://doi.org/10.4047%2Fjap.2016.8.2.75
3. Park, H., & Lee, S. H. (2021). Review on Interfacial Bonding Mechanism of Functional Polymer Coating on Glass in Atomistic Modeling Perspective. Polymers, 13, 2244. https://doi.org/10.3390/polym13142244

MO SCI, LLC
4040 HyPoint North
Rolla, MO 65401
T: 573-364-2338
F: 573-364-9589

Privacy Policy
Terms & Conditions of Sale

Copyright 2025 MO SCI, LLC

Search

https://mo-sci.com/enhancing-glass-bonding-characteristics-with-silanes-and-polymer-coatings/

Firing silicone mastic

Several people have asked over time about the consequences of firing glass with some silicone mastic (or caulking) still attached to the glass. 

I can say with confidence that it does not break the glass.

I can also say, that it really is not a good idea.  Take as much as possible off with knives, then use silicone disolvers to remove the remainder.  These photos show the results of firing silicone residue.

Where there were pieces of silicone, a divot appeared with the black combustion product from the mastic/caulking.  Where there were strips of silicone, a small valley occurred.  The smallest amount of silicone appeared as a dark divot in the glass.  

It is possible to remove the silicone residue with sandblasting.  Other abrasive methods are possible, but much more time consuming.   Once the silicone has been sandblasted away, the glass needs to be cleaned of all the dusts, and covered with a fine layer of fine frit, or if prefered, powder.  But I find fine frit works better, although it requires a full fuse to form a smooth surface.

Glass Frit Seals for Microelectronics

 Glass frit sealing technology provides a superior solution for achieving reliable hermetic seals in precise applications like micro electromechanical systems manufacturing and packaging.

Image credit: Mo-Sci, Llc

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Radiation Shields

 Glass has a use as a radiation shield in medicine, industry, and aerospace.

Image credit: Mo-Sci,Llc

Using Ceramic to Drape

Characteristics

Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials.  This is one circumstance where CoE is actually useful. 

The expansion of the two materials is different. 

• Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104.  
• Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64.  

This is important in the final cooling of the project.  As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool.  This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much. 

Shape

The shape of the ceramic form will have a big effect on the usability of it as a mould.  Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning.  The forms suitable for draping need to have a significant draft to work well.

Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.  

A cube shape unsuitable for draping

Ceramic cylinders with straight sides

Although rounded at the base, the sides are too straight to be a draping mould

The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.

You can experience this trapping effect in a stack of drinking glasses.  Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one.  On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one. 

Effect of Shape

The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.

Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.

However, if you choose ceramic forms with significant draft, you can drape over ceramic.  This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required. 

 

These pyramid shapes have sufficient draft to allow the glass to move up the mould during cooling.

Compensation for Lack of Draft

You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form.  The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling.  This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper.  It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.

Kiln post wrapped in 3mm fibre paper with cap over the post's hole.

The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper.  You can tie off the wire with a couple of twists.  Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire.  If you want further assurance, you can put a bit of kiln wash onto the wire.

Conclusion

The choice of ceramic shapes to drape glass over is very important.  It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling.  You often can use items with no draft if you wrap fibre paper around the sides of the form.

Heat Shielding Glass

Glass coatings have exhibited remarkable bonding capabilities with various metals and alloys in aerospace applications to shield materials from heat.

Image source: iStock

Testing for Stress

Testing for stress is one of the most important elements in kilnforming.  It may not look like there is stress when there is considerable amounts.  The non-destructive tests are outlined in this Power Point presentation, prepared some time ago, to describe why and how stress testing can be conducted.  There is no commentary.