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

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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.

 

Bone Grafts with Glass

 The use of glass in bone grafts.

image credit: Mo-Sci, Llc

 

Bioactive glass

 A description of bioactive glass from Mo-Sci,Llc

Image credit: Mo=Sci, Llc

Bubbles on Single Layer Fusing

“I'm making 3mm French Vanilla sconce covers; …

·        [initially they were] fine, but now 1.5" bubbles form during the full fuse.

·        I pop the bubbles and fill the holes with frit and refire,

·        [The]… edges draw in and distort the design…

·        The shelf is flat,

·        I fire on Bullseye paper, and

·        the 13.5 hour long firing schedule [in F] is:

200 to 1150, hold 30 minutes.

50 to 1225, hold 30 minutes.

300 to 1490, hold 30 minutes.

9999 to 990, hold 60 minutes.

100 to 750, hold 1 minute.

Does anyone know what I can do to avoid the large bubbles? 

A critique of the schedule. 

 This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze. 

 The hold of 30 minutes at 810˚C/1490˚F is excessive. 

·        The temperature may be too high.

·        Ten minutes at top temperature is sufficient in most cases. 

·        A soak of 1 minute would be enough. 

·        The anneal soak at 990˚F is most probably a misprint for                          516˚C/960˚F. 

·        The anneal soak is longer than the half hour necessary, but not a             bubble creating problem.

 It means the schedule could have been:

111˚C/200˚F to 796˚C/1465˚F for 5 minutes

AFAP to 516˚C/960˚F for 30 minutes

83˚C/150F˚ to 370F˚/700F˚, 0 minutes

Off

 

Different firing strategies are possible.

•         Reduce the time at top temperature to no more than 10 minutes. 
•         Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary.  Peek frequently to see when the kiln work is complete.
•         Fire on fibre paper covered with Thinfire to allow air out from under the glass.

These strategies can be mixed as desired, and the reasoning for the strategies is:

• Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
• Reducing the top temperature will increase the viscosity, so              resisting the migration of the glass, and maintain the original            thickness. 
• Also, single layers are prone to dog boning, but there are ways of reducing it.

Ways to reduce the risk of bubbles appearing in general are:

•    Reduce the time at the top temperature,
•    Reduce the top temperature,
•    Provide ways for the expanding air to migrate from under the glass.