Cast Iron Mould Risks

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Concealing Layers at the Edge

Credit: Sheri Coughlin

At tack fusing temperatures the layers of glass making up the piece are evident at the edge with two layer bases. 

 It is possible to conceal the layers at rounded tack fuses with a simple layup. If you cut the top piece 6mm larger than the bottom and place it carefully and evenly over the bottom, the top will form over the bottom, concealing the join of the two layers. This will work at all except sharp tack fusing levels.

A Sintering Project

Ready for firing

The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.

Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches

It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:

• 100ºC /180ºF — 482ºC /900ºF, 60'  =5.8 hrs

• 40ºC /72ºF — 593ºC/1100ºF,10'      =2.8

• 20ºC /36ºF — 665ºC /1230ºF,30     =4.1

• Skip to anneal temperature, soak for 6 hours =6.5

• 6.7ºC /12ºF — 427ºC /800ºF,0'       =8.2

• 12ºC /22ºF — 371ºC /700ºF,0'        =4.7

• 40ºC /72ºF — room temperature,0’ =8.8

• Off                       =40.9 hours total or 1.7 days

This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.

However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.

A second attempt used a faster ramp rates to higher temperatures.

• 200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs

• 60°C /108°F -625°C /1157ºF, 30’     =1.92

• 30ºC /54ºF - 685ºC /1265ºF, 120’    =4.0

• skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)

• 15ºC /27ºF – 427ºC /800ºF, 0’        =3.67

• 27ºC /49ºF – 370ºC /700ºF, 0’        =2.11

• 90ºC /162ºF – 50ºC /122ºF, 0’       =3.56

• Off

•  = a minimum total of 18.5 hours plus natural cooling of the kiln

This schedule used a:

• faster first ramp to a higher (540ºC /1004ºF) first soak

• a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)

• the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)

• a shorter (120’) anneal soak

• and consequently faster cooling rates, which showed no stress after firing

The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.

This difference in scheduling is an illustration of how time and temperature can be interchanged.

It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.

More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Shotgun Annealing

 Shotgun annealing is chosen when the annealing temperature is unknown or uncertain. The name comes from the characteristic spread of the shot pellets to include the target.

To follow this process, pick highest relevant anneal temperature. We know soda lime glass has a range from about 540°C/1004°F to 470°C/878°F. Unless you are firing float glass (which anneals between 540°C/1004°F and 520°C/968°F), you can start the anneal cycle at 520°C/968°F and continue it to 470°C/878°F (a 50°C range). The rate to be used is determined by the amount of time required to anneal the piece according to thickness.

To be safe, a shotgun anneal will need double the time to go through the chosen range that a normal anneal soak requires.

• A 6mm/0.25” full fused piece would normally need an hour soak. So the shotgun anneal rate would be 25C/45F per hour over a 50°C/90°F range.
• A 12mm/0.5” full fused piece would normally need a two hour soak. This implies a rate of one quarter of the range or a cool rate of 12°C/22°F over the range.
• A 6mm/0.25” tack fused piece would need to be fired for twice its thickness, so as for 12mm/0.5”.

Annealing times for different profiles and thicknesses are given in this blog post:  and in this ebook.

If the glass is really unknown or older than fusing glass, a wider shotgun anneal range should be used. This gives a temperature range of 540°C/1004°F and goes to 470°C/878°F, or a range of 70°C/126°F. There is still a requirement for the shotgun process to be double the normal anneal soak.

• So for a 6mm/0.25” full fused piece two hours are required to go through the range, or 35°C/63°F per hour.
• A 12mm/0.5” full fused piece and a 6mm/0.25” tack fused piece will need a rate that takes 4 hours to go through the range, or 18°C/32°F per hour.

Once the slow fall of temperature through the range is complete, there should be a one hour soak to ensure the temperature has been equalised throughout the reduction in temperatures. This is applicable to pieces 12mm/0.5” thick. Thicker pieces need a longer soak at this point.

The final part of the anneal is cooling at a rate appropriate for the thickness and profile. E.g.:

• A 6mm/0.25” full fused piece would be cooled at 83°C/150°F to 427°C/800°F, and then at 150°C/270°F to 370°C/700°F or lower.
• A 12mm/0.5” full fused piece needs a two hour soak, so the cooling rates are determined by that, i.e., 55°C/99°F per hour to 427°C/800°F and then at 99°C/178°F per hour to 370°C/700°F or lower.

There is an alternative process which is used to determine the annealing temperature of an unknown glass. Once the anneal temperature is determined for a glass, there is no need for a shotgun anneal process. This is known as the slump point test. 

Much more on the principles and practices of annealing can be found in my e-book. Annealing Concepts, Principles and Practice from Bullseye, Etsy and stephen.richard43@gmail.com

Avoiding Grinding Lines

 

Sometimes in a completed piece a grey line appears between pieces or at the edge of the whole piece. Most often these are the result of grinding the edges to fit with each other, which leads to devitrification.

In scoring and breaking glass to fit into a fusing project, there are often adjustments needed to the pieces for a good fit, so processes need to be used to minimise grinding.

The obvious preventive is accuracy in scoring and breaking to get a good fit without adjustments. This reduces the need for changing the shape.

Many people use their grinders to make the adjustments required though. Most grinders use a medium or coarse grinder bit. This leaves many pits in the edges of the glass, requiring a significant amount of cleaning to remove glass dust from them. The general practice is to place any ground piece immediately into water to avoid the glass particles drying into the pits. This is rarely sufficient to avoid devitrification, because the surface of the edges are not smooth, enabling devitrification to grow on those rough areas.

Fine grinding heads are available and should be used when adjusting sizes for kilnforming. These need to be cleaned just as for the coarser grits. The results of grinding with a fine grit bit (or to 400 grit with other tools) is usually enough when clean to avoid devitrification.

Another way to eliminate the appearance of grinding lines is to avoid grinding altogether. If adjustments are necessary, groze the glass to shape. The slight irregularities will be accommodated by the movement of the glass during contour and full fuses. Larger spaces between glass can be filled with powder of the same colour. If the joint has different colours, use the powder of the darker or denser colour as it will fill the gaps with less evidence.

Glass in Microelectronics

 Passivation involves depositing a protective material onto the surface of metals or metal alloys to enhance their resistance to environmental factors and glass is an excellent choice for this.

Image source: iStock

In the fast-paced world of semiconductor manufacturing, where precision and reliability are paramount, choosing a suitable passivation material is critical to ensuring the optimal performance of electronic devices. Among the library of viable materials, glass has gained significant attention for its unique properties and versatility. This article looks at how glass is used for passivation and what properties make it highly suitable for the job.

Understanding Passivation in Semiconductors

Before unpacking the specifics of glass as a material for passivation, it is essential to understand the concept of passivation in semiconductor manufacturing. Passivation involves depositing a protective material onto the surface of metals or metal alloys to enhance their resistance to environmental factors.

The layering material can be organic or inorganic and should exhibit excellent electrical insulation and strong substrate adhesion, as well as block the ingress of chemical species. In the case of semiconductors, passivation is crucial to preventing degradation and ensuring long-term reliability.^1,2

Why Use Glass for Passivation?

Glass has emerged as a compelling choice for passivation due to its unique combination of properties. For example, glass can be formulated in numerous ways, with common types including Pb-Si-Al, Zn-B-Si, and Pb-Zn-B. This allows manufacturers to produce glass capable of meeting low and high-voltage electrical specifications; matching the coefficient of thermal expansion of semiconductor materials; and meeting the low temperature processing requirements.^3,4

Glass is chemically durable and thus can provide an inert barrier against external elements, such as moisture and contaminants, which might otherwise compromise the semiconductor’s performance. Moreover, the high transparency of some glasses, such as borosilicate glass, makes them ideal for applications with critical optical properties, such as photovoltaics. This transparency enables efficient energy transmission and absorption, contributing to the overall performance of semiconductor devices and solar cells.^5,6

How are Semiconductors Passivated?

Glass can be deposited onto semiconductors in a variety of ways. Choosing methods for passivation depends on factors such as the semiconductor device’s specific requirements, the passivation layer’s desired properties, and the overall manufacturing process. Methods for achieving glass passivation in semiconductor manufacturing include:⁷

• Chemical vapor deposition (CVD), including plasma-enhanced CVD (PECVD)
• Physical vapor deposition (PVD), including E-beam deposition
• Sputter Coating
• Atomic Layer Deposition (ALD)

In manufacturing, the process of glass passivation is frequently succeeded by chemical procedures, such as the etching of contact windows or the electrolytic deposition of contacts. These procedures may pose a threat to the integrity of the glass.

The chemical resistance of different passivation glasses varies significantly and serves as a crucial factor in determining the suitable glass type and the accompanying etching process.⁸

Comparing Glass to Other Materials

While various materials can be used for passivation, glass stands out for its exceptional stability over temperature, humidity, and time. Literature searches reveal a lack of head-to-head comparisons with other common passivation materials; however, general comparisons can be drawn.⁶

Amorphous silicon (a-Si) films utilized in solar cells present numerous advantages. These include a lower deposition temperature, in contrast to the temperatures commonly employed in cell manufacturing. However, it is essential to note that a-Si films exhibit sensitivity to subsequent high-temperature processes, which are frequently necessary in industrial manufacturing technology.⁹

Similarly, AlO_x passivation films can be applied at relatively low temperatures but can be limited by slow deposition speeds when using specific application methods. This can generate problems for high-throughput techniques, such as solar cell production.⁹

Polyimide, a common passivation material lauded for its strength and thermal stability, is also susceptible to moisture absorption. This can impact the strength and dielectric properties of the protective coating, risking the integrity of the semiconductor.¹⁰

Applications of Glass Passivation

Passivation glasses demonstrate outstanding performance in wafer passivation and encapsulation processes, providing advantages to a diverse range of semiconductor devices, including:⁸

• Thyristors
• Power transistors
• Diodes
• Rectifiers
• Varistors

Glass also has applications in solar cell passivation. In a recent study, researchers developed a method for enhancing borosilicate glass (BSG) passivation using high temperatures before lowering the temperature to accommodate the metallization process. In doing so, they notably improved the solar cell’s efficiency.¹¹

In another study, phosphosilicate glass (PSG) was found to significantly enhance the practical lifetime of minority carriers and improve the overall performance of solar cells, particularly in structures involving nanocrystalline silicon and crystalline silicon.¹²

Mo-Sci’s Expertise in Glass Thin Films

Fueled by the increasing prevalence of smart devices and advancements in the automotive and aerospace sectors, the semiconductor passivation glass market is anticipated to grow consistently in the next few years.³

Mo-Sci’s expertise lies in leveraging the unique properties of glass to create tailored solutions, ensuring the reliability and performance of many applications, including glass seals and glass coatings. Contact us for more information.

Krista Grayson

References and Further Reading

1. Pehkonen, S.O., et al. (2018). Chapter 2 – Self-Assembly Ultrathin Film Coatings for the Mitigation of Corrosion: General Considerations. Interface Science and Technology. doi.org/10.1016/B978-0-12-813584-6.00002-8
2. Lu, Q., et al. (2018). Chapter 5 – Polyimides for Electronic Applications. Advanced Polyimide Materials. doi.org/10.1016/B978-0-12-812640-0.00005-6
3. Reliable Business Insights. [Online] Semiconductor Passivation Glass Market – Global Outlook and Forecast 2023-2028. Available at: https://www.reliablebusinessinsights.com/purchase/1365249?utm_campaign=2&utm_medium=cp_9&utm_source=Linkedin&utm_content=ia&utm_term=semiconductor-passivation-glass&utm_id=free (Accessed on 05 January 2024).
4. Schott. [Online] Passivation Glass. Available at: https://www.schott.com/en-hr/products/passivation-glass-p1000287/technical-details (Accessed on 05 January 2024).
5. Zhong, C., et al. (2022). Properties and mechanism of amorphous lead aluminosilicate passivation layers used in semiconductor devices through molecular dynamic simulation. Ceramics International. doi.org/10.1016/j.ceramint.2022.07.191
6. Hansen, U., et al. (2009). Robust and Hermetic Borosilicate Glass Coatings by E-Beam Evaporation. Procedia Chemistry. doi.org/10.1016/j.proche.2009.07.019
7. Korvus Technology. [Online] The Revolution of PVD Systems in Thin Film Semiconductor Production. Available at: https://korvustech.com/thin-film-semiconductor/ (Accessed on 05 January 2024).
8. Schott. Technical Glasses: Physical and Technical Properties. Available at: https://www.schott.com/-/media/project/onex/shared/downloads/melting-and-hot-forming/390768-row-schott-technical-glasses-view-2020-04-14.pdf?rev=-1
9. Bonilla, R.S., et al. (2017). Dielectric surface passivation for silicon solar cells: A review. Physica Status Solidi. doi.org/10.1002/pssa.201700293
10. Babu, S.V., et al. (1993). Reliability of Multilayer Copper/Polyimide. Defense Technical Information Centre. Available at: https://apps.dtic.mil/sti/citations/ADA276228
11. Liao, B., et al. (2021). Unlocking the potential of boronsilicate glass passivation for industrial tunnel oxide passivated contact solar cells. Progress in Photovoltaics. doi.org/10.1002/pip.3519
12. Imamura, K., et al. (2018). Effective passivation for nanocrystalline Si layer/crystalline Si solar cells by use of phosphosilicate glass. Solar Energy. doi.org/10.1016/j.solener.2018.04.063

Devising Slumping Schedules

A while ago Bob Leatherbarrow gave a presentation to Lunch With A Glass Artist (LWAGA) on slumping schedules. You can follow a recording of the Zoom meeting after joining the Facebook group: Lunch With A Glass Artist – Larry Pile.

The most important point for thinking about the process he brought up is his order of consideration of factors. They are:

• span
• thickness
• viscosity

How big is the unsupported part of the glass. Glass on larger span moulds will begin slumping at lower temperatures.

The thickness has an effect. With the same ramp rate thicker glass will need higher temperature.

The viscosity of the glass also affects the temperature of the slump. Low viscosity glass will slump at lower temperatures than higher ones, e.g., black vs. white.

Then you can begin to think about temperature and time. The objective is to use the lowest temperature to get the slump done in 30 minutes, so there is no marking of the glass touching the mould.

There is a lot more in the presentation to LWAGA. Join the Facebook page by answering two questions to get access.

There is even more information about fusing principles and practices in his book FiringSchedules for Kilnforming, Just Another Day at the Office.   This inexpensive eBook is worth much more than the purchase price!

A lot of information is also contained in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and from Stephen.richard43@gmail.com

Glass Microspheres in Medicine

 

Using Embolic Glass Microspheres to Target Chronic Disease

Sierra Kucko

•

February 5, 2025

Glass microspheres under magnification. Image source: MO SCI.

Glass Microspheres: The Tiny Superheroes of Glass Form Factors

Two main factors contribute to the properties of glass: composition and form factor. While emphasis is often placed on glass composition, the form factor is arguably equal in importance. Glass in the form of microspheres has permeated various industries, ranging from aerospace to medical sectors. MO SCI specializes in the production of precision glass microspheres that have become invaluable to these industries, and their usage is ever-evolving. Whether it be controlling gaps for adhesive bondline spacing, improving the visibility of road markings, or drug delivery devices, glass microspheres fit the bill.

Targeted Treatment with Embolic Glass Microspheres

Biocompatible (and in some cases biodegradable) microspheres are especially appreciated for medical applications, such as transarterial embolization (TEA) or musculoskeletal (MSK) embolization.^1,2

TEA refers to the blockage of blood supply, which may sound like a bad thing, but in many cases, these are lifesaving procedures. For example, a substantial driver for this technology is cancer treatment. One way to combat a tumor or abnormal tissue growth is to cut off its blood supply, which can be achieved through the precise application of appropriately sized microspheres to occlude the fine vasculature ‘feeding’ it.¹

Similarly, MSK embolic microspheres are sought after to prevent the abnormal overgrowth of blood vessels, a consequence of chronic inflammation. This kind of inflammation is part of a pathological loop, whereby the inflammation promotes the formation of new blood vessels that in turn, can feed nerve growth and contribute to chronic, debilitating pain.² Microsphere embolization can therefore be used as a pain management tool, as well. For applications with this level of weightiness, the microsphere size is a chief feature.

Together with the form factor, glass microspheres can be tailored through their composition. First and foremost, any implantable glass must be compatible with the body. Ancillary to this, the composition can be altered to offer additional functionality. Using TAE as an example to put this concept into context, the composition of glass used in this type of application is unique and important.

TAE is a procedure utilized by interventional radiologists. Interventional radiology (IR) is the diagnosing and/or treatment of cancer and other conditions while avoiding major surgery. To achieve this, small tools such as needles, catheters, or wires are utilized in conjunction with radiation like MRI, ultrasound, etc. to apply treatment precisely to the tissue site.^1–3 Personalization and optimization of outcomes is a clinical challenge of any medical intervention, making the accurate delivery and distribution of the embolic particles in real-time indispensable.

Due to the use of radiation to guide the placement, the embolic particle should be radiopaque (opaque to radiation) to ensure that guided delivery to the site can be realized. Compared with glass, this radiopacity is lacking or more difficult to achieve in microspheres derived from other material types.

Partner with MO SCI for Precision Glass Microspheres

Each application of glass is unique and therefore may require unique chemistries and form factors. Glass microspheres are becoming increasingly popular, since their form factor alone may improve the function of the glass (depending on the application) when compared to their powder or frit counterparts.

For applications that require precise microsphere size and composition, it is important to turn to trusted experts. MO SCI produces a wide range of glass microspheres in a variety of chemistries to suit nearly any need. Contact us today to learn how glass microspheres may be beneficial for your application.

References

1.  Pérez-López A., et al. (2022). Embolization therapy with microspheres for the treatment of liver cancer: state-of-the-art of clinical translation. Acta Biomaterialia. https://doi.org/10.1016/j.actbio.2022.07.019

2.  Gremen E., et al. (2022). Safety and efficacy of embolization with microspheres in chronic refractory inflammatory shoulder pain: a pilot monocentric study on 15 patients. Biomedicines. https://doi.org/10.3390/biomedicines10040744

3.  Kishore S, et al, (2021). Transarterial embolization for the treatment of chronic musculoskeletal pain: a systematic review of indications, safety and efficacy. ACR Open Rheumatology. https://doi.org/10.1002/acr2.11383

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