Wednesday 13 March 2024

Heat Up vs Annealing

I am amazed by the effort put into ramp up rates, bubble squeezes, and top temperatures in comparison to annealing.  The emphasis on social media groups seems to be to get the right ramp rates for tack fuses and slumps, bubble squeezes, etc.  Most of the attention is on the way up to processing temperature.

The treatment of annealing and cooling is almost cavalier by comparison.  The attention seems to be on what temperature, and how long a soak is needed.  Then some arbitrary rate is used to cool to 370ºC/700ºF.



Annealing, in comparison to firing to top temperature, is both more complex and more vital to getting sound, lasting projects completed.  Skimping on annealing is an unsound practice leading to a lot of post-firing difficulties.

Annealing is more than a temperature and a time.  It is also the cooling to avoid inducing temporary stress. That stress during cooling can be large enough to break the glass.  This temporary stress is due to expansion differentials within the glass.

People often cite the saving of electricity as the reason for turning off at 370ºC/700ºF.  My response is that if the kiln is cooling off slower than the rate set, there will be no electricity used.  No electricity demands.  No controller intervention.  No relay operation.

Annealing at the lower end of the range with a three-stage cooling provides good results.  The results of Bullseye research on annealing are shown in their chart for annealing thick items.  It applies to glass 6mm and much larger.  It results from a recommendation to anneal at the lower end of the annealing range to get good anneals.  Other industrial research shows annealing in the lower end gives denser glass, and by implication, more robust glass.  Wissmach have accepted the results of Bullseye research and now recommend 482ºC/900ºF as the annealing temperature for their W96.  The annealing point of course remains at 516ºC/960ºF.

Bullseye research goes on to show that a progressive cooling gives the best results.  They recommend a three-stage cooling process.  The first is for the initial 55ºC/º100F below the annealing temperature, a second 55ºC/100ºF cooling and a final cooling to room temperature.

It is a good practice to schedule all three cooling rates.  It may be considered unnecessary because your kiln cools slower than the chart indicates.  Well, that is fine until you get into tack and contour fusing.  Then you will need the three-stage cooling process as you will be annealing for thicknesses up to 2.5 times actual height.

 

Of course, you can find out all the reasons for careful annealing in my book "Annealing; concepts, principles, practice" Available from Bullseye at

https://classes.bullseyeglass.com/ebooks/ebook-annealing-concepts-principles-practice.html

Or on Etsy in the VerrierStudio shop

https://www.etsy.com/uk/listing/1290856355/annealing-concepts-principles-practice?click_key=d86e32604406a8450fd73c6aabb4af58385cd9bc%3A1290856355&click_sum=9a81876e&ref=shop_home_active_4


Wednesday 6 March 2024

Slumping Strategy

A schedule was presented for a slumping problem of a 6mm/0.25” blank.  It consisted of three segments each of a rate of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F.  The cool was done with two long holds at 537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”



My response was that, yes it was fired too high.  Not only that, but the firing strategy, as shown by the schedule, is odd. 

Strategy

The general strategy for slumping follows these ideas.

·        Glass is slow to absorb heat, and in one sense, this schedule accepts that by having short soaks at intervals.  As glass is slow to absorb heat, it is necessary to use slow ramp rates and without pauses and changes in rates.  This should be applied all the way to the slumping temperature.

·        Holds of short durations are not effective at any stage in a slumping firing.  The objective is to allow the glass time to form to the mould with as little marking as possible.  This implies slow rates to low temperatures with significant holds at appropriate stages.  This about putting enough heat work into the glass that higher temperatures are not needed.

·        This kind of firing requires observation for new moulds and new arrangements of glass to ensure the slump is complete.  Once you know the mould requirements and are repeating the layup of the glass, the firing records will tell you what rates and times to use to get a complete slump with minimum marking.

·        The hold at annealing temperature is to equalise the temperature throughout the glass to produce a stress-free result.  Any soaks above are negated or repeated by the necessary soak at the annealing temperature.  The hold there must be long enough to complete the temperature equalisation that is the annealing.

·        My work has shown that annealing for one (3mm/0.125”) layer thicker produces a piece with less stress.  This indicates that a 6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.

The summary of the firing strategy for slumping is:

  • ·        A single ramp of a slow rate to the slumping temperature.
  • ·        Observation of the progress of the slump to determine the lowest practical temperature and hold time.
  • ·        Annealing for one layer thicker that being slumped.
  • ·        Three stage cooling of the piece at rates related to the annealing hold.

Critique

This is a critique of the schedule. For comparison, my schedule for a full fused 6mm blank would be different.

  • ·        140ºC/250ºF to 677º/1250ºF for 30 to 45 minutes.
  • ·        9999 to 482ºC/900ºF for 1.5 hours
  • ·        69ºC/124ºF to 427ºC/800ºF, no hold
  • ·        125ºC/225ºF to 371ºC/700ºF, no hold
  • ·        330ºC/600ºF to room temperature, off.

The rate of the published schedule is fast for a full fused blank and extremely fast for a tack fused blank. This needs to be slowed.  The schedule provides a single (fast) rate of heating, but with unnecessary holds.  The holds are so short as to be ineffective, anyway. There is no need for the holds on the way up to the slumping temperature.  In general slumping schedules are of fewer segments.   This is because glass behaves well with steady slow inputs of heat.

Then strangely, the schedule increases the rate to top temperature.  It does so with a brief soak at 593ºC/1100ºF.  This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF.  This is still in the brittle phase of the glass and risks breaking the glass.  The brittle stage ends around 540ºC/ 1005ºF.

This rapid rate softens the surface and edges of the glass without allowing time for the underside to catch up.  This explains uneven edges.  It also risks breaking the glass from too great expansion of the top before the bottom.

Additionally, the schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest slumping temperature.  The top temperature of this schedule is in the tack fusing range.

There is no need for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that equalises the temperature before the cool begins.  The higher temperature equalisation is negated by the cooler soak at annealing temperature. So, the hold at the higher temperature and slow cool to the annealing temperature only delays the firing by about two hours.  It does not have any effect on the final piece.

The schedule is cooling for a piece of 12mm/0.5”.  This is slower than necessary.  As noted above, cooling for one layer thicker than the piece is advisable to get the most stress free result.  The annealing soak could be 1.5 hours following this idea.  Cooling with a three stage schedule reduces the risk of inducing temporary stresses that might break the glass.  Although the initial cooling rate I recommend is very similar to this schedule, it safely reduces the total cooling time.

  • ·        69ºC/124ºF to 427ºC/800ºF, no hold
  • ·        125ºC/225ºF to 371ºC/700ºF, no hold
  • ·        330ºC/600ºF to room temperature, off.

Using my kind of schedule for the first time will require peeking once top temperature is reached to determine when the slump is complete. It may take as much as an hour. Be prepared to either extend the hold, or to skip to the next segment if complete earlier. The controller manual will explain how.

 More information is given in Low Temperature Kilnforming, An Evidence-based guide to scheduling.  Available from Etsy and Bullseye


Sunday 3 March 2024

Using Glass for Passivation in Semiconductor Applications

 Robotic arm holding a silicon wafer

Robotic arm holding a silicon wafer for semiconductor processing. Image source: iStock.

A blog post by Krista Grayson of Mo-Sci Mo-Sci logorayson

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, AlOx 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.

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

Wednesday 28 February 2024

Refiring and Annealing

A question about re-fusing: 

I have just taken a large piece, with uneven layers out of the kiln, it went in … and fired for double thickness. A small piece has flipped and is showing the white side. … If I cover this with a thin layer of coloured powder frit, does the piece need the long anneal process when I fire it again, please. I will be taking it up to the lowest tack fuse temperature possible [my emphasis], so the rest doesn’t change too much.

When considering the re-firing of a fused piece, even with minimal changes, the schedule needs re-evaluation of both ramp rates and annealing. In this case, the major change is using a sinter firing – “the lowest tack fuse temperature possible”.

Ramp Up Rates

Previously the piece was in several layers.

  • The piece is now a thicker single piece and needs more careful ramp rates.
  • It is also of uneven thicknesses.
  • And you intend to fire to a sharp tack or sinter.

These things make a requirement for more cautious firing. You cannot fire as quickly from cold as forthe original unfired piece. Previously, the sheets could be heated as though separate. They were not hot enough to stick together until beyond the strain point. They now could experience the differential expansion from  rapid heating, which can cause breaks. 

The previously fired piece will need a slower initial ramp rate this time. This is because you are firing for a sharp tack. This is also known as fusing to stick, or sintering. It is not because of a second firing. It is because of the differences in the glass for this firing. You are firing a single thicker piece of uneven layers to a sharp tack.

Looking at Stone* and the Bullseye chart for Annealing Thick Slabs indicates that in general, the first ramp rate should be halved for each doubling of calculated thickness. This is for full fused items. However, this is going to be a more difficult fusing profile - sintering. The calculation for sintering is as for 2.5 times the thickest part of the piece. This factor of 2.5 was determined by a series of experiments that are detailed in the eBook Low Temperature Kilnforming.

You started with firing two layers of 3mm/0.125” at possibly 330°C/595°F. You are now firing the fused 6mm/0.252 piece to a sharp tack. This means you should be looking at firing for 2.5 times or 15mm/0.625”. This implies 240°C/435°F as the maximum first ramp rate. A more cautious approach is to fire to 300ºC/540ºF at a rate of 72ºC/130ºF, as most heat-up breaks occur below that temperature. You should maintain that rate to 540°C/1005°F afterwards. 

Annealing

The annealing time and cool rate will be affected in the same way as the change to a sharp tack firing. Without that fuse profile change, and no change in the profile or thickness of the piece, it could have been annealed as previously. However, changing to a sharp tack means a longer anneal soak is required. This sharp tack annealing is for 2.5 times the thickness or 150 minutes.

Cooling

The cooling rates for this piece are not the same as for the first firing. A sharp tack firing will require cooling rates of:

  • 40ºC/73ºF to 482°C/900°F.
  • 72ºC/130ºF f427ºC/800ºF.
  • 240ºC/435ºF to room temperature

This applies regardless of the fusing glass you are using, as it is the viscosity which is the important factor in cooling.  Viscosity is primarily related to temperature.


Refiring with Significant Additions.

Ramp rate

If there are additions to the thickness, a slower first ramp rate will necessary. If an additional 3mm layer is placed on top of a 6mm base for a rounded tack, you will need to schedule as for 19mm/0.75” (twice the thickest part). This will be 150°C/270°F for the first ramp rate. For a sharp tack, it will be as for 22.5mm/0.825”. The maximum rate will be reduced to 120ºC/216F for the first ramp. This shows the additional caution required for sharper fusing profiles.

Annealing

The annealing will need to be longer than the first firing. The thickness has changed with the additions of pieces for a rounded tack firing. Instead of annealing for 6mm/0.25” you will be annealing as for 19mm/0.75”. This requires a hold of three hours at the annealing temperature and cooling over three stages:

  • The first cool rate is 25°C/45°F per hour to 482°C/900°F.
  • The second rate is 45°C/81°F per hour to 427ºC/800ºF.
  • The last rate is at 90C°C/162°F per hour to room temperature.

If there are additions, plus firing to the lowest possible tack temperature – as in the example - the firing must be as for 2.5 times the actual thickness. Annealing as for 25mm/1” gives rates of:

  • The first cool rate is 15°C/27°F per hour to 482°C/900°F.
  • The second rate is 27°C/49°F per hour to 427ºC/800ºF.
  • The last rate is at 90C°C/162°F per hour to room temperature.

These examples show how dramatically later additions in thickness can add to the length of the firing to re-fire a well-annealed piece without breaking it on the heat-up. It also shows that changing the profile to a sharper tack affects the annealing and cooling times and rates.

 

*Graham Stone. Firing Schedules for Glass; the Kiln Companion. 2000, Melbourne. ISBN 0-646-397733-8

As a side note Stone’s book has become a collectable.


Wednesday 21 February 2024

Go-to Schedules

 It’s a schedule I always use.

This is a frequent statement in response to a firing that has gone wrong.

You don't always fuse the same thing, or the same design, or the same thickness, etc. So why always use the same schedule?

The schedule for the firing each piece needs to be assessed individually. It may be similar to previous firings. But it may have differences. Assess what those differences mean for the firing.  Some factors to consider.

Addition of another layer to a stack in tack fusing makes a difference to the firing requirements. Even if it is only on part of the piece. It needs to have a slower ramp rate and a longer anneal soak and slower cooling.

A different design will make a difference in firing requirements too. For example, if you are adding a design to the edges of the glass, you will need different bubble squeeze schedules than when you do not have a border. It will need to be slower and longer than usual.

The placement of the piece in the kiln may require a re-think of the schedule too. If the piece is near the edge of the kiln shelf, or in a cool part of the kiln while others are more central, the same schedule is unlikely to work. You need to slow the schedule to account for the different heat work each piece will receive during the firing.

If you have introduced a strong contrast of colour or mixed transparent and opalescent glass in a different way, you may need slower heat ups and longer cools.

These are some examples of why the same schedule does not work all the time. It works for pieces that are the same. But it does not work for pieces that are different. And we should not expect it to.

There are sources to help in developing appropriate schedules. Bob Leatherbarrow’s book FiringSchedules for Kilnformed Glass is an excellent one.

Another one is especially good for lower temperature work: Low Temperature Kilnforming, anEvidence-Based Approach to Scheduling. Be aware that I have a vested interest here – I wrote it.


 

 

Wednesday 14 February 2024

Differential Cooling of Transparent and Opalescent Glass

A statement was made on a Facebook group that transparent glass absorbs more heat than opalescent glass. And it releases more heat during cooling. The poster may have meant that the transparent heats more quickly than the opalescent, and cools more quickly.

Yes, dark transparent glass absorbs heat quicker than most opalescent (marginally), and it releases the heat more quickly (again marginally) than opalescent. The colour and degree of transparency do not absorb any more or less heat, given appropriate rates. They gain the same heat and temperature, although at slightly different rates due to differences in viscosity.

An occasional table


The rate of heating and cooling is important in maintaining an equal rate of absorption of heat. The temperature of both styles can become the same if appropriate lengths of heating, annealing, and cooling are used. The slightly different rates of heat gain can give a difference in viscosity and therefore expansion.  This slight mismatch during rapid ramp rates, might set up stresses great enough to break the glass. This can occur on the quick heat up of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact, most heat-up breaks occur below 300ºC/540ºF.

The main impact of differential heat gain/loss is during cooling. Annealing of sufficient length eliminates the problem of differential contraction through achieving and maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most  opalescent glass exhibits different viscosities and rates of contraction. The stresses created are temporary. But they might be great enough to cause breaks during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction rates by maintaining small temperature differentials.

Significance of Differential Heat Gain/Loss

Uneven thicknesses and the tack fusing profile both have much greater effects than the differential cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations.  Cooling at an appropriate rate to room temperature for these factors will be sufficient to remove any risk of differential contraction between transparent and opalescent glasses.

Wednesday 7 February 2024

Comparison of Citric Acid and Trisodium Citrate.

These two substances are useful means of removing kiln wash and refractory mould material from glass. They are important where abrasive methods such as sand blasting are not available or appropriate.

My recent experience with both citric acid and trisodium citrate shows differences in performance. This makes each more suitable in different contexts.

credit: Amazon


Trisodium citrate is the safest option when long soaks are required to remove refractory mould material. The trisodium citrate removes any risk of etching the glass on long soaks. It has been shown by Christopher Jeffree that two-day soaks in this will not etch the glass. It is most suitable for casting work.


Items cleaned with citric acid and vinegar
credit: Christopher Jeffree

Citric acid acts quickly on kiln wash, making long soaks less necessary. Depending on the thickness of the stuck kiln wash and the amount of agitation of the stuck kiln wash, the time required may be only a dozen minutes. It rarely takes more than a few hours.  Citric acid does not work quickly on refractory materials. This makes the trisodium citrate the better choice for long soaks.

 More on citric acid as a cleaner

 More on citric acid

More on trisodium citrate

Wednesday 31 January 2024

Care of Ceramic Kiln Shelves

Mullite kiln shelves
credit: IPS Ceramics


The most popular and easily available ceramic shelves are made from Mullite, Cordierite, and CoreLite. Other hard specialist kiln shelves are available. They are made of other materials. Shelves are also made from other materials such as refractory fibre board, vermiculite, and fire-resistant ceiling tiles. This concentrates on the care of ceramic shelves.

Composition and Characteristics

This table gives some information about the characteristics of the materials involved in these shelves.

 

Name

Thermal Shock Resistance

Brittle

Strength

Composition

CoreLite

Low

Yes

Moderate

Ceramic with a high silica content

Cordierite

High

Yes

Strong, but heavy

Magnesium, iron, aluminium oxide, silica

Mullite

High

Yes

Strong. but heavy

Silica, Aluminium oxide

 

CoreLite is a trade name for an extruded ceramic shelf. It is strong, but brittle. It is subject to thermal shock below 540ºC/1000ºF. This suggests the ceramic has a high silica content as the quartz inversion is at 573°C/1063°F, where the ceramic has a sudden expansion on heating and an equal contraction on cooling. The cooling rate at this temperature is normally slow enough to avoid breakage.

credit: Clay Planet


cordierite - composed of magnesium, iron, aluminium oxide, and silica. hard, brittle, and with low expansion characteristics.

credit: refractorykilnfurniture.com


Mullitecomposed largely of silica and aluminium oxide. It is strong, brittle, and has good thermal shock resistance.

Care

There is enough information from considering the composition of these shelves to indicate they are all brittle and have differing vulnerabilities. These have implications for storage, use and cleaning.

Storage

If storing vertically, take care to avoid setting down on hard surfaces. If they are in a rack, have a separate slot for each shelf. This avoids friction between shelves and possible surface scratches. The most useful material for these racks is wood, or harder materials covered with wood. These racks can be horizontal or vertical.

If it is not possible to have a separate rack for each shelf, do not lean them on each other. Shelves leaning against others or against hard surfaces can become scratched. Provide a cushion against scratches such as cardboard, or thin plywood.

When moving the shelves, avoid setting them down on their corners, or bumping the shelf anywhere against hard structures.

Use

Reduce firing speeds to less than 220ºC/430ºF per hour up to 540ºC/1005ºF, especially for CoreLite shelves. Cordierite and Mullite shelves are not as sensitive, but still can be broken by fast firing rates in this temperature region.

Cover a large portion of the shelf at each firing to avoid uneven heating of the shelf. It is best to evenly distribute moulds and other things that shade the heat from the shelf around the shelf to help avoid thermal shock breaks.

If you cannot or do not want to cover the whole shelf, elevate the mould(s). This helps to keep the whole shelf at the same temperature when only small parts of shelf are covered. It does not seem to matter so much when flat glass is in contact with the shelf. But continue to observe the moderate ramp rates below 540ºC/1005ºF.

It is even more important to elevate damp or heavy moulds from the shelf. These kinds of moulds shade the heat from the shelf immediately below them while the rest of the shelf heats rapidly. This difference in expansion over parts of the shelf becomes too great for the shelf to resist.

Another thing to avoid is cutting fibre or shelf paper on top of the shelf. It often creates long shallow scratches in the shelf. These can be the source of bubbles, but more often, flaws on the back of the fired pieces.

Cleaning

Care is needed to avoid mechanical damage during cleaning. Scraping can create scratches in the shelf. These are difficult to remove or fill smoothly. So, scraping needs to be done carefully.

Any sanding also needs to be done carefully. If you use power tools, it is very easy to create shallow depressions that will be the source of bubbles in future firings. It is slightly more time consuming to manually sand the kiln wash with a sanding screen with or without a holder. But it preserves the flatness of the surface.

If it is decided to wash the shelf primer off the shelf, consider how difficult it is to wash a very persistent baked on substance. It requires thorough scrubbing to remove all the hardened material. Power washers are not advised since the high water pressure can abrade the surface of the shelf.  But if you do decide on washing, you need to air dry for several days afterwards. Then kiln dry slowly to just below boiling point of water. Soak at that point for several hours, or until a mirror held above the open port does not fog up.

There is more information on removing kiln wash here and here.


Summary

Ceramic kiln shelves are hard, but subject to scratches, impact breaks, excess dampness, failure due to uneven temperatures, and to rapid rises in temperature below 540ºC/1005ºF.

Wednesday 24 January 2024

Thickness of Powder Application

 

Why does my powder disappear when I fire?

 

Powder may appear to disappear after firing as Donna Brown found out with the pieces of her work shown here. Glass powder is finely ground glass sheet. The full colour of glass sheet is seen only when the glass is 3mm thick. So, to get the same intensity of colour you need to have the powder nearly 3mm thick.

This image shows the powder application before firing.
Picture credit: Donna Brown

There is not enough powder applied to the honeycomb. Everyone needs to run some tests to see how much powder is needed for strong colour. By running some tests of different thicknesses of powder you will be able to see how shading effects can be produced with powder. You should run the tests on both light coloured and dark coloured bases. Opalescent glass requires more powder than transparent. Opaque powders are better than transparent colours to show on dark colour.


This image shows the result of the firing, showing a thicker application of powder was required to give the full effect.
Picture credit: Donna Brown


In this particular application, I would put the powdered colour down and then the honeycomb grid on top for better definition of the honeycomb.


Fading powders  

Colour dilution   

Wednesday 17 January 2024

Mending a crack

 I had a piece crack due to an annealing oops. I put powder on it and put it back in at a higher temp with a much longer anneal time. It looks great on the front, but I can still see where the crack was on the back. Is it supposed to be like that? I didn't think to put powder on that side.

If you think about why you get crisp lines at the bottom of a strip construction and a more fluid appearance on the top, you will be near the answer of why a repair looks ok on top but shows the crack on the bottom. The temperature on the bottom of the glass is less than on the top at the working temperature. And less again than the air temperature which we measure. This means that the bottom part of the glass has less chance to fully recombine. This, combined with the resistance to movement of the glass along the shelf, results in evidence of the crack being maintained.

Credit: Clearwater Glass Studio


There are some things that can be done to minimise the evidence of the crack. Make sure you know why your piece cracked before you try to mend it. An annealing crack will need different treatment than a thermal shock crack or a compatibility crack. Simply refiring the piece may only make the problem worse.

One approach is to place a sheet underneath. Make sure the broken glass is well cleaned and firmly pushed together. Dams may be useful to keep the glass compressed together. Glass expands both horizontally and vertically during the fusing process. Confining the glass will transfer most of the expansion in a vertical direction. This additional (small) vertical movement may help in forming the glass seamlessly. The broken glass now being supported by an unbroken sheet will enable the movement required to “heal” the crack.



If you do not want to change the surface, you can fire upside down. To do this you need to have a loose bed of powdered kiln wash, or whiting (a form of chalk) that is thick enough to press the textured side fully into the separator. Make sure the glass is pressed together without any separator getting into the crack. One way to ensure the crack does not open is to use a small amount of cyanoacrylate (super) glue which will burn away during the firing.  Put a sheet of clear glass over and fire. Thoroughly clean the face after this repair firing. The ultimate top needs to be fire polished to remove the evidence of the crack, and if it has picked up any marks from the powder.

You could, of course, fire upside down in this way but without the additional sheet, to avoid making the piece any thicker. This may or may not work well. If the base layer is one layer thick, it may pull in at the sides and pull apart at the crack where it is one layer thick.  It is also possible that bubbles will develop in the thin parts of tack glass because of the uneven thicknesses.

A final note. Placing powder on the back will not improve things. The powder will not fully incorporate with the glass and so leave a rough surface without concealing the crack.

Avoiding breaks

To repair or not

The process of repairing