Wednesday, 21 May 2025

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


Wednesday, 14 May 2025

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.

Wednesday, 7 May 2025

Radiation Shields

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

Image credit: Mo-Sci,Llc



Wednesday, 30 April 2025

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.




Wednesday, 23 April 2025

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


Wednesday, 16 April 2025

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.





































 

Wednesday, 9 April 2025

Bone Grafts with Glass

 The use of glass in bone grafts.

image credit: Mo-Sci, Llc


 

Wednesday, 26 March 2025

Bioactive glass

 A description of bioactive glass from Mo-Sci,Llc

Image credit: Mo=Sci, Llc


Wednesday, 19 March 2025

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.

Friday, 14 March 2025

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.

Wednesday, 12 March 2025

Dog Boning in Slumps

I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.



Square single layers dog boned even with increased rim width, and reduction of slumping depth made little difference in the amount of dog boning. 

Rectangular single layers shapes persisted in dog boning on the long side regardless of the rim dimension, and exhibited more dog boning on the long side than in the equivalent single layer square.  Two layer slumping had a decrease in dog boning with increased rim width, but with less effect on the long side.

In general, glass slumped in rectangular moulds is more sensitive the shape of the rectangle than the size of the rim, and very sensitive to symmetrical placing on the mould.  The depth of the mould has less influence than the size of the rim, especially for single layers.  The wider the rim, the less dog boning, in general terms.

Deeper moulds, higher temperatures, longer holds, narrower rims, all increased the dog boning. I conclude slumped square glass looks better because the dog boning is symmetrical.

My solution is to make bigger rims and cut the piece square after slumping. This approach needs cold work to the edges, of course.


The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.

An alternative to the cold working is to round the corners of the rectangles to reduce the amount to draw-in.  A 1cm/0.375” radius curve will reduce the extent of the dog boning, but does not eliminate the effect.

Wednesday, 5 March 2025

Pressing glass


I have been looking for a different way than flows or melts to mix colours and thought glass pressing might be a promising way to achieve what I wanted.


Weight vs Temperature

I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of glass to 3-4mm.  One and then two 40x40cmx15mm thick shelves were placed on top of the glass cullet with 3mm spacers at the corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held for 30 minutes, later extended to 90 minutes.  The thickness stubbornly remained between 5 and 7mm. 

A few other attempts with different times and temperatures gave inconsistent results.  Perhaps the uneven piling of cullet had an influence on the outcomes, but I was still looking for a flow and mixing of colours different to that obtained by melts.

Other experiments were being conducted in parallel, relating to viscosity. These indicated that glass became thinner than 6-7mm at higher temperatures without pressing.  These experiments lead me to think there are four elements controllable by kilnformers in pressing: size, weight, time, temperature.

The same weight of press with the same temperature and time will make small amounts thinner than large amounts, and this is not surprising.  More time with the same temperature, weight, and amount allows some slight decrease in thickness. 

Higher temperatures with the same weight, and time will allow thinner pressings of the same amount of glass.   Viscosity decreases with temperature, so higher temperatures make glass easier to thin.

More weight is required get the same thickness when pressing a greater volume of glass.  Of course, more time and temperature can be added to increase the effect of the weight.

However, the main factor in pressing large amounts of glass is higher temperatures, which results in reducing the viscosity and the resistance to thinning. 

 

Annealing and Cooling

An important aspect of pressing is the annealing requirements.  It is sensible to anneal for a longer time than normal for thick glass, because of the heat retention of the pressing weights. 



This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as for 16mm/0.63” (5/8”).  There is widespread low level stress with 30mm thick pressing weight.

Indications are that extending the annealing to at least 3 times the target thickness is a minimum annealing soak requirement.  Alternatively, if it is possible to remove some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.

 

Veiling

The stress picture above shows there is visual element too.  This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass.  Small volume stacks, which are pressed thin will exhibit less of the veiling.

 

 

Four factors that kilnformers can control in pressing glass to less than 6mm are weight, size, time, and temperature.  The main one is temperature.

Wednesday, 26 February 2025

Stress Analysis of Broken Glass

Will stress still show with polarised filters on cracked and broken glass?


It's not a straightforward answer.

I was looking at some broken fused float glass a few years ago.  I had always subscribed to the idea that a fracture relieves the stress. Not always. The broken float glass had been slumped, and the pieces still showed stress.  This turned out to be a compatibility problem, although both layers were float.  

The stress of inadequately annealed glass is likely to remain visible through the filters, because inadequately annealed glass will have stress distributed across the whole piece.  But glass that has been cooled too quickly and suffered thermal shock, is more likely to show minimum stress because the break relieved most of it.

It is likely stress will show on the tree piece pictured because it has not completely broken a[art. And even when it does break, it may still show a residue of stress.

It is sensible when trying to diagnose the problem to perform a strip test of the glasses for compatibility of the glasses concerned to be sure what is happening. If no stress shows on the test strip, the stress showing on the cracked piece is unlikely to be from incompatible glass, and other factors need to be considered.

Photo credit:  Debi Frock-Lyons 

Sunday, 23 February 2025

Rapid Ramp Rates with Soaks


I have seen many schedules with initial rates of advance interrupted by soaks.  These kinds of schedules that are written something like this:

250°C/450°F to 200°C/482°F, soak for 10 (or 20 or 30) minutes
250°C/450°F to 500°C/933°F, soak for 10 (or 20 or 30) minutes
300°C/540°F to 595°C/1100°F, soak for 10 (or 20 or 30) minutes
300°C/540°F to 677°C/1250°F, soak for 10 (or 20 or 30) minutes
330°C/600°F to working temperature (1450°, 1500° etc.)

When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature.  There are occasions when that may be a good idea, but I will come to those later.  For normal fusing, draping and slumping these soaks are not needed.

To understand why, needs a little information on the characteristics of glass.  Glass is a good insulator, and therefore a poor transmitter of heat.  Glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass.  To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock. 

The soaks at intervals do not protect against a too rapid increase in temperature.  It is the rate of heat input that causes thermal shock.  Rapid heat inputs cause uneven temperatures through and across the glass.  When these temperatures are more than 5°C different across the glass, stress is not relieved.  As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and breaks.  At higher temperatures these stresses do not exist as the glass is less viscous.

If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated.  If the glass survived the rate of heat input between the soaks, it will survive without the soaks.

But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance.  Just consider the time used in the soak and then slow the rate by the appropriate amount.  Take the example above using 30-minute soaks:

250°C/450°F to 200°C/482°F, soak for 30 minutes
250°C/450°F to 500°C/933°F, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167°C/300°F per hour to 500°C/933°F.  This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise.  If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.

But, you may argue, “the periodic soaks on the way up have always worked for me.”  As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work.  Each layer insulates the lower layer from the heat above.  As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.

To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion.  This means a steady input of heat with no quick changes in temperature.  How do you calculate that rate?  Contrary as it may seem, start by writing out your cooling phases of the schedule.  The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece.  If that final cool rate is 300°C/540°F, the appropriate heat up rate is one third of that or 100°C/180°F. 

This “one third speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass.  This rate needs to be maintained until the upper end of the annealing range is passed.  This is normally around 55°C/100°F above the annealing point.

Then you can begin to write the rate of advance portion of your schedule.  It could be something like:

100°C/180°F to 540°C, no soak
225°C/405°F to bubble squeeze, soak
330°C/600°F to working temperature, soak 10 minutes
Proceed to cool segments 

I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze.  This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.

Exceptions.

I did say I would come back to an exception about soaks on the first ramp rates  segment of the schedules.  When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks.  The kind of suspension is when draping over a cylinder or doing a handkerchief drop.  This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air.  It also occurs when supported by steel or thick ceramic.

The soaks are not to equalise the temperature in the glass primarily.  They are to equalise the temperature between the supports and the glass.  A thick ceramic form supporting glass takes longer to heat up than the glass.  The steel of a cocktail shaker takes the heat away from the glass as it heats faster. 

The second element in this may already be obvious.  The glass in the air on a ceramic mould can heat faster than that on the mould.  The glass on a steel mould can heat faster over the steel than the suspended glass.  Both these cases mean that you need to be careful with the temperature rises.

Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage.  However, my experience has shown that periodic soaks in combination with gradual increases in the rates of advance are important, because it is more successful. 

An example of my rates of advance for 6mm glass supported on a steel cylinder is:
100°C/180°F to 100°C/212°F, soak 20 minutes
125°C/225°F to 200°C/392°F, soak 20 minutes
150°C/270°F to 400°C/753°F, soak 20 minutes
200°C/360°F to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance.  This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.


Another exception is where you have a lot of moisture in a mould, for example. You need to soak just under the boiling point of water to dry the mould or drive out water from other elements of your work before proceeding.  This also applies to situations where you need a burn out, of for example vegetable matter at around 500°C/933°F for several hours.

In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.

Revised 23.2.25