The wonders of thick and thin glass films.

Applying Functional Films to Glass Substrates

Posted Krista Grayson on Nov 28, 2022

Source: https://mo-sci.com/applying-functional-films-to-glass-substrates/

Glass is a hugely versatile material. Tempered glass, for example, can be produced simply by changing the heating and cooling process during manufacturing. Changes to the shape of glass lenses alter their optical characteristics, while the introduction of pores into bulk glass enables a range of high-tech applications like bio-scaffolds and catalyst supports. Modifications to the chemical composition of glass – for example, through the use of glass modifiers – can change almost all of its properties, enabling the production of corrosion-resistant labware and high-resistance electrical components.

Depositing films on glass provides a way of augmenting the properties of glass without changing the glass itself. From thin-film solar cells to heating elements integrated directly onto glass surfaces, these films enable products and components which combine the properties of glass with those of other materials and technologies.

Film deposition can largely be divided into two categories: thin film deposition and thick film deposition. As we mention in our article on glass films, there is some overlap between the actual thickness of films in these categories – however, “thin films” and “thick films” remain distinct primarily due to differences in the technology used to produce them. Thin films typically range from less than a nanometer to several microns in thickness and are typically produced by sophisticated processes such as vapor deposition. Thick films, on the other hand, generally range from several microns up to a millimeter in thickness; and are usually deposited in the form of inks or pastes via processes like screen printing or tape-casting.

Depositing Thick Films on Glass

Electronics

Thick films are widely used in electronics: alternating layers of conductive and resistive materials can be deposited and patterned onto a substrate to build up electric circuits. While ceramic substrates are common, it is not unusual for these films to be deposited onto glass instead.

In these applications, thick films are typically deposited on glass via screen printing, forming layers between 5 and 20 μm in thickness. Insulating thick film pastes will often contain glass in the form of frit to provide high resistivities.7 After deposition, these thick films are typically fused at high temperatures before the next layer is deposited, providing a reliable and low-cost route to the fabrication of microelectronic devices.

One alternative application of thick is the production of printed heater elements on glass substrates.8 Directly depositing a heating element onto glass enables the construction of self-defrosting windows or glass appliances (such as kettles and cookers) which provide uniform heating with the modern appearance of glass.

Thick films offer the advantage of versatile and low-cost fabrication, making them ideal for the production of electronic components throughout a wide range of industries. For precision applications, however, thin film technologies provide much greater control over film thickness and surface characteristics.

Depositing Thin Films on Glass

Optics

One of the primary application areas of thin films on glass is in optics: in fact, the most famous application of thin films is probably the household mirror, which is produced by depositing a thin metal layer on the back of a sheet of glass to increase its reflectivity.

Depositing thin films on glass can produce optical interference effects, which result in certain regions of wavelengths being transmitted, reflected, or absorbed.2 Such films modify light by virtue of their nanoscale structure rather than the color of the bulk material itself, enabling optical parameters such as reflectivity and the color of transmitted light to be precisely tuned by changing layer thickness. The wings of some species of butterfly use the same fundamental “optical thin film” principles to produce their striking iridescent coloring.3

Low emissivity (or “Low-e”) glass is a major application of thin film deposition on glass. Produced through successive deposition of thin metal oxide films on glass, it allows the transmission of visible light while reflecting radiated heat (i.e., the infrared portion of the spectrum). Such optical films enable Low-e windows to reflect the sun’s light in hot environments or to prevent heat loss through windows in cold environments.

Similarly, these films enable anti-reflective coatings, which reduce glare in architectural applications as well as in consumer electronics.

High-precision deposition of thin optical films enables the construction of specialist optical filters such as dichroic filters. These filters rely on extremely precise film deposition to transmit or reject specific wavelength bands for precision applications in research and industry.

Electronics

Glass substrates for thin films have a number of special roles in electronics — particularly in the fabrication of transparent conducting films (TCFs). TCFs are a special type of film made of materials that are optically transparent and electrically conductive. They are fabricated by depositing or growing thin films of materials such as metal oxides — or even graphene — on glass substrates (with the glass offering the additional benefit of blocking infrared wavelengths of light). TCFs are applied in a range of devices, including LCD and OLED displays, touchscreens, and photovoltaic panels.4

Depositing conductive traces directly onto glass substrates enables circuitry and functional electronic components to be integrated into glass, with widespread application in aviation, automobiles, and consumer electronic devices such as smartphones.5 Other applications of thin films on glass substrates in electronics include the manufacture of thin film resistors and transparent electrodes produced by sputtering metal films onto glass.6

At Mo-Sci, we are experts in creating custom glass solutions for unique and demanding applications: whether that is glass substrates for a specific thin film application or ultra-pure glass frit for the production of resistive thick film pastes. To find out more about our services and capabilities, get in touch with us today.

References and Further Reading

1. Bach, H. & Krause, D. Thin Films on Glass. (Springer Science & Business Media, 2003).
2. Anderson, A.-L., Chen, S., Romero, L., Top, I. & Binions, R. Thin Films for Advanced Glazing Applications. Buildings 6, 37 (2016).
3. Butterflies Hack Light Waves to Produce Brilliant Color — Biological Strategy — AskNature. https://asknature.org/strategy/wing-scales-cause-light-to-diffract-and-interfere/.
4. Rosli, N. N., Ibrahim, M. A., Ahmad Ludin, N., Mat Teridi, M. A. & Sopian, K. A review of graphene based transparent conducting films for use in solar photovoltaic applications. Renewable and Sustainable Energy Reviews 99, 83–99 (2019).
5. Kim, H.-G. & Park, M.-S. Fast Fabrication of Conductive Copper Structure on Glass Material Using Laser-Induced Chemical Liquid Phase Deposition. Applied Sciences 11, 8695 (2021).
6. Thin Film Applications | Bourns. https://www.bourns.com/pdfs/thinfilm.pdf.
7. Zargar, R. A. & Arora, M. Screen Printed Thick Films on Glass Substrate for Optoelectronic Applications. in Photoenergy and Thin Film Materials (ed. Yang, X.) 253–282 (Wiley, 2019). doi:10.1002/9781119580546.ch6.
8. Radosavljevic, G. & Smetana, W. 15 – Printed heater elements. in Printed Films (eds. Prudenziati, M. & Hormadaly, J.) 429–468 (Woodhead Publishing, 2012). doi:10.1533/9780857096210.2.429.

Square Drapes

Two pyramidical moulds. One stepped and the other smooth.

 This kind of draping mould with flat sides will never work very well as a draping mould.  The draping sides have to compress. This takes a long time and is likely to cause folds in the glass.

 The common experience is that two opposite sides drape first and conform to the mould. This displaces the compression necessity to the other two sides. This "taco" style initial drape is common in all drapes. It is usually observed in handkerchief drapes.  In the early stage of draping two sides of the glass fall, creating a taco shape. With continued heating, those long sides fall and spread the initial draped sides to become almost equal. 

 This taco formation also occurs on the pyramid style mould, giving two flat sides.  The glass on the other sides then fall. As the glass area is now larger on these sides than the mould area, a drape or fold is formed.  Imagine the drapes a square piece of cloth place on a pyramid would create. The cloth has more area than the sides of the pyramid.  The excess cloth creates folds at each corner.  The same happens with the glass.

 This draping fold can be minimised by using low temperatures and long (multiples of hours) soaks.  This allows all the sides of the glass to begin forming at more or less the same time.  I am not sure the folds can ever be completely eliminated.  With extremely long soaks, the drapes will flatten to the rest of the glass. 

 Annealing difficulties are caused by this folding.  It will create thick overlaps.  This in turn will cause the annealing difficulties. There are areas that are much thicker than others.  If you started with 6mm glass, the folds will create areas that are 18mm thick. 

 Making sure this glass - with such large differences - is all of the same temperature will require long annealing soaks.  It will also require very slow cooling segments.

 Square drape moulds are rarely successful. Folds are created at the corners, rather than fully conforming to the mould.

Effect of AFAP Rates

 

 

This graph illustrates the effect of a rapid increase (500C/hr) in temperature on the glass.  The blue line represents the air temperature measured in the kiln.  The orange line represents the temperature between the glass and the shelf.  At an air temperature of 815°C, the temperature of the glass at its bottom is around 750°C.  This is a large difference, even though the glass is in the plastic range.  It means that the potential for stress induced by the firing rate is large.  The graph shows the temperature difference evens out during the annealing soak.

 The fast rise in temperature at the initial part of the firing where the glass is still brittle risks breakage.  The difference in temperature between the top and bottom of a 6mm piece of glass is shown to be 100°C plus throughout this initial phase up to 500°C.  Most breaks due to thermal shock occur before 300°C. This large temperature difference that occurs with rapid rates of advance risks breakage early in the firing.

 As an example, I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.

 With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.

 This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

 More importantly at top temperature, the surface will be fully formed while the bottom is only at a tack fuse temperature. This does have implications for the strength of the piece.  There will be an only tack fused structure through much of the piece, but a full fused structure at the surface.  The potential for breaking in further kilnforming or during use is high.

 In addition to the effects on the glass, there will be effects on the operation of the controller.  Controllers operate by comparing the instructions on firing rate with the air temperature recorded by the pyrometer.  In doing this the variances become smaller with time.  An AFAP firing does not give a lot of time for the controller to “learn” the firing curve.  So, the controller tends to overshoot the top temperature by some (variable) degree.  This makes it difficult to precisely control the outcome of the firing.

 There is some concern that the structure of the kiln will be affected by AFAP firings. This is a small risk.  The expansion and contraction of the kiln materials will occur whether quickly or more slowly.  It is not a major concern.  It is a concern for the glass, though.

 AFAP firings have potentially harmful effects on the structure of the fired glass leading to thermal shock and fragile completed pieces.

Notes on Polarised Light Filters

Polarised light filters are used to detect stress in a non-destructive testing method in kilnforming.  The use of the filters is described in this blog. To produce consistent reliable results, there are certain conditions.

 The light source needs to be diffused in such a way that it is even across the viewing area.  An intense, single point light makes it difficult to determine the relative intensity of apparent stress. Another tip is that you can use your phone or tablet as a source of diffused light and as the bottom filter.  It emits polarised light, meaning only a top filter is needed.

Stress halos from broken and fused bottles

 It is important that the glass being tested is of the same temperature throughout to get a meaningful result.  This was emphasised to me when I was running a series of tests. I got in a hurry to test for stress to be able to start the next trial quickly.  I began to notice inconsistencies in the amount of stress I recorded for results of the series of tests.  Going back to the stressed test pieces, showed different stress levels when they were cold from when they were warm.

 The conclusion is that the glass to be tested for stress must be the same temperature throughout.  Even if it is only slightly warm, the apparent stress will be exaggerated.  It may be that the testing can only be done 24 hours after removed from the kiln.

 Stress will be more evident at points and corners.  The light will be brighter at highly stressed points, and even at extreme stress exhibit a rainbow effect.  More generalised stress is evident in a lighter halo.

Stress points in a drawing square illustrating the concentrated stress at corners

 It is much more difficult to check for stress in opaque areas of a piece.  If there are transparent areas, the stress will show there, although the stress may originate in the opaque ones. To be aware of potential stress in the combination of opaque glass, strip tests must be conducted on samples of the glasses. 

 Remember to include an annealing test too, as the stress test does not distinguish the type of stress.  If the annealing test shows stress, the annealing was inadequate. It is of course, possible that the glass is stressed because of incompatibility.  But the only way to determine that is to fire another test with a longer soak at annealing.

Evaluating Top Temperature Effects

Fire for Your Kiln and Objectives

Credit: FusedGlass.org

 Often temperatures to achieve given effects are shared on the internet to be helpful to others.  Those who receive these need to evaluate the schedules used to achieve the profile at the stated temperature.

 The same effect can be achieved at different temperatures by using different rates and times in getting to the given temperature.  This is summarised as the effect of heatwork.  The longer taken to achieve the top temperature, the lower the temperature can be used.  The amount of time the holds/soaks occupy in the schedule will also affect the profile achieved.  This means that a simple top temperature can only be a point from which to begin exploration.

 It is important to evaluate each segment. What is the apparent purpose? How does it fit with the principles of applying heat and the characteristics of glass?

 When asking for help with a firing temperature, ask for the full schedule.  This will help you evaluate the suitability of the temperature given.

 The variations in schedules and kilns mean you should fire by results not by numbers. Use the rates, temperatures and soaks that will achieve what you want in your kiln.

Making Test Tiles

Creating samples or tests provides both references on firing profiles and knowledge of the characteristics of the kiln. 

General samples

 Sample tiles are normally a series of tiles with the same lay up but fired at different temperatures.  These are likely to be intervals of temperature from a sharp tack to a full fuse.  A suitable interval might be 10°C as it is easy to interpolate between these for a slightly different profile than the tiles show. This is the basic arrangement.

 You can make this more informative by including tiles in the same basic lay up but with hot and cool colours, opalescent and transparent, black and white, strikers, etc.  The addition of these will give a richer bank of information.

 Of course, these tiles must be labelled with glass types and code numbers and the temperature used.  This is not all the information required though.

 Many sample tile sets do not include the firing rate.  The heat work required to attain a specific profile is dependent on time, temperature and hold.  These are the time to get to the working temperature, the temperature, and the soak time.  If you do not record the ramp rate used, you will have incomplete information.  It is not that you have to record the entire schedule.  But the rates and any soaks on the way to the top temperature need to be recorded. This means you can take account of any slower rate of heating, any additional holds on the way up, and the length of the soak at top temperature.  Then when contemplating something more complicated than the conditions under which the tiles were made you have better information.

 It is a good idea to maintain a photographic record of the sample tiles to avoid storage problems.  These can be made from the individual tiles and photographed from several angles. 

 Another way of keeping records - without making tiles for each temperature - is to photograph the tiles through peep holes as the set temperatures are achieved.  This means the tiles need to be placed in the kiln so they can be seen from the peep hole. You will only have a physical sample for the top temperature. The other profiles will have a photographic record.  The firing conditions for these need to be recorded just as for the series of physical tiles.

 This photographic record may not be suitable for your way of working and so require making the sets of multiple tiles.  Both these methods provide a generalised record of heat work to achieve given profiles.  Note that you will need to prepare sample tiles for each kiln, as each has different characteristics. 

Specific samples

 However, there will be cases where the general conditions exemplified by the reference tiles are to be exceeded.  In this case you will need to make a sample specific to the piece you are planning.  This can be a general representation of the piece, or a scaled mock-up. 

 The general test tile may be small scale or relatively large.  It will contain only the components in terms of height and shape that will be in the planned, but more complicated piece.

 A more rigorous method is to make a full scale – or nearly so – mock-up of the piece. This is usually done in clear. Fire it to the proposed schedule to determine the exact effect.

  

Sample making gives confidence in preparing work for the kiln and scheduling to get the desired profile.

Kiln wash beading up

Sometimes kiln wash does not seem to want to stick to the mould.  There are several possible reasons. The main two seem to be a hard spot in the slip cast moulds that we use.  Another is the previous use of boron nitride or other sealant of porous surfaces.

The remedies are different for these two causes.  For hard spots you can add a bit extra kiln wash to the area.  Normally enough separator adheres to the spot to avoid sticking.  This is so even though you can see the spot more clearly than the rest of the mould.

Sealed surfaces present a little more difficulty.  It is possible to carefully sand blast off the boron nitride from the surface using low pressure and very little abrasive.  This works well for textured surfaces, if you are careful.  You can also manually sand the sealant off which works better for regularly shaped smooth surfaces.   The object of both these processes is to remove the sealed surface to reveal the porous material again.  You must remember that you are removing some of the surface of the mould in these abrasive processes.  Once removed kiln wash can be applied as before.

If neither abrasive method works, it does not mean the mould is ruined.  You can continue to use boron nitride.  Or, if you want to avoid the costs of boron nitride, you can sprinkle fine dry kiln wash over the mould.  You should give the mould a final application of boron nitride before using the dry kiln wash.

Kiln Wash Mix

There seems to be a view that the exact consistency of the kiln wash mix is important.  Within limits the mix proportions are not vital.  The general recommendations from manufacturers is one part powder to five parts water – both by volume.  This is a good guide for general use.

 

It is possible to make the kiln wash mix too thick.  If it goes onto the shelf or mould in a pasty fashion it is too thick.  A thick mixture leaves definite streaks and uneven levels that are difficult to smooth and level.  If you get these effects, scrape it off and put it into a jar with more water.  Mix until it is creamy to avoid lumps.  Then add more water until you have a very liquid mix.  It needs only be a little less runny than plain water.

 

Is it possible to have too thin a mix of kiln wash?  I suppose it is, but not likely.  If you feel it is too thin, you only need to add more coats of the mix until the shelf surface is obscured. Often when the mix seem thin, it is because the powder has separated from the water.  It is necessary to stir the kiln wash thoroughly to get all the solids in suspension.  Then frequent stirring during the application is necessary to keep the mix even at both the top and the bottom of the container.  Storing the mixed kiln wash in a clear container will enable you to see if kiln wash is still settled on the bottom.

 

The object of the kiln wash is to provide a separator between the supporting surface and the glass.  It needs to be only a film of separator to be effective.  In fact, if the kiln wash is too thick, it will flake and stick to the back of the glass.  In the case of kiln wash - more is definitely worse.

 

For very absorbent materials such as vermiculite or fibre board, I mix kiln wash thicker – about 1:3.  The idea behind this is to reduce the amount of water the mould absorbs.  With less water in the mould, less drying time is needed, especially with a vermiculite mould, where steam pressure could break the mould.

Achieving Multiple Profiles in One Piece

People ask about whether it is possible to tack fuse additional elements without affecting the profile of the existing piece.

It is as though glass has a memory of the heat it has been subjected to.  For example, a sharp tack will become a slightly rounded tack, even though refired to a sharp tack again.  So, it is impossible to refire a piece to the same temperature or higher without affecting the existing profile.  But it is possible to fire a piece with differing profiles if you plan the sequence of firings.

 

Tack fuse onto existing profile

 

It is possible to add pieces to be tack fused with little distortion to the existing piece through careful scheduling and observation.  There are several requirements.

 •     A moderate rate of advance to the working temperature is required, rather than a fast one. This is because the piece is a single thicker piece with uneven thicknesses.  Also, a slow rise in temperature allows completion of the work to at a lower temperature.  This means there will be less change to the existing profile.

•     A minimal bubble squeeze - or none at all - is required on this second firing.  The added pieces generally will be small, so if possible, eliminate the bubble squeeze.  The requirement is to add as little heat work as possible.

 •     The working temperature should be to a low tack fuse temperature with a long soak. 

 •     Observation is required from the time the working temperature is achieved.  Peeking at 5-minute intervals is needed.  This to be certain that the current tack fuse can be achieved without much affecting the existing profile.  It will be a compromise that you will be able to choose during the firing.  The decision will be whether to retain existing profile and have a sharp tack.  Or a slightly rounded tack and more rounded profile on the original piece.

Planning for multiple levels of tack

It is possible to design a piece with multiple profiles within the completed piece.  You need to plan out the levels and degrees of tack you want before you start firing. 

To do this planning, you need to remember that all heat work is cumulative. In simple terms it means that on a second firing you will start where you left off with the first one. The texture in the first firing will become softer, rounded, or flatter than the second or even the third firing.

Three degrees of tack can be achieved with a little planning.  It works similarly to paint firings.  Some paints fire higher than enamels, and enamels hotter than stain.  You have to plan to fire all the tracing and shading first.  Then you add the opaque enamels, followed by the transparent enamels.  Finally, you add the silver stain.  This is unlike painting on canvas where you build up the image all together.

The same principle is true of a multiple level tack fuse piece.  When creating various profiles in glass, you proceed from firing the areas that will be the flattest first. Then proceed to the areas which will have the least tack last.  This is a consequence of the cumulative effect of heat on re-fired glass.

Plan out the areas that you want to have the least profile.  Assemble the glass for those areas. I suggest that a 6mm base is the initial requirement for anything that is going to be fired multiple times.  Add the initial pieces that will become a contour fuse or a very rounded tack. 

First firing

Put this assembly in the kiln and schedule.  Do not fire to the contour profile temperature.  Instead, you will be scheduling for a sinter or sharp tack. This depends on how many textures you plan to incorporate.  Start with a sharp tack.  Fire at the appropriate rate with a bubble squeeze to about 740°C for 10 minutes and proceed to the anneal cool.   Different kilns will need other temperatures to achieve a sharp tack.

You do not fire to the contour fuse temperature, because the base will be subject to more firings.  Each of these firings will soften the base layers more than the previous one.  This is the application of the principle of cumulative heat work.  When you fire a piece for a second time, there will be little effect until the softening point of the glass is reached. Once there, the glass further softens, giving the effect of a contour fuse.

Any glass that had already achieved contour profile from the first firing will flatten further.  This can be used in cases where the working temperature was not high enough.  Just fire again to the original schedule’s temperature.  Take account of the need for a slower ramp rate to the softening point.

Second firing

Once cool and cleaned, you can add your next profile level of tack fusing to the base.  Note that “level of tack” does not refer to thickness being built up.  It is about the amount of roundness you want to impart to the pieces.  You may be placing this second - sharper – level of tack in the spaces left during the first firing.  Again, schedule to the original approximate 740°C. But remember the base is now a single piece.  You need to slow the ramp rate to the softening point, after which the speed can be increased.  You will not need to retain the bubble squeeze unless you are adding large pieces, or into low areas. 

The second firing will show the pieces added for the second firing to have the profile of the original pieces.  Those pieces having their first firing will have a sharper appearance.

 

credit: vitreus-art.co.uk 

This is a piece where the flower petals and leaves could have been placed for the second firing to give a softer background with less rounded flower details.

 

Third firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Final firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Further notes on multiple firings

It is a good idea to observe the firing, once the working temperature is achieved.  This is to ensure enough roundness is being given to the final pieces being tacked to the whole.  Be prepared to extend the soak if the final pieces are not rounded enough.   Although you should have a good idea of the degree of tack for the final pieces from the previous two firings.

You may need to experiment a little with the temperature and length of soaks at the working temperature.  For example, if the degree of tack is too sharp in the first firing, you can extend the soak or increase the temperature for the next ones. 

If you are firing at 740°C, you may feel you can afford to extend the soak for the subsequent firings, because you are in the lower part of the devitrification range. Consider the risk of devitrification increases with the number of firings of the glass.  The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

Because most of your heat work is happening in the low end of the devitrification range, the cleaning regime must be very thorough.  Any chemicals or soaps used must be completely washed off with clean water.  The piece must be polished dry to ensure there are no water marks left on the glass.

You can, of course, have more levels of tack.  One approach would be to start with a sinter, or tack to stick, firing. And repeat that four or more times.  Another is to increase the working temperature and reduce the length of time soaked there.  The shorter time means there is less rounding of each level, allowing the build-up of many levels of tack.  All of these require some experimentation. 

More information is available in the ebook Low Temperature Kilnforming.

Three firings to the same sharp tack profile will give multiple profiles in the finished piece. 

Making Circles from Fused Squares

Dennis Brady has done a lot of work on predicting the size of circles resulting from stacking squares of glass and taking them to full fuse for enough time to allow flattening of the stacks.  This may be up to half an hour at 815°C for Bullseye.  Some observation will be required.

Stacks of 12mm/ 0.5" squares arranged at 45° to each other and taken to a full fuse:

• 1 layer should produce a 10mm/ 0.375" circle
• 2 layers should produce a 12mm/0.5" circle
• 3 layers should produce a 16mm/0.6" circle
• 4 layers should produce a 18mm/0.7" circle

 Stacks of 19mm/0.75" squares arranged at 45° to each other and taken to a full fuse:

• 1 and 2 layers will not fully round
• 3 layers should produce a 28mm/1.1" circle

 Stacks of 25mm/1" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 40mm/1.6" circle
• 5 layers should produce a 45mm/1.75" circle
• 6 layers should produce a 50mm/2" circle

 Stacks of 32mm/1.26" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 48mm/1.9" circle
• 5 layers should produce a 52mm/2" circle
• 6 layers should produce a 58mm/2.3" circle

 Stacks of 37mm/1.5" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 40mm/1.6" circle
• 5 layers should produce a 45mm/1.8" circle
• 6 layers should produce a 50mm/2" circle

 Stacks of 50mm/2" squares arranged at 45° to each other and taken to a full fuse

• 4 layers should produce a 75mm/3" circle
• 5 layers should produce a 85mm/3.3" circle
• 6 layers should produce a 95mm/3.75" circle
• 7 layers should produce a 102mm/4" circle
• 8 layers should produce a 105mm/4.125" circle

 Based on work done by Dennis Brady

I had a few queries about this regular progression and wondered if it applied to opalescent as well as transparent glass. I set up a few tests in my kiln. I fired them at 400C to 815C for 10 minutes.

I got the following results. 

You can see that the opalescents require more heat work than the transparent. If you are making circles with both transparent and opalescent you will need more time at the top temperature - perhaps 30 to 45 minutes.  This results from the greater viscosity of the opalescent colours.

I also tried making ovals from rectangular pieces oriented at about 25 degrees to each other. You can see they were not successful with a 10 minute soak.  

Bubbles in Bottle Slumps

 Any suggestions on how to avoid getting the oblong bubble under the neck of the bottle? This was my first try and I’m really happy with clarity, no devitrification in these.

I used this schedule:

Fahrenheit                    Celsius

300/1150/30                167/620/30

200/1370/20                111/740/20

400/1450/20                222/787/20

AFAP/950/60                AFAP/510/60

150/800/0                    63/427/0

300/100                       167/55/off

The bubble is kind of cool but not sure what it will do when I put it in a bottle mould.

 

To minimise the bubble, you need a bubble squeeze.  There isn't one of sufficient length or at the right temperature in the schedule. The softening point of bottle glass is approximately 720C. Starting the bubble squeeze at ca. 670C/1240F and progressing slowly (ca.50/90F or less) to 720C/1340F may give a better bubble squeeze. 

Also, the anneal soak is a bit low. Bottle glass and float glass both have annealing points of about 550C. You might make use of a lower annealing soak temperature to reduce the cooling time.  It is usually possible to anneal 30C below the published annealing temperature.  In this case that would be 520C.  

There is pretty thick glass in some places due to the way the bottom and neck of the bottle form. You may want to extend your anneal soak to one for 12mm/0.5”.  The soak time for this is 2 hours.  The first cooling segment would be 55C/100F per hour to 475C/888F if you use 520C/970F as the annealing soak.  The second cool segment should be at 99C/180F per hour to 420C/790F.  And the final rate at 330C/600F to room temperature.  It is important to include all three stages of cooling.  The research for my book Low Temperature Kilnforming (Or directly from stephen.richard43@gmail.com) has shown that to get the best stress-free results  use all three stages of cooling.

Bubbles at the shoulder of the bottle are common.  The change in circumference of the bottle at the shoulder means there is a greater amount of glass to “compress”.  Bottles with tapered circumference at the top of the bottle have fewer problems with creating bubbles.  The abrupt change in size at the shoulder causes bubbles to be more common.  A long slow bubble squeeze will allow the shoulder to form more closely in line with the neck. 

There are other things you can do to help avoid the bubbles. One thing is to insert a thin kiln washed wire into the neck of the bottle. This gives a path for the air to escape and allows you to pull it out, although a mark will be left.  You could also think of drilling a hole in what will be the underside at the shoulder to allow air out to the shelf. It does not need to be a big hole.

Bubbles at the shoulder of a slumped bottle are a common problem. It results from the greater amount of glass that has to slump into the space.  This leaves a cavity.  Slower bubble squeezes can help, as well as various venting methods.

Drop Vase Design for Opalescent Glass

Credit: Missy Mac Glass on Folksy

When making a drop vase in opalescent glass, the design needs to be on the outside.  This will require ensuring the design will be on the bottom when suspended on the drop ring.

It is possible to build the whole piece as normal with the design on the top and fire it.  Then you can turn it over to make sure the design is facing downwards. 

To get a crisper design for the outside the flip and fire technique can be used.  Build from the outer layers to the inner layers.  You are building upside down. Place the design to be seen on the outside of the drop vase down on the prepared shelf first.  Follow this up by placing the inner layers in order from the most outside to the most inside layers.

These instructions rely upon firing the blank first rather than building on the drop ring.

However, you can build on the ring if you need to save one of the two long firings.  Only one modification is required.  Place a sheet of clear down first.  Assemble the design as for a flip and fire technique, i.e., outside layers first, inside layer last.  

This will require a slow heat up to ensure you have allowed enough time for the air to be squeezed from between all the layers and that all the glass at the same temperature before the drop begins.  Sprinkling a fine layer of clear powder over the clear is a good way to assist allowing the air out.  Place the design pieces down before applying the powder. 

This is not the best way to make drop vases, but it can work with care in placing the decorative pieces and applying the powder.

Home Made Frit Maker

Recently, when looking for a small frit maker, no shop had one in stock.  Having heard of making one from plumbing pipes, I went in search of material.  I came across stainless steel pipe and caps.

 

The practical size seemed to be 50mm.  Short sections of threaded pipe were available with matching caps.  That forms the containment cylinder.

 

A threaded 25mm pipe and cap can be fitted loosely into the larger one, and so forms the plunger or piston.

 

There needs to be a handle.  It could be a turned piece of wood to fit the inside of the pipe.  In this case, I obtained a reducing connector to fit a 12mm pipe to the plunger and topped it with another cap. 

The whole was put together in less than a minute, once all the parts were assembled.

The completed frit maker

Galvanised pipe would be cheaper but carries the possibility of introducing zinc into the frit.  Stainless steel risks introducing non-magnetic particles into the frit.  As I sieve out powder from my own frit making before washing, I am not too concerned about steel contamination.  If you want powder, use uncoated mild steel so the contamination can be drawn out with a strong magnet.