Annealing Requirements for Shaped Pieces.

 Experiments related to slumping show that shaped items such as slumped, textured and kiln carved glass need annealing for at least one layer thicker than they are. The annealing for one layer thicker than the calculated thickness provides the most stress-free result for the finished product. Annealing for the calculated thickness does not always produce a stress-free result.  

Full Fuse

 This indicates that an evenly thick 6mm thick piece will get the best result from an anneal as for 9mm.

Texture Moulds

 A piece of glass on a texture mould with 3mm or more differences in height requires careful annealing. The more defined/sharper the texture, the greater care will be required. A 6mm blank on a mould with 3mm variation taken to a well-defined texture needs to be annealed as though it were 18mm thick. A sharp tack requires annealing as for a piece 2.5 times its actual thickness plus another 3mm.  This gives the 18mm/0.75” thickness annealing requirement for the 6mm thick piece.

Kiln Carvings

 The same kind of calculation applies to kiln carved items as for sharply textured pieces. Pieces with less sharply defined profiles can be treated as one of the more common tack fused profiles.

Credit: Vitreus-art.co.uk

Tack Fusings

 A rounded tack fused piece of a 6mm base with 3mm tack elements that is being slumped will need annealing as for 21mm.  Twice the actual thickness plus 3mm giving the annealing requirement as for 21mm/0.827”.

 A contour tack of the same dimensions as given in the first example will require annealing as for 19mm/0.75”. The annealing requirement when slumping is for 1.5 times the thickness plus another 3mm.

In General

 The general approach to annealing shaped pieces is to calculate the thickness for the anneal and add one layer more to get a good anneal for slumped and other formed pieces. 

 The research and the reasoning behind this approach is given in LowTemperature Kilnforming, An Evidenced-Based Guide to Scheduling available from the Etsy shop VerrierStudio and from Bullseye. 

Effects of Dam Materials on Scheduling

 I once made a statement about the effects of various dam materials on scheduling. This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board. I decided to test these statements.  This showed I was wrong in my assumptions.

I set up a test of the heat gain and loss of the three materials. This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams. The dam materials were laid on the kiln shelf with thermocouples between. These were connected to a data logger to record the temperatures.

Test Setup

 The thicknesses of the dams may be relevant. The vermiculite and fibre boards were 25mm thick. The ceramic dam material was 13mm thick.

The schedule used was a slightly modified one for 6mm:

• 300°C/hr to 800°C for 10 minutes
• Full to 482°C for 60 minutes
• 83°C to 427, no soak
• 150°C to 370°C, no soak
• 400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

Temperature profile of the air, ceramic, fibre, and vermiculite during the firing.

Highlights:

• The dam materials all perform similarly.
• This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case).
• There is the curious fall in the dams’ temperatures during the anneal soak. This was replicated in additional tests. I do not currently know the reasons for this.
• The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.
• From the second cool to the finish, the dams remain hotter than the air temperature.

 Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air. This graph is to assist in investigating how significantly different the materials are.

This graph is initially confusing as positive numbers indicate the temperature of the first is cooler than the material it is compared with, and hotter when in negative numbers.

 

A= air; C=ceramic; F=fibre board; V=vermiculite

Temperature variations between air and dams

 As an assistance to relating the ΔT to the air temperature some relevant data points are given. The data points relate to the numbers running along the bottom of the graph.

 Data Point       Event

• 1            Start of anneal soak.
• 30          Start of 1^st cool (482°C)
• 45          Start of 2^nd cool (427°C)
• 65          Start of final cool (370°C)
• 89          1^st 55°C of final cool (315°C)
• 306         100°C

 

At the data points:

• At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.
• At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.
• At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.
• At approximately 450°C the air temperature becomes less than the dams.
• At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 More generally:

• The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.
• Ceramic and fibre dams loose heat after the annealing soak at similar rates – having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.
• Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do. This means that vermiculite is the most heat retentive of the three materials.
• Vermiculite remains hotter than ceramic from the start of the second cool. This variance is up to 9°C and decreases to 3°C by 100°C.
• Fibre board is cooler than ceramic dams until the final cool starts, when there is little variance.  At the start of the second cool there is about 15°C between the two.
• Vermiculite remains cooler than fibre dams throughout the cooling process. This ranges from about 12°C at the start of the first cool to about 3°C at 100°C.

Since we cannot see more than the air temperature on our controllers it is useful to compare air and dam temperatures. The same data points apply as the graph comparing differences between materials.

 

Ceramic-Vermiculite; Ceramic-Fibre Board; Vermiculite-Fibre Board; Ceramic-Air Temperature
This graph shows the temperature differences throughout the cooling of various materials.

• During the annealing soak, the air temperature is greater than the dam temperatures. The fibre and vermiculite boards remain at similar temperatures and the ceramic dam is the coolest.
• The three dam materials even out with the air temperature at the start of the second cool.
• Through the second and final cools, vermiculite dams remain hotter than the air temperature – between about 24°C at start of the final cool and 9°C at 100°C.
• The ceramic and fibre dams are close in temperature difference to the air from the start of the final cool. Their ΔTs are 17°C at the start of the final cool and 6°C at 100°C.

Conclusions

• Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass. Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.
• The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the edges of the glass by the dams. There is the risk of creating unequal temperatures across the glass.
• The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board. The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.        
• These tests were fired as for 6mm/0.25” glass and so show the greatest differences. Firing for thicker glass will use longer soaks and slower cool rates. These will allow the dams to perform more closely to the glass temperature during annealing and cool.

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

• There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.
• The lag in temperature rise of the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C, can be used.
• The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.
• The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool.
• Ceramic dams of 13mm/ 0.5” perform better than 25mm/1.0” vermiculite and fibre board. 
• However, in further tests of 25mm/1.0” thick ceramic dams performed similarly to the same thickness of vermiculite. So, 25mm/1.0” fibre board the best when choosing between the three materials of the same thickness. But 25mm ceramic strips are not common, nor are they needed for strength or weight.
• The performance of the three dam materials tested do not show enough difference in temperature variation to have significant affects on the annealing and cooling at times and rates appropriate to the thickness of the glass.
• It is the thermal insulation properties of the dam material, rather than the density that has the greatest influence on performance as a dam material.

 

 

Damming for Exact Shapes

 Many times, exact dimensions of the final piece are not critical.  When they are and the piece is 9mm and thicker, or has irregular amounts of glass near the edge, damming is required.

 If the dimensions are rectangular, you can use straight edged refractory materials, usually sawn up broken kiln shelves, vermiculite, or fibre board strips.  

 These need to be kiln washed and lined with fibre paper.  The dams should be lined with 3mm fibre paper that is 3mm narrower than the final height of the piece.  This allows a bullnose shape at the edge to form.

 If the shape is a circular or irregular shape the dams can be made from thick fibre board or vermiculite.  The lining of the dams is the same as for rectangular shapes.  

 The use of 3mm fibre paper means that you have to make rectangular shapes 6mm bigger in each direction to achieve the exact final dimensions.  For circular or irregular shapes, the edge will need to be only 3mm larger.  This is because the edge goes around the whole shape, rather than only one side.

 

Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening

Scientific Notes on Annealing

 The course from which this information is taken is based on float glass.  This is a soda lime glass just as fusing glass is.  The general observations – although not the temperatures – can be applied to fusing glasses.  This is a paraphrase of the course. It relates these observations to kilnforming.  The course is IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 

 

Viscosity vs. Temperature for a borosilicate glass
Graph credit: Schott

Viscosity Influence on Annealing

 Viscosity increases with reduction in temperature.  So high viscosity (low temps) cannot release stress; low viscosity (high temperature) cannot maintain shape – it will deform.  The range of viscosity is small.  The viscosity must not be so high that the stress cannot be relieved, nor must it be so low that the glass is unable to retain its shape. (p.6).  This indicates there is an inverse relationship between temperature and viscosity.  This is something we experience each time we fire. 

 The mathematical definition for strain point - high viscosity - is 10^14.5 Poise.   And the annealing point as 10^13.4 Poise, where if the glass is all the same temperature, the stress can be relieved in about 15 minutes.  (p.7-8)  

 As kilnformers we talk of the annealing range in terms of temperature, because that is what we can measure. The annealing occurs within a small range of viscosity. This has a relation to temperature that is not the same for all glass compositions. 

 The definition of the annealing as the range of viscosity at which annealing can occur is important.  

 First, the viscosity value remains the same over many types and styles of glass.  The temperature required to achieve that viscosity varies, leading to different annealing temperatures for different glass. 

 Second, there is a range of viscosity - and therefore temperature - during which annealing can occur.  The annealing point is 10^13.4 Poise, at which viscosity the stresses in glass can most quickly be relieved (generally within 15 minutes for 3mm glass).  However, the stress can be relieved at greater viscosities up to almost the strain point - 10^14.5 Poise. (p.8).  At higher temperatures, the glass becomes more flexible and cannot relieve stress.  At lower temperatures (beyond a certain point) it becomes so stiff that stress cannot be relieved.  Again, those temperatures are determined by the viscosity of the glass.

 

Annealing Soaks

 Annealing can take place at different points within the range.  Bullseye chose some years ago to recommend annealing at a higher viscosity, i.e., a lower temperature.  This has also been applied by Wissmach in their documentation although initially the published annealing point was almost 30°C higher. 

 The closer to the strain point that annealing is conducted, the longer it will take to relieve the stress.  Annealing at the strain point is possible, but it is impractical.  Apparently, it would take at least 15 hours for a 6mm thick piece (p.8). 

 However, the trade off in annealing a few degrees above the strain point – requiring longer annealing soaks – is reducing the amount of time required by the annealing cool, especially for thicker or more difficult items.

 A further advantage to annealing at lower temperatures and slower rates is that it results in a denser glass – one with lower volume (p.3). Arguably, a denser glass is a stronger one.

 

Annealing Cool

 After annealing, the glass should be cooled slowly and uniformly to avoid formation of internal stresses due to temperature differentials within the glass.  Stresses that are unrelieved above the strain point are permanent.  Stresses induced during cooling below the strain point are temporary, unless they are too great.  To avoid permanent stress, the cooling should be slow between anneal soak and strain point (p.9).  Although glass can be cooled more quickly below the strain point, care must be taken that the temperature differentials within the glass are not so great as to cause breaks due to uneven contraction.

 Annealing cool factors for flat pieces are about three times that for cylinders and five times that for spheres (p.26). Or the other way around – spheres can be annealed in one fifth the time, and cylinders in one third of the time as flat glass of the same volume.   This indicates how much more difficult it is to anneal in kilnforming than in glass blowing.

 The industrial cooling rate for float glass of 4mm is 6 times the rate for 10mm although only 2.5 times the difference in thickness (p.27). This indicates that the thicker the glass, the slower the rate of cooling should be.  But also, that there is not a linear correlation between cooling rate and thickness.

 Glass with no stress has a uniform refractive index.  Stresses produce differences in the refractive index which are shown up by the use of polarised light filters.

Source: IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 (available online www.lehigh.edu/imi).

https://www.lehigh.edu/imi/teched/GlassProcess/Lectures/Lecture09_Hubert_Annealing%20and%20Tempering.pdf

Firing Small Pieces

 Do you run small pieces of glass through the whole cycle or just bring it up to your degree posted and cool down?

 

Picture credit: Eva Glass Design

It would appear easy to ignore the need to anneal small pieces.  They can anneal with short heat soaks.  In industry the anneal of sheet glass is 15 minutes for 4mm/0.019” glass.   In kilnforming the 30ºC - 40ºC/54ºF – 72ºF below the annealing point is where annealing is effective.  If you are certain that the natural cooling rate of your kiln is more than 15 minutes for that temperature range, you can simply turn off after top temperature.

However, it is not a good practice unless you intend to confine you kilnforming to small pieces.  All glass needs to be annealed to be sound.  Small pieces may need only 15 minutes and often that can be achieved with the natural cooling rate of your kiln.  But pieces of 6mm/0.25” thick and over 100mm/4” in any direction need to be annealed with longer soaks and slower cools.  This is done with a hold of the amount of time appropriate to your glass and layup.  There is an excellent table from Bullseye that gives the hold times and rates for cooling glass of different calculated thickness. 

Using an annealing soak and a cooling cycle for every firing is a good practice.  This gets you into a habit, so that you do not skimp on the anneal and cool for larger, thicker, or tack fused pieces.  If your kiln cools more slowly than you have scheduled, that's ok.  The kiln does not use any electricity to heat the elements.  No additional electricity cost or wear on the kiln occurs.

Fire Polishing of Frit Castings

Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

• ·        They are relevant to particular kilns. 
• ·        They are also dependent on the ramp rate. 
• ·        The presence or absence of a bubble squeeze is important. 
• ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

 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.