Longer Anneal on Each Firing

 Need for an Extension of Anneal Soak on Subsequent Firings

 Sometimes people recommend extending the length of the anneal soak each time the piece is fired. If nothing significant is added, there is no reason to extend the anneal soak.  If the piece can be fired as fast as the previous firing, the annealing will be the same, not longer.

 The physics and chemistry of annealing glass are the same for re-fired glass - without additions - as they were for the first. Extending the annealing soak seems to be more about reassurance of the kilnformer than a necessity.

 Bullseye research has shown that it IS possible to over anneal, locking in stress. If concerned about inadequate annealing, it is best to reduce the cooling rate. Especially over the first 55°C below the anneal soak temperature.  The testing and recording that I have done for a book on tack fusing shows that any differences in the glass - at the end of the anneal soak - will be relieved in that first 55°C/100°F. The remainder of the cool to 370°C/700°F can be about 1.8 times faster, and the final cool ramp can be 3 times faster than the 2nd stage cool. 

 I have observed that a three stage anneal cool is important to successfully anneal a piece. This has been reinforced by the temperature recordings of many firings. Often at the end of the anneal soak there is a little more than the desired 5°C/10°F difference in temperature across the piece. The recordings show this is relieved during the slow firs stage cool and maintained over the next two cool stages. If the kiln is cooling more slowly than the schedule, no electricity is used.  No kiln time is lost.

An example of the first cooling stage

 The first stage cool is key to a successful stress-free result.  If there are concerns about inadequate annealing, two things are important.  Be sure the right length of soak is chosen for the piece being fired.  Second, reduce the speed of cooling by the rates for a piece at least 3mm thicker.  These rates are available from the Bullseye chart for annealing thick slabs.

 The rates are applicable to other than Bullseye glass.  Only the temperatures need to be changed.

 If no significant changes (other than powder, wafer or stringers) are made to the glass before the second firing, no lengthening of the annealing is necessary.

 More detailed information is available in the e-book: Low Temperature Kilnforming.

The Importance of Three-Stage Cooling

It is common to think of cooling after annealing as a simple single cool rate to an intermediate temperature between annealing and room temperatures before turning off. This most often works well for full fused pieces up to 6mm/0.25. But as the pieces become thicker or more complex, the need for more controlled cooling becomes necessary.

 The aim of annealing is to get the glass to be the same temperature throughout its substance during the annealing soak. This is called the ΔT (delta T).  This difference has been shown to be 5°C to avoid high levels of stress.  Therefore, ΔT=5°C/10°F.  This difference in temperature needs to be achieved during the annealing soak and maintained during the cool.

 The object of controlled cooling is to maintain this small difference in temperature. It needs to be maintained throughout the cool to avoid inducing excessive stress in the glass, even if the stress is only temporary.  

 As the thickness or complexity of the piece grows, the annealing soak needs to be longer and the cool slower. The first cool is critical to the production of stress-free fused glass. That is the fastest rate that can be used in a single or multiple stage cooling. If you use that rate all the way to 370°C/700°F you will need at least 1.3 times longer to get to that temperature than if you used the first two parts of a 3-stage cool. This time saving becomes greater as complexity and thickness demand slower cool rates. It is not only time that is saved.

 The risk of breaks from rapid cooling after the anneal soak and to 370°C/700°F increases with more complex and thicker pieces. Although the stress induced by rapid cooing below the strain point is temporary, it can be great enough momentarily to break the glass. This is so even if the glass meets the ΔT=5°C/10° during the annealing soak.

  

Examples may help understand the cooling requirements of glass that it thicker, or tack or contour fused.

Example 1

A 12mm/0.5” full fused piece needs a two-hour annealing soak, followed by three cooling rates of 55°C/100°F per hour, 99°C/180°F hour and finally 300°C/540°F per hour. The first rate is for the first 55°C/100°F, the second rate for the next 55°C/100°F, and the final rate is to room temperature.

 What happens here is instructive as to the reasons for soaks and cool rates. In this recorded example the ΔT at the start of the anneal is 7°C/12.6°F. During the soak, the ΔT reduces to as little as 2°C, but ends with a ΔT=3°C. The 55°C/100°F cool rate over the first 55°C/100°F enables the ΔT to remain between 3°C and 4°C.  The second cool over the next 55°C/100°F maintains this ΔT of 3°C to 4°C. During the final cool the ΔT varies from 5°C to 1°C.

 

An example of the variation in ΔT during the first 55C/100F of cooling

Example 2

A rounded tack fuse of 1-base and 2-layer stacks gives a total of 9mm/0.375”. Research has shown that you need to schedule for twice the actual thickness for rounded tack fusing - so for 19mm/0.75”.

This requires an anneal soak of 150 minutes, and a first cool of 20°C/36°F. The second cool rate can be increased to 36°C/65°F. The final rate can be at 120°C/216°F per hour to room temperature.

 The ΔT at the beginning of annealing was 7°C/12.6°F and at the end of a 2-hour soak was a ΔT of 1°C/2°F. The first cool ramp was 20°C/36°F per hour and gave a variance of between 2°C/3.6° and 0°. The final cool produced variances of up to 6°C/11°F, ending at 88°C/190°F with a ΔT=2°C.

 The first two stages of cooling save 1.27 hours of cooling time over a single stage cooling of 20°C/36°F to 371°C/700°F. It still keeps the glass within that ΔT=5°C. More importantly, the third stage cooling is able to keep the variance to between 6°C and down to 2°C.

 The natural (unpowered) cooling rate of my 50cm/19.5” kiln at 370°C/700°F is 240°C/432°F per hour. It settles to the 120°C/216°F per hour only at 200°C/392°F. This is a fairly typical cooling rate for medium sized kilns. This rapid cooling at 370°C/700°F creates a greater risk of breakage than the controlled cool.

 

An example of the ΔT during the second 55C/100F of cooling

Example 3

A sharp tack or sintered piece with two base layers and two tack layer stacks on top requires firing as though 30mm/1.18”.

 This needs a 4-hour soak during which the ΔT varied from 8°C to 4°C. The first cooling rate was at 7°C/12.6°F and gave a ΔT variance of 4°C to 2°C. The second cooling rate of 12°C/22°F produced variances of 3°C to 1°C by 370°C/700°F. The final cool of 40°C/72°F per hour gave differences ranging from 5°C to 0° at 110°C/230°F.

 Note that the test kiln’s natural cooling rate does not achieve the third cooling rate until 140°C/284°F.  This shows that turning off the kiln at 370°C/700°F produces a high risk of breakage for thick and complicated pieces.  In addition, the two stage cooling rates saves 3.27 hours of cooling time.

An example of the ΔT during the final stage of cooling to Room Temperature

 The temperature differentials below the strain point can exceed the ΔT=5. The stresses induced are temporary according to scientists. But they can be great enough to break the glass during the cooling. It follows that the anneal soak may have been adequate, but the cool was so fast that excess stress was induced by the differential contraction rates. This stress being temporary, implies that testing for stress in a broken piece may not show any. The momentary excess stress will have been relieved upon cooling completely to room temperature.  (IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 , p.9.)

 

More information on cooling is given in the book LowTemperature Kilnforming; an Evidence-Based Approach to Scheduling.

Annealing Soaks are Related to Cool Rates

Good annealing is important to the success of each firing of a piece. 

This is generally agreed.

 

I do not understand the reasoning of those who use long anneal soaks followed by quick cool rates and early shut offs. I don't understand because reasons are not given. Or the reasons are in the realm of kiln fairies and other mythical beings.

The length of the annealing soak can be determined from established sources. The Bullseye table for annealing thick slabs gives the recommended soak times for evenly thick slabs of glass from 6mm/0.25” to 200mm/8.0”. Use that to determine the annealing soak time.

The soak times do not need lengthening except for pieces of uneven thicknesses. The ebook Low Temperature Kilnforming gives the calculations for variations in thickness and degree of tack. Generally, they are 1.5 for contour; 2 for rounded tack; and 2.5 for sharp/angular tack. Excessively long soaks are not desirable. This is additional evidence that long soaks and quick cools create problems.

The Relationship Between Soak and Cool Rate

Use of the Bullseye table shows that there are cool rates associated with the soak times. These rates for the length of annealing soak need to be used, as they are based on research, rather than fingers in the air or mythical beings.

My experiments have shown the need to control the cooling rates to at least 50C before shutting off. The end of an adequate annealing soak has the glass within 5°C/10°F of each other part (the ΔT=5). The slow cool for the first 55°C/100°F below is important to avoid exceeding that maximum differential. The rate for the next 55°C/100°F is faster and can allow a wider ΔT, as the stresses are temporary. But they can be great enough at any point to break the glass during fast cools. Therefore, the rates associated with the annealing soaks cannot be exceeded safely.

Do not just use "what works" for others. Use information based on research. The only company publishing research is Bullseye. Their research is applicable to all fusing glass with the appropriate temperature adjustments. 

If you use long annealing soaks and quick cool rates or ones that stop at about 370°C/700°F, you risk breakages of your glass. There is no reason to take that risk.  Also long cools from annealing to 370°C take longer than the staged cooling recommended by the Bullseye research.

 

More information is available in the ebook Low Temperature Kilnforming.

 

 

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.

 

 

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.

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.

Annealing Previously Fired Items

“Double the annealing soak time for each firing” and “Slow the rate of advance each time you fire” are common responses as a diagnosis when a piece breaks in the slumping process.  It may come from the fact that once fired, It is now a single piece that needs a slower rate of advance on the second firing.  I’m not sure where the idea of doubling the annealing process originates.

You need to think about why you would slow the rate of advance and double the anneal for each subsequent firing of the piece.  This is an investigation of the proposals.

Thickness determines ramp rates and annealing

Annealing soak lengths and cooling rates are related to thickness and complexity.  If no additions or complications are added between the previous and the current firing, there is no reason to extend the soak or decrease the rate of cooling.

You of course, need to consider what lay-up and process you are using in the additional firing.  Have you added any complexity to the piece in the previous or the current firing?  If so, you do need to consider how those changes will affect the firing requirements.

Fire polishing

The question to be asked is, “if the piece was properly annealed in the first firing and shows no significant stress, why do I need to change the firing?”

The answer is, “you only need to slow the heat up because it is a single piece now.”  You do need to know that the existing stress is minimal, of course. A note on stress testing is here.  If there is little or no stress from the previous firing, the annealing and cooling can be the same as the previous firing.  Nothing has changed. You are only softening the surface to a shine.  The anneal was adequate on the first firing, and it will be on the second.

If you are firing a pot or screen melt, you have added a complexity into the firing. This is because of the high temperatures used in the first firing.  It means you may wish to be more cautious about a re-firing to eliminate bubbles, or for a fire polish for the surface.

Frit layers

If you are adding confetti or thin layers of frit or powder you have not significantly changed the piece.  You can re-fire the piece as though you are fire polishing any other piece of the same dimensions.

Additional layers

If you are adding more full layers in subsequent firings, you need to reduce the rate of advance to top temperature.  You also need to extend the soak and reduce the cooling rate according to the new thickness of the piece.  This is because the piece is thicker, so the rate of advance needs to be slower, the time required to adequately anneal is longer, and the cooling rate needs to be slower.  All of these changes in scheduling are to accommodate the additional thickness.

Tack fusing additional pieces

If you are tack fusing pieces to the top of an already fired piece, you need to go slower than you would by just adding a full layer.  Tack fusing pieces to an existing piece adds a significant complication to the firing.  Tack fusing requires a firing for thickness between 1.5 and 2.5 times the actual total height of the piece.  The complexity added is the shading of the base glass from the heat radiating from the elements. 

For example, if your piece from the melt is 9mm/0.375", it would have been annealed with a 90 minutes soak. The first cool would be at 69C/127F per hour, and the second at 125C/225F per hour with the cool to room temperature at 415C/750F. If it shows no significant stress, you can fire polish and anneal in the same way as your initial firing.

But

If you tack fuse pieces on top, then you need to treat the piece as though it were between 15mm/0.625" (a little over 1.5 the thickness) and 25mm/1.0" (a little over 2.5 times) thick.  This would require a soak of 3 or 4 hours.  A cooling rate of between 40C/72F and 15C/27F per hour for the first cooling stage is needed. The second stage between will need a rate between 72C/130F and 27C/49F per hour. The final cooling to room temperature will be between 90C/162F and 240C/432F to room temperature.

Conclusion

If you have made no significant changes in thickness or complexity, the second firing can be the same annealing as the first firing. If you have altered the thickness or complexity of the piece, the second firing will need to be slower.

Further information is available in the ebook Low Temperature Kiln Forming.

Annealing Range

NOTE: completely revised 31 December 2021

After Bullseye published annealing tables for thick slabs, some people feel they need to use the lower part of the annealing range for all their glass. To determine whether or when to use these tables needs some understanding of the annealing range.

Range

The annealing range of a glass is approximately 40ºC/72ºF on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 33ºC/60ºF. The annealing point is around 510ºC/950ºF for System 96; 516ºC/962ºF for Bullseye and Uroboros for example. The range for a fusing glass will be around 549ºC to 477ºC/1020ºF to 890ºF for fusing glasses. Although the upper half of that range is merely theoretical. The lower end of the range is the strain point.

The annealing soak is to equalise the temperature throughout the glass to within 5ºC. Once the annealing soak is complete, the first stage of cooling begins. This first 55ºC/100ºF below the annealing soak is essential to the adequate annealing of the glass.  And this illustrates the impracticality of annealing in the upper part of the range.  The first cool rate needs to be maintained to at least 55ºC/100ºF below the low end of the annealing range.

To exemplify this. It would be possible to start the annealing at about 550ºC/1020ºF for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for the whole range of 550ºC/1020ºF to 429ºC/805ºF, rather than just the 55ºC/100ºF from the anneal soak point. This would more than double the annealing cool time. This high temperature anneal is a much slower process, which – together with the more rapid relief of stress at the annealing point – is why the top of the range is never used for the temperature equalisation point. It is also why the Spectrum 96 soak above the annealing point was not essential.

Soak

The annealing point is the temperature at which, if all the glass is at the same temperature, the most rapid cooling can take place. To achieve that equalisation temperature (+ or – 5ºC throughout), the glass needs to be soaked at the annealing point for varying lenghts of time relating to thickness and other variables. To complete the anneal and keep the glass within that tight range of temperature, the anneal cool needs to be continued at a steady slow rate.

Lower part of annealing range

Bullseye now recommends the use of 482ºC/900ºF for  the temperature equalisation soak, but have increased the soak time from 30 minutes to one hour. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs and is very conservative for thinner glass. Bullseye have not changed the composition of their glass so the anything annealed at 516ºC/960ºF for things 6mm/0.25" or less is still properly annealed.

Using the bottom end of the annealing range for thick items, means there are a fewer number of degrees of very slow cooling to the strain point. But this lower soak, or temperature equalisation point, requires a longer soak to equalise the temperature within the glass before the slow steady decline in temperature to maintain the temperature differentials within the glass to less than 5ºC.

Bullseye have found that using a temperature a bit above the bottom end – 482ºC/900ºF – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 461ºC/863ºF is the bottom end of the annealing range according to the calculations indicated above. 

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range
However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below
No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).