Showing posts with label Soak times. Show all posts
Showing posts with label Soak times. Show all posts

Wednesday 12 April 2023

Sintering Ramps and Soaks

Sintering (or laminating) is a special form of low temperature kilnforming that requires attention to the ramp rates and the length of soaks. The rates and soak times were determined by the strength of the resulting pieces.

Credit: Researchgate.net


Rate

 The ramp rate has a significant effect on the strength of the resulting piece.

 A moderate rate (150°C/270°F) all the way to the sintering temperature of 690°C/1080°F gives the glass particles time to settle together. It works similarly to a slow ramp rate in slumping.

 A rapid rate (600°C/1275°F) - as used in medicine – to the sintering temperature of 690°C/1080°F is used for float glass particles.

 An alternative to both these is to schedule a rapid rise to the strain point followed by a slow - 50°C/90°F per hour - rate to the sinter temperature.

Soak

The soak time is extremely important in sintering to provide strong results. It is loosely related to the ramp rate, but in an inverse manner. The quicker the ramp, the longer the soak required.

 The moderate rate of 150°C/270°F needs a two-hour soak at the top temperature for maximum strength.

 The rapid rate of 600°C/1275°F requires about six hours of soaking at the top temperature.

 The alternative of a rapid rise to the strain point followed by the slow 50°C/90°F per hour rate requires at least a three-hour soak.

 These results show the ramp rate is important to the strength of the resulting piece. Fast ramp rates require increasingly long soaks at top temperature. Even slowing the ramp rate after reaching the strain point requires longer soaking than a steady rate. This is so even though the steady rate is faster than the two-part schedule to the top temperature.

 These results indicate that heat work is put into the glass throughout the temperature rise. The heat put slowly into the structure below the strain point still has an effect on the sintering of the glass.

 This is shown by the two-part schedule that has a slow ramp rate after the strain point. And even then, the time required is only 0.3hour shorter than for the moderate steady rise and soak. 

There is no time advantage to rapid rises to the strain point followed by a very slow rise to top temperature. The six-hour soak required by fast rises to top temperature show there is a large time disadvantage with rapid rise scheduling of sintering.

More information is available in the ebook Low Temperature Kilnforming and from Bullseye.

Wednesday 8 March 2023

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 desirableThis 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.

 

 

Wednesday 14 December 2022

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 1014.5 Poise.   And the annealing point as 1013.4 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 1013.4 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 - 1014.5 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

Wednesday 12 October 2022

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. 

Wednesday 27 July 2022

Softening the Tack Profile



Often people want a particular profile not provided by the schedules in the controller or the ones they normally use for tack fusing. The question arises as to whether to increase the temperature or extend the soak on a previously fired piece.

You can do either.

You can extend the time or increase the temperature. There are benefits and drawbacks with each.

Increasing the temperature is the choice for a quicker firing. But you have less control.  By increasing the temperature, you will certainly get a softer edge to the glass. You do not know until the firing is finished how much the glass has changed.

Extending the time means that you know a softer profile will be created simply by more heat work being put into the glass.  If you combine the extended soak time with peeking at intervals, you have much more control over the exact profile achieved.  Observation at 5- or 10-minute intervals after the target temperature is achieved, will enable you to get exactly the profile you want. Just advance to the next segment when that profile is achieved. 

The drawback is that the firing takes a little longer and you have to be present at the time the working temperature is reached.  You can schedule that by using the delay feature on your controller.


Note that on any re-firing of a piece you need to be aware that you are firing a single thicker piece rather than the original multiple layers.  This will require a more cautious rate of advance up to the softening point of the glass – generally around 540°C.  After that, the original rate(s), soaks and annealing can be used.

Of course, the considerations of temperature versus time can be applied to an initial firing as much as to a re-friing of a piece.


Observation is the best way to have precise control over the profile of your tack fusing.

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

Wednesday 18 May 2022

Large Uprisings on Slumps



Help!  Looks like my shallow bowl wanted a boob.  [16” diameter pot melt slumped into shallow 20” mould, 4” above floor, fired at 175°F to 1100°F for 20 minutes,  and 75°F to 1250°F for 15 minutes]

What happened?

Sometimes a slump results in an uprising at the bottom of the slump. In this case, and many others, we know the vent at the bottom of the mould was open and the piece was supported above the shelf.  This indicates that everything should be set up for a good result.  Still, this uprising occurred.  It is not a bubble, as the glass is apparently evenly thick throughout the “boob”. 

The usual, and mostly unexpected, cause is too long or too hot a slump.  The firing, if allowed to continue, would result in a larger uprising and eventually a thickening of the piece at the bottom of the mould associated with a related reduction in the dimensions of the final piece.

What has happened is that the glass has become soft enough for it to slip down the sides of the mould. But it has not been hot enough for long enough to allow the glass to thicken.  The glass at the bottom is pushed up to compensate for the slightly hotter glass on the sides of the mould sliding down on the glass at the bottom.

In this case there is a moderate (97°C) rise from a soak of 20 minutes at 593°C to 676°C, soaking for 15 minutes.  This is a lot of heat work at a relatively high temperature for a shallow mould. 

What to do in the future?

Rate of Advance

Consider what you are trying to achieve at each stage of the scheduling of the process.  In this slumping there really is no need to soak at 593°C. It is a nowhere temperature.  The glass is no longer brittle.  It is at the lower end of the temperature range where the glass is softening anyway.  A simple, steady rise in temperature, as at the beginning, of at or below 100°C will be sufficient to bring the whole substance of the glass to the slumping temperature. If the piece is really thick, consider an even slower but steady rate of advance without any soaks.

Soak Temperature

Also, if the slump can be achieved in 15 minutes, it is too hot.  If the slump is complete in such a short time, it will be marked much more than needed by contact with the mould. 

The cooler the glass at the conclusion of the slump,  the less marking there will be.  Yes, the soak time needed to complete the slump will be longer, but the bottom of the glass will be cooler than a hot fast slump.  You should always be trying to achieve the effect you need at the lowest practical temperature.  The slow rate of advance will assist in completing the slump at a lower temperature, as the amount of heat work put into the piece will be greater.

Observe the progress of the firing

Observation is necessary when doing something different.  Some argue that it is necessary in every slump.  I admit that I do not always observe every slump, but this case again illustrates the need to observe each slump. 
By observing at 10- or 15-minute intervals, you will see when the slump is complete.  You may feel you do not have the time to wait for an hour or so for the slump to be complete, or that it does not fit with your activities.  The answer is to arrange the kiln’s schedule so that when the critical part of the process is reached it will fit with a space in your other activities. https://glasstips.blogspot.com/2016/12/diurnal-firing-practices.html

Can this piece be fixed?


You could put the piece on a shelf and take it to a high temperature slump with a significant soak to flatten it.  You will need to observe when the uprising is flat again, and then proceed to anneal.  However, the pattern placed in or on the glass will be distorted to some extent.  The uprising will flatten with a thicker rim around the base of the rise in the glass.  This may be visible. 

My view of these things is to learn as much as you can about causes and prevention and move on.  You advance your practice more quickly by understanding what went wrong and why than by trying (unsatisfactorily) to rectify a failed piece.  Often you can cut the glass up and use the pieces in other projects.

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

Uprisings at the bottom of slumps are often the result of too much heat work (rate, temperature and time).  Slow rates if advance to low temperatures with long soaks backed by observation prevent the occurrence of these bubble-like uprisings.

Friday 31 December 2021

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. 



Wednesday 27 October 2021

Tack fusing multiple layers



The question:

Full fused 6mm base, with 3mm tacked pieces. It is to be tack fused and slumped now.  Does the number of fuse firings affect the rate of advance, and how long a soak will be required to slump it? 





Multiple firings
If properly annealed each time the glass is fired, the number of firings does not affect how the glass should be fired.  This assumes the same number of layers are being fired.


Tack fusing
Tack fusing this piece will need some care.  The portion to be tack fused is 3 layers thick – overlapping white pieces surmounted by the yellow balls.  The base layer is shaded from the heat by the white, which is generally slower to transmit the heat than many other colours. Bullseye suggests doubling the total height and firing for that thickness. Bob Leatherbarrow suggests 1.5 times the total height for creating the schedule.  Firing Schedules for Kilnformed Glass,  p. 124-6

In this case, because of the amount of white, I would go with the Bullseye suggestion.  My researches for "Low Temperature Kilnforming" also indicated that a tack fuse requires a schedule for two times the thickness. Other levels of tack fusing require different calculations.  The total height of 15mm will be treated as 30mm for scheduling purposes.  This is midway between the thicknesses in the published table.  The rates and times in the table are linear. You can calculate a mid-point in the schedule to get the numbers for your piece.  Half the difference between 25 and 38 is 6.5mm giving 31.5mm.  Using the half-way point will be slightly more conservative than using exact calculations.  It is so close as to make no significant difference.

You will notice that the table gives only annealing times and rates.  There is way you can use this table for the getting initial heating rates.  Look at the final cooling rate for the thickness. If the glass can survive the cooling rate given without showing stress, it will also survive that rate of increase.  The mid-point between 90 and 45 is 67.5°C.  This gives an initial rate of advance (68°C) which can be applied for this piece that has so much shading of the base layer. It should allow the heat to transfer through the white to the base layer without great temperature differences between the covered and the uncovered base layer.

As there is a lot of work in this piece, and it is for someone else, you can be cautious.  Introduce a soak at 260°C of about 30 minutes.  This will help to ensure the heat is distributed to the bottom layer.  If you want to be even more cautious, you can introduce a second 30-minute soak at 371°C before continuing to 540°C.

At 540°C you have passed out of the brittle zone of glass and can increase the rate of advance to 167°C per hour.  The amount of heat work you have put into this piece by the slow rate of advance may enable you to complete the tack fusing with a soak at 720°C.  You will need to observe when the appropriate amount of rounding has been achieved. You will then be able to advance to the annealing portion of the firing. 

For this piece, the annealing soak will be for 5 hours with a cool of 11°C per hour for the first 55°C. Then 20°C per hour for the next 55°C and a final cool of  65°C per hour. This anneal and cool will be about 21 hours, in addition to the ca. 21 hours, in addition to about 10 hours heat up, so don’t expect a quick firing.  Plan two days for the tack fuse.


Slumping
Slumping will need care too.  The piece has uneven layers and the same care is required as for tack fusing.  Experimentation has shown me that scheduling for an additional 3mm (1/8") is needed to ensure the piece is thoroughly heated throughout its thickness.  In addition, the white is stiffer than the other colours and will not bend so easily.  This kind of slow schedule means the glass will be at the same temperature throughout as the slumping starts.

Because of the slow rates of advance, you may be able to slump this piece at 620°C with a significant soak time.  You will need to observe when the piece is fully slumped.  Be prepared to advance to the annealing and cool segments of the schedule.  Some times you need to extend the hold time.  Be prepared for this too. The annealing time and cooling rates will be the same as for the tack fusing.

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




Wednesday 13 October 2021

Incomplete definition on texture moulds

An enquiry on incomplete definition in the glass from texture moulds:
I have this texture mould, but I’m not getting much definition. I’m using a single 3mm layer of 96. Do I need to go hotter for longer?
155C - 750C, 15 minutes
Full - 516C, 140 minutes
49C - 371C, 0 minutes
Full - 49C, off

My response:

You have sensibly increased the temperature at a moderately slow rate (for 3mm).  This ensures the glass is evenly heated through by the time it reaches the working temperature.  It is slow enough for you to be confident that most of the air has been squeezed out.

If you wanted to be more cautious about bubbles,  you could introduce a slow increase in temperature - maybe 50°C - from 600°C to 650°C.  You may want to soak there for 30 minutes, although it may not be necessary.  Once that segment is finished you can resume at 75°C to the top temperature.

I would not increase the temperature as you are already at risk of dog-boning the glass.  I would extend the soak time to 180 minutes at 750°C. You need to check frequently after the top temperature is achieved.  A quick peek is all that is required to see if the texture is fully reflected in the top surface.  You may find success by using a lower temperature, say 730°C, but it will require at least an hour more soak time.


The piece above conformed completely to the 12mm depth of the mould with a soak of three hours at 750°C. There was incomplete formation of another test piece at 740°C for three hours. So the 10°C made enough difference for complete formation over this depth.  With less extreme heights, a lower temperature or a shorter soak would be possible.

Once the texture is assured, you need to advance to the next segment.  Or, if it is not achieved by 10 minutes before the end, extend the soak.  Check your controller manual on how to skip to the next segment, or to extend the soak.

As an aside, your annealing soak and cool is very long and slow for 3mm.  You can regain the time used in the slow ramp rate and soak. Review the requirements for a single sheet of glass.  A 60-minute annealing soak and a cool rate of 83°C/hr. to 370°C is an adequately slow anneal cool for a 3mm piece.

You may find more success with a 6mm sheet.  The weight of glass helps it conform to the texture mould.  I have found that a slow ramp rate (about 150°C) to the strain point of ca. 540°C, followed by half that rate to top temperature allows a reduction in soak time to achieve the required definition. This reduction in soak time can be one half hour less than the time required to get good definition on a 3mm sheet.

The strategy outlined here for the scheduling is using the principle of slow and low and long firings.  It is much easier to control the results of the firing by using moderate ramp rates to lower temperatures combined with longer soaks and periodic peeking to check on progress.

If you do not have the time to devote to peeking when the schedule is at the top temperature, you should investigate the method of programming a delay to the start of the firing.  Your controller manual will give the method of using this function.


Texture moulds work well with the slow and low principle of kilnforming.  Long soaks may be required with periodic observation to determine when the process is complete.

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


Wednesday 1 September 2021

Texture moulds



"I could use some help here please. I’ve tried this sun mould 3x and as you can see all 3x I get a hole.  If you could tell me what I’ve done wrong I would greatly appreciate. They were all full fused to 1430F (776C)."
Example of the problem



There are a range of views that have been given on how to make texture moulds work without the glass developing bubbles.

closer view of one example

These are a summary of the suggestions made to the enquirer.

Not enough glass thickness. The view is that glass needs to be 6mm thick to be used on texture moulds, as the viscosity of glass tends to draw glass to that thickness, robbing from other areas making them thin and prone to bubbles.

Glass always wants to go to 6mm.  Not always.  It depends on temperature.  The kiln forming temperatures we use results in a viscosity that tends to equalise the forces at 6 – 7 mm.  Hotter glass will flow out more thinly, until at about 1200C, the glass is 1mm or less thick.

Full fuse two sheets first.  The object is to avoid placing two separate sheets on top of the mould, creating the potential for more bubbles between the sheets, as they may slump into the mould at different rates.

Too hot. As the glass increases in temperature the viscosity is reduced and can no longer resist the air pressure underneath the glass.

Use a lower temperature. The idea is to keep the glass relatively stiff to resist bubble formation.

Bubble squeeze needed to avoid trapped air.  Another way to reduce the amount of air under the glass is to allow the glass to relax slowly at a temperature below which the glass becomes sticky.

Elevate the mould.  The idea is that hot air circulating under the mould will help equalise the temperature of the mould and the glass.

Drill holes at low points. This gives air escape routes under the mould, assuming the mould is slightly elevated.

Go lower and slower.  Use a slower rate of advance toward a lower top temperature with longer soaks to avoid reducing the viscosity, but still get the impression from the mould.


Now for a different viewpoint.

None of the views given above are wrong, but they all (except in one case) fail to consider the fundamentals of obtaining texture from such a mould.

It is apparent that the temperature used was too high because the glass had low enough viscosity to allow the air underneath to blow the bubble.  The suggestions of thicker glass, bubble squeezes, lower temperatures, drilling holes and elevation of the mould are ways of reducing the amount of air or resisting the air pressure.  They are not wrong, but miss the fundamental point.

That fundamental point is that you need to raise the temperature slowly on these texture moulds to allow the glass to fully heat throughout. By doing this most of the air has a chance to filter out from under the glass before it conforms to the edges of the mould.  It is simpler to use the slow advance rather than a quick one with a slow-down for a bubble squeeze.  The glass is more certain to be the same temperature throughout by using a slow rate of advance.  Glass with an even temperature can conform more easily to the undulations and textures of the mould.

Mostly, the recommendations given are to use two layers, or 6mm of glass that has already been fused together.  This gives greater resistance to bubble formation and reduces the dogboning and needling of the edges.

However, you can form in these moulds with single layers.  There are of course certain conditions:
  • You must advance the temperature slowly.  A rate of 100C per hour will be fast enough.
  • You can add a bubble squeeze soak of 30 minutes at about 630C as additional assurance of removing most of the air.  The bubble squeeze is done at a lower temperature than usual, as the glass is less viscous because the slow rate of advance has put more heat work into the glass.
  • The top temperature should not go beyond 720C. Beyond that temperature the viscosity of the glass drops quickly and so becomes subject to bubble formation.


The soak at the forming temperature will need to be long and observation will be needed to determine when the glass has fully conformed to the mould. Quick peeks at intervals will show when the design is visible on the top of the glass. The time will vary by:
  • Mould texture complexity 
  • Type of glass (opalescent or transparent),
  • Heat forming characteristics of the glass,
  • Viscosity of the glass or colour,
  • Etc. 

Be knowledgeable about how to extend the soak or to advance to the next segment of the schedule to take advantage of your observations.

Your observation may show that you can do the texture formation at a lower temperature in future. This will provide results with less separator pickup and better conformation to the mould without excessive marking. 

You will need a long soak in either circumstance. This will be in terms of hours not minutes.  If you do these texture moulds at slumping temperatures, you will probably need at least twice your normal soak.

You can do a lot to fool the single layer glass into doing what you want by using low temperatures and long soaks. See Bob Leatherbarrows's book on Firing Schedules.  He gives a lot of information on how to manipulate glass through heat work - the combination of temperature and time.  You might also consider obtaining my book - Low Temperature Kilnforming.


Most of the search for the right temperature, fails to note that the important element is how you get to the temperature. You can get the same result at different temperatures by using different rates of advance.

Kilnforming is more than temperature, it is also about time and the rate of getting to the temperature. By concentrating on temperature, we miss out on controlling the speed and the soak times. You can do so much more to control the behaviour of the glass at slow rates, significantly long soaks, and low temperatures.