Inadequate Annealing - Effects on Next Firing

Credit:

https://immermanglass.com/about-kilnforming/cracks/

The speculation about breaks caused by inadequate annealing of the piece on the previous firing is common.  I do not know if this can be proved to be inaccurate, but we should think about it.

A parallel condition to this poor annealing is toughened/tempered glass which is under a lot of stress between the inside and outside surface of the glass. As Bob Leatherbarrow mentioned to me, we can heat up the highly stressed toughened glass without breaking it by using moderate ramp rates. During this heat up in the brittle phase, the stress is gradually relieved. It does require the moderate ramp rates, of course. 

This parallel circumstance of heating toughened/tempered glass which is highly stressed raises the question: Why should mildly stressed kilnformed glass suffer breakage, if fired at a reasonable rate? Highly stressed toughened/tempered glass does not.

If we apply the experience of relieving the stress in toughened/tempered glass, you can see how inadequately annealed glass behaves. The under-annealed glass has stress distributed (possibly unevenly) across its substance. As the glass temperature moves toward the strain point it becomes less brittle and the stresses are reduced. By the time the glass reaches the strain point, the stresses from poor annealing are relieved.

Any glass not fired slowly enough for its thickness or layup toward 300˚C/573˚F will break. This has been observed to occur around 260˚C/500˚F.  This most commonly occurs in pieces that are laid up with different thicknesses  across the surface. The heat cannot reach the bottom layers as quickly as the overlying ones. The expansion of covered and uncovered glass - due to the heat exposure - is to different.

Thinking about the behaviour of glass in this way indicates that breaks early in the firing relate to a too rapid ramp rate, not necessarily a previous annealing problem. We should, of course, be checking on the stress in our pieces after each firing. This will alert us to the amount of stress in the piece and so to be more cautious in the ramp rate and in the annealing during the current firing. 

Speculation about inadequate annealing in a previous firing as a cause of breaks is misplaced. The thinking that stress will carry through the heat-up and cause breakage is misdirected. 

More information on this is available in the eBook LowTemperature Kilnforming, an Evidence-Based Approach to Scheduling at Etsy VerrierStudio shop and from Bullseye Ebooks.

Refiring and Annealing

A question about re-fusing was posted:

 I have just taken a large [rounded tack] piece, with … A small piece … flipped and showing the white side…. If I cover this with a thin layer of coloured powder frit, does the piece need the long anneal process when I fire it again... I will be taking it up to the lowest tack fuse temperature possible, so the rest doesn’t change too much.

When considering the re-firing of a fused piece, even with minimal changes, the schedule needs re-evaluation of both ramp rates and annealing. 

Ramp Rates

Previously the piece was in several layers. The piece is now a thicker single piece and needs more careful ramp rates. You cannot fire as quickly from cold as the original unfired piece. Previously, the sheets could be heated as though separate. They were not hot enough to stick together until beyond the strain point. They could withstand the differential expansion that rapid heating causes. 

The thicker, previously rounded tack piece will need a slower initial ramp rate. Looking at Stone* and the Bullseye chart for Annealing Thick Slabs indicates the rate should be halved for each doubling of calculated thickness. A rounded tack firing of two layers should be fired as though twice its actual thickness. This means using a schedule for 12mm/.05” thick rather than 6mm/0.25”. This would be at a rate of 330°C/595°F. 

The first firing was of two layers of 3mm/0.125”. Now you are firing a tack fused piece of 6mm/0.25”. It requires a rate of 165°C/297°F as the first ramp rate. If you started with a rounded tack of two base layers and one tack layer, you may have been using a first ramp rate of 150°C/270°F (for 18mm/.075”). Now you will need to be thinking of 75°C/135°F as your first ramp rate. 

Annealing

The annealing time and cool rate will not be affected in the same way. In the first firing you are already annealing for the two layers forming a single piece of 6mm/0.25”. As there is no change in the profile or thickness of the piece, it can be annealed as previously. The cooling rates are the same as for the first firing. 

Credit: Bullseye Glass Company

Refiring with Additions

Ramp rate

If there are additions to the thickness, a slower ramp rate will be required. For example, if an additional 3mm layer is placed on top of a 6mm/0.25” base for a full fuse the ramp rate will need to be reduced to that for 9mm/0.375”, i.e., 415˚C/747˚F according to various charts. However, I never fire faster than 330˚C/595˚F.  There is too much risk in breaking the glass through differential expansion with fast rates.

 

In this case the firing is for a rounded tack. You will need to schedule as for 18mm/0.75”. The rationale for this doubling of the thickness is in my ebook Low Temperature Kilnforming. This initial rate for 18mm/0.75” will be 150°C/270°F. 

Annealing

This time the annealing will need to be longer than the first firing. The thickness has changed with the additions of pieces for a rounded tack firing. Instead of annealing for 6mm/0.25” you will be annealing as for 18mm/0.75”. This requires a hold of three hours at the annealing point and cooling over three stages. The first two of these stages are 55°C/100°F each. The first cool rate is 25°C/45°F per hour and the second is 45°C/81°F per hour. The last is at 90C°C/162°F per hour to room temperature. 

These examples show how dramatically later additions in thickness can add to the length of the firing to get a well-annealed piece without breaking it on the heat-up. 

 

*Graham Stone. Firing Schedules for Glass; the Kiln Companion. 2000, Melbourne. ISBN 0-646-397733-8

As a side note, Stone’s book has become a collectable.

 

Flows

 

Credit: Marcy Berman

I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.

The schedule for Oceanside was:

• 111°C/200°F per hour to 537°C1000°F for 15 minutes
• 167°C/300°F per hour to 662°C/1225°F for 30 minutes
• 195°C/350°F per hour to 798°C/1470°F for 20 minutes
• 9999 to 510°C/950°F for 120 minutes
• 55°C/100°F per hour to 371°C/700°F off

 Your picture shows a bottom view of the piece - made of cullet pieces - as fired. Two large bubbles show to have been created from the bottom rising through the glass to the top.

 Although a long bubble squeeze will not prevent this, it will help to reduce the number of bubbles, and especially large ones. Because of the number of pieces and the thickness of the glass put into the mould, a longer bubble squeeze would benefit this piece.

 The bubble squeeze can be as you have done this – at a single temperature – with a soak. In this case, I would have used 60 to 90 minutes as the soak.

 The other bubble squeeze method is to start the squeeze about 55C/100F below the top of the bubble squeeze. Most people use a soak of about 30 minutes there. They then proceed at a rate of between 30C/55F and 55C/100F to the top of the bubble squeeze and soak there for another 30 minutes. The rates and soak times will vary according to the thickness or complexity of the piece.

 I dispense with the soak at the beginning of the bubble squeeze on the grounds that at 610/1130F so little movement will be created that it is a waste of time. I would prefer to have a slower ramp rate to the top temperature and a longer soak there. I know the glass will be moving at those temperatures. Many people find the soak at the beginning of the bubble squeeze successful.

 The schedule to the top of the fuse is faster than the rest of the schedule. When I want a piece to flow, and especially, to fill gaps, I slow the rate. In this case a rate of between 100C/180F and 167C/300F would be slow enough to allow the glass to flow to fill gaps.

 I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.

 Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think. 

 But for these flows there always is the possibility of devitrification. You have to plan a method of removing it. Unless the surface is very flat, grinding the top is not a fast way to remove it. Sandblasting is a quick way to remove devitrification. Another way is to sift a thin layer of clear glass powder over the surface. This is an increasingly popular way to deal with devitrification for those without access to sandblasting facilities. When fired again, the powder melts and forms a new shining surface. The piece will need to be fired fire again whether sandblasted or covered in glass powder.

The summary for flows:

• Slow down to top temperature.
• Give sufficient time there to get the flow needed.
• Observe the progress as you near the top temperature.
• Extend the soak or advance to the next segment when the surface is smooth.
• Anneal soak for the calculated thickness.
• Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
• Accept there will be devitrification.

Fixing a Broken Piece

This conversation is reproduced by permission (with some editing out of extraneous information). It is presented as an example of how conducting a critique of your schedule can have dramatic effects on the results of your firing. 

This is the piece as it came out of the kiln.

Picture credit: Ike Garson

You may have seen the photo I posted of a large copper blue streaky piece that has cracked right across. …  I’m wondering if it would be better trying to bring the 2 pieces together instead of opening up the 2 pieces and inserting frit. I was thinking of firing it with a tack or contour schedule.

This is the crack that developed later through the frit and single layer centre.

Picture credit: Ike Garson

I have 4 questions:

A.   Even if I manage to fix it, do you think that fissure line will always be too weak and liable to break off at any point?

The strength of the joint will be dependent on the firing conditions.  To make it strong, the temperature should go to full fuse.  Tack fusing will leave the joint more visible and weaker.  To stop the joint rounding during heat up, you will need to dam the piece tightly to stop the normal expansion of the glass and ensure the glass is forced together during the higher temperatures.

B.     I have some large pieces of clear confetti. Would it benefit using them to bridge the 2 sections from below?

Anything you put on the bottom will have distinct outlines and visibility.  The temperature on the bottom can be 10C or more different from the top surface, which is why you can get crisp lines with the flip and fire technique.

C.    Would clear powder hide the crack or would it always be visible after firing?

Any additions to the top may be less visible, but adding clear powder makes the join more obvious.  You need to use powder of the same colour as the sheet glass.  Since you are using a streaky glass, you can’t use coloured power either as it is very difficult to imitate the steaks even with powders of the same colours. 

More information was given indicating the first contour fuse schedule in Celsius:

1. 260 730 00.20
2. FULL 515 00.60
3. 260 150 End

This is the contour schedule I have used many times successfully but never for a piece during this week.

My critique of the schedule. 

Segment 1.

• ·    It is too fast for the small distance to the side of the kiln. 
• ·    It is too fast for a piece of varying thicknesses. Most expansion breaks occur above 300˚C, so a soak at ca.260˚C will help ensure the glass maintains an even temperature, especially with large differences in thickness. Then you can advance more quickly. 
• ·   There is no bubble squeeze.
• ·   The top temperature seems low for a good tack, or the soak is a bit short.  Long soaks allow the glass molecules to bind at the atomic level firmly. This is the principle used in pate de verre.
• ·   It definitely needs to be on fibre paper covered with thinfire to allow air out.

Segment 2.

• ·   The soak at 515˚C is better done at 482˚C for Bullseye.
• ·   My tests have shown that contour firing a piece like this at rates and holds for 1.5 times the height of the piece is necessary for good results.

Segment 3.

• ·   Also, my tests have shown that a three-stage cooling provides the best result.  Slow cooling keeps the glass within the 5°C difference required for avoiding stress.
• ·   Annealing at the bottom end of the range combined with an appropriate length of soak and slow cooling gives a denser glass than soaking at the middle of the annealing range. 
• ·   The best cooling comes from a three-stage cooling process.  This involves a slow rate for the first 55C, a rate of 1.8 times this for the second 55C, and a rate of 3 times this for the final cool to room temperature.

These points mean that I would recommend you fire for at least 10mm thick.  This recommendation is for a new piece, not a repair. In this repair case and for the conditions, I would choose 12mm as being more cautious. My schedule would look something like:

1. 120˚C to 260˚C, 20’
2. 300˚C to top temperature, 10’
3. Full to 482˚C, 120’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

The anneal soak is for a piece 12mm thick.  The cool rates are for 21mm thick.  This is to compensate for the nearness of the glass to the edge of the kiln.  It will help to ensure the glass does not have excess stress locked into the piece during the cooling.

D. Do you think this schedule would work [for a repair]? It's adapted from a standard tack schedule.. 

1. 222 677 00.30
2. 222 515 00.40
3. FULL 482 01.30
4. 63 371 ENDS

Critique of the schedule.

Segment 1. 

• ·   Too fast given earlier difficulties. 
• ·   Too low for good adhesion unless you use about 10 hours soak. 
• ·   Even at sintering temperature (690°C) you would need 2 hours.  But at sintering temperature you do not alter the surface 

Segment 2. 

• ·   Too slow a cool from top temperature and risks devitrification. Should be FULL.
• ·   You do not need the soak at 515˚C.  It only delays the annealing process.  It seems this idea of soaking at the upper portion of the annealing range was introduced by Spectrum over 2 decades ago. 
• ·   Any advantage that might be achieved by the higher soak is cancelled by the FULL rate to the annealing soak. 
• ·   Go straight to the anneal soak. 

Segment 3. 

• ·   You need a more controlled 3 stage cooling to get the best result.

My schedule for repair would look something like this:

1. 120˚C to 540˚C, 10’
2. 300˚C to 780˚C, 10’
3. Full to 482˚C, 210’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

I am making the assumption that 780˚C is full fuse in your kiln.  Anything less than full fuse will certainly show the crack.

 

A Look at Causes.

• ·  The piece is far enough away from the elements.  It is not on the floor. These are not the causes.
• ·  It is very near the sides of the kiln.  These are always cooler than the centre. There is always a risk of breaking in this case.  Very slow rates are needed. 
• ·  There is a 3.5 times difference in thickness within the piece. This also requires slow rates.
• ·  If the break were to have been on the heat up these elements of uneven heating, and rapid rates are a problem.  But the break occurred after the cool down. So, the annealing soak and cool is a problem. 
• ·  I have suggested some alterations to the schedules to address these things.

 

Fixing for Yourself

• ·   Dam it tightly to avoid expansion within the glass as it heats.  This holds the join together and causes the glass to gain a little height. 
• ·   Place the piece on 1mm or thicker fibre paper topped with thinfire.  This will help avoid a bubble forming in the clear.
• ·   I have suggested a schedule which is slower to ensure no further breaks.  It is slow to the strain point and fast after that. 
• ·   It needs to be a full fuse to fully join the two pieces and ensure it is sound.
• ·   The cool to annealing should be FULL.  Eliminate the soak in the upper annealing range. The effects of the time spent there is nullified by the rapid rate to the main annealing soak. 
• ·   Anneal as for 12mm, but with slower cool rates (for 21mm) to ensure there are no stresses built into the piece by the nearness of the glass to the edge of the kiln.
• ·   These methods and schedules will make it a strong whole.  But the join will still show on the bottom. 
• ·   After fixing, if you are still not satisfied, break it up for incorporation in other projects.

Finally, and unfortunately, I do not think it can be satisfactorily repaired for a client.  The crack will show on the back. You will know it is a repair, rather than a whole. And that will reflect on your feeling about the piece, and possibly your reputation.

 

Conclusion

The commission was successfully re-made from scratch by the artist using some of my suggestions on scheduling. This is the resulting piece.

 

Picture credit: Ike Garson

 

Careful analysis of the conditions around a break are important to making a successful piece in the future. Many factors were considered, but the focus became the schedule. Analysis of each step of the schedule led to changes that resulted in a successful piece with the original vision and new materials.

Writing Slumping Schedules

 

Slumping Schedules

When slumping fired pieces, it is most often appropriate to use a slow ramp rate to avoid too rapid expansion of the glass that might lead to a break. Most glass breaks on the ramp up are above 300°C/573°F. It is in this range that there is a rapid expansion of ceramic. This means a slow rate is protective for both glass and ceramic moulds.

This slow first ramp rate is followed by the rate determined as appropriate for profile and thickness. The table below gives rates and times for different profiles that are 6mm/0.25” thick. Of course, the slumping temperature will be altered for the glass according to the manufacturer’s stated range. The nature of the mould will also have a big effect on temperature and time. The soak times at the slump soak are those appropriate for the mould. The annealing soaks are related to the profile of the glass.

Slumping Schedules by Profile (Celsius) 6mm thick

Flat Fuse and Contour Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for contour:
6	240	20	240		30	9mm

Rounded Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for round tack:
6	150	20	150		30	9mm

Sharp Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for sharp tack:
6	120	20	120		30	9mm

 

Slumping Schedules by Profile (Fahrenheit) .025" thick

Flat Fuse and Contour Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for:
0.250”	432	20	432		30	0.375”

Rounded Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal for:
0.250”	270	20	270		30	0.375”

Sharp Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal for:
0.250”	216	20	216		30	0.375”

 

Rates

It is most often best to use a slow ramp rate to at least 500°C/933°F. This avoids the risk of inducing a too rapid differential expansion within the glass as it heats up. Experiments about the first ramp rate have shown firing as for two layers thicker than indicated by the profile schedule provides the best results. 

The rates for the anneal soak and cool are those that are one layer thicker than determined by the schedule for the profile. This has been shown by experimentation to give the best annealing result – i.e., least stress.

Temperatures

The slumping temperature needs to be altered for two factors:

• ·        the glass according to the manufacturer’s stated range, and
• ·        the nature of the mould.

Many manufacturers are giving recommended temperatures and times for slumping in their moulds. An example is the Bullseye “Quick Tip” which gives suggested temperatures and times for various sizes and natures of moulds that can form the basis of your independent scheduling of slumps. The rates are normally for flat uniformly thick pieces. This will need alteration for tack profile pieces.

Take note of the soak time in these recommendations. If it is less than 10 minutes, it is possible to reduce the temperature by about 10°C/18°F by using a 30-minute soak. This will reduce marking on the back of the glass.

Soaks / Holds

Slumping schedules tend to be more imprecise than many other operations in kilnforming because of variations in moulds and what is placed on them. This, consequently, makes observation of the slump more important. It is needed from a point below the target temperature – say 22˚C/40°F – to ensure the slump is stopped when it is complete, or extended if not. The controller manual will give the information on how to do both of these operations. In general, schedule slower ramp rates for thicker pieces in combination with the half hour soak. This means for each thickness greater than 6mm, the top temperature can be reduced slightly and still achieve a full slump.

The schedules here are applicable for pieces up to 9mm actual thickness.

Slumping of thicker pieces needs to apply the underlying scheduling method:

• ·        Apply the rate for two layers thicker for the advance to 260°C/500°F.
• ·        Continue the next ramp rate as for two layers thicker than calculated up to the slumping temperature.
• ·        For annealing, also select the rates and times for one layer thicker than indicated by the profile.

For example:

• ·        Rounded Tack of Bullseye, 12mm/0.5” thickness
• ·        Schedule for 25mm/1” (2 times multiplier)

Celsius schedule for up to 9mm actual thickness:

Segment >	1	2	3	4	5	6	7
Rate	150	150	ASAP	15	27	90	off
Temp	260	Top	482	427	370	RT
Time(mins)	20	30	240	0	0	0

and in Fahrenheit:

Segment >	1	2	3	4	5	6	7
Rate	270	270	ASAP	27	49	162	off
Temp	500	Top	900	800	700	RT
Time(mins)	20	30	240	0	0	0

 

A further example:

• ·        Sharp Tack of Bullseye, 0.5” thickness
• ·        Schedule for 31mm/1.25” (2.5 times multiplier)

Celsius schedule for up to 9mm actual thickness:

Segment >	1	2	3	4	5	6	7
Rate	78	78	ASAP	11	20	65	off
Temp	260	Top	482	427	370	RT
Time(mins)	20	30	300	0	0	0

and in Fahrenheit:

Segment >	1	2	3	4	5	6	7
Rate	140	140	ASAP	20	36	117	off
Temp	500	Top	900	800	700	RT
Time(mins)	20	30	300	0	0	0

 

These examples show that considerable differences in scheduling are needed for different tack profiles. It also shows longer annealing soaks and slower cooling rates are required for sharp than rounded tack pieces.

More information is given in the e-book Low TemperatureKilnforming. 

* Of course, the slumping temperature will be altered for the glass according to the manufacturer’s stated range. The nature of the mould will also have a big effect on temperature and time.