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.

 

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.