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 below 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.

Slumping Schedules by Profile (Celsius)

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)

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	540		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”

 

*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.

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 relating to the first ramp rate have shown firing as for two layers thicker than indicated by the profile schedule provides the best results. It is then possible to increase the rate as determined by the profile schedule.

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 for 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 difficult to devise 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 20°C/36°F – to ensure the slump is stopped when it is complete.  If it is not complete, the soak can be extended. The controller manual will give the information on how to do these operations. In general, you 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 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.
• ·        Increase the rate after that to one for a single layer thicker than calculated all the way 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)
• ·        Initial ramp rate for 31mm/1.25” (two thickness greater)

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)
• ·        Initial ramp rate for 38mm/1.5” (two thickness greater)

 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 Temperature Kilnforming. and at Bullseye eBooks

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.

Steel Moulds

Credit: Slump and Fuse

Do steel moulds need to be kiln washed for slumping?

Some prominent people in the kilnforming milieu like to promote the extremes of the craft. The argument seems to be that the glass does not get hot enough to stick to the metal at slumping temperatures. It could be argued in the same way that it is possible to slump glass on bronze or unglazed ceramic moulds.

This proposal may come from glass blowing where glass is pressed into metal moulds. The difference is that the glass is in contact with the metal for a short time. And in industrial processes the steel is water cooled.

Also, the higher the slumping temperature, the “softer” and “stickier” the glass becomes. The metal is also heating up and expanding, unlike in glass blowing. If the glass adheres to the metal at all, the greater contraction of the metal during cooling will ensure the glass is stressed and likely break. Therefore, it is usable only in low temperature slumping – below about 630˚C/1167˚F – or for short times. The break patterns that occur when slumping on bare steel show glass most often sticks to the steel and becomes crazed or even shatters on cooling.

Bare steel as a slumping mould is always a risky practice. Just because it can be done - or is done - in glass blowing and other industry settings, does not mean it should or can be done in studio settings. The practice comes with high risks of failure.

To be safe, a separator needs to be used between any supporting structure and the glass. Why risk glass into which you have put significant amounts of work for a few pennies worth of kiln wash, fibre paper, or boron nitride?

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.

 

Spikes on Frit Castings

Credit: The Crucible.com

It is frequent to have castings from frit with spikes, needles, or prickles around the edges. 

Causes

These spikes result from the glass touching the edge of the mould or separator during the hottest part of the firing. The glass particles first begin to compact as the glass rises toward the fusing temperatures. As the temperature increases toward the casting temperature it begins to expand both horizontally and vertically from that compact mass. As it cools, the glass sinks down and retreats from the edge. This movement leaves some small bits of glass stuck to the sides. The glass contracts as it cools, leaving the spikes as it contracts from its hottest state. 

Avoidance

The usual recommendation is to mound frit in the middle and let it flow to the outside. Still, the glass flows to the outside of the mould at casting temperature and it touches the sides. Leaving the risk of creating spikes. Accurate measuring of the amount of glass to charge the mould with is important. With the right amount of glass, the mould will not be overfilled and so, reduce the spiking. 

Measuring the weight of glass for the mould is not difficult. In many cases, the manufacturer of the mould has done the work for you. If you need to calculate the weight of glass required for the mould, it is not difficult. A method is given here. In short, you use a dry fill of the mould. Measure the volume (using the metric system) and multiply by the specific gravity to get the weight in grams. 

Larger chunks of glass tend to produce fewer spikes than smaller frit. Usually longer soaks at top temperature are required to fully form the glass with smaller frit. It is also possible to drip glass into the mould from a pot suspended above the mould. Accurate measurement of the weight will still be important. But add 100gms/4oz. to the amount to allow for the glass that will stick to the pot.

My view is that with dams, it is better to use a straight sided shape with fibre cushioning around the outside. When annealed and cool, clean it well. Then fire polish with a slow ramp to 540°C/1000°F followed by a quick ramp to the fire polish temperature. This will polish the sides of the piece that were in contact with fibre paper.

Long Anneal Soaks

Credit: Bullseye https://www.youtube.com/watch?v=-8hK9Klprvc

Long anneal times seem to be becoming popular. At least they are being recommended frequently by people in Facebook groups. They are recommending very long anneals to solve stress and breakage problems.

Are they effective?

This Bullseye video on some results from annealing research shows (at 13:00 minutes) that excessively long soaks can cause more stress than the recommended length does. The video shows a 1” slab annealed for 4 hours (the recommended time) has less stress than one annealed for 16 hours.

The thoughts are that this effect results from the cooler space under the shelf and glass than the top during long soaks. This induces temperature differences between the bottom and the top of the glass, if not across the surface. The recordings show that during the anneal soak the temperature at the bottom of the kiln is less than above the glass.  This difference on a long anneal soak is larger than the ΔT=5˚C required for a good anneal.

The remedy shown by the video is to introduce heating elements under the shelf, which are separately controlled. This is impractical and is not needed in smaller kilns. The solution for these smaller kilns is to use slower and graduated cooling rates from the end of the annealing soak – not longer annealing soaks. 

The slower rate can be selected from the table. Whether you choose the rates for one or two layers thicker, relates to your perception of risk. Do not extend the length of the annealing soak when you use the slower cooling rates.

This cooling process has been researched by Bullseye and is effective to keep the glass within the temperature distribution requirements. It is a three-stage process. Stage one is to 427˚C/800˚F. The rate for the actual or calculated thickness is given in the Bullseye table (see below).

Stage two is to 371˚C/700˚F. This is normally 1.8 times the rate of the first cool.

Stage three is the cooling from 371˚C/700˚F to room temperature. This can be up to 6 times the first stage cooling rate. However, I find that a final cooling rate of 330˚C/600˚F is faster than most kilns can achieve.

I do programme this final cooling into the schedule. 

• It does not use more electricity unless the kiln is cooling faster than programmed. 
• It does not cause the relays it click in and out if the cooling rate of the kiln is slower than programmed. 
• It does protect the glass from too rapid cooling, or peeking. This is so especially from 100˚C/212˚F, when we are inclined to want the glass to cool faster than the closed kiln allows. 
• The sound of the relays operating indicates the kiln is open too much for the safety of the glass.

 

The Bullseye table Annealing Thick Slabs shows the recommended soak times and cool rates for each cooling stage. It is applicable to all fusing glasses. After annealing for the appropriate time at the temperature for your glass, use the rates and temperatures from this table.

Prevention of Spikes at Corners

Often, after fusing rectangles we are left with sharp points at the corners. How can we prevent it?

 

Nipping corners off rectangles, especially opalescent and the underlying base pieces, is a standard practice to avoid sharp points on corners of a finished piece.

The principles of this relate to how glass contracts when cooling.

The glass expands in the fusing and then contracts on cooling. The hot glass can attach to a rough bit on the paper or shelf and be stretched in the cooling. This results in a sharp needle point.  There is more glass at the corners than at the sides to contract. This leads to the creation of more sharp points at the corners.

Only a small portion of the corner needs to be nipped off as shown in the first picture. It will not affect the dimensions of the piece. It will not affect the appearance of the corners. Except, of course there will be no sharp points.

The fix is simple. Nip a bit of the corner off before assembly. Also no more than 10 minutes at top Temperature should be needed.

Full details are in my eBook Low Temperature Kilnforming.

 

Muriatic acid as a cleaner of kiln wash

Muriatic acid is a common name for hydrochloric acid.   Let’s look at what is being cleaned off first.

The main components of kiln wash are hydrated aluminia, kaolin, and colouring. Colouring burns away, hydrated aluminum is inert at kilnforming temperatures, Kaolin begins a non-reversable change from hexagonal plates to a crystalline form at about 600C/1100F and completes it by 900C/1650F. Now consider the characteristics of each element. 

Aluminium Oxide

Aluminium oxide is widely used for its hardness and strength. It is only slightly softer than diamond. In its hydrated form it is a separator between glass and supporting structures. It has excellent refractory characteristics with a melting point of 2,072 °C/3,762 °F. But it is insoluble in water and all solvents. It is largely impervious to acids. 

Kaolin

Kaolinite structure, showing the interlayer hydrogen bonds in white.
Source: Wikipedia

 

Compared with other clay minerals, kaolinite is chemically and structurally simple. It consists of layers, each bound together by shared oxygen ions. The layers are bonded via hydrogen bonding between oxygen on the outer face of one sheet and the other. … The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's non-swelling character.

When moistened, the tiny plate-like crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape.   Source: https://en.wikipedia.org/wiki/Kaolinite

It is this slipperiness that makes it a good carrier of the aluminium hydrate. However, kaolin begins a non-reversable change from hexagonal plates to a crystalline form at about 600C/1100F and completes it by 900C/1650F. It is the crystalline form that sticks to glass. So, it is the clay (kaolin) that needs to be removed from the glass.

Hydrochloric acid as a cleaner of kiln wash

Glass is almost impervious when it has a minimum of modifiers. Glass which has a minimum amount of [modifiers] and is almost entirely SiO2 is remarkably chemically inert and reacts only with very strong alkaline (bases) materials.   Source: https://www.quora.com/How-come-hydrochloric-acid-does-not-burn-through-the-glass-bottle-that-its-stored-in

Note that coloured and fusing glass have a significant level of sodium and potassium modifiers. This means that fusing glass is subject to attack by hydrochloric acid. 

Safety notes on hydrochloric acid

Being a strong acid, hydrochloric acid is corrosive to living tissue and to many materials, but not to rubber. Typically, rubber protective gloves and related protective gear are used when handling concentrated solutions. Solutions of less than 25% cause skin irritation, serious eye irritation and respiratory irritation. Over 25% causes severe skin burns and eye damage. It is also a precursor of many illegal drugs. Serious safety gear is required to handle even 10% solutions. 

Even then:

“Clays are not truly soluble in HCl acid, [but] exposure to HCl acid does affect the structure of clay minerals. Hydrochloric acid cleans clay minerals by removing free iron oxide from the surface. … The dissolution of kaolinite clay in hydrochloric acid solutions has been carried out in the presence of fluoride ions. Leaching in the presence of fluoride ions activates the clay for leaching, making higher extractions possible at lower roasting and leaching temperatures. Acetic acid [vinegar] is less effective.”   Source: Stability of Clay Minerals in Acid, by D E Simon and M S Anderson. https://onepetro.org/SPEFD/proceedings-abstract/90FD/All-90FD/SPE-19422-MS/68436 

This piece of research shows that hydrochloric acid is most effective in combination with fluoride and heat.

Other reported research from Researchgate shows:

“Kaolin and other clays are partly soluble in acidic solutions (organic or inorganic acids in water) but the … solubility is never complete. Increasing the acid content doesn't … increase the solubility.” Philip G Jessop, Queen's University. 

       “Potassium hydroxide … will get kaolinite dissolved with a white residue for selective leaching. … The most aggressive solvent is hydrofluoric acid which "kills" almost all silicates [including kaolin]. … For the kaolinite group … use hydrazine as solvent.” Harald G. Dill, Leibniz Universität Hannover. 

Hydrazine is highly toxic unless handled in solution. Hydrofluoric acid may dissolve the kaolin, but it also dissolves the minerals in glass. Both these chemicals are extremely dangerous. 

Conclusion

It is not advisable to use hydrochloric (muriatic) acid as a cleaner of the kaolin in kiln wash from glass. 

There are other much safer methods which use a chelating action rather than attempting to dissolve the almost insoluble kaolin. These are citric acid for brief (less that 24 hours) soaking, or trisodium citrate for longer periods.

Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx