Drop Rings

Mould

It is possible to purchase drop rings of various sizes. It is also easy to construct one from vermiculite board or ceramic fibre board. Merely cut a circle of the desired radius from the board. Leave at least 50mm of board outside the circle, and more for thinner boards.

Kiln wash the top and inner sides of the drop ring

Glass

The glass should be larger than the hole in the ring. This will vary by radius of the hole. The glass will need to be from 50mm larger diameter than the hole for smaller holes to 100mm larger diameter for holes over 300mm.

Glass should be at least 6mm thick for the first 100mm of drop and an additional 3mm for each 50mm more. So, a drop of 200mm would require glass of 12mm thick


Temperatures

The temperature rise should be no more than 150C per hour to about 675C for 6mm glass and less for thicker glass. Remember the glass is much closer to the elements than normal and it is easy to thermal shock the glass.

With close inspection you can see that the edge of the glass rises from the mould as it sinks in the middle.

The outside edges of the glass rise from the mould as the centre begins to drop in the centre.  As the glass gets hotter, this raised edge settles back on to the mould.  If the glass is really near the elements, there is a small risk the glass will touch the elements.  No harm will be done to the kiln, but the glass edge may have some needles.

The rate and amount of slumping is controlled by temperature, span (the width of unsupported glass on the mould) and time. The higher the temperature the faster a piece will slump and the thinner the walls will be. However you can slump at lower temperatures by holding the temperature for a longer time to reduce the thinning of the sides.

Also note that the wider the span, the faster the glass slumps.

If you slump at high temperatures with a drop ring the sides of the bowl tend to be straight and steep. The strain is limited to the region immediately inside the rim. Therefore the glass tends to thin next to the rim and the colours are diluted. If you slump at a lower temperature for a longer period of time the strain is distributed over the entire unsupported area. This results in a more rounded shape for the bowl and even thickness of the glass across the bottom of the bowl.


Experiment

Finding the right combination of time and temperature requires a bit of experience and guess work. If you want a rounded bottom, heat the glass to the point that it starts to bend on the mould and wait for 30 minutes. If it has slumped about 1 inch in that time wait another 30 minutes. You are looking for a slumping rate that is acceptable. If it hasn't moved very much then increase the temperature 15C and check again in 15 minutes. Keep moving temp up and waiting for 15 minutes until the piece has completely slumped. This might take several hours.

If you want straight sides keep heating the piece rapidly.

Stopping
When the piece has slumped to the desired shape, flash cool the kiln to about 30C above the annealing point to stop movement in the glass. Extend the annealing soak and increase the length of the annealing cool time (reduce the rate of temperature fall) over normal slump firings of the same thickness.

Glass falls through drop rings in relation to the size of the glass on the drop ring, the size of the opening, the temperature rise rate and to some extent the colours and amount of opalescent glass used. 

Getting the Right Firing Temperature

“what temperature should I use to get a tack fuse that is just less than a contour fuse?”

This is the kind of question that appears on the internet often.  Unfortunately, no one can answer the question accurately, because it depends on some interrelated variables.

Kiln characteristics

Top or side elements, size of kiln, relative size of piece, all have an effect. Also no two kilns even of the same model have exactly the same characteristics.

Ramp Rate 

How quickly or slowly you fire has a big effect on the temperature and soak needed to achieve the desired result. This is the effect of heat work.

Temperature

There are no absolute temperatures for a given effect, given the above two variables.

Soaks

The length of time and the number of soaks will affect the temperature required to achieve your effect.

OK. So, what can I do?

Observation

The only certain way to get the effect you want is to observe.

Set a schedule, guessing the top temperature and length of soak.  Know your controller well enough that you can extend the soak or end the segment by advancing to the next.  Your manual will tell you how to do this.

Peek at intervals from 10-15C below the selected target temperature. Peek at 5min intervals until the effect is achieved.  Advance to the next (cooling) segment.  Record the temperature and length of soak at which the effect was achieved.  On subsequent firings you can experiment with reducing the temperature by 5C – 10C with a 10-minute soak.  Observe and record the temperature and effect as before.

The reason for going for a 10-minute soak rather than longer is to avoid holding at the target temperature for a long time, as that can help induce devitrification.  The reason for a soak at all is to achieve the minimum of marking on the reverse or picking up kiln wash or kiln paper on the back.

If effect is not achieved by the end of the soak, extend it by using the appropriate key or combination of keys.  Keep observing at five-minute intervals until the effect is achieved.  Advance to the next segment and record both the temperature and time.  The objective is to get the heat work done with a 10-minute soak, so you will need to increase the temperature on the next firing.  The amount of increase will depend on the length of soak required to get the desired surface on the previous firing.  The longer the soak, the more temperature you need to add.  You will need to repeat the observations and recording until you find a temperature that will achieve the effect with a 10-minute soak.

Use the lessons from the observations to lower temperature, extend soak, raise temperature, reduce ramp speed, or reduce soak as required.  It will also help you judge on other pieces the approximate temperature and time required for the new layups or new moulds.

Slumping Breaks

“Why does my full fused disc break when I slump it?”

There are several possibilities. The two main ones are annealing and ramp speeds.

Inadequate annealing in the fusing stage can lead to a very fragile piece when being re-heated.  If there is significant residual stress in the fused piece, it is much more sensitive to heat changes during subsequent firings whether full, tack, or slumping/draping. It is important to thoroughly anneal any piece at every firing.  If you are firing a different layup or contrasting colours and styles, you should check for stress using polarising filters.  

The slump – or drape – firing needs to be much slower in temperature rise than the fuse firing.  You now have a thicker piece which takes longer to absorb the heat evenly. 

If your piece is tack fused, it needs an even more slow rate of advance.  Sometimes this needs to be as though the piece were two to four times the actual thickness of the piece.  The more angular and pointed the tack fused elements, the greater the reduction in firing speed.  This post gives guidance on how the piece is designed and its thickness affects the rates and soaks in tack fusing. 

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

Soak

Kilnformers seem keen to reinvent terminology and then wonder about imprecise language being used in the field. Much of the terminology for kilnforming is already available from ceramics. It makes sense to continue to use that terminology where it applies.

A soak at a stated temperature is the same as "hold" at the same temperature.

The concept of soak is more useful than the term “hold”.  “Soak” implies the temperature is held at temperature to allow the heat to soak into the glass. And that is the purpose of a hold.  Using the term “soak” brings this purpose into the thinking about scheduling.  It is related to the concept of heat work. 

Using the concept of heat work allows you to use a slow rise to a temperature for a short time to get the effect you want.  Or to rise to a temperature in the normal way but with a long soak.

This is how you can get a tack fuse at 750C with a long soak – say 30 mins - as at higher temperature for a shorter soak – say 780C for 5 minutes. This the concept of heat work in practice.

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

Rates of Advance with Soaks

I’m sure I have written about this before, but a repetition will not hurt.

I have seen many schedules with initial rates of advance interrupted by soaks.  These kinds of schedules that are written something like this:

250 degrees C to 200C, soak for 10 (or 20 or 30) minutes

250 degrees C to 500C, soak for 10 (or 20 or 30) minutes

300 degrees C to 1100C, soak for 10 (or 20 or 30) minutes

300 degrees C to 1250C, soak for 10 (or 20 or 30) minutes

600 degrees C to working temperature (1450, 1500 etc.)

When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature.  There are occasions when that may be a good idea, but I will come to those later.  For normal fusing, draping and slumping these soaks are not needed.

To understand why, needs a little information on the characteristics of glass.  Glass is a good insulator.  It is a poor transmitter of heat.  Therefore, glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass.  To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock. 

The soaks at intervals do not protect against a too rapid increase in temperature.  It is the rate of heat input that causes thermal shock.  Rapid heat inputs cause uneven temperatures through and across the glass.  When these temperatures are more than 5°C different across the glass, stress is induced.  As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and breaks.  At higher temperatures these stresses do not exist as the glass is less viscous.

If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated.  If the glass survived the rate of heat input between the soaks, it will survive without the soaks.

But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance.  Just consider the time used in the soak and then slow the rate by the appropriate amount.  Take the example above using 30-minute soaks:

250 degrees C to 200C, soak for 30 minutes

250 degrees C to 500C, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167 degrees C per hour to 500 degrees C.  This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise.  If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.

But, you may argue, “the periodic soaks on the way up have always worked for me.”  As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work.  Each layer insulates the lower layer from the heat above.  As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.

To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion.  This means a steady input of heat with no quick changes in temperature.  How do you calculate that rate?  Contrary as it may seem, start by writing out your cooling phases of the schedule.  The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece.  If that final cool rate is 300 degrees, the appropriate heat up rate is half of that or 150.  If you are in the habit of turning off the kiln at 370°C, you can use the cooling rate that is scheduled to get you there.  Normally, you would double the rate you used to get to 370°C as the rate to room temperature.  So, the rate to 370°C is the same as half the final cooling rate.

This “half speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass.  This rate needs to be maintained until the upper end of the annealing range is passed.  This is normally around 55°C (110°F) above the annealing point.

Then you can begin to write the rate of advance portion of your schedule.  It could be something like:

150°C to 540°C, no soak

225°C to bubble squeeze, soak

300°C to working temperature, soak 10 minutes

Proceed to cool segments 

I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze.  This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.

Exceptions.

I did say I would come back to an exception about soaks on the rate of advance segment of the schedules.  When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks.  The kind of suspension is when draping over a cylinder or doing a handkerchief drop.  This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air.  It also occurs when supported by steel or thick ceramic.

The soaks are not to equalise the temperature in the glass primarily.  They are to equalise the temperature between the supports and the glass.  A thick ceramic form supporting glass takes longer to heat up than the glass.  The steel of a cocktail shaker takes the heat away from the glass as it heats faster. 

The second element in this may already be obvious.  The glass in the air on a ceramic mould can heat faster than that on the mould.  The glass on a steel mould can heat faster over the steel than the suspended glass.  Both these cases mean that you need to be careful with the temperature rises.

Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage.  However, my experience has shown me that periodic soaks in combination with gradual increases in the rates of advance are important, because more successful. 

An example of my rates of advance for 6mm glass supported on a steel cylinder is:

100°C to 100°C, soak 20 minutes

125°C to 200°C, soak 20 minutes

150°C to 400°C, soak 20 minutes

200°C to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance. 

Note:

This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.

Another exception is where you have a lot of moisture in a mould, for example. You need to soak just under the boiling point of water to dry the mould or drive out water from other elements of your work before proceeding.  This applies to situations where you need a burn out, of for example vegetable matter at around 500C for several hours.

In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.

Firing Schedules for Wissmach 96

Petra Kaiser is reporting that there are people finding cracks in white W96, which she cannot be replicate.  However, they are using strange firing schedules.

The most popular one appears as follows, in Celsius, with my comments.

166°C per hour to 232°C and hold 20

166°C is relatively slow. It is a rate I would use for a fused 6mm piece.  An unfired two-layer piece I would fire at 200°C to the bubble squeeze.  There is no effect in soaking for 20 minutes at this temperature.  If there is a worry (often expressed) that there will be thermal shock unless you let the glass catch up, slow the rate of advance to 134°C.  This is of course excessively slow for a two-layer piece. 

If, however, you are tack fusing onto two un-fused layers, then 166°C may be appropriate, as you are shading parts of the base from the heat of the kiln. But the soak is not necessary.  It does not do anything useful.

166°C per hour to 538°C and hold 20

As the rate for this segment is the same as for the first, I repeat the soak is not necessary.  If the glass survived the first 200°C at this rate, it will survive the next 300°C too. 

This rate for two layer pieces could be increased to 200°C without damage.

The 20-minute soak at this temperature again does nothing useful.  If the glass survived to this point, you can continue the temperature rise to the bubble squeeze at the same rate as in this segment.

278°C per hour to 621°C and hold 30

Although this rate is not excessive, there is no real reason to speed the temperature rise.  If you use 200°C from the outset to the bottom of the bubble squeeze, no time will be lost in getting to the bottom of the bubble squeeze.

However, this schedule leaves out the important second part of the bubble squeeze.  This is a slow rise to about 50°C above the start of the bubble squeeze process. 

Insert an advance of 50°C per hour to 670°C with a 30-minute soak

278°C per hour to 788°C and hold 15

788°C is a temperature given in the Wissmach tutorial on firing schedules.  However, Petra Kaiser has found that 771°C with a 10-minute soak is sufficient for a full fuse (or 765°C with a 12-minute soak).

The speed at which you reach the top temperature affects what you need to use as the top temperature.  This rate of less than 300°C will not require more than 771 as a top temperature. However a faster rate will require a higher temperature, and with it potential bubble problems, over firing, needling, and inconsistent results.

afap to 527°C and hold 120

This seems to come from the old Spectrum 96 schedules where a temperature equalisation soak was established above the annealing point.  Even if it were necessary, two hours is excessive.

The temperature equalisation of the glass should occur at the annealing point. Therefore, this segment is unnecessary.  And should be replaced by an AFAP to 510°C

55°C per hour to 510°C and hold 120

If the previous segment is eliminated, the rate in this one should be AFAP to 510°C with a soak of 30 minutes for a full flat fuse of 6mm.  There is no need for a longer temperature equalisation soak, as this is enough time for all the glass to be within 5°C of each part.

If you were tack fusing, a soak of an hour would be sufficient for a single layer of tack on a 6mm base.

28°C per hour to 399°C and hold 1

This rate is appropriate for a piece of 19mm.  A 6mm piece could use a rate of 80°C per hour.  A tack fused piece as described above could have an annealing cool of 60°C per hour.

Depending on the natural cooling rate of your kiln, it is possible to turn the kiln off at this point.  If you kiln cools off faster than the cooling rates given above, then you do need to programme a second stage cool.

  

55°C per hour to 93°C and hold 1

This is excessively slow for a 6mm thick full fused piece – a possible rate would be 200°C per hour.

The one-minute holds in these two down rates are only required where your kiln controller will not accept “0” as the number.  If the controller will accept 0, then use that, as 1 minute will not do much of anything, except confuse.

Writing and evaluating  schedules

When you are writing or looking at others’ schedules, review what is happening to the glass at various temperatures.  This excellent guide tells you what is happening to fusing glass at various temperature ranges.  Float glass has some different characteristics.

Combine that knowledge with what you are trying to achieve in the firing.

Guidelines on temperatures

Time versus Rate 

Assessing pre-programmed schedules

Making your own schedules

Over Annealing

Sometimes the statement is made that you can never over anneal.  This statement is true only under certain circumstances. 

Annealing

The statement is also dependent on the understanding of what anneal means.  Annealing is the process of stabilising the temperature, ensuring the piece is at the same temperature throughout, and then gradually cooling the piece to avoid heat shock.  This is to point out that annealing is both the soak and the slow cool.

Long Soaks

Long soaks at the annealing stabilisation temperature can be injurious to your piece if the temperature in your kiln is not even.  This can mean that one or more parts of your piece are at different temperatures. This sets up stress within it.

Placing

You can reduce the possibility of stress by placing the piece at the centre of the kiln or avoid placing the piece in the cool spots of the kiln. 

Cool Rates

Another method of avoiding locking in the stress to the piece is to reduce the cooling rate to less than normal.  This will reduce the temperature differential within the piece.

Mass Being Cooled

In all this you need to remember that the anneal cool rate is relative to the mass of material to be cooled.  Therefore, a thick piece needs a slower annealing cool than a thin one.

But it is not just the thickness of the glass to be cooled.  You need to think about the mass of the kiln shelf or mould that supports the glass.  An example is that glass on a ceramic shelf needs slower cooling than one on a fibre board shelf, because the mass of the shelf needs to be taken into account as well as the glass. Connected to this is whether the shelf is on the floor - slower cooling - or supported on posts, allowing air to circulate under the shelf.

Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:

• Temperature
• Soak Times
• Edges
• Devitrification
• Annealing
• Testing

It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

Temperature

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics. 

Soak times

A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

Edges

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

Devitrification

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Annealing

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

Testing

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.

Pot Melts – Weight of Glass Required

Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.

50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass

Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.

Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.

For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.


Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).  

If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres.  As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.

If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.

To make a 1 cm slab of the same size you need 250 grams of glass.

To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).

To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.

A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.