Annealing Range for Unknown Glass

It is possible to anneal unknown glass with some degree of certainty by using what is known as the slump point test.  This will not be as accurate as a factory determined test, so you do have to extend the range over which you do the annealing.  

The annealing of glass with unknown characteristics is possible in two ways - shotgun and calculated.  The examples here are for 6mm thick glass.  The soak and cooling times need to be extended for thicker glass.  

Both the shotgun and calculated approaches exemplified here assume glass of 6mm thickness.  For thicker glass the soak time needs to be extended and the anneal cool rate slowed more than indicated above.  Using the Bullseye chart for annealing thick slabs will give you an indication of the relationship of thickness to speed.

1)  One is the traditional shotgun approach – pick an arbitrary, but slightly high temperature, and soak for a minimal amount of time there. Then go very slowly through the next 55°C.  This may be as slow as 25°C per hour, followed by a doubling of that rate for the next 55°C. Then double again to 300°C or less.

2)  By using the slump point test and the calculations, you will be sure of the annealing point/temperature equalisation point within 10°C.  The approach here would be to soak for half an hour at the calculated temperature, followed by a slow drop of 50°C per hour to 55°C below annealing soak and then at 100°C/hr to 110°C below your chosen temperature equalisation point. The final cooling could be at 200°C to room temperature.

2a) An additional tweak to the slump point test calculations is to use the Bullseye concept behind their recommendations for thick slabs.  Using their concept, you reduce the calculated annealing point by 30°C from the calculated annealing point to do the temperature equalisation soak at the lower end of the annealing range.  Having calculated the annealing point, you reduce that temperature by 30°C and soak for  a longer time of 60 minutes and at a slower rate as noted in the chart.

In using the chart for unknown glass you substitute the calculated temperatures, but continue to use the rates and times indicated.  An example:

• You have calculated that the annealing point is approximately 535°C.
• Subtract 30°C from that to get a equalisation temperature of 505°C.
• Assume the piece is uniformly 12mm thick or 6mm tack fused (when you want to use rates for  twice the actual thickness to account for the difficulties in tack fusing). 
• For a 12mm thick piece your soak time at 505°C will be two hours.
• The cooling rate for the first 55°C is given as 55°C per hour according to the chart. Therefore the first cooling segment will be 55°C from 505°C to 450°C.  The second will be 99°C per hour from 450°C to 395°C.  The third rate will be 330°C per hour from 395°C to room temperature.

You can see that the times and rates are taken as given by the chart (as determined by the thickness of your piece), but the temperature set points are determined by the calculations for the glass you have tested.

When determining what temperature you should use to anneal a glass about which you are uncertain of its characteristics, you can use one of two basic approaches.  Pick an arbitrary temperature and soak for some time there and then proceed slowly in 55°C segments to about 370°C.  A second more certain method is to use the slump point test to determine the annealing point and then apply the Bullseye chart for thick slabs for the soak times and cooling rates.

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

Use of Sal Ammoniac block

A block of sal ammoniac is an excellent aid to keeping your soldering iron tip (or bit) clean and able to hold a small blob of solder.

A description of what sal ammoniac is and the safety precautions in its use are here. 

• Place the block in such a way that it cannot slide around as you rub your iron over it.
• Place your hot soldering iron tip on the block until it begins to smoke. 
• Move your iron slowly back and forth along the block.  Initially, the block will be black from the contaminants coming from the soldering iron bit.  As you rub the bit along the block, it will begin to clear. As it does, you can add a touch of solder and turn the bit over to check whether there are still any black spots on the face of the bit. 
• If these spots are persistent, you can use a brass wire brush to help clean the contaminants off.  
• Add a touch of solder and return to rubbing along the block.  
• Repeat this check until the whole bit is bright and holding a small blob of solder.  
• This completes the tinning process on one side.
• Repeat this process for the other side too.
• Leave a small blob of solder on each side of the bit to protect the bit from oxidising.  

This cleaning process should be done at the end of each soldering session if the bit is not clean.   The frequent quick wipe of the bit on a damp sponge or a ball of brass swarf cleans the bit as you work.  The sal ammoniac block is for cleaning persistent contaminants off the bit.

Pot Melt Temperature Effects

Image credit: Craft Gossip

When firing a pot melt, you have to consider how high a temperature is needed.

Viscosity reduces with higher temperatures which increases the flow and reduces the length of soak, although there are often some undesirable opacifying effects at prolonged higher temperatures.

The size of the hole is also relevant to the temperature chosen. The smaller the hole, the higher the temperature will have to be to empty the pot in the same amount of time. Of course, you can just soak for longer at a lower temperature to achieve the desired object of emptying of the pot without changing the temperature.

Using the same principle, the larger the hole the lower the temperature required to empty the pot in a given amount of time.  So, in general the larger the hole in the pot, the faster it will empty, given the same temperature.

The temperature used to empty the pot will need to be between 840C and 925C (1546F and 1700F).  The problem with temperatures in the 900C to 925C range is that the hot colours tend to change, e.g., red opal tends to turn dark and sometimes become brown. Some transparent hot colour glasses also opacify. There is also the possibility that some of these glasses will change their compatibility with others in the range.

The best results seem to come from temperatures in the 840 to 850C range with longer soaks than would be required at 925C.  Also remember to give melts a longer than usual anneal as they will be thicker than 6mm at the centre - sometimes as  twice the edge thickness, which will require annealing for twice the thickest area.

Revised 2.1.25

Soldering old lead

This is normally only a requirement when repairing old windows. Usually either to join new lead to the old, or to repair breaks at the original solder joint.

You will need to clean the lead down to the bright metal at the joints. This is more than a rub with steel wool. You need a glazing nail or other sharp pointed tool to scratch through the oxidisation layer.  The corner of your lead knife, or in cases of mild oxidisation, a brass wire brush might do. But not a steel one, as that may scratch the glass and any painting.  

Do not clean the oxidisation off the lead elsewhere. That is a protective layer already formed which leads to the longevity of the came. It is best to leave oxidised lead alone rather than expose the metal to further oxidisation.

Getting to the bright metal where you want to solder the joint means the flux can act appropriately and help the solder form a secure joint.  Otherwise only a weak, cold joint is possible.

Note that you always need to use dust masks or other breathing protection.  You need to have the work area well ventilated and need to do a damp wipe down of surfaces to reduce the amount of lead oxide in the work space.

Using Cut Running Pliers Without Cushions

There are a wide variety of cut running pliers for different purposes.  A description of some of them is here.

This post is to describe maintenance and use of this kind of cut runner.

The plastic covers that come with these cut runners eventually wear out.  The replacements are hard to find. There are things you can do other than buying a new pair just for the shields.

You can dip the jaws in tool coating compounds such as Plastidip.  This does not last as long as the plastic, but is easy to re-do.

You can wrap the jaws in tape.  Electrical tape, duct tape or even self-adhesive elastic bandage will do the job. Again, not long lasting, but easy to replace.

Or

You can use the cut running pliers without any covering on the jaws.  “You can’t do that. You will crush the glass!” is the response I hear.  You can use them bare. I do, and so can you.

The key is in the adjusting screw.  It is there not just to tell you which is the top of the pliers; it has a function too.  That screw adjusts the opening of the jaws to the thickness of the glass. 

A simple way to ensure you have the correct opening is to put one corner of the jaw on the edge of the glass with the jaw opening less than the glass is thick. Then tighten the screw until you feel the handles of the pliers begin to open.  Then you have the right opening for the thickness of the glass. 

It ensures you cannot crush the glass, as the jaws will not close at the centre to be less than the glass thickness. 

You also have a more direct feel of the glass without the spongy connection of the plastic. You can sense the glass beginning to bend just before the score runs due to the gentle pressure of the jaws of the cut runners on either side of the score.

Whether you use the cut runners with or without cushions on the jaws, it is important to keep the adjustment screw lubricated so you can adjust the width of the jaw opening for different thicknesses of glass.

Pot Melt Schedule

I usually use a schedule like this for pot melts:

• 100C/hr to 220C (180F/hr to 438F) for 20 minutes ; approximately the crystobalite inversion temperature – to be kind to the pot.
• 220C/hr to 570C (396F/hr to 1090F) for 20 minutes; approximately the quartz inversion temperature – again to be kind to the pot.
• 220C/hr to 677 (396F/hr to 1250F) for 30 minutes; the bubble squeeze temperature to allow larger bubbles to escape from the pot before melting begins.
• 330C/hr to 850C (595F/hr to 1564F) for 120 minutes;  to ensure there is plenty of time to empty pot. 
• AFAP to 805C (AFAP to1482F) for 30 minutes; to allow thickness equalization and also to allow bubbles to pop and seal.
• AFAP to 482C for 120 minutes; this temperature is for Bullseye, but substitute the annealing temperature for your glass.
• 55C/hr to 427C (100F/hr to 800F), no soak (for 6 to 12mm thickness)
• 99C/hr to 370C (180F/hr to 700F), no soak.
• 200C/hr to 150C (360F/hr to300), end.
• Allow to cool to room temperature in the kiln

Revised 5.1.25

Pot Melt Contamination

Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash most often sticks to the back of the melt.

If you put only fiber paper – Thinfire, Papyros, or standard 1mm or 2mm fibre paper – at the bottom, the dripping glass will tear and move it about.  It also tends to incorporate fibers from the refractory papers into the melt.  It is best to avoid fibre papers of any kind on the base.  Using fibre paper around the edges of dams, if you use them, is better than simple kiln washing of the dams.

From wikihow

If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature to shine up the surface. You will need to flip this over and fire again, if the original top surface is what you want to present.  Or if you like the new shiny surface, use it as is.

If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness, and removal of contaminants.


There is another thing you can do to avoid kiln wash contamination.

The best solution appears to be to put a disk or rectangle of glass on top of fibre paper. It can be clear or any colour you wish, but needs to fill the area enclosed by the dams. This seems to keep the fiber paper from tearing and being incorporated into the glass, even though the base will have the fibre paper marks.

It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, in addition to 3mm fibre paper arranged so that it is 3mm narrower than the expected final thickness, or any excess glass may stick to the dams. The means of arranging the fibre paper around the dams is given here. You may need to grind the marks off the edge of the disk, but this is much easier than grinding it off the bottom.

Separators sticking to Opalescent glass

It is worth thinking about how fast you fire pieces, especially where your current working temperature and rates of advance are giving difficulties.  One common difficulty is where opalescent glass picks up kiln wash or fibre paper and partially incorporates it, requiring a lot of work to remove it. 

At higher temperatures opalescent glass seems to incorporate some of the separator, especially near the edges.  It does not seem to matter whether kiln wash or fibre papers are used – there is frequently a little pick up.

The difficulty is achieving the profile you want without the higher temperatures.  This is where heat work concepts can assist.  Glass reacts to the heat applied, rather than simply the temperature.  Heat is a combination of time and temperature.  Rapid rates of advance require higher temperatures than slow rates of advance to achieve the same effect.

These facts should make you consider slower rates of advance to achieve the work at a lower temperature and so pick up less of the separators.  Perhaps you could consider a rate of advance of 150°C or 200°C instead of 330°C once you have passed the bubble squeeze temperature.  This would allow you to have a full fuse at ca. 800°C or even a little lower instead of 816°C (for Bullseye).  You will need to observe to find what is the appropriate temperature for the effect you want.  This will apply both with different rates of advance and with different lay-ups.

Limits to the “Low and Slow” Concept

I frequently advocate using slow rates of advance and low temperatures to achieve the results desired with a minimum of marking in forming, or a minimum of firing difficulties during the fusing part of kilnforming. 

But there are limits to this both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will affect the brittle nature of the glass, for example.  If your temperature is below the strain point of the glass, virtually no change will occur even with very long soaks.  The graph below shows the slumping range is from the annealing point (glass transition temperature) to about 180C above the annealing temperature.  After that temperature the glass is prone to devitrification (the beginnings of crystallisation). 

This shows the the slumping range of a specialised glass rather than the soda lime glass that kilnformers normally use.

In this graph, the glass has an annealing temperature of about 600C, which is higher than that for float glass and much higher than for kilnforming glasses.  The glass transition temperature range for existing fusing compatible glasses is around 510C (+/- ~10C).  Float glass has a higher annealing point of around 540C (+/- ~ 10C). Following the research behind this graph, stable slumping temperatures would be in the range of about 510C to 690C (+/- 10C).  

It is important to be aware that the annealing point is determined mathematically as the glass transition point.  This is the annealing point at which temperature the glass can be most quickly annealed. The practical research conducted by Bullseye has shown that a temperature equalisation soak in the lower part of the annealing range is a good solution to the the practicality of balancing adequate annealing with the use of the kiln time.  The annealing point temperature and that which you use to equalise the temperature within the glass may be quite different.

Even where it is possible to achieve an effect at a low temperature, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 12 hours. This is not practical.

In addition to practicality, there is the physical limitation.  If you slump below the glass transition point, you will be unable to properly anneal the glass and therefore produce an unstable item.  It will contain stress from this inadequate annealing leading to an increased fragility.

The balance required between the rate of advance and top temperature means that you will need to do your own experiments to find where the practical limits to using heat work are for you. The more patient you are, the lower temperature you can use.

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

Analysis of Breaks during Fire Polishing

The analysis of breaks in fire polishing can be difficult.  The temperature and heat work are minimal, so the edges can look sharp, which would indicate that the break occurred on the cool down.

But this is where you really need to feel the edges.  If they feel very sharp, then you can be more confident that the break occurred on the cool.  But if there is even the slightest smoothness to the edge as you feel it, the break probably occurred on the heat up to fire polish.

In this picture, there appears to be an annealing break, because of the hooked ends of the break.  That is typical of a break due to inadequate annealing.  It is important to know when the break occurred, so that appropriate remedial action can be taken for future firings of similar pieces.

To determine if the annealing break occurred because the initial anneal was inadequate, it is important to do a touch test. Just looking at it will not be enough.

If the edges were even slightly smoothed, the anneal break occurred on the way up.  This would mean that the anneal of the original blank was not adequate, assuming a reasonable rate of advance was used for the thickness of the piece.

If the edges are razor sharp, the break occurred on the way down, indicating that the anneal after the fire polish was not adequate.  This would mean that in future the annealing needs to be done more carefully on fire polished pieces.

Being too quick to apply a diagnosis of a break during a fire polish can lead to the wrong conclusion, and so the incorrect alteration of future schedules.

Schedules for Steep Drapes

I have been asked for a schedule for draping in the context of a tip on steep straight sided drapes.

What you are trying to do with a steep drape is two things. One is to compensate for the heat sink that the glass is supported by, and the second is to compensate for the relative lack of weight at the outer edge of the glass.


The supported glass transmits its heat to the support, leaving it colder than the unsupported glass. This often leads to breakage due to heat shock at much lower temperatures and slower rates of increase than glass supported at its edges. My experience has shown that - contrary to what I recommend for other kinds of firings - a slow rise with short soaks at intervals up to the working temperature works best. The reason for these slow rises and soaks is to try to get the support and the glass to be as nearly as possible at the same temperature throughout the rise in temperature. The soaks help ensure the mould is gaining heat without taking it from the glass.

The other problem with steep drapes is that the edges of the glass begin to drop more quickly than the area between the support and the edge. This leads to the development of an arc that touches the mould side near the bottom before the glass between the edge and the and the support. Extended soak times are required to allow the glass to stretch out and flatten. If this is done at high temperatures, the glass will thin - possibly to the extent of separating.

So the requirements for a firing schedule on this kind of drape are slow increases in temperature with soaks to avoid thermal shock, and an extended soak at the (low) forming temperature.

Whether using steel or ceramic moulds, I use a slow rise in temperature to 100C with a soak of 15 minutes. I then increase the rate of rise by 50% for the next 100C and give a 15 minute soak there. For the next 200C I raise the temperature at twice the original temperature rise, again with a 15 minute soak. The glass and mould should now be at 400C. This is still at the point where the glass could be heat shocked, so I only increase to 2.5 times the original rise rate but use this rate all the way to forming temperature.

Each kiln has its own characteristics, so giving schedules is problematic. 

•  A side fired kiln will need slower heat rises than a top fired one. 
• The closer the glass is to the elements, the slower the rate of increase needs to be. 
• The kind of energy input - electric or gas - has an effect. 
• The thickness of the glass is also a factor in considering what rate to use. 
•  The size of the glass in relation to the size of the support is important - the greater the differential, the slower the heat rise should be. 

So in making a suggestion on heat rises, it is only a starting point to think about what you are doing and why you are doing in this way.

I have usually done this kind of draping in top fired electric kilns where the elements are about 250mm above the shelf, and about 120mm apart. In the case of a 6mm thick piece about three times the size of the support area, I use 50C/hr as my starting point. This is one third of my usual rate of temperature rise. However you must watch to see what is happening, so that you can make adjustments. You should observe at each of the soaks, so you know how the glass is behaving. It will also help you to pinpoint the temperature range or rate of advance that may be leading to any breakages.

On steep slumps, the temptation is to use a high temperature to complete the drape. This is a mistake as the glass will be more heavily marked and tends toward excessive stretching and thinning. What you really need is a slow rate of advance to a relatively low temperature. If you normally slump at about 677C, then you want to do this steep, straight sided drape at 630C or less. It will need a long soak - maybe up to an hour. It will also need frequent observation to determine how the drape is progressing. So plan the time to make yourself available during this forming soak.

Annealing is done as normal, since the mould and glass are more closely together and will cool at the same rate.

The original tip on the set up of a steep straight sided slump is here.