Soldering Iron Maintenance

“How do I maintain my soldering iron?  I see so many different methods online that I find it confusing.”

Regular cleaning

There at least two reasons for regular cleaning of the solder bit.

The first is to avoid the build-up of carbon and other contaminants which impedes the transfer of heat from the soldering bit to the solder and surfaces to be joined.

Many soldering stations come with a sponge which, when wet, is used to quickly swipe the iron's tip clean. A small amount of fresh solder is usually then applied to the clean tip in a process called tinning.

The second is to maintain the soldering bit in good condition.

The copper that forms the heat-conducting bulk of the soldering iron's tip will dissolve into the molten solder, slowly eroding the tip if it is not properly cleaned. As a result of this, most soldering iron tips are plated to resist wearing down under use. To avoid damaging the plating, abrasives such as sand paper or wire brushes should not be used to clean them. Tips without this plating or where the plating has been broken-through may need to be periodically sanded or filed to keep them smooth.

To avoid using abrasives, cleaning with sal ammoniac is recommended. This comes in a block. You rub the hot soldering iron bit on the surface. As the surface becomes hot, it begins the cleaning process, noted by the smoke rising from the block. When the block under the bit becomes clear, the bit will be clean and can be tinned as above. If this is done at the end of each session of soldering, the bit will last longer and will be ready for soldering immediately when you next need to use it.

Turn off the Iron

The most important element in the deterioration of soldering iron bits is long idle times. This is where you leave the iron on, and not in use, for a long time.

Have everything ready when you start soldering, so the iron will be used continuously, and will not sit there building up heat, while you get ready to use it again. An idle iron will keep heating to its maximum capacity and, without anything to transfer the heat to, it will start burning off the tinning after a short while. If you will not be using the iron for a while turn it off until you are ready again.

Tinning

If a bit has not been properly tinned, solder will not wet to it. Without solder on the bit heat transfer from the bit to the work surface may become extremely difficult and time consuming, or even impossible.

You will understand that proper wiping and continuous wetting is important and a lot easier than continually having to clean and re-tin the bit, especially at the risk of damage to the plated surface because of accidentally scratching, or over abrading it.

When you notice that an iron is not performing as well as it did when it was new you will find that poor thermal transfer from the element to the work is usually the cause. Improper care and maintenance and the lack of a periodic cleaning of the bit can cause a layer of oxides to form, which will inhibit the transfer of heat through the bit.

These factors are reasons why keeping a film of solder on the bit (tinning) is important in maintaining the long life of the soldering bit.

Courtesy of American Beauty Tools

Cleaning the whole Bit.

Each soldering bit has a shank which fits into a heating collar of one kind or another.  The bit should be removed at periodic intervals and the build-up of oxides should be cleaned from the shank.  The oxides inhibit the transfer of heat from the elements to the soldering bit.  This cleaning work, of course should be done when the iron is cool.  You can use fine abrasives on the shank to remove the oxides.  You can also make a tube of fine sand paper to clean the inside of the heating collar.  This should not be done on ceramic heated soldering irons such as the Hakko.

Wattage

Another element in the maintenance of soldering irons is to have an iron of high enough wattage to readily melt the solder and be able to reheat fast enough to maintain the necessary melting temperature. An iron with enough power will reduce the strain on the heating element of the iron and the strain on the user while waiting for the iron to catch up.

For example, an 80-watt iron is sufficient to solder with, but it will continue to get hotter, as it has no temperature control until it becomes too hot for stained glass soldering, often causing breaks in the glass. An iron of this type is often used with a rheostat in order to prevent overheating while it is idling. However, this reduces the power to the iron and so increases the time needed to recover sufficient heat to continue soldering.  Also, a rheostat only slows the heat up, it does not limit it, so eventually the iron will still become too hot if left to idle.

Most temperature-controlled irons seem to be produced in 100 watts or higher. These irons attempt to maintain a constant temperature. Their ability to do so depends on the wattage and the amount of heat drained from the bit during soldering. The temperature-controlled irons are normally supplied with a 700°F bit (identified by the number 7 stamped on the internal end of the bit) and is sufficient to melt solder without long recovery times. You can obtain bits of different temperature ratings, commonly 800°F and 600°F. The 800°F bit is particularly useful when doing a lot of copper foil soldering, because it recovers to a higher temperature, allowing much more continuous soldering action.

You can also get several sizes of tips for different detail of work.  Upon first sight a fine tip would be useful for fine copper foil work.

But fine tips loose heat quickly, requiring the user to wait while the tip regains the required heat.  A 6mm to 8mm wide bit is useful to maintain the heat during the running of a long bead.  Of course, the bit is wider than the bead being run, but the solder has enough surface tension, while molten, to draw up into a bead the copper foil without spreading – unless too much solder is being applied. Really big bits of 12mm or larger are not practical – long initial heat up times, and too much area is covered, even though there is enough heat stored for really long solder beads.

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.

Draping over steep moulds

Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

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.

Slumping Tack Fused Glass Stringers

After you have tack fused your stringers, you will have a fragile blank, about four stringer layers thick. This will need to be handled gently and watched carefully during slumping.

When you slump, you should fire very slowly. Although thermal shock is not likely, the thin pieces have a tendency to crack when they bend into the mould. A safe firing schedule would be to advance at no more than 150C/hr to 540C, then increase the temperature at about 55C per hour toward 677C. Keep watching until the piece slumps into the mould, then advance to the next segment of the programme and anneal as usual for 6mm thickness.

For your first attempts, it's a good idea to use a shallow mould. After you get a feel for the process, you can achieve deeper slumps.

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

Tack Fusing Glass Stringers

The most time consuming part of tack fusing stringers is laying the stringers out. Most stringer bowls start with around four layers of stringers. The stringers need to be arranged in rows. It is often necessary to use a small amount of glue to keep the stringers in place as they are arranged. Some people glue the stringers directly to a piece of paper (normal or thinfire) to make them easier to arrange.

Making an 450mm square piece will take around six tubes of stringers.

If you want a piece where the individual strands of stringers are visible rather than fully fused, you will need to fire to as low a fusing temperature as possible. The precise temperature will, of course, vary by kiln. Most kilns will achieve tack fusing results in the range from 700C to 730C. Fire as quickly as you would like to around 675C, then increase the temperature very slowly, 50C per hour or less. You need to watch closely as the temperature approaches 700C. When the top layer of stringers begin to sag, start cooling the kiln. Firing too high will lead to a flat piece with no feel of the individual stringers.

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

Temperature Equivalents of Orton Pyrometric Cones

The pyrometric cones used by ceramicists can be very useful for checking the temperatures within your kiln. Bullseye have a test described on their website for discovering the eveness of heat distribution in the kiln. The Orton cones can provide an alternate means of testing. This process will also test the accuracy of the temperature readings of you controller/output.

You need to place the cones on supports all around the kiln. Small cones, wich are most useful for this purpose have their own supports built in. The behaviour of the cones will indicate both the temperature achieved - if you fire them according to instructions - and where the hotter and cooler parts of your kiln are located.

You do need to make visual observations to determine when the cone has matured. So you begin checking about 20C - 15C below the indicated maturing temperature. What you will see is the point of the cone bending down. When the point of the cone is pointing directly down, the maturing temperature has been achieved.

You can now check the temperature that is recorded by your read out. Write that down some where. Switch the kiln off now, if you want to see what temperature differences there are within your kiln. You do not need to do any controlled cooling. When cool enough, you can open the kiln and observe where the temperature has differed, by the extent to which the cones are pointing down. If the cone has completely conformed to the edge of its support, it has been over fired. Those that do not point directly down, have not reached the maturing temperature.

The cone numbers that are useful for kiln forming are 022 - 011. Remember that to achieve the temperatures, the cones must be fired at the indicated rate. Any other firing rates will not give accurate temperatures, as the cones are measuring heat work.

Large Orton Cones fired at the rate of 60C/hr over the last 100C will give the following temperature equivalents:
019: 676
018: 712
017: 736
016: 769
015: 788
014: 807
013: 837
012: 858
011: 873

However if you fire large cones at 150C/hr over the last 100C, you will get the following temperature equivalents:
019: 693
018: 732
017: 761
016: 794
015: 816
014: 836
013: 859
012: 880
011: 892

You of course, get different temperatures for the small cones of the same numbers. The small cones must be fired at 300C/hr over the last 100C.
022: 630
021: 643
020: 666
019: 723
018: 752
017: 784
016: 825
015: 843
014: 870
013: 880
012: 900
011: 915

If you decide to use self supporting cones, the evidence you are looking for is slightly different. In this case, the cone has achieved the heat work when the point is level with the base. If you fire the self supporting cones at 60C/hr for the last 100C you will get the following temperature equivalents:
022: 586
021: 600
020: 626
019: 678
018: 715
017: 738
016: 772
015: 791
014: 807
013: 837
012: 861
011: 875

A wall chart is available from the manufacturer

Glueing Glass Pieces

The best solution is to avoid the use of glue completely. If you cannot, use as little as possible and make sure it burns out cleanly.

The glues to which kiln workers have normal access, do not survive to tack fusing temperatures. Therefore they can only be considered as a means to get the glass assembly to the kiln. The glue will not hold the pieces in place until the glass begins to stick, so the pieces must have a stable placement. If not, the pieces will slip, roll and move once the glue has burned out.

The second requirement of glues is that they burn out without leaving a residue.

Glues that have been used with little or no residue include:

Powdered CMC that can be disolved in warm water

-CMC (carbylmethylcellulose). It is a cellulose based binder used in a wide variety of industries, including food. For our purposes, it is also used in the ceramics industry and is often called glaze binder. It is a main constituent of "glas tac" from Bullseye. This can be made up into a viscous solution to catch and hold frits and other sprinkled elements in place.

- PVA (Polyvinyl Acetate) is water-based glue. It is sometimes known as school glue. It can be diluted to about 10parts water to 1 part PVA. This is sufficient to hold the glass pieces together with only a drop for each piece of glass. It does not work so well for small sprinkled elements.

One of many brands of  Ethyl Cyanoacrylate
 glue

- Super glue burns off with no concerns about cyanide. It should be used sparingly and also works best for pieces of glass.

One of many hair laquers in pump spray bottles

- Hair lacquer is normally applied as drops at the edges of the assembled pieces and so can be used to hold pieces of glass as well as sprinkled elements.

In all uses of glue the principles to remember are:
- Use the minimum to hold pieces together while getting the work into the kiln.
- Put the glue at the edges of the glass or where its combustion gasses can escape easily.
- And in all cases, you need to test to see if a residue is left on the glass at full fuse when using a new glue.

An alternative to glue is frit as described here.

Effect of Heat on Sandblasted textures

This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for other glasses, but illustrate the principle of how heating temperatures affect the glass.
Temperatures in degrees Celsius.

650 Blasted surface softened, evened, less "brutal".
690 Blasting still opaque but less "white"
700 Blasting becoming too sheeny but still okay for certain effects.
740 Blasting now subtle and glossy

Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point

Polishing with Cerium Oxide

If you want to go beyond cork in your polishing, cerium oxide will give an optical polish.

You need to grind your glass at 800 or higher grit, or use the cork belt after 400grit. Any rougher surface will not give a smooth polished surface. It will only polish the high spots.

Many do not like to use cerium oxide as it is messy. Especially so on a wet belt sander as the speed is really too fast for the use of polishing pastes. The speed sprays the slurry all over the place.

You need a felt wheel or belt to which you apply the cerium oxide. First you mix the cerium with water to a yoghurt consistency and apply that to the wheel or belt. Begin polishing and add more water and cerium paste as the polishing surface dries. You will notice this as the glass will begin to drag. Do not delay, add more of the paste before continuing. Otherwise you will heat up the glass and risk breakage.

His Glassworks has good descriptions and videos on use of cerium oxide.

It is helpful to mark the glass with a paint or china marker before starting the polishing process to show the areas that are to be polished. This enables you to see what work has been done without completely drying the piece.

For large surfaces you will need to use a horizontal grinder with a polishing pad attached, or a hand held polisher.

An alternative is to use “trizact” belts that are about 4000 grit. These achieve a polish that is very good, if not as optical as with cerium oxide.

Viscosity Changes with Temperature

This image is taken from Pate de Verre and Kiln Casting of Glass, by Jim Kervin and Dan Fenton, Glass Wear Studios, 2002, p.27.

It shows in graphic form how the viscosity of glass decreases with increases in temperature. The temperatures are given in Fahrenheit.  

The coefficient of expansion also changes with temperature. 

This graph is also from Kervin and Fenton

 It is these two forces of viscosity and expansion that must be balanced around the annealing point to give a stable and compatible range of fusing glass.

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. 

Mould Cleaning

There are a variety of moulds available to kilnformers – slumping/draping, texture and casting are currently popular ones.  Each has a slightly different maintenance regime.

Slumping and Draping

Slumping and Draping moulds are the easiest to maintain, as they are not taken to high temperatures. Normally one application of kiln wash will last very many firings.  The kiln wash needs to be renewed when bare spots appear on the mould.  Some people immerse their moulds in water to wash off the old kiln wash.  This is excessive and requires a long slow drying time for ceramic before you can re-apply the separator.  I’m not even sure the practice is good for ceramic moulds.

Normally, you only need light abrasion such as with a green washing up scrubby to clean off the old kiln wash. You can also use a small nylon brush to take off the old kiln wash and prepare it for a new application.

If you are using boron nitride on your slumping or draping mould, you need to brush off the old separator each time you fire the mould. And then renew the boron nitride surface to ensure there is no sticking.

Texture Moulds

Texture moulds require cleaning before applying any additional separator to avoid blurring or obscuring the textures of the mould.  It is best to use a kiln wash that does not have a lot of china clay in it, such as Primo, to allow easy brushing of the separator off.  If you use a kiln wash with little or no china clay, you will need to clean and re-apply each time you prepare to fire the mould.  Boron nitride works well for texture moulds, but also needs to be carefully brushed off the mould before re-applying the separator in preparation for the next firing.  This is both to avoid blurring the texture and to ensure there is sufficient separator to avoid sticking.

Casting

Casting moulds that are intended to be re-used multiple times are best coated with boron nitride.  The boron nitride should be lightly brushed off after each use to ensure the detail is retained, and then re-coated.  A nylon brush is good for this.

The materials and purposes of moulds have an effect on the separators used and the methods of maintaining them clean.

Wire for Fusing

Although there are other ways to combine wire with glass, one popular method involves fusing wire inside the glass. This technique generally fuses and seals the wire between two layers of glass, so it is important to select a wire with the right characteristics. The main characteristics are:

1. The wire must be capable of withstanding the heat of the kiln.

2. The wire must emerge from the kiln in a relatively pristine condition, or at least can be easily cleaned.

3. The wire must also retain the desired flexibility and pliability. If it's too soft or brittle it may not support the piece.

4. The wire must not react with or contaminate the glass. In most cases colour changes and metal flakes are not desirable.

5. The wire must be of a small enough diameter to avoid causing excessive stress within the glass.

6. It is a bonus if the wire is reasonably priced or even inexpensive.

This post gives the characteristics of some types of wire for fusing.