Thinfire* and Devitrification

There are reports that Thinfire causes devitrification by rising over the edges of the piece.  There as many saying they have no difficulties with the Thinfire curling.  This indicates there are several factors that may be at work.

If the Thinfire curls over the edge of the glass while firing, it will deposit a fine powder on the edge and perimeter of the piece.  This gives an ideal condition for devitrification to form.

Bullseye recommends placing dams or other kiln furniture on the edges of the paper to resist any tendency for the paper to curl.  Of course, if the paper is put upside down, it is much more likely to rise over the edge.  The smoothest surface should face upwards. Now that Bullseye prints their logo on the bottom, this is unlikely to be a problem.

Cutting the paper to the size of the piece is initially an attractive idea.  However, it does not account for the expansion beyond the initial footprint that glass goes through while heating to the working temperature, and before it contracts to its final size.  The Thinfire must be cut larger than the piece. The amount depends on the thickness of the piece.  6mm larger may be adequate for a 6mm thick piece.

Bullseye does not recommend using Thinfire under multiple small pieces of glass because the paper can shrink and move, disrupting the glass placement on the kiln shelf.  Instead using kiln wash as the separator may be better in these circumstances.

There are other things that can affect the deposit of the separator powder from the Thinfire onto the glass.

Venting – It seems to be good practice to open the peep holes or leave the door/lid slightly ajar during the heat up.  These should be carefully closed once the smell of the binder burning out disappears.  This is usually around 500°C.  The idea here is that the combustion products from the binders are allowed out of the kiln without settling on the glass.  I do not find this necessary, but many do, so it is worthwhile trying it out.  When the smell of the burnout of the binders ceases close the lid slowly and place the bungs gently into the peep holes to avoid disturbing any dust within the kiln.

Opening the kiln or ports - Opening or closing the kiln above ca. 500°C, if done quickly, will create a draft that will distribute the powder around the kiln.  Some of this will land on the surface of the glass. Other parts of the Thinfire will be moved up onto the edges of the piece.  This dust and the pieces of Thinfire will create nucleation points for devitrification.  Always open or close any part of the kiln slowly when there are powders or anything else which can be disturbed by a gentle waft of air.

Over firing - Another element that can bring Thinfire onto your pieces are a too hot a firing.  During high temperature firings, the glass will expand and thin more than usual.  During the cooling phase, the glass will draw back to being 6-7mm thick. This means the glass will have expanded over the Thinfire and drawn some of it back onto the edges as it thickens and retreats.  The solution for this is to reduce the top temperature and possibly lengthen the soak time, but do not do both at the same time.  First see what a lower temperature with a 10-minute soak will do.

Of course, if you are not having problems with Thinfire or Papyros, continue your practice as normal.

*I have used the term “Thinfire” almost exclusively throughout, but remember all these notes apply to Papyros too.

Copper inclusions

Inclusions of metals can be achieved with care.  Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass.  This note provides some things you might consider when planning to include copper in your fused pieces.

The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin.  Copper leaf can be very faint if a single layer is used.  Placing several layers of leaf improves the colour, but often provides wrinkles.  In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.  

Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles -  sometimes a single large one.

Copper can provide several colours.

Copper sheet normally turns burgundy colour when oxidised.  This means that there is enough air reaching the copper to oxidise it to deep copper red.  This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.

To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing.  Reduction of air contact with the metal helps to retain the copper colour.  There are two methods I have used.  Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface.  Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.

In certain circumstances you can get the blue green verdigris typical of copper in the environment.  This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation.  The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures.  Experimentation is required to achieve this consistently.


The spaces under and over the copper give the opportunity for bubbles to form. 

This means that the copper needs to be as flat as possible for one thing.  Burnishing the copper can have a good effect on reducing the undulations in the copper.  Thinner copper is easier to make flat than thicker.  If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat.  Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal.  This remains the case even after burnishing to be as flat as possible.

The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass.  The spread of the powder over the glass assists in giving places for the air between layers to escape.

These two things combined with a long slow squeeze can reduce the amount of bubbles you get.  It cannot totally eliminate them.

Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.

Foiling Space

There are a lot of views on what amount of space is required between copper foiled glass pieces.  Some say the pieces should be tight, others that a consistent space is needed, and some who say that variable spaces are fine.

It is necessary to consider what holds a foiled panel together.

Adhesive

The foil is supplied with an impact adhesive which helps keep the foil attached to the glass before soldering.  However, the heat of soldering deteriorates the adhesion of the glue.  If you must take a foiled piece apart you will find that the adhesive is sticky rather than firm. Also, the adhesive will continue to degrade during the life of the object.

Solder

The solder bead is significant in creating the matrix required to hold the panel in one piece.  The bead on each side holds the glass in place and resists deformation away from a single plane. This resistance is significantly reduced if there is not a fin of solder connecting the two beads.  The beads and the fin of solder form an “I” beam which together resists movement of the glass.

Strength

To form that “I” beam there does need to be space between the foiled pieces. It does not need to be wide, but it does need to be enough to wiggle the pieces.  This will allow the solder to flow from one bead to the one on the other side, forming a strong “I” beam.

In vertical panels, the glass is the strong element.  The solder lines serve to hold the matrix together.  Where people indicate the strong border will keep the whole panel from falling apart, they are correct in part. But, if there is not a sufficient “I” beam between each piece, the whole panel is subject bowing, either from wind pressure, vibration or mechanical pressure from handling.  Therefore, you cannot rely on the border to make your panel strong and long lasting.

Dissent

Some take the view that there will be enough unintentional spaces created between pieces to allow the fin form between beads intermittently.  But the gaps in the “I” beam due to tight fitting pieces will make it much weaker than a continuous bridge between beads.  The existence of gaps puts greater pressure on the solder that does bridge between beads.

An example was provided for me in a lamp brought in by client which spontaneously fell apart one evening.  (Not made by me, I add). The upper band of glass remained attached to the vase cap, but separated from the rest of the shade.  Fortunately, it fell straight down and only a little of the bottom edge was broken.  Investigation showed there was very little solder between pieces, although there was a good bead on each side of the lamp.  The lamp pieces separated, in different places, at the foil-glass interface and elsewhere at the foil to foil interface.  This indicates there was little or no solder where the foil remained on the glass, as the adhesive is much weaker than even a thin fin of solder running between the inner and outer beads. This case is an example of the need for a fin of solder to be formed between the beads on either side to provide a strong, long lasting object.

Heat Cracks

There is sometimes a fear expressed that tight fitting of foiled pieces can lead to heat fractures when soldering due to expansion.  Yes, when soldering pieces with a lot of variation in width, you do need to move reasonably quickly. Come back later to improve a bead if you need, to avoid overheating the glass.  Even the thin copper foil can transmit heat along its length, which reduces direct heat transfer to the glass.  Mostly, breaks occur from dwelling too long in one place with the soldering iron. It may be better to tin the foil all around the suspect piece just before running the bead.  This will warm the glass around the edges in preparation for the greater heat of laying down the bead.

The main point is that the solder needs to connect the beads on either side of the glass to provide a stable, strong and long-lasting piece.

revised 28.12.24

Marker Pens

A lot of us use marker pens on our glass to determine cut lines, indicate areas that need grozing, etc.  These pens have a variety of names – felt tips, Sharpies, paint pens, fibre tips, permanent markers, laundry markers, and many other generic and trade names.

Most, except the paint markers, contain water or spirit based colours. Many of these pigments are reputed to burn away during the firing of the glass. 

Paint markers and the ones that contain metallic colours rarely fire off.  They are more likely to fire into the glass.  Some people take advantage of this fact to quickly add marks that will survive the firing.

I no longer trust anything to burn off. Even if the marks do apparently burn away, the residues are sites for devitrification to begin.

I clean all my marks off before firing.  It only takes the marks to be fired into a favourite piece to convert you to cleaning. If you use paint markers on black glass or coloured felt tip marks on clear, clean it all off before firing.  This removes the chance that the pigment will remain throughout the firing and ensures the glass is spotless when it goes into the kiln.

AFAP firings

As Fast As Possible (AFAP), sometimes referred to “as soon as possible” (ASAP) firings need caution.  Those who use  this AFAP rate, apply it only above ca. 540⁰C/1004⁰F or higher.

This is possible for small pieces in smaller kilns.  It is often desirable for pieces under 100mm.  In the case of smaller items, the heat can be distributed across and through the pieces easily.  There is no need for the same caution as for larger or thicker pieces.

But

There are effects on the glass and kiln that AFAP rates have, and need to be considered when setting the schedule for the firing.

Effects on glass

Bubbles

An AFAP rate softens the upper surface of the glass early and before bottom can catch up. This leads to greater possibilities of creating bubbles, as the surface is more easily moved by the air underneath.  So, the air can push upwards rather than be pushed by the weight of the glass from under and escape out the sides. 

Dog boning

The characteristic dog boning of thinner glass is increased, as the temperature overshoots, allowing the glass to become much less viscous, so the surface tension of glass can take over to draw the glass in to create a greater thickness.  This “robbing” of glass occurs both from the interior and edge.  The interior glass becomes thinner and so less able to resist bubble formation.

Effects on the Kiln Control

The controller is comparing the relationship between the energy input and the temperature achieved all the way through each firing, even though you fired the same piece yesterday.  The controller is constantly (well, about once a second) comparing the actual rate of temperature increase or decrease with the programmed one.  When there is a difference, power is applied. On the way down there is no input of energy unless the cooling is too fast, so there are no concerns about the controller having to catch up.

Temperature overshoots

If you programme AFAP, especially in a small kiln, you will get overshoots in temperature.  This is because considerable time is required for the controller to determine the continuing energy requirements for the rate set.  In small kilns, the upper temperature can rise quickly as there is less kiln mass to heat than in a larger kiln.

Energy

Also, the amount of energy required at the higher temperatures is greater than at the lower ones. This means the controller must constantly adjust the rate of energy input at different temperatures.

Both these factors combine to give overshoots of the top temperature, sometimes by as much as 20⁰C.  

Temperature drops

During the soak time at top temperature, the kiln will attempt to adjust the energy input to maintain an even temperature. The result of this constant comparison is that the temperature drops considerably below the one set. The controller then overcompensates and goes over the set point again.  It continues bouncing above and below with less and less variation as the soak proceeds, because the controller is “learning” the heat input required.

This bouncing of the temperature results in less control over the results of the firing.  This is especially so when there are voltage variations in the electricity supply.

Revised 34.12.24

Cooling the Kiln

image credit: getradianlife

“Why does my kiln take so long from boiling point to room temperature?”

The rate at which a kiln cools is dependent several factors:

• The mass of the kiln. Some kilns have dense insulation bricks.  These are very good at holding heat, and release it slowly.
• Its insulation characteristics. Other kilns have light weight bricks or fibre insulation. Both these materials have less mass and can release heat quickly at high temperatures, but much less slowly at lower temperatures.
• The environment. The temperature of the surroundings has a big effect at lower temperatures.  The amount of air movement around the kiln also influences the cooling rate at these lower temperatures.

The physics of heat transfer determine the cooling rate. if all other factors are the same, the rate of temperature fall is faster when there is a greater temperature differential.  And it is slower where the temperatures are closer together.  You can see this by comparing the rates of fall at 800⁰C and at 300⁰C.  It is much faster at the higher temperature and slower at 300⁰C.  You will also notice that the kiln cools more slowly at the lower regions when the outside temperature is high than when low.

Rather than waiting hours or days for the kiln to get 
to room temperature, there are some things you can 
do.

·        Open any vents or peep holes your kiln has. Not only are peep holes good for observing the progress of the kiln work, they are important in cooling.  Their relatively small size insures that there is not such a great air exchange that could cause thermal shock.  The temperature at which you do this is relative to the thickness or variation in thickness of the pieces in the kiln, of course.

·        Open the kiln lid/door a little. As the temperature fall rate reduces, you can crack the kiln open a little.  Many times, you need to put a prop under the lid to keep it open only a little.  Again, this should only be done at a low enough temperature to avoid any thermal shock to the glass.

·        Create greater air movement around the kiln.  You can of course create greater air circulation around the kiln by opening doors and windows, or by a fan.  If you use a fan, it is best to avoid direct air current from the fan onto the kiln. This is because when the vents or lid are open, dust can be spread over the glass and throughout the studio.  If using a fan, it is best to have the kiln closed.  Some kilns have powered ventilation to speed cooling, but these are usually industrial.

How do I tell if I am cooling too fast?

The risk of opening your kiln after the end of the second part of the annealing cool (generally around 370⁰C) is thermal shock from the relatively cool air contacting the glass and cooling one part too much, causing a break or fracture.

You can select how fast a cool rate is safe for your piece and programme that into the controller down to room temperature.  Doing this does not use any more electricity than simply turning the kiln off.  The controller will only put more energy into the kiln if it is cooling more quickly than the rate you set. 

And this is the point of programming to room temperature.

When you vent your kiln, and have the controller set for a cooling rate, it will only add more heat if you have opened the kiln too much.  If you hear the controller switch on the elements, you know to reduce the size of the opening, because it is cooling faster than you set the rate to be.  This makes for a safe, but more rapid cooling than just letting the kiln cool with no ventilation.

"My controller shuts off when I open the kiln."

If your kiln does not allow any opening of the lid/door without the controller switching off, you need an alternative.  In this case, you will need to take note of the temperature drop over set periods to learn if the temperature is falling too fast or too slowly.  Usually 15-minute intervals are all that is required.  Record the temperature at the switch off and before venting the kiln. Vent the kiln. Fifteen minutes later record the temperature. Multiply the difference by four to get the hourly rate.  If that rate is above the one you intended, close the venting a little.  If it is less, open the venting a little more. Then record the temperature after another quarter of an hour. You continue to do this until you are satisfied you have settled on the rate of cooling you intended.

You must exercise patience with the cooling.  

The larger, thicker, more important the piece is, the more caution is required. 

revised 30.12.24

Kanthal vs. Nichrome

Both Kanthal and Nichrome are high temperature wires.

Kanthal

Kanthal is the trademark (owned by Sandvik) for a range of iron-chromium-aluminium (FeCrAl) alloys used in resistance and high-temperature applications. The first Kanthal alloy was developed by Hans von Kantzow in Sweden.

“Kanthal alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %). The alloys are known for their ability to withstand high temperatures and having intermediate electric resistance.”  So, it is often used in kiln elements.

“Kanthal forms a protective layer of aluminium oxide (alumina) when fired.”  This layer resists further oxidisation of the elements when firing.  Aluminium oxide is an electrical insulator with a relatively high thermal conductivity.  Ordinary Kanthal has a melting point of 1,500°C.

“Kanthal is used in heating elements due to its flexibility, durability and tensile strength.” Its uses are widespread, with it being used in home appliances and industrial applications as well as glass and ceramic kilns.  As an aside, it is  used in electronic cigarettes as a heating coil as it can withstand the temperatures needed in this application.

Based on Wikipedia https://en.wikipedia.org/wiki/Kanthal_(alloy) and other sources.

Nichrome

Nichrome is an alloy of various amount of nickel, chromium, and often iron.  The most common usage is as resistance wire.  It was patented in 1905.

“A common Nichrome alloy is 80% nickel and 20% chromium, by mass, but there are many other combinations of metals for various applications.”  Nichrome is silvery-grey, corrosion-resistant, and has a high melting point of about 1,400°C.

It has a low manufacturing cost, it is strong, has good ductility, resists oxidation and is stable at high temperatures.  Typically, nichrome is wound in coils to a certain electrical resistance, and when current is passed through it, the resistance produces heat.  This is probably the most common material used for kiln elements.

When heated to red hot temperatures, the nichrome wire develops an outer layer of chromium oxide, which is stable in air, being mostly impervious to oxygen.  This protects the heating element from further oxidation.  However, once heated the nichrome wire becomes brittle and must be heated to red hot before bending.

Based on Wikipedia https://en.wikipedia.org/wiki/Nichrome and other sources.

Flat shelves

Can I use a pizza stone or a tile for the shelf?

Yes. but, you need to be consider how flat the stones are.

Choose the flattest, smoothest stones you can find.  Take a ruler or other straight edge with you to select the flattest.  Hold the straight edge vertically, and look for light coming from between the edge and the surface of the stone.  Choose the ones with the least light showing.  A more accurate method is to sprinkle black powder on the shelf and pull the straight edge across the shelf.  Any black areas left are indications of depressions and their size.

Determining how flat the stones are

You can make the stones very flat and smooth when you get them to your studio.  Put the surfaces together face to face and move one against the other in a circular motion.  After minute or so of grinding, lift and take note of the areas which are showing the effects of the grinding. Where the stone has not been affected, are the low spots.  The number and depth of the low spots will determine whether you wish to continue to even out the variations in the surfaces.

Grinding

You can speed the grinding by putting a slurry of grit between the two surfaces.  You can use a coarse grit of 100 or less in the grinding. Place a small pile of the grit and make a depression in which to put the water.  Mix into a runny paste.  And place the other stone on top and begin to move the upper stone in multiple directions.

Keeping the grinding surfaces damp will prevent any dust from the grinding getting into the air. You will hear a difference in sound when the slurry begins to dry out.  This is the time to add a spritz of water to the grinding materials.  As you check from time to time, you will see the areas that already are ground and those that are not yet evened.  The grit will remain in the depressions and be clear from the higher areas.  Push the grit onto those clear areas to continue the smoothing and flattening process.  Continue until the surfaces of both stones are smooth and flat.  This probably will not take much more than a quarter of an hour.

It is advisable when smoothing ceramic or glass materials to wear a dust mask. The dust from both are irritants, although not carcinogenetic.

Drying

When the stones are smooth, they need to be carefully dried.  If you have the time, you can leave them to air dry for a few days.  Even then you need to fire them to just below the boiling point of water and soak there for several hours.  Keep the vents open, or the door/lid propped open slightly.

Firing

It will continue to be important to fire up slowly to keep the stone from breaking from thermal shock.  The most rapid expansion of the ceramic is in the 200⁰C to 250⁰C range. This means that the rate of advance of firings should be slow until 250⁰C has been passed, no matter what the glass might survive.

Also look at mullite and corderite shelves.

Revised 23.2.25

Drapes over cylinders

Draping glass over cylinders or similar shapes presents some ordinary problems in a problematic combination.

• ·        In general, the glass is a long rectangle
• ·        The glass is supported on a long thin part of the mould
• ·        The glass is usually high in the kiln
• ·        The mould is heated unevenly
• ·        The material of the mould influences the way the glass is heated
• ·        The characteristics of the glass interacting with the mould material

Narrow glass

Especially in smaller kilns, a long rectangle will receive uneven heat.  The short edges of the glass are nearer the sides of the kiln than the long edges are.  This means that the ends nearest the sides are in relatively cooler parts of the kiln in a top fired kiln.  It is the opposite in a side fired one.

Long thin support

A drape on a cylindrical mould means the glass is supported on only a long thin part of its substance.  This further increases the temperature differential in the glass.   The unsupported glass receives both radiant heat and heat transmitted through the air, allowing the unsupported glass to heat faster than where the glass is in contact with the mould.

Elevated glass

Glass high in the kiln – the effect of placing glass on top of a cylindrical mould – heats more unevenly than on the shelf. 

Uneven mould heating

The mould directly under the glass will be shaded from radiant heat, but will continue to be heated by convection of along the lower sides.

Mould material

The two common mould materials are steel and ceramic.  These gain heat at different rates.  The steel generally heats more quickly. The ceramic is usually thicker, so with a greater mass, and the heat transfers more slowly through the ceramic than an equivalent mass of steel.

Glass characteristics

Glass is a good insulator of heat.  This means that heat transfers to the mould supporting the glass more slowly than through the air.

The question becomes how to overcome or at least alleviate these limitations.

Relatively narrow glass sheets that extend near side elements will heat those narrow edges more quickly than on the long sides.  Top fired kilns often have the opposite problem, as the short sides may be in the cooler part of the kiln. The usual solution is to reduce the rate of advance, or to baffle the hot parts.  Either of these should work well in this circumstance.

The long thin support of the glass creates the problem of a heating differential.  The glass may be in contact with half a centimetre of the mould all along its length. The glass and mould heat at different rates.  The normal solution to this is to slow the rate of advance.  The slower rate of advance can be combined with periodic soaks 100⁰C intervals.

Elevated glass

Glass high in the kiln needs special care, as the heat is more uneven there than most parts of the kiln on the heat up.  A general rule of thumb is that the radiant surface temperature given by the elements evens out at a distance from the elements.  This distance is determined by the distance between the elements.  The radiant temperature evens at a distance that is one half the distance between the elements.  If your elements are 100mm apart, the radiant temperature will only be even 50mm below the element.  Any glass closer than this will require slow schedules to overcome this uneven heating.

Uneven mould heating

As described earlier the mould will be heated by convection current of the hot air, rather than directly the radiant heat from the elements.  To reduce this difference, the rate of advance needs to be slow.

Mould materials

Although there are other materials, steel and ceramic are the most common materials from which moulds are made. Steel gains heat much more quickly than ceramic.  In the forms used for glass draping, ceramic has much more mass to heat than steel.  Steel also transmits the heat more quickly.  This means that a steel mould can give a hot line under the glass, and ceramic a cool line.  Reduction in the rate of advance will assist in overcoming this differential heating.

Scheduling

Experience has shown that a very slow rate of advance to a soak of 20 minutes at 100⁰C will allow the temperature to equalise between the glass and mould.  However, too fast a rise after that will cause thermal shock possibilities.  So, increase the rate of heating by 50% to another 20 minute soak at 300⁰C.  Follow this by a rate twice the initial rate to 500⁰C for another 20 minutes as a precaution.  Then proceed to fire at a normal rate.

These precautions are not necessary on the annealing cool as the glass will be in contact with the mould.

Glass characteristics

Glass is a good insulator, so the heat passing to the mould will be less than through the air.  With steel, this will give a hot line and with ceramics a cool line.  Slowing the rate of advance will help reduce this differential.  Experience has shown that placing a sheet of 1mm fibre paper over the mould will also help to reduce the effect of the temperature differences.  You can place a sheet of Thinfire or Papyrus over the fibre paper to retain as smooth a surface as possible.

Summary

The best defence to the thermal shock of glass on a cylindrical mould is to reduce the rate of advance with periodic soaks to equalise the temperature.  The addition of fibre paper to the cylinder is an added protection against uneven heating from a hot or cold spot on the mould.

But why does the glass break at right angles to the length of the mould?

I have talked of the long thin contact line between the mould and the glass. “Why does the glass not break along the length of the glass?” I hear you ask.

In thermal shock, the break will occur on the line of least resistance.  In these cases that is on the short sides.

Glass Cutting Surfaces

There are several considerations about your surface for cutting glass.

Make sure you are putting the glass on a flat surface. If the surface is uneven, it will give difficulties in scoring and breaking.  This means that large sheet timber is an excellent surface.  These boards need to be securely screwed down to the bench structure to avoid any warping.

There is some advantage to having a slightly cushioned cutting surface. This will help accommodate glass with a lot of texture and those sheets that have slight curves in them. 

In this example the user has placed corrugated cardboard under the glass for cushioning, but with a hard surface underneath

Consider ease of cleaning.  As you score and break glass, small shards will be left on the cutting surface.  The tell-tale squeaks as you move the glass indicate there is other glass under the sheet. These shards and any other small almost invisible things under your glass can promote unwanted breaks. Also, if there is glass or other grit on the surface, it may scratch the glass. So make sure you brush the cutting surface clean frequently.

An example of a ready made cutting bench.  It has the advantage of being easy to clean and compact when not in use. 

Think about the size of sheets you will be cutting.  Large sheets often have minor imperfections in texture, or some bowing.  These benefit from a slightly cushioned surface. It also allows the sheets to be put down onto the surface with more confidence that it will not break in contact with the bench top.  But if you are cutting mostly smaller sheets, they benefit from a smooth hard surface to support the whole of the sheet especially when cutting long thin or curved pieces.

An example of a large cutting bench with composition board top surface

Some of the materials used are sheet boards (such as marine plywood, MDF, and other composition boards), short pile carpets,  thin rubber or foam sheets, dining table protectors and pin boards. 

All these are useful for cutting each with advantages and disadvantages.

• Carpets and foam can trap shards of glass, so have to be cleaned very carefully to avoid retaining sharp glass within the pile or foam.
• Smooth, wipe-able surfaces avoid trapping glass, but can be slippery. Choose one with a non-slip surface.
• A slightly cushioned surface is good for large sheets
• Smaller sheets of glass are best cut on smooth hard surfaces, providing support for all of the glass sheet.

You can also consider, as in the example above, the use of different cutting surfaces on top of the larger smooth and hard surface.  This allows adaptation to the needs of your glass without duplicating surfaces.

Before scoring, clean the glass on both sides, to ensure any sounds you hear when moving the glass relates to glass shards on the bench rather than grit on the glass.  At the very least, clean along the cut line, as this makes the action of the cutter smoother. The grit on the glass actually interrupts the action of the wheel, so you get a staccato effect in the score line.

Soldering Fumes

Exhausting Soldering Fumes

Health and Safety

The health and safety of working with lead and solder are a great concern of many people.  Greg Rawls, the acknowledged expert in glass working health and safety, puts soldering and lead work in perspective.

"Soldering lead came for stained glass does not usually present an inhalation hazard if the area is well ventilated and you are using an iron and not a torch. With normal soldering, you are melting the lead at temperatures that are NOT hot enough to create a fume.

Lead fume is the inhalation exposure issue. Fumes are very small respirable particulates that are made with heat. Liquid chemicals give off vapours.

Avoid exposure by ventilating the area when soldering, especially if using a torch instead of an iron. Open a window and turn on a fan!  Wash your hands thoroughly when finished working with lead. There are specific products for this purpose.

Use a P95 or P100 respirator when concerned about lead exposure." 

http://www.gregorieglass.com/chemicals.html

There are commercially made fume traps which often have an activated charcoal filter and can be effective.  A simple desk top fan blowing away from you can be effective in well ventilated areas, if you are working on your own. (otherwise it blows the smoke toward others.)

An example of a fan drawing fumes away from the person soldering

Making a fan

Exhausting fumes while soldering is a safety issue. If you happen to have an outdoor screened-in studio a simple fix can be had with a computer fan! You can scavenge such a fan from an older used computer ready for disposal. Simply cut four timbers 50mm square or 25mm x 100mm to fit around it as a box. Attach a long electrical cord to it with an approved plug. Attach a screen to both sides. Plug in. An additional feature is to attach an activated charcoal filter (as used for cooker hoods) to the front of the fan. This removes particles and some fumes.

Positioning

Always set a fan to draw fumes away you, generally pointing it so that it is blowing the fumes in the same direction as the larger air flow in the studio. A very large fan doesn't always do the job alone, since the fumes seem to rise and find your nose. However, with an additional small fan sitting right next to where you are currently soldering, the fumes just move away.

Wearing an appropriate dust mask as illustrated by the Bohem Stained Glass Studio is a good practice.

However, the main health hazard is not to lungs, but to eyes.  This blog explains the hazards of flux fumes.

Revised 30.12.24

Needling in Bottle Moulds

Sometimes people experience sharp, needle-like points on the bottle after it is slumped.

Causes

As the bottle expands and softens, it conforms to the surface of the mould.  When the cooling begins, some parts of the glass are trapped in the tiny pits of unevenness that always exist in the mould or in the separator.  As the glass retreats, the glass is stretched until it breaks off, leaving the sharp needles.

Prevention

Remedies relate to separators and temperatures.  This of course, assumes you already have a good coating of kiln wash or similar separator on the mould.

Separators

These additional separators can be fibre paper or powders.  Thinfire laid on the bottom of a bottle mould can provide additional separation between the bottle and the glass.  This works, because with a slow rate of advance, the Thinfire will have turned to powder before the bottle begins to slump. This powder will not interfere with any designs on the mould.  Papyros will work on smooth moulds, but not so well with textured bottle moulds, because of its more fibrous nature.

This use of powered paper indicates that you could use a cheaper solution.  Just dust a fine film of kiln wash on the mould.  I do this by placing the powdered kiln wash in a sock and shake the sock above the mould.  This will allow an almost invisible layer of fine powder to fall onto the mould.  This is enough to provide an additional layer of separation between the glass and the mould.

Temperature

It is quite common for people to slump bottles at tack fusing temperatures to do both the flattening and the slumping at one firing. This is quite hard on the mould and softens the glass enough to promote the needling. 

It may be better to use two firings – one to flatten using tack fusing temperatures, and one to form the bottle at slumping temperature.  This lower temperature will avoid the needling, as the bottle will not soften enough to form the needles during the slumping. The reason many people avoid this is because the bottles tend to devitrify on second firings.  If you do this two-stage slumping, you will need to apply a devitrification solution to the upper surface of the flat bottle to try to prevent it.

You can take a different solution to the two-stage firing.  As lower temperatures reduce the possibility of needling, you can simply soak for a longer time at the slumping temperature than a normal one stage tack and slump.  You will need to peek in at intervals to determine when the slump is finished, of course.  After a few firings though, you will get a good idea of the amount of time required to complete the slump. An additional advantage is that at the lower temperatures, devitrification is less likely.