Cordierite/Mullite vs. pizza stones or tiles

Description of the materials

Cordierite refractory shelves are generally combined with mullite to achieve low expansion rates.  These are most often manufactured as solid slabs, although there is an extruded version with hollow channels along the length, given the trade name corelite.

Cordierite is magnesium, iron and aluminium in a cyclosilicate form (or rings of tetrahedra).  It is named after its discoverer, Louis Cordier, who identified it in 1813.

cordierite/mullite shelves

Mullite is combined with cordierite in small amounts to increase strength and reduce the amount of expansion. It does this through the formation of needle shapes that interlock and resist thermal shock. It also provides mechanical strength.

Mullite was first described in 1924 and named for an occurrence on the Isle of Mull, Scotland, although it occurs elsewhere, usually in conjunction with volcanic deposits.   

Pizza Stones and Tiles

Pizza stones are a variant of baking stones where the food is placed on (sometimes heated) stones.  Baking stones are a variation on hot stone cooking, one of the oldest cooking techniques. The stones are normally unglazed tiles of varying thicknesses.  What is said of pizza stones also applies to tiles.

Characteristics

Pizza stones  

Ceramic tiles and pizza stones are essentially the same things.  Some tiles may be thinner, especially if they are not large. In both cases, the ceramic is a poor heat conductor and the thermal mass means care needs to be taken in rapid heating and cooling of tiles and of baking stones. These are dry pressed which give a coarser surface texture than cast shelves.  All these ceramics are generally fired at about 1100C, so they can withstand kiln forming temperatures.  They are adequate as small shelves, but will deform over larger areas over time.

Cordierite-Mullite kiln shelves and furniture.

This formulation of materials has an extremely low coefficient of thermal expansion that explains the outstanding thermal shock resistance of these kiln furniture materials. They are also strong although heavy. Cordierite/mullite shelves are sintered, to allow the mullite needles to form, and fired at 1400C+, higher than tiles (which are most often fired at about 1100C).

This material can be cast, dry pressed or extruded. 

Cast shelves are the cheapest of the methods and provides a smooth surface.  These are used for kilnforming glass, and low temperature ceramic firing. 

Dry pressed shelves have a higher temperature resistance than cast. For this reason, these are often marketed as ceramic shelves, even though the cast shelves are fine for smaller areas.  These are more expensive than the cast shelves.

Corelite, a brand name for extruded shelves with hollow channels, is often used where larger shelves are required, as the weight is less than the solid cordierite. Extruded shelves are ground smooth after forming.

pizza stones

Preparation

Pizza Stones and Tiles

Due to the thermal mass of pizza stones and the material's property as a poor heat conductor, care must be taken when firing.  Firing quickly can break the stone or tile.  The stone or tile should be fired slowly to just under the boiling point and soaked for a couple of hours to eliminate any dampness in the material.  This probably should be done each time kiln wash is applied.  Because it is porous, a baking stone or tile will absorb any liquid applied, including detergent. They should be cleaned with a dry brush and then plain water if further cleaning is necessary.

Pizza stones and tiles should be checked for having straight and level surfaces. It is not a priority for these to have flat surfaces as for glass and ceramics shelves.  If by placing a straight edge on the surface you can see slivers of light, the shelf needs to be smoothed.  You can do this by grinding two of the proposed shelves together with a bit of coarse grit between.  This best done wet to avoid the dust getting into the air.

Cordierite

Cordierite/mullite shelves do not need this level of preparation, unless they have been stored outside.  It is possible to kiln wash and air dry for a few hours before placing glass on the shelf and firing.  This difference is the low rate of expansion (CoLE 19, if you are interested).

corelite shelves

Corelite

The extruded corelite shelves are made with cordierite/mullite, but are more delicate due to the hollow channels along their length.  They should be fired slowly to just under the boiling point of water to eliminate the moisture.  It should be fired to 540C with a pause before going to the top temperature.  The shelf should be supported at 30cm intervals under the shelf to minimise breakage.  The whole surface of the shelf should be filled rather than having just one heavy piece; again this is to minimise breakage.

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.

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.

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 shrds 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 the best solution.

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.

Foiling Tight Curves

Foiling tight curves often leads to splits in the foil.  This note gives information on how to generally avoid the splits and a repair method when they do occur.

Make sure the piece of glass is free of dust and oil and that there are no sharp ledges. Find the flattest part of the glass piece to begin the foiling.

To get the foil to stick to the curve and fold over onto the glass with the minimum of tears, use your fingers first to burnish the sides of the foil onto the piece of glass beginning on the outside curve.

As you move to the inside curved portion gently and progressively ease the edge of the foil with your finger toward the surface of the glass. Easing and burnishing one side of the curve at a time will give you better results toward getting the foil to stretch onto the glass without tearing the foil.

Finish by burnishing the foil down using a fid. This helps keep the foil firmly adhered to glass through the soldering.

Dealing with splits
Tears in the foil line happen. Clean up the broken foil lines with a craft knife and the solder line will look nicer when project is finished. You can also patch the tears by placing a small section of foil over the broken foil.  Place the scrap on one side, burnish it, and fold it onto the edge.  Ease the foil onto the second surface and burnish all the surfaces again.  Then trim the scraps that extend beyond the rest of the foil with a craft knife.

Foiling and Soldering Small Pieces

There are several approaches to dealing with small pieces in copperfoiling:

No-foil approach
One approach is to have some of the pieces held in place by over-beaded solder without foil on the tiny piece, but it is patchy at best and likely to lose pieces in the long term.

Bevel approach
A very good and strong approach is to partially 'bevel' the edges of each piece on both faces. Grind at 45 degrees until the very edge is only 1 mm thick. Then use foil that is 4 mm wide for 3mm thick glass. For 4 mm glass, you will use 5.4 mm foil. Make sure that the foil covers only the bevelled edges and does not extend outside them.

Solder into the 'V' formed by the bevelled edges. Don't over-fill the joints as you don't want solder outside the 'V'. It also is best if the panel is supported underneath the area being soldered by a wet sponge to more quickly cool the solder.

With the solder contained by the 'V', the solder lines will be of constant width throughout the piece. Best to practice this technique on some scraps before you start the main job.

This approach will minimise the amount of light blocked by the foil - important with tiny pieces - while still providing the strength of fully foiled pieces.

Triming approach
If you have to have really small pieces, just foil them as you would any other piece, and burnish it as normal. Then take a very sharp craft knife (Exacto or similar) and trim the foil so that just a little tiny bit of foil is on the front and back of the piece.

No glass approach
Tiny pieces are really tedious to work with. So if the piece is going to be black or really dark, for example a small hummingbird's beak, or a bird’s eye, don't bother with glass but just fill the space with foil and solder.

Lead free Solder

There are some problems to overcome when using lead free solders. 

One is that all, except for expensive compositions, lead-free solders have a higher melting temperature than tin/lead compositions.  The table in this link shows the melting temperatures.

Most lead-free solders have a wide pasty range, so careful attention needs to be paid when selecting the composition, if you want a eutectic, or nearly so, solder.

Some eutectic solders are:

65% tin, 25% silver with a eutectic temperature of 233C.  It is known as “Alloy J” and patented by Motorolla.

99.3% tin, 0.7% copper has a eutectic temperature of 227C. It is expensive.

96.5% tin, 3.5% silver has a eutectic temperature of 221C.  This is slightly lower than the tin/copper composition but more expensive.  It is also likely to rob copper from the soldering bit, although it is easier to solder with as it has excellent wetting properties.

Lower eutectic temperature solders are available:

91% tin, 9% zinc has a eutectic temperature of 199C.  It corrodes easily and has a high level of dross.  This makes it a poor choice for copper foil work.

42% tin, 58% bismuth has a low eutectic temperature of 138C.  It is a well-established solder, but it is expensive.

48% tin, 52% indium has the lowest eutectic temperature of 118C, but it is very expensive.

Copper bearing solders

Another problem is that a solder without lead, robs copper from the soldering bit/tip, and even more so at the higher temperatures lead-free solders normally require.  One means of avoiding the rapid deterioration of the soldering bit is to use solder with a small amount of copper included in the composition. As little as 0.5% can be useful.  Normally, nothing greater than 1% is required to extend the life of the soldering bit.

Eutectic copper bearing solder

However, only one of the commonly available solders is eutectic. This is 99.3% tin and 0.7% copper with a melting temperature of 227C.

Copper bearing solders and pasty ranges

Other copper bearing solders are available. Most of them have high temperatures and wide pasty ranges making them less useful for copper foil work.

  

Near eutectic solders

97.25% tin, 2% Silver, 0.75% copper has a small pasty range of 217C – 219C, making it a nearly eutectic solder and suitable for copper foil, except for its high melting temperature.

91.8% tin, 3.2% Silver, 0.5% copper has a pasty range of 217 – 218C, also making it a near eutectic solder suitable for copper foil; again, except for its high melting temperature.  With its high silver content, the solder is expensive.

95.5% tin, 3.8% silver, 0.7% copper has a pasty range of 217-220C.  This also has a small pasty range, but may be similar in cost to the 91.8% tin composition.

95.5% tin, 4% silver, 0.5% copper has a pasty range of 217 – 225C.

95.5% tin, 4% silver, 1% copper has a smaller pasty range of 217 – 220C, but may be more expensive.

Other copper bearing solders 

94.6% tin, 4.7% silver, 1.7% copper has a wide pasty range of 217 – 244C.

96.2% tin, 2.5% silver, 0.8% copper, 0.5% antimony has a

smaller pasty range of 217 – 225C and may be slightly cheaper because of the reduced silver content.

  

95.5% tin, 4% Copper, 0.5% Silver has a pasty range of 217 – 350C and is the usual lead-free plumbing solder.  The high melting temperature of 350C makes it unsuitable for most copper foil applications.

97% tin, 0.2% silver, 2% copper, 0.8% antimony has a high melting temperature and wide pasty range of 287 – 318C., which makes it unsuitable for copper foil.  It is known as “Aquabond”. 

95.5% tin, 4% silver, 0.5% copper has a pasty range of 217 – 225C.

95.5% tin, 4% silver, 1% copper has a smaller pasty range of 217 – 220C, but may be more expensive.

94.6% tin, 4.7% silver, 1.7% copper has a wide pasty range of 217 – 244C.

96.2% tin, 2.5% silver, 0.8% copper, 0.5% antimony has a

smaller pasty range of 217 – 225C and may be slightly cheaper because of the reduced silver content.

Lower temperature copper bearing solders

94.25% tin, 2% silver, 3% bismuth, 0.75% copper has a pasty range of 205 – 217 which is smaller than many of the other copper bearing solders.

90.7% tin, 3.5% silver, 5% bismuth, 0.7% copper, with a pasty range of 198 – 213C, has a lower melting point than many other copper bearing solders.

93.4% tin, 2% silver, 4% bismuth, 0.5% copper, 0.1% germanium has a relatively small pasty range of 202 – 217C, but because of the incorporation of rare earth metals may be expensive.

Cleaning the Kiln of Dust

Dust is promoter of devitrification. You should do the most you can to keep your kiln free of dust.

Dust can come from the kiln lining materials.  Regular gentle vacuuming of the kiln surfaces will help prevent particles from falling on to you work or other surfaces in the kiln.

It can come from the separators you put in the kiln.  I often see pictures of used fibre paper at the side, or under, the kiln shelf.  This should be cleaned out after each use to provide clean firing conditions.

The main reason for this obsessive cleaning is that dust particles within the kiln will be disturbed by the air movement involved in closing or opening the kiln lid or door. There also is air circulation within the kiln during the heating and cooling phases, although it is not as much as when opening the door/lid.  These disturbed dust particles will settle on the glass and defeat your cleaning of the glass.  

Eutectic Solder

This a term for solder which becomes liquid and solid at the same temperature.  How is this possible?

An explanation is given by Wikipedia:

" … each pure component [of a homogeneous mix of materials] has its own distinct bulk lattice arrangement. It is only in this atomic/molecular ratio that the eutectic system melts as a whole, at a specific temperature (the eutectic temperature) the super-lattice releasing at once all its components into a liquid mixture. The eutectic temperature is the lowest possible melting temperature over all the [possible] mixing ratios for the involved component species.

Upon heating any other mixture ratio, and reaching the eutectic temperature, … one component's lattice will melt first, while the temperature of the mixture has to further increase for (all) the other component lattice(s) to melt. Conversely, as a non-eutectic mixture cools down, each mixture's component will solidify (form its lattice) at a distinct temperature, until all material is solid."

[https://en.wikipedia.org/wiki/Eutectic_system]

When soldering with 63/37 solder, the solder is heated above its melting (liquidus) point and so remains liquid for a short time until is reaches its solidification temperature.  The important element is that this is the lowest temperature that a mixture of materials can melt.  In the case of lead/tin solder, it 183C.  Other solders have different eutectic temperatures, e.g., a 96.3% tin and 3.7% silver solder has an eutectic point of 221C.

Slumping an Unknown Shaped Glass

A request for suggestions on how to slump found glass that had been shaped by some method was received. The request included a schedule for flattening - open side down – in a mould.

My response:

I would not attempt to do both the actions in one step. Flatten first, slump second. 

Before you start the flattening, clean it well, as any dirt trapped will be permanently imbedded.

During the slumping onto a flat surface, watch to see when it slumps during the flattening. When the form definitely begins deforming, note that temperature. The rate of advance should be moderate – no more than 150C per hour.

Observe the progress of the slumping.  When it begins to deform and change shape this will give you the slumping temperature. Record this temperature as this will be the temperature at which to conduct the slumping of the flattened form.

The temperature at which the deformation begins, minus 40C, can be taken as the middle of the annealing range. This will give you an idea of the annealing temperature as this method is not exact, but good enough to get an adequate anneal.  You can begin your annealing at this temperature without worry of it being too high.

Annealing Point and Range

A question has been asked about whether the statement that “annealing longer never hurts” is true.

To understand why this statement is not always true, you need to be aware that annealing is not just the soak at the stated annealing point.

The annealing point has a mathematical description, but in lay terms it is the temperature at which the stresses in the glass are most quickly relieved.  Annealing at this point is only possible in large industrial processes.  It is reported that float glass manufacturers can anneal glass in 15 minutes because of excellent temperature control in their lehrs.  For those of us who do batch annealing such speed and accuracy is not achievable.

As we cannot achieve such accuracy with our kilns, annealing for kiln formers consists of a temperature equalisation soak at the annealing point and then slow cooling through the lower strain point.  That is the point where the glass becomes so stiff that no further annealing is possible. 

Most kilns have relatively cool areas.  They mainly are in the corners and at the front of top hat or front-loading kilns.  You should know where these cool spots are.  They can be checked for by a simple test as described in Bullseye Technote 1.   This will enable you to know if and where any cool spots may be.  In smaller pieces, you can just avoid those areas in the placing of your pieces.

Annealing of large pieces, parts of which must be in the cool areas, is possible.  But not with excessively long anneal soaks.  If the kiln has temperature differentials, a long soak will impose those variations in temperature upon the glass. This means that the glass will begin its annealing cool with variations in temperature across the piece.

During the anneal cooling, research at Bullseye Glass Company has shown that to achieve as stress free a piece of glass as possible, the temperature variation across and through the piece should not vary more than 5°C. This is relatively difficult to achieve if you have cool areas in your kiln.  But it is possible.

To alleviate the possible difficulties of temperature variations in the kiln, the anneal soak should not be extended beyond that recommended by its thickness.  What should be extended is the anneal cool. The rate of cooling should be slowed to the rate for a piece at least twice the thickness of the current piece.

If it is a tack fused piece, this reduction should be for a piece four times the thickness of the thickest piece you are annealing.

The conclusion is that it is possible to anneal too long, if the piece is large and the heat in the kiln is not uniform. If you are concerned, remember that the soak at annealing point is to equalise the temperature throughout the substance of the piece. The annealing cool - the first 110 degrees Celsius - is very important. If you are concerned, it is best slow that rate of decrease dramatically. This provides a safer option for an adequate annealing of large pieces.

Flat Kiln Shelves

A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon.  Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.

It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.

A magnified view of a shelf surface that is not perfectly even

You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler.  The light areas are the places where the surface is lower than the rest.  If these are few and small you can make corrections in the surface of the tile by grinding.

You can make sure they are flat by putting two tiles back to back and grinding them together. The initial grind will show you the high spots as they will have the grinding marks there. 

You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, scribble with a paint marker over one and let it dry.  Then add grit between to grind again. When all the paint marks have come off they are both flat on the back.

This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles.  This also works for larger kiln shelves.

Preventing Devitrification on Cut Edges

“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”

Devitrification occurs where there are differences in the surface.  This means that the surfaces exposed to the heat must be both clean and smooth.  It is not enough for only one of these to be the case, both are required.

First, the sawn edges need to be clean.  A good scrub with a stiff bristle brush is essential.

Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.

Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.

1 – Grind

This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap.  The grinding should go down to at least 400 grit before cleaning and arranging to fire.

2 – Clear glass

This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface.  You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.

Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead.  In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.

Composition of Glass

Glass can do most anything. From bottles to spacecraft windows, glass products include three types of materials:

• Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.)
• Fluxes help formers to melt at lower temperatures.
• Stabilisers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart.

Chemical composition determines what a glass can do. There are many thousands of glass compositions and new ones are being developed every day.

Formers
Most commercial glass is made with sand that contains the most common former, Silica. Other formers include:

• Anhydrous Boric Acid
• Anhydrous Phosphoric Acid

Fluxes
But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F). So fluxes are required. Fluxes let the former melt more readily and at lower temperatures (1300°C, or 2370°F). These include:

• Soda Ash
• Potash
• Lithium Carbonate

Stabilisers
Fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. So stabilizers need to be added. Stabilisers are added to make the glass uniform and keep its special structure intact. These include:

• Limestone
• Litharge
• Alumina
• Magnesia
• Barium Carbonate
• Strontium Carbonate
• Zinc Oxide
• Zirconia

Based on an article from the Corning Museum of Glass

Float Glass

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

More information on float glass in the kiln is here.