Thermal Shocking Ceramics

When firing glass in ceramic moulds, and especially ceramic pots for pot melts, you should be aware of the temperatures at which the ceramic material quickly expands and contracts.

There are refractory ceramics which are not as sensitive as the kind of ceramics we are using in most kiln work.  The ceramics we use as moulds are not refractory materials and contain, among other things, quartz and crystobalite. These two elements are important, as they have considerable effect on the survival of the pot or mould during the firing.

The effects are called inversions.  This is because the rapid expansion experienced upon the heating is reversed as rapid contraction on the cooling of the ceramic.

The first element to be affected by the heat up is crystobalite.  This element has a sudden expansion of 2.5% at 226°C.  This does not seem to be much, but compare it to the expansion of glass at this temperature - .0085% - almost 300 times that of glass at the same temperature.  And of course, the ceramic contracts by that amount when it reaches 226°C on the cooling.

The second element affecting the heat up is quartz.  There is quite a bit of this in clay.  The critical temperature for this is in the 570°C to 580°C range.  The expansion and contraction is not so great here – only 1% - but it is still more than 100% that of the glass, and in a critical range for the glass on the cooling.   

More information on the quartz and crystobalite inversions are given here.

The importance of these inversions for us are to remind us to be careful at these temperatures of about 225°C and 570°C - 580°C to prolong the life of the ceramic pots and moulds that we use.  

It is probable that 150°C per hour is as quickly as we should increase the temperature when using ceramic moulds or pots.  Some thought should be given to the cooling of the moulds too.  They should not be taken from the kiln while hot nor subjected to draughts of relatively cold air.

Leading Small Circles

Putting came around small circles such as lenses and small bullions often leaves an irregular curve. There is a way to avoid this.

Use oval or round came to reduce the kinking of the leaves of the came. As there is less material at the edges of the leaves of oval came, there is less kinking than on flat came, where the thickness of the leaves is constant.

Begin to form the lead round the circle, about half way. Then take the circle out of the came and cut, at a right angle to the length of the lead, at an angle from top to bottom. The degree of the angle is not important at this stage, only that you can repeat the angle – so it must be fairly shallow and natural for you.

Put the circle back into the came and continue to form the came round it until you meet the angled cut at the beginning. Again at right angles to the length of the came, cut a repeat of the angle.

Then fold this end toward the other end. Push the two angled ends together. If they slip up and down from each other, the came is too long. Open the came and cut a sliver off.

Try again until they meet with very little “slippage”.

Then the piece is ready to put into the panel. Place the join at a lead joint so you don't have an additional solder spot.

This technique can be used for small ovals too.

Tin Bloom

Using float glass sometimes produces partial clouding as though devitrification were present. Although float glass is prone to devitrification, not all the cloudy film on the surface is due to devitrification.

Float glass, which these days, is almost all clear smooth glass, gets its name from the process of floating the glass on molten tin. The tin in compression gives an apparent devitrification effect which is called tin bloom.

it is different from devitrification, to which float glass is particularly subject. Devitrification sprays and solutions will not have an effect on this surface defect called tin bloom. 

When the tin layer is stretched, it does not create a tin bloom on the surface.  Therefore, it is important to have a means to detect which is the tin surface.  Always fire the glass with the tin in the same relative location to each other.  I.e., on several layers of glass have all the tin side down or all up, but not mixed. 

This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show.  The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.


When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed.  Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.

Leading - Establishing the perimeter

The first thing to be established about the panel is the placing of the came that goes around the edge of the panel.

Fix your cut line cartoon to the work board.  Usually a long strip of masking tape on all the edges will be sufficient.  To establish the placing of the battens, which will form the frame for the leading process, you need to determine the spacing from the cut line.

This shows the initial battens in place and ready for the final two battens to be put in place before soldering.

To determine the size of the off-set of the battens you should cut a short piece of the came you will be using for the outside and use that as a guage.  Place the heart of the came on the outside cut line near one end and move the batten to the side of the came.  Nail that end of the came to the board.  Move the guage came to the other end of the cut line and do the same with the batten as you did for the other end.  Establish one other batten at right angles in the same way.  Then you are ready to place the cames.

Make a straight cut across the came to be used for the outside and put that trimmed end into the corner and along the vertical wood strip. The lead should extend beyond the cut line to accommodate the length of the upper horizontal came. The minimum length must be longer than the width of the perimeter came that will butt against it. If it is even longer, the extra can be trimmed off after the leading is complete or after soldering.


Next butt a trimmed piece of perimeter came along the horizontal wood strip. This one should be shorter than the cartoon. It should be half the width of the perimeter cames to allow the vertical came to butt against it. The reason for having the vertical cames running from bottom to top is that there is a fraction more strength in the heart of the came going all the way to the bottom of the panel, rather than resting on the flanges of the came.

This is how the finished perimeter cames will appear:

These perimeter cames should be held in place with horseshoe nails. Try placing the nails only where a lead line will be soldered in order to cover any nicks the nails might make. Alternatively, you can place the nails at the ends of the perimeter cames to keep them from sliding vertically or horizontally.


If you want to have mitred corners, this post will show you the method.

The next stage of placing the first pieces of glass is shown here.

Leading acute angles

Most of us like flowing lines in leaded glass windows, but these often give very acute angles to be leaded up. One way is to avoid creating intersections by using passing cames.  

But, if the cartoon does not allow for passing cames in acute joints, you have to consider how to cut the came to butt well against the next came. The easiest, but most time-consuming method is as follows:

Determine what the length of the came must be to reach the end of the joint.

Mark your lead there.

Determine what the shortest part of the came will be at the joint and make a faint mark there too.

Cut the came at the first (longest) mark.

Use your lead dykes to cut the heart out of the lead, leaving only the flanges. This is done from the end to just beyond the faint mark you made to indicate the shortest part of the joint.

You then need to smooth the two flanges where the heart was. You can use a fid or your lead knife to draw over the rough interior of the flanges. This enables the flange to be inserted below the came already in place, or to slide the new came over the modified came.

You can trim the upper came flanges immediately to conform to the angle of the joint or do it when the whole panel is leaded. Make a mark with a nail or your lead knife along the edge of the un-modified came. Then raise the flange and use your lead dykes to cut the flange along the line. Fold the flange down to butt against the passing lead and it is ready to solder.

False Lines in Leaded Glass

False lines are used in leaded glass where the design calls for an angle that cannot be cut into the glass. This includes right angles and even more acute angles. E.g., the petals of a fuchsia flower. 

The design would call for an angle of about 60 degrees. This is impossible to achieve through hand cutting. So the glass is cut in a curve and the cames on the side and bottom of the petal have their hearts cut out so they overlap each other. 

In the example above, the red petal points would be cut rounded, so that the clear glass below can be rounded as well.  The came or foil is extended beyond the glass to give the visual points required.

The overlap is then trimmed to the shape of the outside of the petal. When soldered, the appearance is of the glass being cut at the angle required for the flower.

At other times, the requirement is for a line to go into a piece of glass, but not all the way across. As in this stained glass panel by Justin Behnke.  The hanging lines are those on the lower left of the panel, giving a great flow to the whole.

Again you cut the heart out of the came, and overlay the smoothed lead onto the glass. You can use just a little silicone to hold the lead in place until you finish cementing. After this you can lift the piece of came and use silicone or epoxy resin to firmly attach the came to the glass. You do not want to do this before cementing as any excess of glue will be made dirty by the cementing process and be very difficult to clean up.


There are also times when you may want to have a silhouette, you can cut it out of lead foil and solder it into place. This allows intricate shapes to be made when a dark representation of the shape is required. If the panel can be seen from both sides, the overlays should also be on both sides. These should be glued to the glass just as for cames.

Further information on removing the heart of lead came are given in this post on leading of acute angles.

These principles can be applied to copper foil too.

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