Glass Stuck to Element

First consideration you need to think about when you discover glass stuck to an element is the nature of the metal of the elements.  Once fired, kiln elements become brittle.  This means that they are likely to break if disturbed when cold.  So, you need to make sure you absolutely must do something to rescue the kiln.  It may be that you can just leave the stuck glass alone.  Where the glass is, and how much of it, is stuck to the elements is important when considering what to do.

Where

This brittleness of the elements means that the location of the glass in relation to your firings needs to be considered.  If the glass is on an element below your normal firing position, you can think about just leaving it.  This applies to glass stuck to the side elements too, unless you are in the habit of firing very close to the side elements. The heating elements of the kiln form an external layer of oxidisation that protects the inner metal.  This means that small amounts of glass will not affect the operation of the elements, nor your future pieces.

If the glass is stuck to top elements, you are likely to be more concerned about future drips of the glass onto your future work.  The glass is not likely to become hot enough to detach or drip onto your work except at extended full fuse or casting temperatures.  This means that you can observe the progress of any possible drip at each firing and only remove the glass when it begins to begin to hang down from the element.

How Bad

How much glass is stuck to the element?  Normally, if it is only a small amount, it can be left.  Ceramics kilns often have a bit of glaze (a glass carrier of the colour) stuck to the elements and continue to be fired for years without damage.

If there is a lot of glass stuck to the elements you will need to remove most of it to avoid dripping onto future work. 

Methods of Removing

In most cases where there are significant amounts of glass stuck to the element, it is on the brick or fiber lining of the kiln too. 

My recommendation is to heat the glass just a few centimetres from where it is attached to the element. Use a hand-held blow torch to do this. When the glass is red hot - enough to begin moving - you can pull it away between the lining and the element with long handled tweezers.  Do not attempt to pull it off the element right away.  You can later chip the glass off the lining without damaging the element as the connection is separated.

As the element has begun to be warmed by the heat used to separate the glass on the lining and the element, you can continue to warm the element, moving the torch in a slow waving motion at least 10cm each side of the stuck glass.  When the glass and element are red hot, you can begin to pull the glass off with long handled tweezers, bit by bit.  Keep re-heating the element and glass as much as necessary so the temperature does not drop below cherry red.  This ensures the elements continue to be flexible and will not break.

Of course, glass can be melted onto its kiln furniture and there are different considerations for those circumstances.

Holes vs. Elevation of Moulds

Drilling holes and raising the mould are not the same. They achieve different things.  Drilling holes allows air out from between the mould and the glass.  

There are some things you need to check about the vent holes in moulds.

            Are the holes in the mould at last touchdown point(s)?

Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch the surface of the mould.  On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch. 

 On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.

The vent holes in this could also be at the other two angles in addition to those at the top and bottom of the picture.

Are there holes in the side of mould to allow air out from under the mould?

If there is one or more, there is no need to elevate the mould.  The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould. But you can be safe by elevating the mould on pieces of 1mm or 3mm fibre paper.

This mould has side vents, although the holes at the base may be a little large.

Are the holes clear?

This is more important.  If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air.  The vents need to be checked on each firing to ensure they are open.

  

Does the mould need holes at all?

There are shallow slumpers and other simple moulds - such as a wave mould or any cylindrical mould form - that do not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.

More information can be found in this and related blog posts.

Large thick bubbles at the bottom of the glass

Not all large bubbles at the bottom are the result of the lack of holes.  Sometimes they are the results of too fast or too high a firing. Some notes on this are given in this blog entry. 

What does elevating the mould do?

The purpose of elevation is not allow air to escape from under the glass, although that may be a by-product.  Elevating the mould allows marginally more even cooling of the mould and glass if it is on a thick kiln shelf. It will not create any problems, but you need to be careful about how near the elements it will place the glass.  The elevation does not need to be more than 25mm, just as for the shelf above the floor of the kiln.

Baffles in Side-Fired Kilns

The object of using baffles in side fired kilns is to keep the direct radiant heat from the edges of the piece(s) being fired.  If the edges receive direct radiant heat, they increase in temperature more rapidly than the interior of the piece.  This means the edges become sticky and seal before upper layer of the interior begins to conform to the lower layer.  This seals air into the piece.

fusedglass.org

The materials and placing of the baffles is important.

Baffles can be made from almost anything that can withstand the heat of the firing.  There is an argument that light-weight materials such as fibre board, vermiculite board, or fibre paper should be used to reduce electricity costs.  Heavier pieces such as brick and kiln shelf pieces require more energy to heat them up.  They then of course, store heat that needs to be released on cooling, so slowing the cool down and increasing the risk of devitrification.

The placing of the baffles is important too.  Baffles protect the glass edges from radiant heat until the general heat of the kiln can come into effect over the whole of the piece.  This means that if the baffles are placed against the elements at shelf level, the element above can still give radiant heat to the edges.  Therefore, baffles placed near the glass are better.  They protect the edges from radiant heat at whatever level the side elements are placed.  This is more important for pieces that are further from the edge of the shelf, than those nearer the edge, as the centrally placed glass can “see” the radiant heat from the upper elements. 

Elevation of Moulds - Kiln Forming Myths – 15

All moulds must be elevated to allow air out from between the glass and mould.

This is not a big problem, as it will not create any problems, but it does show a lack of thinking about the mould itself. 

There are some things you need to check.

Are the holes in the mould at last touchdown point(s)?  Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch down in the mould.  On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch.  On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.

Are there holes in the side of mould to allow air out from under the mould?  If there is one or more, there is no need to elevate the mould.  The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould. 

Are the holes clear?  This is more important.  If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air.  The vents need to be checked on each firing to ensure they are open.

Does the mould need holes at all? There are a number of shallow slumpers and other simple moulds, such as a wave mould or any cylindrical mould form, that to not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.

Thinning a Melt

There are two basic methods, both use gravity, but one uses additional weight.

Gravity

In this you take advantage of the forces of gravity and the fact that heat reduces the viscosity of glass.  The universal belief is that glass tends towards 6-7mm thick. Yes it does, but only under the times and temperatures we give during fusing.  Those who have seen the results of relay stuck on for hours will know that glass will become thinner than that. A kiln stuck at 1200C for several hours will produce glass that is less than 3mm thick, although stuck to the shelf.

The practical approach is to give the glass plenty of heat work by reducing the usual rate from bubble squeeze to top temperature.  Also increase the top temperature, and give the glass time to flow as it moves slowly.

If your melt is 12mm at the centre and 6mm at the edge you need to take that difference into account when setting the initial rate of advance. A rate of about 90C/hour up to the softening point should be slow enough to avoid thermal shock.  You do not need to hurry from there onwards, because the glass needs to be hot throughout to move easily.  A rate of 200C, or less, per hour would be fast enough.  The top temperature should be set around 810C and for at least half an hour, perhaps an hour depending on the diameter of the piece.  Periodic observation is advisable.  When the reflections seem fairly straight from one edge to the other, it is as flat as it will get using this process.

Anneal for a piece of 12mm, even though the piece is no longer that thickness, because the glass has been through a high temperature process and the compatibility of some of the glasses may be a little less than originally.

Note that this process should be done on a kiln washed shelf.  Thinfire or papyrus will get caught up in the moving glass.  The coarser fibre papers will inhibit the flow of the glass.  You need to expect to do considerable cleaning of the glass afterwards.

Pressing

The other method is to use weight above the glass to thin it more quickly and certainly to the desired thickness.  Place a kiln washed shelf with the kiln wash facing toward the glass.  The weight of the shelf above presses the glass outwards more evenly than a free flow will.

Put solid spacers of the thickness you want the glass to become.  Remember that ceramic fibre used as spacers will thin when the binder has burned away. So, a 6mm stack of ceramic fibre paper will be less than that at the end of the firing.  The larger the pieces of fibre paper you can use, the less the effect will be, as the weight of the shelf will be distributed over a wider area. 

The same kind of firing schedule can be used on the way up as in the gravity only method, but you need to approach the annealing differently.  With two shelves and the glass between, you should be thinking of annealing for something in the region of 25mm. 

Do not do this pressing on top of your normal shelf, as the temperature differential between the exposed shelf and the part of the shelf covered with 12mm of glass and 15mm of shelf will be pretty large, leading to thermal shocking of the shelf. 

Stainless Steel Preparation

Preparing stainless steel rods and moulds for kiln work is done slightly differently from ceramic moulds.

Just to ensure that the steel is of the right grade, I fire it in the kiln to about 720C. This ensures that if the steel is not adequate for the high temperature work, you will find out that it spalls before the glass is put on top. It also has the advantage of removing any dirt and oils on the surface of the metal.

The separator that you need to put on the steel can be done cold if you use MR97 or other boron nitride coating. Its main advantage is that it can be put on cold and also that it has a very smooth surface. This should be put on thinly, or it will come off onto the glass.

You can also put standard kiln wash on the metal. The metal needs to be dry and clean. It could be sandblasted if desired for a bit of extra “tooth”, but is not normally necessary. Heat the metal to about 120C – 150C in the kiln. Remove it from the kiln with tongs or similar thing to grasp the hot metal. Spray or paint the kiln wash solution onto the hot metal. Return it to the kiln as necessary until you have a coating all over the metal. It does not have to be even all over, but noes need to have all of the metal covered.

If the kiln wash boils off the metal, it is too hot. So turn the kiln down a bit.

If the kiln wash runs off without sticking at all, the metal is not hot enough and needs to be returned to the kiln to heat up.

It is best to avoid applying the kiln wash to the metal in the kiln, as water and the hot elements do not mix well.

Screens for Melts

You can buy various stainless steel screens such as barbecue grids for supporting glass melts. The grids need to be of stainless steel. Type 304 is the most common, but there are other grades which work at high temperatures too [link to stainless steels]

You can make your own grid as Cynthia Morgan does. This provides a more flexible arrangement for various effects. 

Instead of imbedding the rods into the brick, you could also place them on top. Place a kiln brick or other kiln furniture on the ends of the rods to secure the metal from moving. Then you can put the glass on top of the rods without them shifting as the glass is placed.

Stainless Steel for Kiln Uses

The reason for using stainless steel is that it differs from carbon steel by the amount of chromium present and reduces the spalling. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the greater volume of the iron oxide this tends to flake and fall away (spall).

Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure, and due to the similar size of the steel and oxide ions they bond very strongly and remain attached to the surface.

There are a number of grades of stainless steel. Some of the ones that perform better in hot conditions are:

300 Series—austenitic chromium-nickel alloys. Austenitic steels have a cubic crystal structure. Austenite steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the extremely cold to the melting point of the alloy.

Type 304—the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506 (not to be confused with A2 tool steel).

Type 304L—same as the 304 grade but lower carbon content to increase weldability. Is slightly weaker than 304.

Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.

Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.

Type 316—the second most common grade (after 304); for food and surgical uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304.

Type 316L—is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications, as well exclusively in the fabrication of reactor pressure vessels for boiling water reactors, due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.

Type 316Ti—variant of type 316 that includes titanium for heat resistance. It is used in flexible chimney liners.

Type 321—similar to 304 but lower risk of weld decay due to addition of titanium.

400 Series—ferritic and martensitic chromium alloys

Type 439—ferritic grade, used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.

Type 446—For elevated temperature service

500 Series—heat-resisting chromium alloys

Based on Wikipedia 

First Firing

Even if this is not your first kiln there are a number of things to do when starting.

The first is to read the manual. Obvious, but in our enthusiasm to get started, reading seems boring. It is essential to understand what the manufacturer wants you to do and to understand how the kiln and controller work.

Then, you can prepare kiln. Test fire the kiln empty to make sure it works and burn out any binders remaining in the kiln materials. You can do this firing at about 400C/hour as there is no glass to damage. Fire to around 800C. Then you can shut off, or programme a dummy anneal. This also ensures you know how to work the controller.

While waiting for the kiln to complete the fast test firing, read the manual again.

When the kiln is cool, apply kiln wash to the bottom (if it is brick) and lower sides, below any side elements. If the bottom or sides are fibre, no kiln wash is required. Kiln wash the shelf and any kiln furniture too.

Now is the time to test for how even the heat is in your kiln. Arrange the kiln furniture around the shelf as described and put glass over. The kiln furniture can be any refactory material, even folded 3 mm fibre paper will be strong enough to hold the small pieces of glass above the shelf. Fire the kiln as described in the Tech Note 1.

Now try out the suggestions in the manual, especially the programming of the controller, even if it has pre-programmed schedules. Look at Bullseye and Spectrum sites to get sample schedules. Enter these as trial schedules.

Then you should be ready to fire the first piece of glass. Place the glass in the kiln, programme it, and record the information about the firing. Now turn the kiln on for the first real fuse.

Black Specks in Mesh Melts

The first time you use a mesh for a melt, it doesn't spall until it cools. By that time, the glass has hardened enough that any black specks of metallic oxidisation just land on the top of the melt and can be brushed away.

But, once a mesh has been fired previously, it can spall and drop little bits at any time during the firing process, so some of the bits get embedded in the glass.

The only way I have found to prevent this is to sandblast the mesh between firings to remove any loose flakes of metal. This is time consuming enough that you may wish to use a new piece of mesh for each melt. The alternative is to ensure you are using stainless steel as the grid.

There are several options for grids.

Kiln maintenance

Before or after each use

Vacuum the inside of the kiln. Use a low suction setting, especially on fibre walls and ceilings. Stronger suction is possible when cleaning the brick floor.

Check on the kiln furniture – including shelves, boards, supports. Are they kiln washed and without scrapes, scratches, gaps? Has the kiln wash been fired to full fuse temperature?. In both cases, clean the used kiln wash off the shelf and renew.

Check that the shelves and other kiln furniture are without cracks.

Clean kiln furniture of dust and debris.

Check the level of any item placed in the kiln, e.g., mould, with a spirit level.

Example of a small 2-way spirit level

Monthly

Electrical parts: check the elements and their connections (normally at back or side). The screws on the connectors for the element tails should be tight. If they are badly corroded , they need to be replaced.

Any support pins or wires should be firmly seated in the brick work or supported by sound hangers.

Check the level of the kiln and internal shelves on a a regular basis and every time the kiln and its internal furniture is moved.

Supporting Overhangs on Moulds

In general, the blank should be no larger than the thickness of the glass over the mould. So a 6mm blank would have no more than 6mm overhang.

In the case of steep sided moulds, the glass should be entirely within the mould to avoid any hangup on the edge, leading to uneven slumps and needling on the edges.

But, if you need the glass to be the size of the mould, you can make a collar to go around the mould, which will support the glass while it begins to slump into the mould.

Make a donut shape that will fit around the mould (whether round, oval or rectangular) and extend beyond. Support the collar on kiln furniture to be as high or slightly higher than the top of the rim of the mould. This makes a kind of drop out ring, allowing the glass to drop into the mould.

Donut ring suitable for placing around a circular mould

This arrangement is suitable for placing around a mould of the same diameter as the interior of the ring

Make sure that the collar is well covered with kiln wash to ensure the glass can move along the fibre board. This includes both the surface and vertical edges of the collar.

As the glass softens and begins to fall into the mould, the glass at the edge does not have the weight to bend down and so raises off the collar and begins to slip into the mould.

And finally, you need to ensure that the mould is not so steep as to trap the glass inside. This is more of a concern on steel with its greater expansion and contraction than ceramic.

A steel mould likely to trap the glass inside with its vertical sides

Why do kiln shelves break?

Kiln shelves are made of clay – a very hard clay, sometimes called mullite. So when firing you need to remember that like other ceramic materials it can be heat shocked.

examples of broken shelves

The recommendation is that you put the shelf on supports to keep it above the base of the kiln and allow air to circulate around both the top and bottom of the shelf.

The question remains, why do the shelves break. There are at least two reasons: physical impact and thermal shock. It is possible to knock the shelf while moving it around the studio. This impact does not always cause a break, but sometimes creates a stress point that later can develop into a crack and break. You can sometimes see the start of the crack from the edge of the shelf. In this case, you can either continue to use the shelf with support under the crack or dispose of it immediately, because at some point during a firing it will separate.

The thermal shock that causes the break occurs because (usually) uneven cooling. It seems the shelves are pretty resistant to rapid heating, but less tolerant of rapid or uneven cooling. In general un-dammed fusing and using moulds elevated a little from the shelve do not create that uneven cooling.

However placing a large refractory mould directly on the shelf can promote cracking either immediately or on subsequent firings.

Broken shelf with casting moulds laded to one side 

The main culprit in any breakage seems to be large or heavy and damp refractory moulds directly on the shelf. The mould is giving off water vapour which cools the immediate area around the mould. So as the temperature rises, the covered part of the shelf stays cool, in addition to being shielded from the general heat of the kiln while the uncovered parts of the shelf rise in heat. At some point the temperature differences in the shelf are too great for its strength to resist. The solution is to remove the shelf from the kiln and place the mould, slightly raised, on the floor of the kiln. The bricks, being softer, do not react in the same way as shelves to uneven heating.

For thick fusing with dams all around, it seems best to do this on a shelf that almost fully covered with glass and dams. This promotes more even heating and cooling of the shelf than having a small part of the shelf covered. It does mean having different sized shelves, but then you may already have some of them due to the breaking of other shelves. Just cut the broken shelf to the size you want on a tile cutter.

Placement of Pieces for Firing

Placing pieces in the kiln, especially in oval and side fired kilns, is not about filling the kiln completely. Kilns have hot and cold spots, and the arrangement of the elements can have an effect too.

The first thing to determine with a new kiln - and immediately after any alterations to the kiln - is where the hot and cool areas of the kiln are. There is an extensive guide to this on the Bullseye site. In short, the method is to place strips of glass on short kiln furniture all around the kiln at the level(s) you will be firing. These strips should be of equal size and the kiln furniture the same distance apart. Take the temperature slowly up to slumping temperature. Observe when the visible glass pieces begin to slump. Let that continue until they are about half way down. Then proceed to the anneal. When cool you can open the kiln and see the areas where the glass has slumped most – the hotter areas – and where it has slumped least – the cooler areas. This will give you information on areas to avoid if you want an even finish all around the edges.

If your kiln is side fired, you need to consider the shelf placement in relation to the elements. The best arrangement is to have one element below the shelf and the shelf between elements so the radiant heat is not directly onto the edge of the shelf as that may lead to breaks.

Put glass on the shelf as centrally as possible. If the glass must be near the elements, baffle the glass from the direct radiant heat from the side elements.

Storing Moulds

Requirements

• protection – some form of container is required to maintain the life of the mould. A container also prevents the kiln wash or other separator from being rubbed off.
• ease of access – It should be easy to get to the moulds, especially as some can be heavy.
• flat vs vertical – some like to store moulds in boxes vertically rather than horizontal with them stacked one above another.
• In either case you need shelves of the right depth and height to store and support the moulds.
• custom made vs bought in – Moulds you have purchased normally come in their own boxes which can be retained for storage for a number of years. You can also make light weight boxes either from old boxes or card cut to size and taped and glued together. More robust boxes can be made from thin plywood or thin pressed board.

Mesh Melt Grids

You can construct your own grid rather than relying on the barbecue manufacturers to use high grade stainless steel.

You can buy stainless steel rod and cut it to the lengths you need for your kiln or project. Push them into soft fire brick at each end. Do the same with another pair of fire bricks and put one pair at right angles to the other.

In this way you can vary the intervals to suit yourself for different projects.

You could also put the rods on strips of mullite and weight the ends so the rod does not roll about while assembling them or when firing.

Alternatively, you can buy the grids available for various purposes. Not all are high grade stainless steel, but normally the spalling happens at a temperature that is low enough to avoid incorporation with the glass.

Glass bending

The purpose of glass bending is to achieve a mark-free curved surface. This is usually done with glass that has smooth surfaces - normally clear but it can be applied to any smooth coloured glass too.

There are at least two methods – moulds and free bends.

Mould

Using a mould enables you to achieve the shape you want with the least observation. However, you need to be careful to use the lowest temperature to achieve the shape to avoid marking the glass. You also need to measure the outer circumference of the original shape, as you are bending, not stretching the glass as in most other slumping operations. A mould is most useful when the shape is not a radius curve. Metal can work very well and because you are using low temperatures, a dusting of alumina hydrate or talc will act as an effective separator. Of course you can use ceramic or fibre as a mould too.

To be able to use the low temperatures required, you need to take advantage of the weight of the glass.  This means the glass needs to fall into the mould, not drape over it.  Relatively fast rates of advance can be used, as you are normally bending one layer of glass.  However from the annealing point upwards the rate should be slowed to allow all the glass to heat throughout and enable the bend to occur at low temperatures.  Observation will be required to determine when the bend fully conforms to the mould.

Free drop

A free drop is similar to an aperture drop, but using a channel rather than a bounded opening. To do this arrange a channel of the appropriate width plus 10mm to allow the full curve to form at the edge. Kiln shelves that are cut into strips, or lengths of fibre board - both supported on kiln furniture - will provide a good channel. Apply kiln wash to horizontal pieces forming the channel or cover in fibre paper. Place a witness at the appropriate height to ensure you can see when the glass has reached the depth/curve required. Initially, this seems to be extra work, but the expense of making a metal mould far outweighs the time taken to set up a free bend for simple radius curves.

Temperature

The temperatures required for glass bending are 40C-80C above the annealing point of the glass. For simple curves start with the 40C above annealing whether using a mould or channel . If after two hours, nothing has been achieved, advance 10C and soak for another two hours. Repeat as necessary. Lots of observation is required. Remember that the wider the aperture and the thicker the glass the less time and temperature is required to achieve the result. After the first bend you will know the combination of temperature and time required for the depth and width of any other piece like this.

Stopping the bend

If you are using a mould, you simply advance to next segment. With a free drop you can also simply advance to next segment as the movement is so slow. But if it is a deep bend - more than a simple radius curve - advance to next segment and open kiln until annealing temperature is reached. Then close the kiln and anneal.

Anneal for the thickness of the glass. No special annealing is required.

For the free drop, when cold, cut off the excess to the size required for the opening.

Fibre Board for Kiln Shelves

Some advantages of fibre board:

• It is light weight making it easy to move.
• Fibre board has very good cooling characteristics as it doesn't hold the heat the same way a mullite shelf does.
• The board is fragile, but with care can last years.
• Fibre board shelves do not thermal shock as ceramic based shelves can.

Considerations for use:

• If it will be moved it needs hardening.
• It needs repeated sanding and hardening for a really smooth surface. Alternatively you can smooth on something like batt/kiln wash or alumina hydrate for each firing.
• It needs to be supported on kiln posts at 100 mm intervals.

You need to use dust masks when ever working with fibre board.

Some disadvantages of fibre board:

• The board can warp over time even when supported every 100 mm.
• The board will warp over time if placed on the kiln floor.
• The board needs to be thick - at least 25mm, thicker for larger kilns.
• It can't be scraped clean of batt/kiln wash.
• Ceramic fibre board is possibly not much cheaper than mullite shelves.

Thick Uneven Pieces

Occasionally fused pieces come out of the kiln with one side thicker than the other. There are several things that need to be done for the present piece and for the future.


Level

First check how level your kiln is. The best for this is to begin with a check of the bed of the kiln. Check the level in four directions – left-right, front-back and the diagonals. If it is practical, wedge up the legs of the kiln to make the bed of the kiln as level as practical.

Then check how level your shelf is. Put in your shelf supports and then place the shelf on them. Again check with a spirit level the four directions. Place pieces of fibre paper under or on top of the supports to level the shelf. It is only after these checks have been made that you can consider firing your piece to help it return to an even thickness.  As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.

Variation in Thickness
Now that you know the shelf is level, you need to consider what the variation in thickness across the piece may be. The firing schedule needs to be more conservative than just for the thickest part. As the thinner parts will heat through more quickly than the thickest parts, you need to fire less quickly than you normally would for the thickest area. A rule of thumb – not always correct of course – is to add the difference of the thick and thin areas to the thicker and fire for that calculated thicknesses. This will make the firing schedule slower and so allow the thicker part to be the same temperature as the thinner. For example, a piece 6 mm at one side and 10 mm the other would have a difference of 4 mm. Add this 4 mm to the thicker 10 mm and then fire for 14 mm.

Temperature and Soak
You also need to consider the top temperature to use and the length of soak required. Glass flows relatively slowly at kiln forming temperatures. The conservative approach – one that allows further work if necessary – is to use the previous fusing temperature and extend the soak by at least twice the length of time on the previous firing, even perhaps to a couple of hours.

Bubbles
One thing that will happen is that the bubbles that previously were near the surface will rise and burst giving pin holes on this extended soak. So you should consider cleaning the bottom and putting the top face down on a separator between the shelf and the glass.   This will reverse the direction of flow for the bubbles. Few if any will break through the new top and there should be no pin holes when flipped.

Further Firings
When the piece is cool, check it for the even-ness of the piece all around. If it is not even enough, you will need to consider re-firing again. If you decide to do so, you should go no faster than the rate of advance as previously – probably even slower - but consider raising the temperature or extending the soak. Remember that achieving the heat work required at the lowest temperature is the guide line for kiln forming. So an extended soak should be preferred over a higher temperature, unless there are strong indications that a higher temperature is required.

Fire Polishing
Of course, you will now need to throughly clean the face down side and re-fire to fire polish the original top. The rate of advance should be the same or slower than the firing to even the thickness. Once you have achieved about 600C, a soak of about 30 minutes will ensure that the glass is thoroughly heated through. Then you can advance at a quick rate to the fire polish temperature with a soak of no more than a minute. This allows the surface to change without giving the rest of the glass time to begin to move.  Of course, a thorough annealing is required.

This procedure for re-firing  can be used when re-firing pieces for any reason. You only need eliminate the considerations on the uneven thicknesses.

Kiln Furniture

You do not have to buy all your kiln furniture. Any refractory material – any heat resistant material that will stand up to the forming temperatures – that you have from broken shelves or used material will do.

The advantage to home made kiln furniture is that you can have exactly the shape you want. There are a number of possibilities.

Example of broken shelves

 Broken kiln shelves can provide supports and dams. They can be cut with a tile saw to give long strips that can be used as dams. Smaller squares and rectangles can be stacked to give height to other supports for the glass.

Cutting a shelf with a hand saw

25mm vermiculite board

Vermiculite in the form of a pressed board provides a medium strength kiln furniture. It can be cut with a wood saw, although it dulls cutting tools quickly. The board can also be carved with wood working tools although it is very abrasive, requiring tools to be sharpened before use on wood again.

25mm ceramic fibre board

Fibre board is not as strong or rigid as kiln shelves and vermiculite are, but is much more adaptable to curves and undulations. They can be cut with a knife and used with or without hardener – colloidal silica. If the hardener is used some kind of separator will be required to keep the glass from sticking. Dust masks are required for working with ceramic fibres.

Soft fire brick

Soft fire brick can be cut with a hand saw into many shapes and and sizes. They are suitable as supports and braces for dams.  There are also higher density bricks that are much heavier to resist the movement of dams.

3mm fibre paper

Fibre paper can be cut and stacked to provide shims to level shelves and moulds. Shapes can be cut into stacked layers of the material to provide dams for irregular shaped projects.