Multiple Layers of Kiln Wash and Fibre

Recently, there have been confirmations of multiple of layers of kiln wash on the shelf under pot melts, frit stretches, and flows.  Ten, and even sixteen layers are mentioned. Also reported are two layers of 1mm fibre paper as a separator for the same processes.

These practices are excessive and wasteful.

 

Kiln wash   

·         Once fully covered, the shelf does not need additional layers. 

·         Stir the kiln wash mix each time you dip the brush.

·         Apply thinly.

·         Use only enough coats to evenly cover the shelf.

 

Fibre and shelf papers are not recommended to be placed on the shelf for high temperature processes.

·         The shelf papers can become incorporated within the glass as it moves along the shelf.

·         Fibre papers inhibit the movement of the glass in unpredictable ways.

 

If you do use fibre papers anyway:

·         Use only one layer.

·         Place a clear sheet of glass over the fibre paper to allow better flow during the firing.

·         A disc of clear glass also helps to separate opalescent glass from the shelf.

 

More layers of kiln wash or fibre paper does not make a better separator.

How Many Times Can You Fuse?

Is there a limit to how often a piece can be refused either to add on new pieces or to break up and reuse?

 

Bullseye test their glass to ensure it can go through three firings.  After that you are on your own.  This limitation has been generally accepted within the kilnforming community.

Multiple Firings

Many people report that they fire their glass many times.  I have fired Bullseye glass up to seven times using powders.  The first five firings were to contour fuse.  The final fuse firing was to full fuse.  And then there was a slump firing to make up the seven firings.  These multiple firings assume that the temperature is not taken above full fuse for any of the firings.  The annealing for each of these firings remains as for the calculated thickness of the piece.  No additional annealing time is required for multiple firings without significant changes.

You do not need to consider firing multiple process at once.  The possible number of firings is more than enough to achieve multiple processes.

High Temperature Work

However, the many firings of my piece would not have been possible with high temperatures or long soaks.  The high temperature firings are those that go to casting temperatures (835ºC/1540ºF) and above. These are temperatures for things like pattern bars, melts, and castings.  The glass can change its nature enough to give problems at these temperatures, especially with long soaks.  

If you do multiple firings at high temperatures, I recommend annealing to be from one and a half to two times the expected thickness.  And the rate of cooling will need to be in line with the length of the anneal soak.

Even with care, there can be problems.  I created a thick bowl from transparent glass, of which I was proud.  After an afternoon in the sun while on display months later it showed a crack developing.  I am still convinced - by other similar work surviving – that I annealed it properly. It exhibited minimum stress after the final flat full fuse firing. It was not checked after the slump.

Multiple firings of items with soaks at high temperatures are less likely to survive.  The number of firings possible can be determined only by experience.

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 being 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.

High Temperature Wire for Screen Melts

You can use high temperature wire for screen melts. This is variously described as Kanthal or nichrome wire.  It is the same kind of wire used in the heating elements of your kiln.

wire with bent ends

To use the wire, you lay or weave the wire and support it on both ends.  Weaving the wire provides more support, but is not necessary, as the wire is strong enough to support a lot of glass.

first line of wires pushed into board

You need to have the wires as tight as the supporting material will allow. Straightening the wire before beginning to fix them will help, as will thicker wire.

The wires need support at each end, which can be brick, cut up shelves, or strips of tile.  If you do this, you can form a dam or vessel in which to put the glass without fear of it spreading over the edge.

I use fibre board for the support and just bend a right angle into each end of the wire to push into the board.  These can be arranged in any configuration, although for ease of illustration, I have used a rectangular arrangement of wires.

A grid of wires ready for kiln wash

Put the completed screen over a tray or sheet of plastic to collect the excess kiln wash.  Mix the kiln wash very thick and pour over the wires. I put the board with wires into the kiln to heat to about 200°C to help the wash stick.  I repeat a few times.

Make sure you coat the area surrounding the screen to avoid the glass sticking to the supports.

When the kiln wash has dried, knock off the stalactites of wash on the underside of the wire to prevent any excess kiln wash being incorporated into the final piece.

Place on kiln washed supports, and put the glass on top of the screen.

This is a relatively quick and inexpensive means of providing a custom shaped screen.  

One disadvantage of this over stainless steel rods, is that it is difficult to get enough kiln wash to stick to the wires to be able to pull them out easily.

Mandrels for Screen melts

In creating screen melts, the steel or other support left in the project can leave such a degree of stress that the piece will began to fracture over time.  The use of thin stainless steel rods as used in mandrels for bead making is an alternative, as they can be pulled out.

The separator used on the mandrel can be bead release.  If you have it that will work very well. This illustration shows a bead maker coating a mandrel from a bottle of mixed bead release.

If you do not have bead release on hand, you can use kiln wash.  To give the thick coating required to easily pull the steel out you need to mix the kiln wash differently. 

The normal mix of kiln wash would be 5 parts water to one of powdered kiln wash.  As you want this to be thicker so it will stay on the mandrel, you can mix it in a 3:1 ratio.  This will be sufficiently thick to keep it running off the mandrel and be able to extract it after kiln forming.

Mandrels prepared for bead making.  In coating them for a melt, you need to have the whole length coated.

To avoid the mess of pouring the wash over the mandrel, you can fill a stringer tube with the mixture and dip the mandrel into it. You can place the end of the mandrel into a polystyrene insulation block or a bit of clay to let it dry as done by bead makers.

Once dry, you can arrange these coated mandrels in any shape of grid you choose.  Lay them across your supports whether fibre board or brick with about 25mm on the support at each end.  Lay all of one direction down first and the follow with the second, or more layers.  Place you glass on top of the grid created and fire.

Devitrification on Repeated Firing

 Devitrification is defined as the crystallisation of the glass, making it a non-vitreous substance.

Molecular level difference between vitreous and devitrified silica
from Digitalfire.com

You can see that there is not much difference between the the two states of the glass in structure, but mainly the arrangement of molecules.

The appearance of devitrification has a range of appearances from a mild smeary look through a dull surface to a crazed, crumbly aspect in severe cases. 

Mild devitrification

Medium level devitrification requiring abrasive cleaning

Causes of devitrification are related to slow changes of temperature (up or down) and most importantly nucleation points such as dust, oils, or cleaning residues. So, thorough cleaning is most important. 

Causes in repeated firings of the same piece relate to:

        Cleaning. 

It is important to thoroughly clean the piece before each subsequent firing.  Many times abrasive cleaning such as sandblasting is important to clean out impurities from the previous firing.  The resulting surface from any abrasive cleaning requires further cleaning with lots of clean water and a thorough drying with clean cloths or paper.

        Slow cooling or heating. 

Devitrification normally occurs in the range of 670⁰C to 750⁰C. This is the reason for the rapid rates of advance in this temperature range rather than other factors.  It can form both on the rise and on the fall in temperature. Slower rates in the devitrification range allow enough time for the crystallisation to begin.

        High temperatures.

Both high temperatures and long soaks can promote devitrification.  It is not just the slow rise or fall in temperature, but long periods at high temperature can lead to devitrification even though other precautions have been taken.

Changes in the composition

High temperatures and many repeated firings of the piece can lead to changes in the glass.  Some metals and fluxes are more likely than others to change composition or oxidise at extended soaks at high temperatures.  This can reduce the ability of the glass to resist devitrification.

Prevention/Correction

Prevention relates to thorough a) cleaning and b) firing rates.

All correction of devitrification relates to the modification of the surface.  If the problem is only at the surface, you can use either abrasive cleaning or the addition of fluxes to the surface, or a combination of the two. 

Where you have a mild dulling of the surface due to devitrification you can apply a flux.  This softens the surface by reducing the melting temperature of the glass and so reverses the crystallisation at the surface. The devitrification solution can be a proprietary spray such as Super Spray. Be aware that some sprays use lead particles as the flux, so are inappropriate for pieces intended to be food bearing. You can make your own devitrification solution by dissolving borax in distilled water.  When the devitrification is wide spread or deep, abrasive cleaning is required.

Abrasive cleaning can be by hand with sandpapers or diamond pads.  Be sure to keep them damp.  This keeps dust from rising, and the sanding surfaces clean for better working.  Sandblasting can be quicker, especially on uneven surfaces or where there are deep imperfections.  The surfaces resulting from abrasive cleaning need to be scrubbed clean with sufficient water, and then polished dry as for a finished piece.

It is possible to combine both these methods to be more certain of a shiny finish.  When combining, you need to do the abrasive cleaning first, then the wet cleaning and finally add the devitrification solution.

A fourth possibility is to sprinkle a fine but consistently thick layer of clear fine frit or powder over the piece.  This, when fused, provides the new surface concealing the devitrification below.  Again, this must be done at a full fuse, so it is not applicable to items you wish to remain tack fused.

However, if the devitrification has progressed to a crazed appearance, it is so deep as to be almost impossible to reverse.  The piece will also probably have developed incompatibilities. So the only real option in crazed pieces is to dispose of them.  They will not be useable in combination with any other glass. They will make any glass with which they are combined subject to devitrification and possible breakage.  These are pieces which truly cannot be cut up and re-used.

Dams for Melts

There are a number of commercial moulds, dams and rings to contain pot and screen melts.

You can, of course, make your own. A simple one is to use 10 or 15 mm fibre board to contain your pot or screen melts. Cut the size and shape of hole you want into the board and that will contain the glass.

You can place this directly onto the kiln washed shelf. No fibre paper is absolutely required unless you want to. You can weight the board by placing the supports for the screen or post directly onto the board.

If you want to use the board more than once, you need to harden it with colloidal silica and fire it. Then you always need to put a separator on it at each firing to ensure it does not stick to the glass melt.

This process allows you to make custom shapes and sizes without great expense. With a bit of ingenuity, you can provide your own textured bottom to the melt.

Glasses at Risk of Compatibility Shift

Many people take their fusing glasses beyond the tested parameters of the manufacturers in pot and screen melts and combing and casting operations. It has been speculated that there are compatibility shifts of hot colours and of opalescents.

Reading, and some experience, lead me to the belief that is the colouring minerals that are the key to which glass will shift in compatibility. Colours made with sulphur and selenium are more likely to opalise and also change their compatibility at extended times at high temperatures. Extended time is in the region of an hour or more. High temperatures are those over 850ºC

The colours at most risk of compatibility shift seem to be:

Reds

Oranges

Browns

Ambers

and a few bright and olive greens, but not dark greens.

http://www.warmtips.com/20070207.htm

Of course testing, using polarising light filters, is required to determine which will remain compatible after long, high temperature firings.  A method of testing is given here.

High temperature compatibility shifts are discussed here.

Compatibility Shift at Higher Temperatures

People experience breakages of their pot and screen melts that do not seem to have anything to do with annealing or glass sticking to the shelf. The common suggestion is that there has been a compatibility shift of the glass. This view is re-enforced by the opalisation of the transparent hot colours experienced by most.

Bullseye indicates in their glass notes that some colours are not suitable for high temperature work. This probably applies to other fusing glasses too. My experience leads me to believe that this compatibility shift occurs with all the opalescent glass colours as well as the hot ones. Further work will appear soon. is required to determine if there are any general indicators of the kinds of glass that are likely to develop incompatibility at high temperatures.

If you are concerned about the lack of durability of your piece due to possible incompatibility, you need to include tests with the firing. To make this test, place a piece of each colour used in the melt on a double layer of clear. If you are using a single base piece, ensure you leave space between the colours. It is best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces. Put all those pieces somewhere within the kiln out of the way of the area the melt will occupy and fire the lot together.

When cool, take all the pieces from the kiln and check the test pieces for compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for the halo showing the degrees of incompatibility.

If any or all, of the the pieces show stress, check the clear stack for stress. If the clear also shows stress, the annealing has been inadequate, rather than just the compatibility shift. Ideally, this process should be conducted in every firing.

Performing these tests will give you confidence in the durability of your piece, as it will show the levels of stress in the finished piece.

Bubble Reduction in Casting

There are several things that can be done to reduce the number and size of bubbles in casting.

• Fire higher - to 830ºC instead of 815ºC - and soak for at least four hours. This allows more bubbles to rise to the top and burst. If there are still more bubbles than wanted, increase the soak time.

• Stack the glass in the centre of the mould, allowing a few centimetres from the mould walls. This allows the glass to spread and flow from the bottom and up the sides, reducing the likelihood of trapping air. If you have more than one stack, keep the same space between the stacks as the mould walls.

• Make sure that the way you stack the billets or casting plates so there is a smaller space at the bottom of any cavity than at the top. The reverse allows the glass to soften and seal in the air in the space.

• You can construct a mould to make billets of the general shape of the final object. This of course, is much more work, needing two moulds.

• A major thing to avoid is the use of frit, especially at the bottom or deep in the mould as bubbles will collect around each piece and lead to a multiplicity of bubbles throughout the casting.