Kiln Characteristics Investigation

Many people ask about the best kiln to buy.  Sometimes they mean the cheapest, but mostly they mean the best for their favoured processes. To get the best from your proposed kiln, you should be aware of its characteristics and how it fits your proposed kilnforming practice.  There are a range of factors that interact to give the special conditions of your kiln.  They range from the purpose, the materials of construction, the placement of heating elements, how it opens, and its shape.  All these can affect the degree of even heating of the kiln bed or shelf.

Kiln types

There kilns for many purposes. Some of them are powder coating of metals, enameling of metals, vitreous painting of glass, glass forming, ceramics, casting of glass and metals, lehrs for annealing, and furnaces among many others.  

Large powder coating kiln

Large enameling kiln

Jewellery enameling kiln

Electric glass painting kiln with multiple shelves

Example of a sheet glass annealing lehr

For our purposes we are concerned with the glass and ceramics kilns.

Ceramics vs Glass Kilns

In general ceramics kilns are made to lose heat slowly, while glass ones are designed to lose heat relatively quickly.  There are many glass kilns based on ceramic ones.  You should be aware of the differences between kilns designed exclusively for glass and those based on ceramics kiln designs.

Small ceramic kiln

Small glass kiln

Construction Materials 

The materials used in constructing kilns are refractory insulation and a steel structure of a design to hold all the refractory materials together. 

Refractory bricks for glass kilns are light weight and usually designed for temperatures under 1200°C (dense bricks rated much higher are normally used in ceramic kilns). 

Light weight refractory brick

Bricks tend to be used in most glass kilns on the floor as well as the walls (some smaller ones use only refractory fibre).

Small fibre kiln

Kilns derived from ceramics tend to have brick walls and lids.  Most kilns designed for kilnforming have fibre walls and lids.  In the cases of top hat opening kilns, fibre is a necessity to reduce the weight of the lid.

Fibre board and fibre blanket are used widely.  The floor tends to have a floor consisting of steel, fibre board on top and brick on top of the board. Fibre blanket tends to be used on the walls and ceilings of rectangular glass kilns. Oval and circular ones tend to have brick walls and ceilings.  The use of fibre board and blanket walls and ceilings leads to a more rapid cooling than those with brick ones.  This will affect the scheduling of the kiln firings.

The steel used to contain and support the refractory materials is important.  Many kilns use mild steel in sheet form to fill the spaces between the heavier structural support steel.  The higher quality kilns use stainless steel sheet, even though they may use mild steel for structural support.  The stainless steel lasts much longer than mild steel, especially when there is liable to be moisture involved in the kiln processes, such as pate de verre or casting.

Opening Method

This post gives a description of the common methods of opening the kiln.  

The purposes for which you want to use the kiln relate to the firing characteristics needed.

Top opening

Top opening kilns have the advantage of depth, normally with elements around the sides.  This makes them good for casting, but not so good for processes that need observation or manipulation.  The depth is most useful in casting  and deep slumping work, but requires a lot of experimentation to make use of multiple shelves in one firing.

Front opening kilns have the advantage of being able to observe the whole depth of the firing, if you protect yourself from the heat that will be dumped from the kiln.  They often have elements on the sides which is an advantage for drops and melts (when observation is necessary).

Top hat opening kilns are those that have the whole heating chamber hinged at the shelf level.  These are very good for placing of work, as you can work directly above the pieces.  These are one of the best types of kiln for combing or any other manipulation of the glass during the firing. You can also observe by opening the kiln a little during the firing.

A range of top hat and a bell kiln

Bell kilns are those where the whole of the heating chamber lifts above the bed.  These are often equipped with two bases which can be wheeled in turn under the chamber which is lowered before firing.  These tend to be very large kilns.

Small gas fired kiln

Heat source

Most kilns are heated with electrically powered elements, either exposed or in quartz tubes.  The quartz tube contained elements provide more even heating than the exposed ones.  The most even heat is provided in gas fired kilns, although these are generally more expensive and less widely available.

Element Placing  

The location of the heating elements can have a significant influence on the way you fire your glass.

·        Top fired kilns are generally the easiest to use as the glass is most affected by radiant heat.

·        Side fired kilns provide the radiant heat to the edges of the glass first, before the air temperature can begin to affect the surface of the glass.  This means more caution is required in the heat up of the glass.  However, side elements are very useful in drops and casting processes.

·        Some kilns have both top and side heating elements.  This provides flexibility in heating up and in cooling evenly.

·        A few kilns have elements around the sides but below the shelf.  This promotes even cooling of glass from both the top and bottom. It is most useful in dealing with the cooling of thick slabs.

Kiln sizes and shapes

Kiln sizes have an effect on the behaviour of the kiln.  Smaller kilns (depending on the refractory materials) generally heat and cool quicker than large ones.  The mass of a larger kiln takes more energy to heat up and more time to release the heat than smaller ones do.  This will influence the scheduling for different sized kilns.

 

The shape of the interior of the kiln affects the distribution of heat within the chamber.  Rectangular kilns tend to have cooler corners than circular ones (as there are no corners).  Oval kilns tend to give space for longer pieces and reduce the cool corners.

 

The height of the kiln also affects the heat distribution within the kiln.  Taller kilns are cooler at the bottom than the top, even with side elements.  They are especially good for casting and drop processes.  Deeper kilns, even if rectangular, require more energy to complete any given process, because of the distance between the radiating elements and the glass.

Hot and cold spots can be tested for by using this method.  The actual operating temperatures can be tested by the use Orton cones to measure heat work. This depends on the speed used to get to the process temperature.

There are many factors that make up the characteristics of kilns. The main ones are style, construction materials, opening method, shape and depth. These need to be considered in relation to the kind of kilnforming you intend doing, to make the selection optimum for your practice.

More information is available in "Your New kiln" from Etsy shop VerrierStudio: https://www.etsy.com/uk/shop/VerrierStudio

or direct from stephen.richard43@gmail.com

First Firing of your New Kiln

First Firing of your New Kiln

I have just been reviewing information on kiln elements. I have discovered the reason you need to do your first firing with the kiln empty of everything. No kiln wash, no kiln furniture, nothing. Vacuum the kiln to take out any dusts from travel.

The element forms a protective layer of aluminum oxide during the first firing. If there are elements of kiln wash, dusts, or glass, this will inhibit the ability of the oxide coating to be uniform. The uniform coating of the elements is important to the long life of your elements. There are other things of course, but this is the initial, essential element of preparing you kiln for use.

After this first firing you can add the other elements of kiln wash, furniture, and even glass.

In summary, fire your kiln clean and bare. No kiln wash, no furniture.

Sam Smith adds: [This] applies to kilns made with Kanthal A1 elements. Those are the good ones which last pretty much forever. Cheaper quality kilns can have nichrome elements which do not develop the coating. The firing the kiln empty allows the oxide coating to form. If you do a firing where combustion takes place such as firing fibre paper or shelf paper you should realize those combustion products are attacking your element coatings and it may be worth while venting the kiln and or firing the kiln up empty after the firing in order to protect or allow the development of a new layer of coating covering the kanthal. Kiln wash us is cheaper and safer for the long term life of your kiln elements. Smart people only purchase kilns with Kanthal elements.

Kiln Maintenance

Switch off the kiln before doing any 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 a brick floor.

Example of vacuuming around elements

Example of vacuuming lid without elements

An alternative to vacuuming the elements is to use the air compressor hose at low power to gently blow out any dust settled in the element grooves.  Do not do this for fibre insulated kilns, only brick.

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 newly placed in the kiln - e.g., mould, or shelf replacement - with a spirit level.

Two examples of two-way spirit levels

 Check on the conditions and placement of the thermocouple.

Check on the elements.  Some may be sagging or hanging out of their channels.  Use tweezers to bring the coils closer together.  This shortens the length of the element and it then can be pushed back into the channel.  It may not have to be done after each firing, but checking will catch things before sagging becomes a major problem.

When the shelf paper is exhausted lift out the thicker papers and vacuum the shelf.  The Thinfire and Papyrus papers can be vacuumed directly or gently swept up and placed in a container for disposal.  Do not introduce any moisture to help reduce the dust.  This is not good for the kiln or you, as it could induce shorting out of the elements.

Monthly

Electrical parts: check the elements and their connections (normally at back or side).

First unplug or switch off the power to the kiln.

Check the screws on the connectors for the element tails are tight. Loose connections cause the wire to vibrate at the connection during the power phase. They heat up enough to melt the wire at the connection. For a single element kiln, it will simply lose power.  In multiple element kilns the remaining elements work much harder to achieve the temperature and provide uneven heating.

If the connectors are badly corroded , they need to be replaced.  This can be done without replacing the elements. Unscrew the connectors and put new ones on.  If the connector is fused to the element wire, you need to cut the wire as close to the connector as possible to maintain a length of wire for the new connector to be fixed.

Check the condition of leads and plugs supplying power to the kiln.  Make sure they are sound, not frayed and not kinked. Replace any frayed parts.  Take out any kinks in the power supply cable.

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

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

Making a schedule of maintenance checks and noting on it the dates checked is a good idea for those who need reminders.

Kiln Cleanliness

Problems with finished pieces can be caused by an untidy or dusty kiln interior.  Pieces can be affected by devitrification or specks of refractory material in or on the fired project.

Dust is a common problem.  Kiln wash, fibre papers and combustion products all produce particles that are collectively referred to as dust.

Vacuuming the kiln regularly is a good practice to keep the dust down.  It is best if the vacuum has a variable suction control to avoid damage to the refractory materials which make up the kiln.  It is best to use a brush attachment rather than the bare hose.

Dust on the brick or fibre board floor of kilns can be vacuumed easily if you remove the shelf.  Usually you need to use the most suction available to pick up heavier particles such as glass frit along with the dust that accumulates on the floor.

Dust also accumulates on the sides and top of the kiln too.  If you have brick sides and tops, you can continue to use the high suction.  You need to be careful around the elements so that you do not bump them.  This is where the brush attachment is most useful, as you can gently brush out any accumulated dust and any loose particles from the brick.

If you have fibre sides or top, the high suction setting on the vacuum will pull fibres from the refractory material.  You need to use a low setting to avoid damaging the insulating materials.  Gently pass the brush attachment along the insulating fibre and along the elements.

This vacuuming of the kiln does not need to be done on every firing, only at regular intervals.  It is also a good time to check the condition of the elements and condition of the interior of the kiln.  Any element tail connections can be checked for tightness.  The condition of the bricks can be checked as you vacuum. 

Of course, if you are going to fire an important piece, it is a good idea to make sure the kiln is clean before you start. But daily cleaning is not required.

It is not only the interior structure of the kiln that needs to be clean.  You should be checking the cleanliness of your kiln furniture too.  Make sure you keep the shelves dust free and regularly kiln wash them.  Check the kiln posts for flaking kiln wash and dust.  Clean off any dust or loose material and re-coat as necessary.  

And while you are doing all this cleaning, you could vacuum the outside of the kiln too.

Kiln Elements - Aging

As elements age, they generally increase in their resistance. This increase in resistance decreases the amount of amperage and, so, the amount of heat given off by the elements. This explains  why older kilns sometimes go so slowly and may not reach their maximum temperature.

There are several factors which affect the longevity of elements and so have implications for firing practices.

• ·        Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.
• ·        Allowing the wires to become tightly wound increases overheating of sections of the element.
• ·        Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.
• ·        Firing close to the elements allows fumes to contact the elements.
• ·        Subjecting elements to reducing atmospheres will age the elements quickly.  This would be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are carbon, wax, halogens (such as chlorine or fluorine), molten metals (such as zinc, aluminium), lead glazes, alkaline metals, borax compounds.

All these elements attack the element coating.  And each time you fire the slight difference in expansion between the core of the wire and the coating creates cracks in the coating.  The exposed core forms new coating to fill the gaps.  This over time reduces the thickness of the element wire.  As the wire thins, the resistances increases, causing more fissures in the coating to occur, accelerating the aging process.

The next in this series is about how firing practices can affect the life of elements.
Firing Practices

Other relevant posts:
Nature of elements
Maintenance

Kiln Elements

Questions arise on whether old elements can become inefficient or hold a “memory” of previous firings. Old elements increase in resistance so decreasing amperage and consequently reducing the temperature that can be achieved or speed with which the kiln can reach the top temperature required. It not a "memory" of heat or temperature.

This series of blog posts will look at the nature of elements, their aging, effects of firing practices, and maintenance of the elements.

Nature of elements

A kiln element is a wire designed to have considerable resistance to electricity passing along it so creating heat.  The measurement of the amount of resistance is in units named ohms.

Most electric kiln elements are made of Kanthal A-1, a trade name of Sandvik.  This is an alloy of iron, chrome and aluminium. During the first firings the wire develops an aluminium oxide layer on its surface.  This helps protect the rest of the substance of the wire from further corrosion.

It is critical to the life of the elements to develop this aluminium oxide coating and is the reason people are told to fire their kiln clean and empty.  Any contaminants, including dust can inhibit the formation of the protective oxide locally.

The new kiln elements begin to achieve their protective coating when they reach 1000°C.  The kiln does not have to reach that temperature, just the element.  But it is advisable to fire toward 950°C at a moderate rate of about 250°C per hour and soak there for half an hour to ensure the coating is firmly in place. Your kiln may never be fired as hot again, but you will be sure the elements are properly prepared.

It is important to remember that the elements are flexible when heated initially, but after a few firings become stiff and brittle.  After the initial firings, you need to ensure the elements are still well seated in their groves.

A well-designed kiln will have the largest diameter element wire and the largest distance between the coils (runs cooler). The thicker the elements, the greater the stability and the longer the life.  When elements get above 925C they become very soft.  As they soften, the coils begin to collapse, causing the distance between the coil turns to lessen. When the distance between coil turns is small the element will overheat in those areas.

The next in this series is about the aging of elements.
Aging of elements
Firing Practices
maintenance

Element Coatings

You will notice that after the initial few firings of your new kiln that a grey residue forms on the elements.  This is a protective layer.  It is a surface oxidisation that protects the underlying metal from further corrosion. 

Kiln elements are generally made from Kanthal or Nichrome wire. 

Kanthal wire is an alloy of iron, chrome and aluminium.  The aluminium oxidises to provide a protective layer of aluminium oxide.

Nichrome wire is an alloy of nickel (the main element) and chromium in various proportions for different applications. It is the most common heating element for high temperatures. The chrome forms a protective layer of chromium oxide at red hot temperatures.  But once heated, it becomes brittle, so it can be manipulated only when hot.

This layer is not a chemical reaction to the things you put into your kiln.  It is the necessary protective layer to give long life elements. This coating should not fall from the elements unless it is disturbed by bending, abrasion or impact. If it does, check for damage to the elements and look closely for any break.

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.