Kiln Choices

There are a lot of considerations when you are preparing to buy a kiln. Often the advice to people buying a new kiln is to “buy the largest you can afford,” or “x kiln is great, and they have good service.”  These are general advice, not directly applicable to your needs.

More important is to think about buying a kiln that suits your kind of work. This might be:

• Flat and shallow sumps
• Small and detailed work
• Powder and other work that needs detailed assembly and little movement
• Deep slumps/ tall drapes
• Drop-outs and melts
• Large scale panels

These kinds of work are determinants for the size and depth of kiln you require. Even if you were to later decide on a larger kiln, the first kiln will continue to be valuable. And having a choice of kilns means you can use the one most suitable to the work.

The way you assemble your work will affect your choice of how the kiln opens. The most common styles are:

• Front door     
• Top lid    
• Clamshell/Top hat

Each has its advantages for different types of work.

Kiln depths are variable. Shallow kilns are easier to load. Deep kilns give more possibilities for casting and drops. It is possible to raise shelves on posts in deep kilns for flat work, making deep kilns flexible for both kinds of work.

The scale of your work will have a big effect on the kiln size. The larger the scale the bigger the kiln will be needed. But be careful to avoid “buying the largest you can afford” attitude. Kiln sizes vary:

• Tiny
• Small            
• medium  
• large
• extra large

 These are some of the considerations that have been organised into two grids. They are an attempt to organise by the kind of work you intend to do with this kiln - drops, casting, jewellery, etc. It then lists the choice by kiln characteristics such as size, opening method, insulation, etc. Some of these characteristics will not be relevant to you at this time, but may be later.

A range of kilnforming styles are given across the top and kiln characteristics down the side of the grid. Where the kiln is very good for the kind of work given at the head of the column a “Y” is entered.  Where the kiln is definitely not suitable a “N” is given. Where neither of these are given, the kiln will do the work acceptably, but not in an optimum manner.

 

 

This second grid relates to controls and various features that kilns have added to the basic kiln. It provides you with a checklist of items that might be desirable and allows you to compare different brands of the same shape according to the additional features they have or can have added.

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

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.

First Kiln Selection

Glass fusing works best in top fired kilns.  Glass casting and some tall work are best with side or bottom elements too.  Compromises can be made of course.  The comparison of glass and ceramics kilns is important to understand.  

Kinds

Most of the following types of kilns are available for glass purposes.

Front loading.  These are good basic, multi-purpose kilns with good viewing properties.

Bell kiln.  This is where the whole of the heating chamber lifts up from the firing bed.  This is more common with very large kilns and is usually combined with lifting gear.

Clamshell kiln.  This is where the firing chamber is hinged, usually on the long side.  This kiln provides access from three sides. It can become too large to reach to the back of the kiln, so these tend to be rectangular.  The lid can also become too heavy for ease of movement and support.

Top loading.  Often called a coffin kiln, there are very good for casting or deep work, but are hard on your back while loading.  They need peep holes at appropriate levels to be able to monitor progress of the firing. These tend to have smaller floor areas than the clamshell.

Car kilns.  These are those where the firing chamber lifts like a bell kiln, but has the firing base on rails or tracks to move multiple firing bases under the firing chamber.

Modular kilns.  These are normally rounded kilns where each ring is controlled separately and can be placed on top of one another.  This is good for large heavy castings, as the refractory and glass reservoir can be placed on the base and the rings built up around the work.

All these kilns come in a variety of sizes.

Choose a kiln relevant to your current work.

The first thing you must decide is the kind and scale of work you intend to do in the near future.  It is too difficult to predict how your work might progress based on experiences with your current work.  It is better to by a smaller kiln that is ideal for the current work and then move to a different kiln, if necessary, or a kiln for different styles or scales of work.

The general advice is to buy as large a kiln as your budget and space and electrical installation will allow.  This remains the case with some precautions.  Think about how often you will fire - daily, a few times a week or a few times a month.   Think about how long it will take to fill the kiln.  A large kiln can take days or even a week to fill with small works. This would really limit the variety of things you could do in that period.  You would have to wait to slump until you had enough things fused to fill the space.  Indeed, you would need to have more moulds than if you had a smaller kiln.

I’m sure you can envisage a time when you will want to work larger than at present, but your first kiln will not become redundant.  It will continue to be useful throughout its long life.

Factors in the choice

Size. As already alluded, the size needs to fit with your current style and scale of work. 

Access.  How big a kiln can you get through the doorways?  How much bigger than actual external dimensions will the packaging make it upon delivery?  It is no use buying a kiln that must be taken apart, or all the packaging removed, to get it into your studio.  Of course, the wider the entrance(s) to your studio the easier it will be to get a larger kiln.  If you really need to have a large kiln, you might have to alter or move your studio space.  You also need to think about the kind of access to the studio.  Does the kiln have to come along the side of the house? Is the path paved or gravel? Stairs? Lift size? Parking for the delivery vehicle?

Space. The kiln also needs to fit into the space you have.  You will need about 15cm all around the outside dimensions for safety purposes.  This applies to ceramic kilns also, even though they routinely reach higher temperatures. The skin of the kiln does get hotter than is comfortable for your hand, but normally not hot enough to burn paper. You can reduce the front to back storage space by putting the kiln on wheels.  But the 15cm saved is not worth the time required to once again ensure that the kiln and shelves are level each time you move it. 

Accommodation also needs to consider access around the kiln to place work in the kiln, especially if you build elements in place on the shelf. 

Location within the studio is important, as the kiln needs to be near a power supply and in a place where it is away from the movement within the studio.

Power supply.  The nature of your power supply will also determine what size of kiln you should buy.  Note both the wattage and amperage required for the kiln and determine whether your household supply can cope with the energy requirements.  Usually a kiln can be run on household supply until it reaches the 1 metre² size, where three-phase power is required to have efficient use of the electricity.

Wattage. Kilns below the 1 metre² (approximately 1 square yard) in size have a need for at least 0.6 -1.2 watts per cm², or 4-8 watts per inch².  Once the kiln is larger, more power is required per area to accommodate the greater mass of the kiln.

Insulation.  All kilns require insulation.  This can be fibre or light weight brick, or a combination of the two.  These insulating bricks can be red hot internally, but only warm to the touch on the outside.  Generally, the refractory fibre – whether board or blanket – requires less energy to heat and cools more quickly in the critical devitrification range.  Most often the kiln floor will be made of brick to provide a firm base to support the kiln furniture.

Features

All kilns come with a range of features, many of them relevant to the size, but not all have the same ones, or the ones important to kiln forming.

Viewing ports.  These are variously called vents, ports, bung holes, etc.  Their importance is at least three-fold. 

·        These provide an opening(s) for you to view the progress of the firing, so you can add more time or heat, or skip to the next segment when adequate heat work has been completed earlier than expected.

·        They provide a means of venting the kiln.  This is important in the burn out of any fibre paper binders, and in allowing enough air to promote the oxidisation and maturation of the hot enamel colours.

·        These openings allow the kiln to safely cool more quickly at lower temperatures, say 300°C, but lower for thicker or more delicate pieces.

Opening.  The way the kiln opens is an important consideration.  Some kilns do not allow the kiln to be opened at all during firing.  This is not a desirable feature on a glass kiln.  It is important to have a switch that will turn the kiln off after a certain degree of opening, so that no contact can be made with a live element. 

·        A front opening kiln allows maximum flexibility to view the progress of slumps, drapes, tack and full fuse kilnforming.  It should have a switch to turn off the power to the elements after a certain degree of opening.

·        A top loading kiln allows you to add glass during a casting process, but is not suitable for working the glass during firings – E.g. combing, manipulation of a slump or drape.  This type of kiln occasionally has no allowance to open the top without turning off all the power to both the controller and the elements.  Avoid this, or have it changed.

·        A clamshell or bell kiln allows maximum accessibility during the loading phase and the forming stages.  Although a lot of heat is dumped forward, it is the easiest to use for combing and other manipulation of the glass during the firing. Again, this kiln needs a lid operated switch to cut the power to the elements when opened beyond a certain point.

Controller.  Although essential, controllers are often given as options, especially on smaller kilns.  There are at least two reasons for this.  There are a variety of controller styles and costs.  The buyer may already have their own controller, or wishes to specify the kind.  Controllers are significant costs involved in smaller kilns – sometimes being at least one-sixth of the price.  In general, the more features a controller has, the more it costs.

Controllers are often classified as “three-key”, or as full number pad.

·        The three-key controller – even if they have many more than three keys – is one where the numbers must be cycled through by holding an up or down arrow to change the numerical information.  This includes the programme number, segment number, time, rate, temperature, and sometimes other information. 

·        The full number pad controller will allow direct entry of numbers at each segment of the programming.  It will often have additional features, such as calculating the firing cost or kilowatts used, elapsed time, additional capacity for more saved programs, ability to control different areas of the kiln heating, etc.

Extras 

There are often things which will be worth considering purchase along with the kiln, but are not usually included in the base price.

Stands.  Smaller kilns range from table top - which do not need stands at all – through medium sized – which have optional stands – to larger ones that come with the stand integral to the whole kiln. Unless you intend to move your kiln about, it is not necessary to buy one of the metal stands. Even so, most of these stands come without wheels, so check that they do have wheels already attached.  If you will not be moving the kiln, you can use a wooden table with a refractory fibre board between the stub legs of the kiln and the table surface.  If the kiln does not have stub legs, you can set it on 4 house bricks. 

Kiln furniture. This consists of the refractory props and dams that will be needed in kilnforming.  The most essential are short (2.5cm) kiln posts to support the shelf.

Shelves.  Most shelves require a mullite/cordierite shelf to fire on.  This is a robust shelf that does not have the quartz/crystobalite inversions that ceramic shelves and tiles used for shelves have.  It is a good idea to buy one of these to fit your kiln at the time of purchase. Smaller kilns can use fibre board or vermiculite board as the shelf.  These can be purchased later.

Extractor fans.  These are available on many kilns. They are unnecessary on smaller kilns as they cool quickly anyway.  Larger kilns in a production environment may need quicker cooling, and these arrangements are very useful in those circumstances, but not others, as most kilns will cool in 8 – 16 hours without drawing air through the kiln.

There are a lot of other considerations in buying a kiln, but these are among the important ones, especially in selecting the first one.

Peeking Without a Vent

What can I do if my kiln does not have a plug?

To understand thoroughly what is happening to your glass while firing, observation is key.  This means that an observation port is an ideal feature of any kiln.

However, many kilns are made without ventilation or observation ports.  This means that several possibilities need to be considered.

The easiest is simply to open the door or lid a small amount to make a brief observation.  This means that you have to set up the piece to be fired in such a place it can be seen from a small opening of the door/lid.  This brief opening of a small space will not normally cause any problem to the glass or kiln.  At the higher temperatures, you need to take personal safety precautions against the heat and light from the kiln.

It is possible to be more radical and drill an observation port through the metal casing and brick or fibre lining of the kiln.  This is then filled with a piece of fire brick or roll of fibre blanket.  This is sufficient to insulate the heat from the external part of the kiln.  This port should be about 50mm diameter to give a decent field of view.

A further refinement is to place a quartz viewing window in the hole you have drilled.  This viewing piece will become very hot, but not visibly red.  So, you must provide some insulating cover over the window.

But best of all, is to purchase a kiln with a viewing port in the first place.

Selecting a Kiln

Kiln Forming – Selecting a Kiln

You have been doing some fusing and slumping and now want to get into kiln forming in a serious way. So you need to get a kiln.

The basic kiln choices are ceramic vs. glass. The brand, model and size are up to you. But there is some helpful information on the advantages of each kind of kiln in this post.

Another consideration is the shape of the kiln. Generally the greater the area of the shelf in relation to its size, the better it will suit fusing and kiln forming. Oval kilns seem to waste some space, although they do not have cool corners like the rectangular ones do. Relatively deep and round kilns are best for casting and high temperature work.

Even before you buy the kiln you need to think about where you will be installing the kiln and that will have an effect on the model and size. Some considerations are here.

Think about the kind of work you want to do. This will change with time, but you cannot anticipate that now. Will you be doing jewellery scale, detailed work, lots of forming work, high temperature or even casting work. Each of these have different requirements.

Small kilns are best for jewellery and detailed work – they can be fired quickly and will reach the top and annealing temperatures with a minimum of delay.

If you tend to work larger then you should consider a kiln of about 40 cm square to start with, although smaller kilns will work if they have enough height.

Slumping and kiln forming put a premium on height. If you are going to be doing a lot kiln forming you should consider a kiln with at least 25cm from base to elements. Ex-ceramics kilns can be good for this.

If you are going to be doing a lot of high temperature work, such as casting, pot melts, pattern bars, etc. you might want to consider a brick lined kiln such as a ceramics one as they retain heat by design longer than those designed for glass.

There are a lot of models, so it is up to you to find the combination of style, shape, size and price that suit your present needs.

Installing Your New Kiln

You have your new kiln unwrapped. Now where to put it?

First read the manufacturer's recommendations. If you are still uncertain there are a number of things you should think about.

One of these is getting access around it. You need to be able to work around three sides ideally if it is a top loader, two sides for smaller kilns is usually enough. If it is a front loading kiln you only really need to consider the space in front of the kiln. You are going to be carrying pieces, even shelves to the kiln so you need enough space for you and the shelf in front of the kiln. At the sides you only really need enough space for yourself.

Getting access is part of the consideration of distances from other things. Often people are worried about the heat that the kiln will be giving off. Kilns are well insulated to reduce the costs of firing, so the heat release is slow. Still, you want at least 300mm space from anything inflammable.

You also will want to think about the support surfaces. Sometimes the kilns come with their own stands, but usually these are to raise the kiln to working height. You may want to protect against any (unlikely) meltdowns, so you should put the kiln on steel, ceramic or concrete surfaces. There are a number of table top models and in these cases a large ceramic tile or ceramic fibre insulation under the kiln is an entirely adequate safety precaution.

You will need a place to put things down just before loading the kiln, so placing the kiln near adequate flat surfaces is important.

You also should think about putting the kiln out of the main traffic areas of the studio to avoid disturbance to the kiln or the rest of the studio activity.

Now that you have the ideal location for the kiln you have only begun.

You need to make sure the kiln is as level as possible. The first stage of this is to make sure the casing is relatively level. Use of a spirit level on the top front and sides is probably enough. Put hard spacers under the legs to level things up. You can if you want, level the internal base of the kiln instead of the casing. Many find that more re-assuring. Then you need to put the kiln furniture to hold up the kiln shelf into the kiln and the shelf on top of that. This is the part that really needs to be level. Spend time on it. Place pieces of ceramic fibre under the shelf supports as required to get things really level. A circle or three-way level is good for this purpose. The shelf needs to remain level to get good, consistent results. Any time you move the kiln, the shelf, or the supports, you need to check the level of the shelf.

Once you have the shelf level you are ready to do a test fire. Normally you need to have a firing without anything in it to burn out binders used in the making of the kiln. There is no reason that you cannot have the furniture (shelf and supports) in the kiln for this first firing as they need to be test fired too. In addition you can run a test to discover where the cool spots are in your kiln (every kiln has them). Look up and follow this technical note on how to run a test for discovering how even the heat is within your kiln.

Once you have run your test firing, you will want to protect the kiln floor from any spills of hot glass and the glass from sticking to your furniture. If the manufacturer has given you some kiln wash with the kiln mix it up about 1 part powder to 5 parts water and lightly paint the floor of the kiln - not the walls. The kiln furniture needs this too as does the shelf. This note on applying kiln wash will give you information on how to do it. 

It is important that you have some protective gear to do the work with kilns. At first and for fusing temperatures, you need eye protection and gloves. You need to look frequently and briefly into the kiln to monitor the firings, especially at the start of your career. For this you need eye protection. Sun glasses will not do as you need protection against infrared rather than UV light. There are a number of things that will do from welders' goggles to special lenses as used by bead makers. Use them! Every time. You will need gloves, at the start leather gloves with sleeves going half way up your forearm (such as welders' gloves) will do. Later and for higher temperature work you will need better and much more expensive gloves, sleeves, and body protection.

These things will get you off to a good start.