Placing of Pieces in the Kiln

 Distance from Sides of Kiln

 

"Is there a rule of thumb for interior size of kilns and piece size? (i.e., “allow for X inches between the piece and kiln walls on all sides”).  I’m thinking about how to determine piece size limitations when shopping for a kiln."

I don’t know of a formula, or rule of thumb, to determine the amount of space required between the glass and the kiln walls.

I have only been able to determine the spacing required after I have purchased the kiln.  Each kiln has different characteristics. 

The most obvious is whether the kiln is fired from the side or from the top.  More space is required with side fired kilns.  The radiant heat from the elements tends to heat the edges of the glass before the centre becomes equally hot. This requires more space or baffles between the elements and the glass.

Top fired,  with enough distance to get even distribution of heat

Side fired. Red arrows indicate the important infrared heating.
Blue arrows indicate the less effective ambient heat.

There is less concern about uneven heating with top fired kilns.  But as each kiln is different, you must test the heat distribution around the kiln.  Bullseye Tech Note #1 has a good method.  This will show where the temperature is less than the rest of the shelf.

In general, rectangular kilns are cooler in the corners.  Round kilns do not have the same characteristic, but may still have uneven temperatures, due to the configuration of the elements.  Smaller kilns seem to have more even temperatures than large kilns, which tend to be cooler along the sides.  Kilns with a ring element below the shelf seem to have the most even distribution of temperature.

I had a large kiln 2 metres by 1 metre which had a requirement of 50mm/2” from the edge to even the temperature.  A recently purchased 50cm square kiln has almost perfectly even temperatures across the whole shelf.

[The illustration is taken from the ebook Low Temperature Kilnforming, available from Bullseye and Etsy.]

The required glass distance from the side will depend on side or top elements and size but no formula is available.  Testing for heat distribution is necessary once you have the kiln.

Replacement Kiln Vent Plugs

 

Replacement Kiln Vent Plugs

Accidents happen.  Sometimes the lightweight plug for the vent of a kiln gets dropped and broken.  You can replace this, whether brick or ceramic.

A quick solution is to roll up some fibre blanket or thick fibre paper into a roll large enough to fit into the hole.  This is enough to keep the heat from escaping and stop outside air flowing in.  If you leave excess outside the plug hole, it allows you to pull it out and view the interior as usual.  This will last quite a while and can be renewed easily.

A more permanent solution is to shape soft fire brick.  This can be shaped with a cheap saw. The brick is soft but very abrasive. So, use a cheap saw or an old one.  I keep an old saw especially for shaping bricks and vermiculite.  You could use 50mm/2 inch vermiculite in this way too, but firebrick this thick or more is easy to obtain.

Cut a cube from a fire brick.  This usually is about one third of a full brick.

Mark all around with a pencil how thick the shoulder (the outer part of the plug) should be. 25mm or 1 inch is thick enough.  It is possible to have it thicker if you wish.  The important element is that the outside part of the plug should not over balance the neck that fits into the vent channel.

Present the brick to the vent hole and twist a little, this will leave a mark to determine the diameter required. Alternatively, measure the inside diameter and draw this onto the end of the brick with a soft pencil or charcoal.

One end of brick cube marked, although a little off centre

Saw down to the shoulder mark on each of the four corners. Then make it eight corners. Test how well this adjustment fits to the hole.  It is probable that it is too big on the corners, but ok on the flat sides.

The first four corners sawn off to the shoulder

This is the time to use a wood rasp to round the multiple corners. Repeat the testing for size and adjusting until the plug fits the hole.  

The plug should not be tight.  It needs only a loose fitting so that it is easy to remove and put in.

Fit too tight to slide easily into the vent.

Finally, with 60 grit sandpaper round off any remaining corners.  Test and sand more off until it slides easily into the hole. This should not take more than a quarter of an hour to complete.

 

Fit just right. It slides in and out easily.

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.

How Close to the Edge

“How close to the edge of my shelf can I place a large piece?”

It depends in one sense how thick the piece is.  A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge.  Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature. 

But

The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece. 

This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. - http://www.bullseyeglass.com/methods-ideas/technotes-1-knowing-your-kiln.html

The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.

If you have greater differences in temperature than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.

I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.

Controlled cooling

It is sometimes stated that you can simply turn the kiln off below 370C and let the kiln’s natural rate of cooling take over the cool down.

This works for most flat 6mm pieces in most kilns, but as you work thicker or with greater contrasts in thickness, lots of tack fused elements or in a small rapidly cooling kiln, you do need to control the cooling toward room temperature.

The first thing you need to know is the natural cooling rate of your kiln.  

The rate of cool is not just about the annealing soak. The soak at annealing temperature is to equalise the temperature throughout the blass to have a differential of not more than 5C. 

The rate of cool is about avoiding thermal shock, too. The glass needs to maintain the temperature variation to less than 5 degrees Celsius difference throughout the glass as it cools.  This requires a slow controlled cool.  

You may program a cool of 100C to 370C thinking that the kiln will maintain that rate or less.  If the natural cooling rate of your kiln at 370C is 200C/hour, you risk thermal shock due to the rapid increase in the cooling rate.

You really do need to know the natural cooling rate of the kiln from the point you turn the programmer off to room temperature to be safe from thermal shock.

The alternative to turning off at 370C is to program the schedule all the way to room temperature.  The kiln will use no energy unless the kiln cools too quickly on its own.  At which point the program will kick in to slow the cooling of the kiln.

Finding Your Kiln’s Natural Cooling Rate

You need to observe how your kiln behaves while cooling without any power to be sure when you can safely &turn it off and let it cool without power.

Assuming you have programmed your kiln for a shut off at 370C, you need to observe every quarter hour or so to record both time and temperature.  From those observations you can calculate the cooling rate at the various temperatures.

Say at 6:00 your kiln was at 370C;

At 6:15 it was at 310C;

At 6:30 it was at 265C;

At 6:45 it was at 230C;

At 7:00 it was at 200C;

At 7:30 (you missed the quarter hour) it was at 160C;

At 8:00 it was at 140C;

At 9:00 it was at 125C;

At 10:30 it was at 110C.

To calculate the rate, you divide the temperature difference by the proportion of an hour between observations, as demonstrated in the following table.

Kiln Name/Description
Size
Shelf composition
Amount of glass
Observations
	Time	Temperature	minutes	Proportion	temperature	Rate of
1st	06:00:00	370	difference	of an hr	difference	cooling
2nd	06:15:00	310	15	0.25	60	240
3rd	06:30:00	265	15	0.25	45	180
4th	06:45:00	230	15	0.25	35	140
5th	07:00:00	200	15	0.25	30	120
6th	07:30:00	160	30	0.50	40	80
7th	08:00:00	140	30	0.50	20	40
8th	09:00:00	125	60	1.00	15	15
9th	10:30:00	110	90	1.50	15	10

Although this is an example, it shows how the cooling rate slows down as the kiln cools. 

If you were cooling a flat piece 12mm thick, you might get away with turning the kiln off at 370C, as a flat piece can cool as quickly as 300C/hr.

If you were cooling a piece 19mm thick, the natural cooling rate of the above kiln is too fast. 19 mm thick pieces need a cooling rate of 150C/hr, so according to the figures above you need to programme this kiln down to 230C to get the appropriate final cooling rate.

If it is a tack fused piece with a 6mm base and areas of two layers of tack fusing, you should fire as though it is 24mm thick.  In this case, the final cooling rate needs to be 90C/hr.  For the kiln in the example above, that rate is not achieved until below 160C, so that is the minimum temperature for switch off.

This method can be used for any temperature range.  For example, you may want to know the rate of cooling from the top temperature to the annealing temperature.  This method will work there too. You may want to record the temperatures more frequently than every quarter of an hour though.

Further information is available in the ebook Low Temperature Kiln Forming.

You really need to know your kiln’s natural cooling rate before you can be confident of switching the kiln off at 370C.  This blog shows a method of determining the natural rate of cooling.