Newcomers to glass, wishing to purchase a first kiln, may look to the pre-owned market, so these notes will outline how to assess what may be on offer.

In past years, Australia had a buoyant kiln making industry, but this has now shrunk to only a few, so that the craft market is now supplied almost exclusively by imports from America.

Because of different approaches to kiln design in the two countries they will be considered separately.

Australian made kilns

With minor exception, almost all Australian made kilns were for the pottery market, so a brief outline of pottery firing is in order, as it may have a marked effect on the condition of the kiln.

Pottery clays are fired in two main temperature ranges, stoneware in the vicinity of 1300ºC. and earthenware in the vicinity of 1100ºC. Clays and glazes used in these processes produce fumes which, when they condense at low temperatures can cause severe corrosion of unprotected metal components. They will also attack mineral or rock wool insulation when this is used near the shell (case).

Most of the small electric kilns are made to operate to either 1100ºC (2010ºF) or to 1300ºC (2370ºF).  There is no guarantee that a kiln designed to operate to the lower temperature has not been taken higher.

The major difference is the in the elements; their design depending greatly on the temperature to which the kiln is to operate. A certain diameter of wire can handle a higher current at 1100ºC than it can at 1300ºC. A kiln designed to operate to 1100ºC, if operated to 1300ºC, will suffer accelerated breakdown of the heating elements.

However, elements of even higher current rating than those fitted to the 1100ºC kiln can be installed if it is to be used for glass to about 900ºC. Depending on the condition of the rest of the kiln, it is often beneficial to fit new and higher powered elements if using it only for glass.

As seen previously, locally made kilns were either front loaders or top loaders.

They were roughly cubic in shape, IFB lined, having elements supported in grooves in the walls. Many did not have elements in the door. 

They usually had a mild steel case, and this was often attacked by the corrosive fumes from the clay. Sometimes stainless steel panels were inserts in a mild steel frame. This looked fancy but didn’t protect the frame from attack.

There are also kilns made for copper enamelling that have elements on only two sides. These are generally too small and have insufficient power input for glasswork.

It is usually not cost effective to fit elements to the other sides on kilns having elements on two or three sides.

Control of temperature could be by means of a Simmerstat and by observing pyrometric cones, or by a kiln sitter. Later, a temperature switch was fitted which turned off the power when top temperature was reached. A lower level of control is adequate for pottery than for glass, and fitting a better controller is usually necessary if converting a kiln from pottery to glass use.

American made kilns

American  design of pottery kilns has generally been toward multi-sided top loading kilns, with elements in the walls.

These products are mass produced by a small number of makers and sold here under maker or resellers brand names.

Diameter is determined by the number of bricks forming the wall: 6, 7, 8, 9 bricks in a circle. The number of rows of bricks determines the height. They are deep in relation to their floor area.

There have been a number of different methods of lid construction used over the years, including IFB, varieties of fibre reinforced castable, or vacuum formed fibre products.

Control of these kilns could be by a ‘kiln sitter’ that uses a pyrometric cone, matched to the work being fired, to turn off the kiln at the appropriate time. They often had built-in controls to regulate the application of power to the elements at various stages of the firing, depending upon whether the firing was bisque, glost, stoneware. None of these are directly applicable to glass, so some wiring and control modification is desirable if reasonable glass firings are to be obtained. Subsequently, programmable controllers were fitted.

With the growth of the glass hobby market in America, cut-down versions of the pottery kilns became available, with the same programmable controller that is fitted to pottery kilns.

See Programmable controllers elsewhere for more on them.

The lack of a viable Australian small kiln maker results in the bulk of the local demand being met by imports from a number of American makers, including Evenheat, Paragon, Skutt and others.

Assessing the condition of a pre-owned kiln

The case or shell

• Look for rust on the case. This may be at joins of the case, around rivets or screws. Are screws tight or is the steel rusting around them? Are the actual screws or rivets rusting? Some small amount of rusting is acceptable around the vent hole but too much can point to bad corrosion of other areas of the shell. With mild steel cases or frames, corrosion starts on the inside and may be well advanced before it appears on the outside.
• Are there areas of overheating of the paintwork? This can indicate a breakdown of the insulation.
• How does the door or lid fit? Is it buckled, sagging, are the hinges secure or loose? Does the catch lock securely?
• Does the door interlock work? Does the actuator align with the switch? Does it look like it has been re-shaped or altered in some other way?
• On a front loading kiln, how extensive is the discolouration of paintwork or metal above the door? This can indicate a badly fitting door.

Brickwork

• How severe is cracking in the brickwork? Hairline or gaping open?  Hairline cracks are normal but only re-bricking can repair wide gaps. Filling the gaps is not effective, as the filling will not penetrate far.
• What is the condition of the brickwork supporting the elements (bottom lip of groove)? Are the edges in good condition or crumbling away?  A small amount of damage is acceptable. Sections of missing lip can be replaced but it requires much patience and some skill. See Insulating Fire Brick for more.
• Major repair of brickwork is usually undertaken by re-bricking.
• American style multi-sided kilns have the bricks retained by a stainless steel band. They aren’t mortared in position so can be dismantled if the element is removed and the band loosened. Damaged bricks can be replaced if replacement bricks can be found. The machining and size of the brick is crucial so make sure you have the correct replacement item before attempting repair.
  
• Oval shaped kilns called ‘coffin’ kilns can suffer damage to the bottom row of wall bricks; large runs of the lip can break away, leaving the element resting on the floor.

Elements

• Are the elements on all sides of the kiln?  Anything less is not suitable for glass. In particular, do not even consider kilns with elements on only two sides.
• The element spiral should appear evenly spaced with the coils vertical and not leaning over on their side. Adjacent coils should not be touching. The spirals should be in the grooves and not crawling out of them. A slight amount of displacement is acceptable but you are cautioned against trying to push them back in or straightening them. The element wire becomes extremely brittle with use (particularly at pottery temperatures) and could well break if efforts are made to return them to their original position. See elements for more.
• How pitted is the surface of the element wire? New wire is shiny and smooth, but it quickly dulls. Extended use causes the surface to become pitted and cratered. 
• An extension of the life of the kiln can be obtained by pinning the elements in position.

Elements can be replaced at reasonable cost. It could be advantageous to consider fitting new elements of higher power if you are contemplating using a kiln designed for pottery temperatures for glass work.

Bead annealing kilns

An exception to the rule regarding elements on all four sides are kilns used for bead annealing. This style of kiln is usually long, narrow, and not very high, with elements on only the two long sides; ideal for accepting mandrels but not really suitable for firing flat glass. For safety, elements should be shrouded to prevent metal mandrels coming in contact with the elements.

Rated voltage

American voltages are different to that used in Australia. In Australia it is 240Volts whilst the American is 110Volts; with a High Voltage of 220Volts.

Whilst kilns currently being imported suit the Australian voltage, some dubious methods were previously used to most easily make them work here.

One model of Skutt kiln, the small Hot Start model CK668, is particularly suspect. See the special note later in this paper.

Be particularly careful when replacing elements in American kilns. 

Power, Instruments and controls.

• Check that switches operate through their full range.
• Are labels readable? At the least, the function of all controls should be clear and there should be no risk of them being used incorrectly because of illegible or missing labels. 
• Are all controls properly secured?
• Is the power lead and plug in good condition?
• Apply power to the kiln. Turn it on. Let it run for a short time and check to see that any temperature-indicating device shows a rise in temperature.
• Are the elements working. A simple method of testing is to place a small piece of paper on each element and then plug in the kiln and turn on the power for a couple of minutes. Scorched or burnt paper indicates a functioning element.
• If fitted with a temperature cut-off device, set it to a low figure and verify that it does cut-off when the set temperature is reached. 50ºC is enough for testing.

A kiln which is otherwise in good condition can have the most up-to-date programmable controller fitted at reasonable cost. See  Programmable Controllers for more.

Kiln furniture (shelves & props)

• The shelves should be about 20mm to 30mm smaller than the kiln floor in each direction.
• There should normally be at least two shelves with a pottery kiln, plus six or so props. (shelves are supported on three props, not on four.)
• Shelves or props with hairline cracks or which make a dull sound when tapped are of no value.

Literature

Are there any operating instructions? This can be of more importance with some imported kilns having many knobs but little labelling. It is especially important with American kilns with either programmable or three button controllers.

If no literature can be supplied, try and establish from whom you can get information, spares and service.

IN BRIEF

• ELEMENTS AND CONTROLS CAN BE EASILY REPLACED OR UPDATED, AT REASONABLE COST.
• SUBSTANTIAL REPAIR OF DAMAGED BRICKWORK OR RUSTY OR BROKEN STEELWORK CAN BE COSTLY.
• FITTING ELEMENTS IN AREAS WHERE THEY ARE NOT ALREADY INSTALLED IS RARELY COST EFFECTIVE.

Special note.

I was recently asked to advise on fitting a better programmable controller to a Skutt kiln. It was in its original packaging and contained literature identifying the importer, as well as makers instructions stating that it was designed for use in Australia.

The information detailed below relates to that particular kiln, and it is not known whether any others with similar defects have been released here.

Labels identified the kiln as follows;

Hot Start by Skutt.

Hot Start CK668.

Serial number 104696 with Glassmaster LT controller

The compliance plate read 240Volts 15Amps 1800 Watts.

Instructions accompanying the kiln stated that it  must be fitted with a 15Amp power plug. That would obviously be an Australian 15Amp plug. 

Whilst the compliance plate stated it was rated at 15Amps 240Volts, it was found to be fitted with an element suited to the American 110Volts and not the 240Volt rated element it should have had. This higher voltage would drastically overload the element and lead to its premature failure.

 This kiln is fitted with the Bartlett 3 button controller which uses ON-OFF control, so that kiln temperature rises or is maintained in a series of bursts followed by drops until the next burst arrives; causing the temperature profile to appear as a zig-zag.. In this case the overloaded element made the swoops and drops more violent than normal

 At one point the indicated temperature rose from 93^oC to 105^oC in a few seconds and then dropped to 101^oC before the next power burst.  At soak, supposedly 796^oC, the temperature fluctuated rapidly over the range 792^oC to 814^oC.

The compliance plate carried incorrect information. Watts is Volts multiplied by Amps. 240Volts x 15Amps is 3600Watts, not 1800. To achieve the 1800Watt rating stated on the compliance plate would require a current of 7.5Amps (1800 divided by 240 equals 7.5)

.Such a kiln with the proper element could be fitted with a 10Amp plug and be used on any General Purpose Outlet (power point) in Australia; instead of a special outlet costing hundreds of dollars to install.

 At the end of the second firing the element was falling out of the groove in the lid in two places. This was undoubtedly because of the excessive power overload on the element. The element wire maker, Kanthal, recommends a power loading no higher than about 4 watts per square centimetre at kilnforming temperatures. In this case it was about double what it should be.

How could such an experienced kiln builder get it so wrong?