Factors impacting on kiln design

 By and large, glassies are badly served by kiln builders. They are often presented with what the kiln builder believes they want, with little chance of making any input. It is small wonder that those with a measure of mechanical aptitude undertake to build their own kiln.

It matters little whether the work be Art or Commercially centred; other than for the artist having more control over the nature of the works they will create.

Detailed below are some of the matters to which they should attend in making their ordering or building decisions.

Type of kiln.

There is little doubt that for most slumping fusing and small mould work a top hat kiln is far superior to other types. This superiority comes at the price of complexity and higher cost.

For larger glass casting, where a front loading kiln allows easier access, there are still a number of kiln builders in Australia who can build good kilns of this type.  Essential is elements on all four sides and desirable is elements in the floor.

One style of Top-hat especially  suitable for massive glass casting is the Bell kiln. This has a flat hearth and a circular or square hood which is tall in proportion to its hearth area. Can be either electric or gas fired. Contact me if interested in learning more.

Top Hat kilns.

Let’s assume that they will be used for flat glass work, so will be top fired and the internal height will be about 300mm between hearth and elements.

Top temperature will be generally around 800°C but to allow for some experimentation an upper limit of 900°C will be assumed. Because the firing cycle is short, a low thermal mass lining will be most effective.

They are the easy decisions. The rest are a bit harder.

Size of hearth.

This is a major decision and one which will impact on and be influenced by a large number of factors, including;

•    Available power.

•    Size and volume of work,

•    Space available

•    Access restrictions

•    Budget

Available power..

A major limitation on the size of kiln will be the power available in the premises. Whilst not using enormous amounts of power, firings can often benefit from a large concentration of power for a short period. This is especially the case with architectural work, to minimise devit.

As a general guide, kilns with a large hearth area and a hearth to element clearance of about 300mm should have installed capacity of about 9kWatts per square metre of hearth area, rising to around 12-13kW for smaller kilns.

As examples,

•    4000 x 1400 hearth, 9kW per sq. metre.  70Amps 3 phase

•    2000 x 920 hearth, 12.5kW per sq. metre.    32Amps 3 phase

•    1300 x 675 (Riley FS-1), 13.1kW per sq. metre.    16Amps 3 phase

•    650 x 650 (Riley FS-3) , 14kW per sq. metre.   25Amp single phase

Sometimes there will be severe limitations on available power and these values need be reduced.

•    One client had available only 20Amps single phase, so reluctantly a 20Amp FS-3 kiln was supplied. It took 3 hours to reach soak temperature with a 600mm circle of 10mm glass and mould, but was so much better than alternatives that it was considered a success.

•    One long running kiln with a 3000 x 1500 hearth uses 8kW per square metre. 48Amps 3 phase. This because the building had only 50Amps installed. The kiln was fired at night, with special provision to allow for annealing during the morning.

Size and volume of work

This can vary widely. Large pieces require a large hearth, but often a client will want a large kiln to fire many small pieces or, sometimes, just to have a large hearth; maybe to have the biggest.

A fairly common request has been for a 3000 x 2000mm hearth. This is a nice round number and my response has generally been to ask, Why so wide?.

Rarely will it be to fire a single piece of glass approaching 2 metres wide. If the need for firing such size pieces exist, so well and good. If just to get more smaller pieces in a firing; then there are better arrangements of width and length.

Two problems associated with wide hearths are;

•    Security of element support, and

•    Access to centre of hearth for setting out designs and loading glass.

In large kilns with low power density the element support tubes do not need to be of large diameter. The longest readily available length is 1500mm. As explained elsewhere, (see Elements) achieving a secure join with small diameter support tubes can be difficult and is a frequent source of trouble.

•    My recommendation is, that unless there is a specific need, limit hearth width to 1400mm and allow the use of element support tubes without joins. It is axiomatic that the elements run in the shortest direction. This allows for a greater number of lower power elements, and reduces the percentage power loss should an element fail during a firing. Hopefully that doesn’t happen, but with the Kiln God/dess around!!

The centre of a 1400mm wide hearth is reasonably easy to reach without the need to support oneself on the hearth. This simplifies the laying out of designs in bedding powders and the loading of glass without significant disturbance of the design.

Hood support and access

There are numerous ways in which the hood can be lifted off the hearth, They include;

1.    Overhead gantry using electric or hand winch lifting.

2.    Fixed straddle frame allowing the hood to be lifted above head height for secure operator access. Hi-lift hood.

3.    Fixed straddle frame with the hearth on wheels for rolling out from under the hood. Rolling hearth.

4.    Straddle frame on wheels allowing the hood to be rolled away from above the hearth. Rolling hood.

Overhead gantry using electric or hand winch lifting.

This can be by fixed or swinging monorail with a trolley supporting an electric winch or chain block. It will be a single point lift, so the hood will need to be well balanced to ensure it stays level whilst being moved. Some sort of guide arrangement is desirable to ensure proper alignment of the hood with the base before it makes final contact.

 The monorail can be secured to the building or carried on A frames at each end.

 Fixed straddle frame allowing the hood to be lifted above head height for secure operator access.

A typical version of this style is the Riley Hi-lift design shown in Basic kilnforming/The kiln, a brief outline, where the entire kiln is a single structure which can be picked up by forklift and moved in one piece. One large Hi-lift kiln was recently moved between buildings and placed on a mezzanine floor, requiring only the disconnection of power cables.

The hood has four point lift and fixed guides to ensure correct registration with the hearth. Special safety catches are needed to allow the operator to work beneath the suspended hood.

Fixed straddle frame with the hearth on wheels for rolling out from under the hood. Rolling hearth.

The hearth is mounted on wheels which run on guides to ensure its correct positioning below the hood. One or two hearths can be used, with one hearth being entered from each end or side.

Straddle frame on wheels allowing the hood to be rolled away from above the hearth. Rolling hood.

Again, one or two hearths can be used, with the hood and frame moving between the two.

In both Rolling hearth and Rolling hood designs hand or power lift can be used. Because of the complex electrics, power lift can cost about $1500 more than hand lifting for a typical large kiln.

Both designs require some form of guides on or in the floor to ensure proper mating of hood and hearth.  A common method is to use Vee grooved wheels running on an inverted angle iron strip. In recent times, problems have been experienced with Work Safe inspectors deeming the tracks a safety hazard. The alternative of steel edged slots embedded in the floor can be expensive to install; especially in rented premises.

One disadvantage of the rolling hearth arrangement is that vibration whilst the hearth is being moved can cause the glass to flatten textured or embedded designs. This is less likely to happen with the rolling hood arrangement.

Requirements for access and for safe handling of glass.

Not to be overlooked is provision of adequate space for access and for loading glass. Loading can be more critical than unloading as the glass is sharp and texturing or designs on the hearth may require much more delicate handling.

Hood construction.

Low thermal mass hood linings are discussed in All about kilns/Ceramic fibre linings, but there is one aspect which should be mentioned here.

There are two methods of construction;

•    Layered or wallpaper construction, and

•    Stackbond construction.

Where the hood seals to the hearth a considerable amount of wear takes place. The layered construction requires a wear layer of blanket to be fitted and this requires maintenance and replacement from time to time.

The stackbond method presents a cut face to the hearth brick so that it is compressed under the weight of the hood, improving its effectiveness as a seal.

In one kiln  having made over 2000 firings the seal is as good as when built.

In another case, the hood was lowered onto a glass case and the lining was severely pushed out of shape; but was easily made good using a couple of wood paddles. Such an incident with a layered lining would have required considerable repair.

I am a strong advocate of stackbond construction as it provides an almost limitless life at glass temperatures, gives a better hood/hearth seal and allows for a more secure element installation than layered construction.

It is slightly more costly to install, but this spread over the long maintenance free life is not great.

Hood / hearth sealing.

Although it has been discussed elsewhere, the design in the hood/hearth interface area is so important that it should be repeated here.

Even in a completely sealed kiln, cold air will be sucked in during cooling. This is because it reduces in volume as it cools. A likely place for it to enter is where the bottom edge of the hood meets the hearth.

Were this cold air to impinge on the glass it can create stress sufficient to cause cracking at some future time. There have been very costly failures of major architectural installations due to this kiln design fault.

It is a feature of Riley Glass Kiln designs to create a depressed lip around the hearth proper so that cold air will be deflected upwards across the wall and not be able to impinge directly onto the glass.  See Crash cooling your kiln.

•    It is bad practice to extend the surface of the hearth right to the outer edge of the hood.