The kiln: A brief outline

The kiln is to the kilnformer what the oven is to the cook. It is a tool, albeit the most expensive item in the toolbox; but still a tool. As when choosing all tools, it helps if one has a reasonable understanding of the purpose for which the tool is being chosen.

Cooking glass is different to cooking cakes, or pottery. Not all that much different, but enough that a kiln designed for cooking clay may not do the best job with glass. 

Glass needs even heating all over; it does not like being hotter in one area than another. It requires slow cooling through the annealing range; so pottery kilns with Simmerstat and Kiln Sitter controls cannot do the job properly.

How heat behaves in kilns

First, a few points about firing glass and the way heat behaves in kilns:

• Hot air rises, so tall kilns will be hotter at the top than at the bottom.
• In the home oven, two identical cakes on different shelves will rarely cook the same. The top one will often be cooked before the other and may be taken out first: with the  bottom one being left in a bit longer.
• So with glass: except that one piece cannot be taken out before others. To remove glass before it is cool would almost certainly result in it breaking. Minor differences in temperature can alter the appearance of fired pieces, so that items from the bottom of a tall kiln can appear different in detail to those from the top.
• Glass needs to be heated as evenly as possible from all sides; so kilns should have elements on all sides, or evenly across the lid or roof.
• Most kilnformed items are large in area and low in height, so the ideal glass kiln has similar dimensions; a large hearth or shelf area and low height.
• Ideally, glass is fired on a single shelf in a kiln; so that it is all heated to the same temperature.

A few words on fuel

All that is needed to cook a cake  is a source of heat, which can be wood oil gas or electricity. Of these, electricity best meets the needs of most kilnforming activities. It is clean, easily controlled, comparatively hazard free. However, it is restrictive of the amount of energy which can be packed into a space so, for the firing of heavy or thick glass items a gas fired kiln will sometimes be used.

Properly controlling a gas fired kiln can be complex, and takes time to learn. Potters using gas kilns have already gone through this learning process, so will sometimes fire their glass in their gas fired kiln. Still others may have no option, living or working where sufficient electric power is not available.

Forget the wood or oil fire: the glass gets too dirty.

Choosing a kiln

There are three main types of kiln, based on the way in which access is gained:

• Top Loader. Square or multi sided with a hinged lid.
• Front Loader. Like a fridge with a hinged door 
• Top-hat. Shallow upside-down box suspended above the hearth and raised for access.

A  variant of the Top-Hat is the Clamshell, where the top is hinged on one side and supported on gas struts. With the exception of the top-hat, most of the kilns sold as being for kilnforming are modifications of pre-existing pottery kiln designs.

Top Loader

Small top loading hobby kiln

New kilns on offer, especially for hobbyists, are primarily of American origin. The brands will differ only marginally one from another, mainly in the type of controller and in the degree of sophistication of firing control. They will have a lift-up lid, be mounted on short legs and have elements in the lid and sometimes also in the wall.

Coffin kiln

To accommodate larger glass pieces they may be elongated with rounded ends as in the unit shown above; commonly called a coffin kiln.

Australian top loader

The traditional Australian top loader was a square or rectangular box with a counterbalanced hinged lid. They were designed for pottery firing, so were deep with basic firing controls. The elements were only in the walls. Rarely made now, but pre-owned units are sometimes on offer.

Front loader

Front loader 

Is the type of kiln which was traditionally made in Australia for pottery. The elements are always mounted in the walls: not always in the door, occasionally also in the floor. When firing pottery they are used with a number of shelves; often three or more.

When firing glass one can rarely use more than two; sometimes only one. Smaller versions, often having elements in only two  sides, are used for copper enameling and similar crafts. These are really not suitable for kilnforming but can be used for bead annealing.

Glass needs a more even distribution of heat than pottery, so kilns with elements on less than four sides aren’t really suitable. It is rarely cost effective to try adding extra elements where they weren’t installed originally.

Taller front loaders are the preferred style for glass casting, as they facilitate the loading of heavy moulds; but they must have elements on the door, and preferably also in the floor. Kilns of a similar style but sold as suitable for glass firing are offered by American makers and some are appearing in Australia.

Bead annealing kiln

To meet the demand for an annealing kiln for bead makers, a style of squat front loader with a lift-up or guillotine door has become available. They usually have elements on only two sides so are a second-best choice for kilnforming; although they can be used for slumping and fusing provided that one can accept the effects of the uneven heating.

Top-Hat kilns

Consist of a flat hearth covered by an inverted box or hood, with provision for raising the hood by jack, hand or power winch. They are the style preferred by commercial kilnformers and can have hearths many square metres in area. They usually have ceramic fibre insulation in the hood with the elements spaced evenly across the roof.

Because they are generally used for shallow slumping and fusing, they have an internal height of only about 300mm or so.

This style of kiln can have the hood suspended in a number of ways:

• It can be suspended above the hearth from the building structure or an overhead gantry by chain block or power hoist.
• Can be suspended from a wheeled frame so that the hood can be lifted and then wheeled clear of the hearth: either for greater access to the hearth or to allow the hood to be used on two hearths.

Riley Hi-Lift kiln

Can be built as an integral, self contained structure including hood, hearth support frame and lifting gear.

FS-1

They can have a hearth which rolls out from under the elevated hood for easier access to the glass. They can be large like the Riley FS-1 shown above, or be small like the Riley GS single phase craft kiln seen in  ‘Making a platter’.

Clamshell type

A cross between the top loader and the top-hat styles, with the lid hinged along one long side. In some designs the lid may be like that in the top-hat kiln except that it is hinged instead of lifted. In others, the walls may be split, with part incorporated in the base and the rest forming part of the lid. They can be small like the Skutt GM22CS or large commercial units like the one above which is installed at the Canberra Glassworks.

Kiln controls

Firing pottery requires control of temperature only to the end of the heating stage, after which the power can be turned off and the kiln allowed to cool down naturally. The glass firing process is more complex, and requires control of temperature during cooling to ensure the glass is properly annealed. This can be done in a pottery kiln, but it requires constant supervision and is so much simpler in a kiln fitted with a programmable controller: and with satisfactory results being so much more likely. 

If buying a new kiln one must accept the type and level of control chosen by the kiln maker.

If considering a pre-owned kiln, and especially an older pottery kiln, there is the opportunity to make design and control changes to better suit it to its new purpose.

Information on 'Evaluating a second hand kiln' and on updating the controls or other features can be found in the section 'all about kilns'.