The kiln in depth

Kiln is one name for what is essentially a box with heating elements to introduce the heat, and insulation to slow down its loss to the atmosphere. Collectively they can be referred to as heat enclosures and come in a vast range of shapes and sizes; and with lots of different names depending very much on the terminology used in a particular industry. Kiln is the accepted term in the ceramics industry.

 They are used in two kinds of processes:

• Continuous, where the product is moved continuously through various temperature zones on conveyors or through pipes.
• Periodic or batch, where the temperature of both the product and the enclosure are raised and lowered conjointly.

Glass firing is a periodic process and suffers from all of its inherent inefficiencies.

How heat travels

Before delving too deeply into kilns, first a look at how heat moves around in the kiln.

Heat is energy. It flows only from hotter to colder; not the other way.

There are three ways in which this occurs, called the modes of heat transfer:

• Convection, where heated air expands, becomes lighter and rises to allow colder heavier air to flow in underneath. The heat in the hot air can transfer to colder air or to solid objects with which it comes in contact.
• Conduction, where heat flows from the hotter to the colder parts of solid materials. Thus, it flows from the hot side to the cold side of a piece of glass on a hearth or shelf; or through insulation from the inside to the outside.
• Radiation, where the heat as electromagnetic rays travel through space to heat any object they encounter. Radiated energy can travel in any direction. Thus, radiant energy from an element in the roof can heat glass below it.

As well as flowing into the glass and the shelves and moulds, it also flows into and through the insulation to the outside shell and then to the atmosphere. It is important to remember that insulation doesn’t stop the flow of heat; it only slows it down.

Speed of heat flow

The speed at which heat flows is dependent on the method by which it is moved.

When transferred by convection the rate is slow; relying as it does on the small forces induced by slight pressure differences. This is the principal mode of transfer at low kiln temperatures, and explains why temperatures are so uneven in a kiln early in a firing.

The rate of heat transfer by conduction is dependent more on the nature of the material than on the temperature difference inducing the flow. In any case, conduction is responsible more for heat loss out of the kiln than for heat circulation within the kiln. 

Radiant energy travels at the speed of light.

Where the heat flows

Let’s look at where the heat goes during a firing. Imagine that the only control is an ON-OFF switch and the switch is turned ON. The kiln will heat up as fast as it can.

Fig 1

When first turned on, the temperature inside and the outside will be the same. There is no difference to make the heat flow outwards, so it is all retained inside to rapidly heat the load and the inside of the kiln. The kiln heats rapidly, as in Fig 1

Fig 2

As the firing progresses the rising temperature forces heat through the insulation to the outside. Some will heat the glass and kiln furniture, some will heat the insulation itself (heat storage) and the rest will continue on to heat the shell and the surrounding air (heat loss). There is less available to heat the load and the kiln temperature rises more slowly, as in Fig 2.

Fig 3

As the kiln temperature continues to rise, the rate at which heat is lost will also rise. Eventually, the heat will be flowing out through the insulation as fast as the element is putting it in, so there’s none to raise the temperature of the load.

There will be a balance between the heat input and heat loss and the kiln temperature cannot rise any further. Rate of Rise (ROR) will be zero.

Fig 43-04

Fig 4 shows the heating curve for the kiln, as curve A.

If more powerful elements are fitted, the kiln will be able to heat faster, as well as being able to reach a higher temperature before the balance point between heat input and heat loss is reached; resulting in the curve B.

Curve A could be labelled ‘lower power’ and curve B ‘higher power’. 

If the kiln had more effective insulation then heat loss to the atmosphere could be reduced. The kiln would heat faster and reach a higher temperature before there was a balance between heat input and heat loss.

Curve A could describe a kiln with poor insulation whilst curve B one with more effective insulation. 

Of course, taking forever to reach soak is no good to the kilnformer, nor to the glass.

In practice, soak temperature should fall well down from the peak temperature of the kiln, so that it will have a reasonable rate of rise at soak temperature. See Kiln power below.

Stored heat

All of the energy which has raised the temperature of the glass, kiln furniture and the insulation is stored heat.

The quantity for a particular firing won’t vary with changes to the firing time, but only with changes in soak temperature. Once having been heated to a particular temperature the materials cannot absorb any more energy, irrespective of how long they are held at that temperature. Changing the top soak temperature can increase or decrease the total amount of energy stored.

It all must be lost to the atmosphere to allow the kiln to cool down.

In heat balance calculations it is stated as a single value, as heat storage for the firing.

Lost heat

Fig 43-05

The temperature difference between the inside of the insulation, (called the hot face) and the outside shell of the kiln (called, unsurprisingly, the cold face) causes heat to flow outwards.

The greater the temperature difference the greater will be the quantity of energy which will be lost from the cold face to the atmosphere. It can be thought of as 'heat pressure' and this energy flow will continue as long as the temperature difference is maintained. Therefore:

• Much more energy will be lost during a slow firing than during a fast firing to the same temperature.
• The higher the firing temperature the greater the energy loss per unit time.

In heat balance calculations, heat loss is stated as quantity of energy lost per unit area per unit time.

Heated air

As air is heated it expands. Between room temperature and 800°C (1470°F) it expands to about 3.7 times its original volume. This heated air must escape from inside the kiln, and when it does it takes heat with it.(In pottery firing to stoneware temperatures it expands over 5 times)

It can escape through vents, at gaps between door and body on a front loader or between body and lid on a top loader. In a top-hat it will escape around the bottom of the hood.

This heated air cannot be retained, because the kiln cannot be sealed and pressurised.

Because the air inside the kiln is lighter than the surrounding air, it will rise. Any gaps or openings which will allow cold air to enter and hot air to escape will give rise to what is called the chimney or stovepipe effect. The expanding cold air in the kiln acts like a pump, encouraging the escape of more hot air. For more, see Rapid cooling the kiln.

Insulation

There are two types of insulation which can be used; Insulating Fire Brick (IFB) and Ceramic Fibre (CF)

In general, it can be said that a given thickness of CF will have lower heat storage and lower heat loss than will IFB.

However, other factors can have a great influence on which material is used in a particular situation. These can include;

• Need for element support
• Difficulty of installation.
• Susceptibility to damage.
• Personal preference.
• Health and safety restrictions
  
• Material and labour costs. 

In small kilns it is common practice to use grooved IFB wherever element support is needed, as in the walls and lid of top loaders or the walls of front loaders.

In larger kilns a composite lining may be used, with grooved IFB where elements are needed and stackbond CF for the remainder of the walls and roof.

In larger glass kilns, where elements are roof mounted, the walls and roof will generally be CF. The CF will usually be in the form of blanket, but will sometimes be fibreboard.

The choice of insulation will usually be made by the kiln builder or designer.

There’s more on these matters in Insulating Fire Brick and Ceramic Fibre linings.

Kiln efficiency

All of the matters mentioned above will have an impact on the efficiency of the firing.

A measure of the efficiency is the proportion of the total energy used which actually  goes into heating the load.

The amount of energy required to heat the glass can be calculated from its mass, its specific heat and the temperature rise.

Specific heat  is a measure of the amount of energy required to raise the temperature of unit mass of a solid by unit temperature. It varies with the material; with glass, brick and refractory materials being close to double that of metals.

Consider the platter shown in the general content section of the site.

Its mass is about 1.5kg. (3.3lbs)  Based on an energy cost of 15 cents per kWh (kilo Watt hour) the cost of the energy to heat the glass to 800°C (1470°F) would be 6 cents. At the same energy cost per kWh the total cost of the firing in that kiln could be from $1 to $4, depending on how fast was the firing and on how efficient was the kiln.

Even if one regards the mould and the kiln shelf itself as essential parts of the load, then the energy cost to heat the load would be about 28 cents.

These kilns are called periodic, and the efficiency isn't very high.

How insulation works

As has been seen, the bulk of the energy used during a firing goes into the insulation or through it to warm the studio; described as heat storage and heat loss.

Reducing heat storage is achieved by lowering the total mass of the insulation or by using insulating materials which have the lowest specific heat (amount of heat energy needed to raise the temperature of unit mass by one degree). Great advances have been made with low temperature insulation, but little change has been made in high temperature insulation.

To reduce heat loss, the rate at which the heat flows through the insulation must be reduced. Consider if one must make a cross town trip by public transport. The greater the number of changes from bus to tram to train involved, then the greater the time for the trip.

A similar approach is taken with kiln insulation. IFB contains myriad air cells dispersed throughout. As heat is flowing through it has to constantly change mode of transfer from conduction to convection as it encounters solid material or air cell.

With CF, there again is much changing of modes of transport from fibre to air to fibre. With fibre linings there can be a more free flow of air than with IFB, but this can be retarded by the use of membranes such as aluminium foil between the layers. See Ceramic fibre linings for more on this.

Kiln Power

A kiln of a given size can be fitted with elements of different power handling capacity.

Thus, the single phase Riley FS-2 has been fitted with 20Amp, 25Amp and 32Amp elements.

The choice in each case was to meet the specific need of the user; influenced mainly by the power available.

The most common arrangement was 25Amps, and at that power input it performed quite satisfactorily. In a normal domestic situation having 50Amps or 80Amps available, a kiln drawing that current poses no great restriction on activities.

In one case, 20Amps was all that was available and the kiln replaced an existing brick lined front loader. The heat-up time was long, but still shorter than the kiln it replaced. In addition, the size of item which could be fired was increased by over 50%. 

It is not always possible to take advantage of the faster heating rates of a more powerful kiln. However, when it can be done there can be substantial reductions in firing time and in power consumption.

In tests on two identical GS-1 kilns, one 15Amp and the other 20Amp, with identical float glass loads fired to the same pattern, the 20Amp kiln used 10% less power and reached soak temperature in almost 30% less time than did the 15Amp unit.

The saving in time was almost all above transition temperature.

Kiln cooling

 Fig 43-06

When the power is turned off there will be a big difference between inside and outside temperatures, so the heat flow outwards through the insulation will be rapid; but will slow down as the kiln cools and the temperature difference between inside and outside becomes less. This is shown in Fig 6.

The rate at which the kiln will cool will depend on the amount of heat stored in the load and the insulation; as well as on the efficiency of the insulation. For more infomation see rapid cooling proceses.

Pottery kilns

Kilns for glass work have evolved from those designed for pottery. They were generally cubic in shape, because that gives the best ratio of volume to outside surface area. A tall narrow kiln will have a greater surface area than a square one of the same volume. It is from the outside surface that heat is lost, so the lower the surface area the lower the heat loss.

This is OK for potters, because pots also tend to be cubic in shape, so they can be packed in to get the most efficient load density.

Australian made kilns were generally square, and were made in a range of sizes having increasing shelf size accompanied by a corresponding increase in height. They were generally front loaders, with the occasional top loader.

As well as front loaders, American makers had adopted the multi sided top loading  design, making kilns of varying capacity by stacking rings one-on-top-of-another; and also by changing the number of bricks in each ring. The top loader is the cheapest to produce, but is poorly insulated. The outside reaches dangerously high temperatures during a firing.

Glass is not so accommodating. Most pieces are thin or shallow and relatively large in area, so the conventional pottery kiln was not the best shape for kilnforming.

It has long been known by glass artists and commercial kilnformers that the top-hat was the most effective design for glass kilns, but they are more expensive to build than other designs.

Pre-owned kilns

The bulk of those on offer will be pottery kilns with controls appropriate to that medium and to the accepted practices of the time the kiln was built. The various types of control which have been used are discussed in Simple Kiln Controls.

Choice of kiln

A glass kiln should allow the kilnformer to:

• Automatically fire the glass to a comprehensive firing pattern
• Not require constant supervision
• Allow monitoring of the progress of the firing
• Allow for crash cooling procedures with optimum safety.

The first two are met by having a fully programmable controller and adequate power input.

The third requires clear vision of the progress of slumps and edge development; viewing from the side.

The fourth is met by being able to vent hot air away from the operator.

It is generally accepted amongst professionals that the Top-Hat kiln is the best type for kilnforming, meets all these needs as well as allowing the glass to be safely worked whilst hot.

Lacking suitable kilns at an affordable price the hobbyist must compromise.

With the exception of the top-hat, most of the kilns sold as being for kilnforming are modifications of pre-existing pottery kiln designs.

As a result, the predominant type of new kiln available to kilnformers in Australia is the top loader. Most will be imported from the USA, bearing brand names such as Evenheat Paragon Skutt.

New kilns for kilnforming should be fitted with a programmable controller, and these are discussed in Programmable controllers elsewhere in this section.

However, one aspect which should be mentioned here is the difference in approach to programming and kiln control between American and Australian kiln builders and users.

The Australian approach has been for the kiln builder to provide controls which would allow the user to exercise full control of the firing, on the assumption they had a reasonable understanding of the firing process. See The kilnforming process explained.

The American approach is more toward offering a series of button selectable pre-programmed options which can be used by those with little understanding of the firing process.  This is sometimes referred to as the ‘let’s cook a chook’ approach, as with ‘easy-to-use' microwave ovens.