Woodrow Kilns – [Promotion]
Perhaps the question should be qualified by asking “Best at what?”
We could ask “Which gives the best firing results, which has the best efficiency or which is the best and easiest to use?” The argument could also extend to include “Which offers the best service life, the best safety or the best (lowest) repair cost?”
Best Firing Results
There are basically two issues here – heating rate and heat distribution. Obviously the best result is achieved by the kiln with the most even firing chamber. But, several factors affect this and identifying them, and then understanding them, is crucial.
When getting to know a new kiln, it should be test-fired, with and without work pieces, using pyrometric cones placed strategically around the chamber. A kiln log should be used to document all firings. The early logs will establish heating rate capability and heat distribution.
The initial unloaded result should give a fast even firing, without the influence of the load. By comparison, the longer, loaded firing may depend on operator-influenced variables such as the load distribution, the load mass, moisture content and chosen heat rate. Other factors will include the wall thickness of the pieces and the complexity of the shapes. All of these will affect uniformity and how well the kiln can maintain the chosen heat rate. The chosen heat rate should always be within the capability of the kiln, otherwise the firing will be virtually uncontrolled, and reproducible results may be almost impossible. This will apply particularly if the load mass is different from one firing to the next, which it generally is.
In regard to heat distribution, consider the concept of natural heat rise, for example, in an electrically heated kiln. Some of the element heat will heat the chamber air, resulting in an initial convection air current upwards. Most electric kilns have elements equally spaced in the walls from bottom to top and are of equal power, so, typically, most kilns will have a cold spot in the bottom and a hot spot in the top. However, by careful distribution of the load mass, a clever operator may be able to make a bad kiln fire perfectly, just as an inexperienced operator may make a good kiln fire badly, through poor distribution. How can heat distribution be improved? Variable element spacing where the elements are closer together at the bottom and further apart at the top, is one simple solution, helping to offset the effect of natural heat rise.
Similarly, in a gas-fired kiln, the gas flow path through the kiln is critical to uniformity.
Generally, a downdraft firing kiln, with a flue in the floor or a lower wall, will provide better uniformity than an updraft firing kiln where the flue is in the roof or upper wall. Again, even in a well-designed gas kiln, it is important to have an understanding of how the draft may effect the uniformity – too much may cause the gases to ‘short circuit’ the chamber, not travelling to the full chamber height before being dragged back down by the flue.The kiln size will also influence the minimum number of burners required – too few can mean the difference between mediocre and excellent. It may come as a surprise to know that most kilns are not ‘even’ for the whole firing, but only come in to uniform temperature as the air begins to circulate towards the end of the firing. This can be compensated for by slowing the rate down towards the end or by ‘soaking’ at the end.
Alternatively, a multi-zone temperature control system can be used so that each level or zone is brought up to temperature together. The time taken to get to temperature, particularly over the last hour, is important. The relationship between the time and the temperature is called ‘heat work’. To illustrate this, try putting your hand into a domestic oven at 200°C for 5 seconds – OK.
Now try the same for 1 minute – Ouch! The difference is the ‘extra heat work’, basically a measure of the temperature and the time. This is why the pyrometric cone manufacturer has two different deformation temperatures on the side of a box of cones. One temperature is for a rate of 60°C/hour, the other for 150°C/hour. The slower rate will have a lower deformation temperature, but the same heat work.
Understanding these basic firing issues is one of the most important aspects of kiln control.
As we get to know the firing limitations of the kiln, we can deliberately choose say a slightly slower ultimate firing temperature that we know that the kiln can achieve, irrespective of the load in the kiln, and in this way the whole of the chamber can always be made to reproduce a result that was achieved with a similar piece, months or years earlier.
Best (Easiest) Operation
Today’s modern kilns can be fitted with a variety of controls. Some are better than others.
The modern digital controller has given us access to low-cost, very accurate control of temperature and time (heating rate). The best controller is probably the simplest and most user-friendly for the individual, and the one that will do the job. A simple single-program controller takes just 15 seconds to program the temperature, heating rate and soak time, while a more complex multi-program multi-stage unit requires a little more time and patience.
The use of simple electronic solid state relay switching devices (SSRs) has also helped improve efficiency by enabling shorter, more frequent switching cycles and a smoother heating curve (less overshoot when following a programmed heating rate). Older electro-mechanical relays with necessarily longer, less frequent switching cycles result in coarser ‘saw tooth’-like heating profiles. Each overshoot and resultant fall basically represents wasted energy. The weight of hot-face and back-up materials is also an important consideration. Assuming that they can cope with the temperatures, the lighter, more aerated materials will provide better insulation and also reduce the amount of stored heat in the kiln walls.When more heat is wasted to the walls, the kiln is less efficient and more costly to run. For example: pre-fired, vacuum-formed fibreboard is three times lighter, and provides firing costs approximately half of those of the similarly rated insulating brick.
Best (Longest) Service Life
A kiln is like an investment in a ceramicist’s future. It must last, be reliable, economical and easy to repair. The lining materials that make up the hot-face and back-up insulation package should be able to comfortably withstand the maximum firing temperature, with a little to spare. The strength of the hot-face material is also important, both in its ability to withstand repeated expansion and contraction during firings (without breaking down or crumbling), as well as it’s ability to hold together when the kiln requires transport from the factory, or later for relocation.
Over the last 30 years, brick kilns have received serious competition from ceramic fibre, initially with ‘fibre blanket’-lined kilns. While efficient and cheap to construct, these have proved to lack service life, largely due to poor strength and integrity and an inherent propensity to constantly shed fibres. An alternative stack-bonded construction offers longer life because of less shrinkage and more strength, but is more expensive. Since the eighties, we have witnessed the success of pre-fired (i.e. preshrunk) rigid, vacuum-formed, fibreboard-lined kilns. These have been shown to exhibit long life, in excess of 20 years. Similarly, the older, mild steel-framed constructions, which suffer badly from rust (from the inside out) have been largely replaced by aluminum or stainless steel (rust-free 300 series). Some manufacturers use highly polished but low-grade 400 series stainless steel, which looks great when new, but will still rust. The popular lightweight and easy to transport aluminum frames should incorporate plenty of natural ventilation between the case and insulation to whisk away moisture.
Best (Lowest) Repair Cost
In addition to their long life, rigid, vacuum-formed, fibreboard linings are inexpensive to repair, although they may require different skills to those required for more traditional materials. Like its brick counterpart, the fibreboard may incur damage through impact or through melted heating elements or melted workpieces, but can be repaired using simple fillers composed of the same fibre and a suitable binder. If the damage is major, the whole vacuum-formed board panel can be replaced fairly quickly. By comparison, repair of individual bricks is not always easy or successful, particularly if the original brickwork was fully mortared and not just dry jointed. Re-bricking of the whole area or wall can be a major job and very expensive.
Best for Health and Safety
It is a known fact that most silica-based clay products, when heated to temperatures over 1000°C and allowed to cool slowly, will change to a state commonly known as crystobolite, a potentially carcinogenic material if ingested into the lungs. This is not a concern with dense ceramic products like dinnerware and artifacts; however, the inherent lightweight and fragile nature of clay-based refractories and insulations (which is achieved through the deliberate trapping of air to form a heat barrier) is a concern. Lightweight brick becomes crumbly or fractures, fibreboard breaks if panel sizes are allowed to become too large and, worse still, fibre blankets can shed fibres continuously and easily.
The introduction of high-tech surface coatings over the last ten years has seen a significant improvement in the reduction of airborne fibres and surface brick dust. The best of these surface coatings, applied relatively thinly, can provide dust-free brick and fibreboard surfaces for periods of up to 10 years. Progressive kiln builders are now including these products as a standard feature in their new kilns and offering aftermarket re-coating services.
While many of the practices which are in use today will, no doubt, continue for some time, there are always going to be new developments. Kilns offered today for home studio use are beginning to look more like appliances than heavy industrial type pieces of equipment.
Currently under review are new construction techniques using space age insulations (super lightweight carbon fibre) and heating technologies (microwave). Typically, not all will make the grade as viable alternatives to our current technologies. Some kiln builders now offer multi-purpose kilns, e.g. dual purpose kilns for ceramics and glass with switch-selectable heat pattern zones. More financial flexibility now allows simple lease plans, making it easier to equip our studios and easier to unload equipment that is no longer needed or to update to more modern versions.