Section: Firing, Subsection: General

Description

By understanding what sorts of change art taking place in the ware at each stage of a firing you can tune the curve and atmosphere to produce the best possible ware.

Article

A kiln is not a microwave oven and firing one is not like heating Chinese noodles! It is not a rote timed process where you can just set the oven and go shopping. Firing a kiln is much more like baking an angel food cake. It requires awareness of kiln contents, the process, and the objective. Inflexible schedules are out; flexibility and sensitivity are in.

As a potter or industry, you are basically making rocks in your kiln; metamorphic rocks. You are changing the form of matter just the way a metamorphic rock has been changed from another by the forces of heat and pressure. As I have highlighted elsewhere, there are physical and chemical things that happen in the kiln. The physical changes give us the headaches, but the magic of the chemistry makes it all worthwhile. Let's review the simple stages in a firing.

The Stages of a Firing

Final Drying

The ware has to dry in preparation for bisque or single fire. If you don't have a dedicated drier, then you are using your kiln as a drier. If your drier does not exceed the boiling point of water, then you are using your kiln as a drier. If your ware sits in the studio or plant after drying, then its hydroscopic nature results in the absorption of water from the air and once again, your kiln is the final drier. Whatever the case, all water has to come out and even though a piece looks and feels like it is dry, there can still be plenty of water present. If just 2 or 3 percent needs removal, a typical industrial periodic kiln setting could have hundreds of pounds of water that must escape, all of which expands a thousand times when it turns to steam. Needless to say, this makes for a damp atmosphere during this stage and proper ventilation is a must. Many modern driers (an kilns) have fans that impose a virtual hurricane of draft on the ware in the kiln to remove this water efficiently.

In a fine grained clay (one containing bentonite or ball clay), it requires time to vent the moisture out, especially if ware is thick. If you fire too fast at this early stage, the water within boils, generates steam, and just blows the piece apart. If you heat just a little slower, a few chunks will be blown off at sites of thicker cross section. A little slower yet and maybe just a few cracks. Still slower and only micro cracks that will weaken the ware and encourage failure in later stages or during use. Slower yet and you have it right. Slower yet and you have some margin for getting it right on a continual basis.

Another matter, which must not escape your notice, is the drying of glazed biscuit ware. The absorbent biscuit can pick up considerable water during glazing and this must not be driven out too quickly. A rapid warm up will loosen the glaze from the biscuit, causing it to fall off or crawl during the firing. A moisture-laden atmosphere during early stages can even re-wet fully dried ware, and the subsequent sudden drying and associated steam and pressures associated with rapidly increasing temperature will likewise compromise the fragile glaze-body bond.

How do you tell what firing schedule is right? Experiment. There are a number of variables that make it very difficult to establish rules. The most important are the weight of the ware, the density of the setting, the water content of the clay and its ability to vent this water, and the air flow within the kiln to remove the vapor as it is generated.

Large hand-built sculptural pieces weighing hundreds of pounds can require weeks or even months of protected air drying. These must be fired over two or three days, most of this time at the boiling point stage. Lighter industrial ware like mugs can be humidity force-dried in special chambers in minutes and fired in hours. In general, for dry ware and good airflow in the kiln, most ware, including large porcelain items, can be brought through this stage in several hours. Ware that is not dry may require much more time. In a worst case scenario, namely a densely packed electric kiln having no airflow and large pieces that have not been boil dried, this stage could take 24 hours or more.

Whatever the case, as long as you understand the importance of thoroughly dry ware, air flow in the kiln, clay venting ability, and density of pack, you will be able to adjust matters to encourage success. In this stage, it is not so much a matter of even firing but speed and atmosphere. It is all just common sense.

Before continuing, I would like to mention the matter of clay bodies. Certain ill-conceived clays are much more vulnerable to failure during this stage of firing. Clays, which lack particle size diversity, made only from ball clays, kaolins, and very fine ground feldspar and silica, do not perform well even if grog is included. On the other hand, natural native materials with limited processing will sometimes tolerate very fast firing. I have seen clays that can survive to 2300

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Firing: What Happens to Ceramic Ware in a Firing Kiln