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In this plant, they are firing a 20-hour cycle. In sanitary ware this is considered fast firing. This kiln is a combination drier and furnace, it is 96 meters long. It has a pre-heating zone, firing zone, and cooling zone. In this plant, after the firing zone ware is cooled quickly, from 1200-800C, to ensure a glossy glaze surface. Each car takes 13 hours to reach a peak temperature of 1200C. The panel displays the temperature of 24 points in the tunnel.
These "one piece closets", shown in the background photo, weigh up to 45kg (100lb). In this factory, they fire cold-to-cold, in a tunnel kiln, in about 20 hours. Total shrinkage of the product is 12% (so the moulds are made 12% bigger). They are porcelain so dry hardness is fairly low - but they need to dry and shrink while doing so - without cracking. And, as porcelain, they have a high firing shrinkage - yet need to fire with minimal warping and no cracks - despite the weight. They also have to perform in use every day for decades (they must withstand a minimum 400 kg load). The glaze has be to durable and it absolutely cannot craze, shiver or leach. The engineers, mold makers and kiln designers that make these possible are amazing. But so are the production personnel - not just because of conditions in the plants (e.g. heat, humidity, noise) but also the need to keep costs down yet still produce a good product. That means incorporating as much local material, dry and fired scrap as possible. While some plants have pressure-casting lines, the cost is high so most still use traditional plaster molds - it is heavy and demanding work.
Glossary |
Sanitary ware
A type of porcelain zircon-glazed ceramic that includes bathtubs, sinks, toilets, etc. |
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