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Fun with Fire
As an impressionable young ceramic engineer still wet behind the ears, I remember going out with my boss to look for gas leaks in the plant. His method of choice was to grab a propane torch and get close to wherever the smell was coming from to see if he could get it to light on fire. He had a bunch of pseudo-technical arguments for his method about flammability limits and air/fuel ratios and why it wouldn’t result in an explosion, but I think mainly he liked scaring his co-workers and impressionable young ceramic engineers. The slightly maniacal look in his eyes didn’t help matters much.
We may not admit it, but most of us in the business of turning dirt into tile have just a little bit of pyromaniac in us. Putting on those thick, silver gloves to rescue test samples from the kiln reminds us that we’re playing with fire… literally. The first place every tour group wants to congregate is the kiln. They want to look down that tunnel of white-hot gas and see the tile being born. Harnessing all of that energy in a safe and efficient manner is quite an achievement of man over the elements.
Apart from just being cool to look at, firing tile to that high of a temperature is important to the many properties that make tile such a versatile and long-lasting product. The typical porcelain composition is based on three ingredients. Clay is the sticky stuff that holds everything together long enough to go into the kiln, and silica is the crystalline backbone of the tile-but feldspar is the star once the tile gets into the kiln. This mixture of sodium, alumina and silica turns from stone to glass in the presence of a lot of heat, reacts with the silica, and turns the green piece into a durable, fired tile.
During the transition, many important things are happening. All of the organic materials from the clay and impurities are burned away, leaving a tile that is free from volatile organic compounds (VOCs). (I’ll spend more time on this topic in a later column.) The inorganic pigments that were incorporated into the body and/or applied to the surface of the tile react and form the correct color, which will remain forever. The pores in the tile fill with glass, and if the porosity drops to less than 0.5%, the tile becomes a porcelain. Most importantly to the end user of the tile, the entire thing becomes very dense and durable so it will show very little wear for decades to come.
It is at this critical phase of the process that many things can go wrong as well. Tile can warp too much as it goes through the kiln. It can shrink too much or not enough. If the kiln has too much oxygen (or not enough), the colors of the pigments may not develop correctly. The surface can come out too shiny, or not shiny enough. If it is cooled too quickly, the tile can crack. In short, the same step that makes tile what it is, can also render it unacceptable.
Controlling the firing of the tile is very important. Modern production kilns have hundreds of sensors that monitor temperatures, pressures and valve positions. Each piece is individually measured after it is produced to make sure it is the right size and flatness. The process is constantly monitored and fine tuned. Modern roller-hearth kilns are efficient, infinitely controllable and amazingly high-tech ways to play with fire.
Next time you open a carton of tile or walk on a tile floor, think about it being forged in fire by a slightly maniacal-looking ceramist who uses fire to turn a square glob of clay into a beautiful, durable floor. If it weren’t for fire, it wouldn’t be tile.