Wednesday, January 23, 2013

Glass is Hard Pt III

Two extreme stories regarding glass chemistry. In the first, after bringing in a new furnace (the furnace had been torn down and rebuilt with new brick, this must be done periodically) the glass coming out of the furnace was foaming. It was full of bubbles. After more than a month of researching the problem, someone though to ask the old melter (the person in charge of running that furnace) about the problem; he had retired between the last rebuild and this one. He had no idea about what to do, but he did comment that whenever he brought in a new furnace, he always through a silver dollar in for luck. In the second story, one of the company’s CRT making glass furnaces suddenly started have well above average product breakage at the customer. Coincidentally, they also had a problem with their glass molds; molds that were supposed to last for weeks were lasting only days.

The first story was told to me and may be apocryphal, the second, I actually witnessed and was a participant in reaching resolution. They were both essentially the same problem. In the first story, it can be hard to appreciate, but commonly parts per billion of some element or another, can have a large effect in stabilizing or processing the glass. Much of glass development is taken up not with developing new glass chemistries but in determining how a glass will react with the impurities it picks up from the furnace. Parts per billion, below the resolution limit of most x-ray machines commonly used for measuring glass chemistry, can have a determining effect on the glass suitability for a particular application. In the second story, one of the recouperators, the part of a glass furnace that captures the heat and sends it back to the furnace, had collapsed and the furnace was being fired with just one recouperator. Although the glass chemistry as was closely monitored before and after the collapse, the glass chemistry measurements usually do not include one of the most important elements, Oxygen. When the recouperator collapsed the melter changed the way he fired the furnace with changed the oxidation state of the glass. The glass chemistry before and after the change was the same but no one was looking at the oxygen.

Fusion glass, used as LCD substrates, is made in platinum lined furnaces to minimize an impact refractory contamination can have. However, raw materials can still have a huge impact. For most "dirt is dirt and sand is sand" however for glass and ceramic making, many times, even when the mining location is changed within the same mine, it means requalifying the process.

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