Some info:Pure quartz sand has the advantage that, even when very hot, it is not very conductive. The purpose is twofold:1. Any ionic contaminants will depress the melting point2. Ions move around, making molten glass conductiveRegular soda-lime glass becomes conductive at only red heat (around the same temperature where it begins to anneal). Compare to these data: http://www.qsiquartz.com/techinfo.html Note, this is high purity stuff, maybe more pure than what's used in the fuse. Or alternately, if they did use very pure quartz, that might be explain part of the cost of those beasts!I don't know if they used quartzite exactly (maybe you know this), but there are many sources of high purity silica sand. Probably, anything that's good enough for making colorless glass is suitable here?Geology nitpick: quartzite is a rock. Rocks are made of minerals. The principle mineral in quartzite is quartz (naturally).As for the bronze rectangle at the top, maybe just a convenience for assembly? A machined part is needed for threaded holes. I'd think it could just as well be a band of heavy copper or bronze (heavy enough to handle the screws and current), bent into shape to pick up the fuse element. Or press-in inserts, but maybe those can come loose over time, which would obviously be a bad thing here. But, whatever, it's not like these things are heavily cost-optimized.The soldered joints are probably a combination of soft solder and braze ("silver solder", but not "silver-bearing solder" which is just soft solder with silver in it..). Copper, bronze and silver are all excellent metals to join with any of these alloys. I don't like that they soft-soldered (by the looks of it) the one end cap, but I guess it must stay cool enough that that's not a problem. Using higher-melting filler inside definitely helps with soldering, as you can join the inside stuff with a higher melting alloy, and not worry about it coming apart when you do the outside.Cool to see the melted/vaporized silver condensed around all the sand grains.Tim