Si3N4 Bonded silicon carbide brick is a different type of refractory substance, in which Si3N4 exists between SiC grains in needle-formed or fibrous crystalline form. Dec 28, 2018 #7 The flat surface doesn't should be glass, in reality glass is just not that hard. It could be something rather hard http://erickkctu128.huicopper.com/the-evolution-of-fuji-silicon-carbide-guides