Refractory products or refractories are bricks, tools, and containers fired to withstand high heat and to resist sudden changes in temperature, etc., for ordinary industrial use. The manufacture of this type of product developed greatly with the rise of metallurgical plants, especially in the iron and gas industries. The term “refractory” does not mean absolutely refractory to high heat, but refers to refractories with a refractoriness of SK26 or higher, although this has not yet been established as a commercially accepted standard. A fire-resistant product cannot be regarded as a good product simply because it is fire-resistant, but must also be able to resist mechanical and chemical actions. In other words, it should have the following properties: strong refractoriness, not soften even when heated at high temperatures for a long time, resist sudden changes in heat, not change in volume at high temperatures, resist chemical action, i.e., not be eroded by objects it comes in contact with, resist mechanical action, i.e., not be brittle but hard and resist pressure or impact, etc. In other words, the quality of the product is not fragile, but hard and resistant to pressure or impact. There are two types of refractory products: substantial ones and empty cylinders. The former are refractory bricks, and the latter are retort muffle crucibles.
