Brick won't work, if you're going to build a wall, you have to plaster it on the outside, otherwise after ten or twenty years those bricks will loosen.,"Charcoal is even more expensive, three coins per hundred jin. In winter, it costs three and a half coins.",He pulled out another thin slate and showed it to everyone: "There are also these kinds. The foreman at the quarry said that these slates can be used for heating floors."。