BOOK VIII.	275
	to the ore which has been roasted, while it is still hot, in order to make it softer and more easily broken ; for after fire has dried up the moisture in the ore, it breaks up more easily while it is still hot, of which fact burnt limestone affords the best example. By digging out the earth they make the areas much larger, and square ; walls should be built along the sides and back to hold the heat of the fire more effectively, and the front should be left open. In these compartments tin ore is roasted in the following manner. First of all wood about twelve feet long should be laid in the area in four layers, alternately straight and transverse. Then the larger pieces of ore should be laid upon them, and on these again the smaller ones, which should also be placed around the sides ; the fine sand of the same ore should also be spread over the pile and pounded with shovels, to prevent the pile from falling before it has been roasted ; the wood should then be fired.
	Lead ore, if roasting is necessary, should be piled in an area just like the last, but sloping, and the wood should be placed over it. A tree trunk should be laid right across the front of the ore to prevent it from falling out. The ore, being roasted in this way, becomes partly melted and resembles slag.