Glass is made of mixture of many substances melted together in a large furnace. The most important substance is silica which is found in sand. Potash(钾碱), obtained from wood-ashes, is also necessary. To these two substances are added lime, lead, soda and charcoal. Acids are used to remove iron and other impurities. Which might discolor the glass. The heat of the furnace is so intense that the mixture of these various materials is changed into a boiling liquid. How is liquid glass formed into beautiful or useful objects? Let us imagine that we are watching things being made in a glass factory. It is a small factory in which the work is still done by craftsmen and not by machinery. Here is a man holding a long iron tube called a blow-pipe. The tube has a wooden mouth-piece to protect the man's lips from the heat. He puts the other end of the pipe into the molten glass and picks up some of the mixture on the end of the tube. Then he robs the lump of glass to and fro on a metal table, rather like a person rolling pastry. As he rolls it, he shapes the lump roughly into the required form. Next he lifts the pipe, puts the wooden end to his lips, puffs out his cheeks and blows as you do when you are blowing up a balloon. But of course he has to blow much harder. When the air is blown down into the lump of molten glass a gorge bubble is gradually formed. The craftsman needs both powerful lungs and great skill to ensure that the bubble is of exactly the required size and thickness. Now you understand why so many hand-made glass objects are round in shape. Handles and feet must be added while the glass is still plastic. When the object is shaped to the craftsman's satisfaction, it is allowed to cool off. The cooling must be done very gradually. As you know, glass is cracked by extreme and sudden changes of temperature. Some of the loveliest and many of the most useful things in the world are made of glass. It is most surprising, in view of the great technical skill required, that glass making is a very ancient art. It was practiced thousands of years ago by ancient Egyptians although it was many centuries before men discovered how to make glass transparent. Glass Making To remove impurities from the glass,【46】. Craftsman rolls the molten glass【47】on a metal table. Glass objects are made【48】. The mouth-piece of the blow-pipe is【49】. All glass is【50】.