Glassmaking involves two main steps: (1) heating and mixing raw materials to produce molten glass, and (2) forming the molten glass into the desired shape. Most glass then receives further treatment to produce the final product.

Glass manufacture begins with the careful selection and measurement of raw materials. The most important raw material is sand. Other raw materials used depend on the type of glass being made.

Broken glass, called cullet, is usually added to the raw materials. It promotes the melting of the raw materials as they are heated. Most cullet is waste from glass-forming operations; some is obtained from recycled glass products.

Melting is done by batches or as a continuous process. With the continuous-tank furnace, the most common type of furnace used for melting, raw materials are fed into one end and molten glass is withdrawn continuously from the other end.

The raw materials and cullet are heated until they have melted into a spongy mass full of bubbles. The temperature of the melt is then increased to make it more fluid, allowing the bubbles to rise to the surface and escape. The glass at this stage is clear and homogeneous. Soda-lime glass, the most common type of glass, is initially heated to about 2,550 F. (1,400 C.) and then rapidly heated to about 2,800 F. (1,540 C.). Most other kinds of glass must be heated to higher temperatures.

When the glass is withdrawn from the furnace its viscosity is too low for it to hold any form. As the glass is cooled, it flows less easily and can be formed. Any of several processes can be used:

Blowing. In machine blowing, blasts of compressed air are used to force gobs or ribbons of glass into molds. Bottles are made by machine blowing.

In hand blowing, a mass of molten glass is gathered on the end of a four- to five-foot (120- to 150-cm) pipe called a blowpipe or blowing iron. The glassmaker blows through the pipe, giving the material a hollow, balloon-shaped form. The glass is further shaped with various metal tools. It is usually reheated several times to keep it pliable while it is being shaped.

Drawing. In producing flat drawn glass, a horizontal wire called a bait is lowered into the molten glass and then raised. Glass adheres to the wire and is drawn upward as a continuous sheet. Once hardened, the glass is cut into sheets for use in windows and inexpensive mirrors. Flat drawn glass has a fire-finished surface that forms naturally as the molten glass cools. Drawn glass tubing is made by forcing molten glass along a ceramic cone, called a mandrel, that forms the glass into tubing.

Floating. Molten glass is allowed to flow onto a bath of molten tin and is then allowed to cool. The tin has a low melting point and remains liquid at temperatures at which the glass hardens. The surface of the metal leaves the glass with a very smooth surface. Float glass is used in windows and other flat glass products.

Pressing. A measured amount of molten glass is placed, by machine, into a mold, and a metal plunger presses the glass outward to fill the mold. When the glass has cooled and is firm enough to hold its shape, the mold is removed. Pressing techniques are well-suited for rapid mass production. Many of the glass objects made for everyday use, such as drinking glasses and heat-resistant glassware, are produced by pressing.

Rolling. As the molten glass leaves the furnace, it flows between sets of large, heavy rollers. This method is typically used for flat glass that does not require a fine finish, such as glass made with a figured pattern on its surface.

After forming, glass is usually heat-treated, either by annealing or by tempering. It may also be decorated.

Annealing. Stresses build up in glass objects as they are cooled from forming temperatures to room temperature. These stresses greatly weaken the objects and may cause them to fracture spontaneously. In annealing, glass products are reheated in ovens called lehrs, and then allowed to cool slowly under controlled conditions, so that the stresses do not recur.

Tempering. Tempered glass is tougher (more resistant to breakage) than ordinary glass. In tempering, controlled, uniform stresses are deliberately set up in the surface of a glass object. The glass is heated to just below the temperature at which it would begin to soften and is then quenched (rapidly chilled) to stress its entire surface uniformly. Tempered glass cannot be cut or drilled. If a piece of tempered glass is pierced in any way, the uniformity of the stress is destroyed and the whole piece shatters into small pieces.

Decorating. Glass objects can be decorated in many ways. Some glass objects are painted or glazed. Others are etched with hydrofluoric acid, either to produce an artistic design or to frost the entire surface. Designs can also be cut or ground into the surface.