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.
Making Molten Glass
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.
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.
Adding chemicals into the mix, with particular chemicals creating specific colors, makes colored glass. We often call these frits and use them in both coloring a batch or in a fusing technique.
When the glass is first withdrawn from the furnace its viscosity is too low for it to hold any form. As it cools, it flows less easily and can be formed.
Blowing. In hand blowing, a mass of molten glass is gathered on the end of a five-foot stainless steel 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 and wooden tools. It is usually reheated several times to keep it pliable while it is being shaped.
Pressing. A measured amount of molten glass is placed into a form, and a metal plunger presses an engraved mold into the glass blob leaving behind the raised design.
After forming, the glass ornament is heat-treated, by annealing, and later may also be decorated.
As glass cools from forming temperatures to room temperature is shrinks and solidifies. Uneven cooling would weaken the glass due to stress so we use annealing ovens, known as a Lehr, to control the cooling process overnight.
Glass objects can then be decorated further. Some glass objects are painted or glazed. Others are etched with hydrofluoric acid or soda blasting, either to produce an artistic design or to frost the entire surface. Designs can also be cut or ground into the surface.