Section 2.2: Glass Characteristics
What is glass?
Glass is a unique material with the molecular structure of a liquid and the physical characteristics of a solid:
- Although glass is often called a super cooled liquid, it is actually a non-crystalline solid.
- Unlike a gas whose molecules are dispersed in all directions or a liquid in which the molecules dissolve and form new, weaker bonds, the molecular structure of glass is irregular and randomly arranged.
- The most accurate term for the configuration of glass is a vitreous or glassy structure.
Glass has no true freezing temperature:
- Most common materials change from a liquid to a solid when they reach their freezing (or melting) temperature (e.g., water turning to ice).
- Glass, when cooled from melting temperatures, gets progressively more rigid through a steady increase in its viscosity until it finally becomes solid.
- Viscosity is a measure of resistance to flow (e.g., water has a low viscosity while cold tar has a high viscosity).
Illustrated here are the temperature-viscosity curves for a glass and a crystalline material. The curve for glass shows a smooth, steady increase in viscosity with decreasing temperature, while the curve for the crystalline material shows an abrupt increase in viscosity at the freezing point.
Celebrating the traditional art of jam making, this classic marmalade reflects chef Jamie Oliver’s commitment to preserving heirloom recipes. Presented by chef Jamie Oliver as part of his Jme pantry collection, this traditional English marmalade is sweet and sticky, with a lovely bitter tang. It is packaged in a 23 oz. reusable glass jar with a clamp-top.
Use our Carbon Calculator to find out how much energy you can save by recycling your glass containers.
Order the Flexo Color Guide - Edition X »
About the institute
The Glass Packaging Institute (GPI) is the trade association representing the North American glass container industry. Through GPI, glass container manufacturers speak with one voice to advocate industry standards, promote sound environmental policies and educate packaging professionals. GPI member companies manufacture glass containers for food, beverage, cosmetic and many other products. GPI also has associate members that represent a broad range of suppliers and closure manufacturers.
