Glass has four separate and sometimes intersecting glass zones. There is no freezing or melting point in glass.
As this material is heated, it turns more fluid. When cooled, glass becomes more firm. Glass behaves differently at various temperatures and these zones are established by these behaviors.
Read through these different zones to see what is happening to glass as it transpires through the different stages of heating and cooling. Remember to always use compatible glass to achieve the best results.
Cold/Brittle Zone – Glass is considered cold from room temperature to approximately 700-900 degrees Fahrenheit. This zone can vary depending on the glass and kiln being used during fusing. This is also considered the brittle zone, because if heated or cooled too quickly in this zone, glass will break. In the brittle zone, glass swells when heated or shrinks when cooled. This breaking is caused by stress or thermal shock. Because glass does not have flexibility, it is subject to breaking if exposed to quick or irregular changes in temperature.
Transitional Zone – Glass begins to change between 900 degrees Fahrenheit and 1250 degrees Fahrenheit. The
is at the lower point and the softening point is at the upper point of this temperature. As the glass is changing, it has the features of setting up pudding. Glass is less sensitive to thermal shock during this zone, and heating rates can be increased. The annealing point is somewhere in between the lower point and the upper point. It is important when cooling off the glass to take it slowly through the annealing point to relieve any internal stress that has been built up by the heating process.
Workable/Pliable Zone – This zone is between 1250 degrees Fahrenheit and 1350 degrees Fahrenheit. During this zone the glass will begin to act somewhat like taffy as it slumps and moves as gravity pulls on the pliable glass. Glass during this zone can be flattened, slumped or stretched. The glass is slackened enough that it can be maneuvered. Thermal shock does not affect the glass during this stage, and it can be heated or cooled rapidly without fear of breakage. The consistency becomes stick and it can adhere to itself or any other hot objects that might come in contact with the glass. Glass gets more adhesive as it gets hotter.
Molten/Fluid Zone – Above the temperature of 1350 degrees Fahrenheit, glass becomes more fluid and takes on the consistency of syrup or honey on a cold winter’s day. Fusing of two or more pieces happens at around 1500 degrees Fahrenheit to 1600 degrees Fahrenheit. Thin glass will bead up into a blob, while a pile of glass will flow if not restricted.
Learning more about the activity that transpires in these diverse glass zones will assist in understanding the glass fusing process.
Return from this page to one of the following pages:
Fusing and Slumping
Glass Zones to Glass Fusing Made Easy