Information on fusing recycled glass

Fusing Recycled Glass

Material:  Recycled Glass

Issue: It is very simple to fuse recycled container glass to make tiles and relief plaques.  However, information on raw material characteristics, temperature profiles, and operational issues has not been widely disseminated.

Best Practice: At temperatures considerably below melting, particles of metal, ceramics, or glass begin to bond through a process called sintering.  Sintering of soda-lime glass begins at about 1250° F. Sintering can be used to create solid glass objects by heating glass particles to above the sintering point and holding at that temperature until a dense body forms.

The basic steps are described below:

1)      Obtain a mold that will withstand temperatures of at least 1600° F.   Coat the mold with a release. Commercial “shelf paints” are available.  A mixture of 40% kaolin and 60% alumina hydrate also works well.

2)      Fill the mold with glass pieces. Glass of any size can be used.  However, the most consistent success has been obtained with glass that has been crushed to a uniform shape and screened to a size 16 mesh and finer.  There is a trade-off here between glass grain size and cleanliness.  It is easier to obtain a glassy, translucent product with larger grain sizes, but smaller grain sizes minimize contamination.

3)      Fire the kiln to fusing temperatures.  For green or amber container glass, the following schedule is a place to start:

      500°F/hour                                to 1100°F         
(organic contamination should be gone by this point)

      1000°F/hour                              to 1600°F                                    
(or go as fast as the kiln will rise)

      soak 5 minutes                          at 1600°F                  
(this is the critical time & temp – experiment)

      Natural cooling                          to 1020°F

      Soak 15 minutes                        at 1020°F           
(may need longer soak for thick or uneven pieces)

      70°F/hour                                  to 950°F

      Cool 300°F/hour                        to 250°F

      Open kiln to cool to ambient

Each kiln acts differently.  The maximum temperature that works best in a particular kiln may be anywhere from 1500 to 1600°F.  Fusing can also be done in cone kilns.  Fire at medium speed until organic burn-off then at maximum speed and try cone 010, 011, 012, and 013.  If the fused tiles have bubbles or bloat, it may mean that organics need more time to burn out, that other contamination is present.  Try soaking at 1100°F for 10 to 20 minutes.

Container glass “devitrifies” very easily.  This means that the glass changes from a glass back into a crystalline material, with two results: the glass takes on a matte finish; and it becomes much stiffer. Slower temperature profiles and long soaks appear to make devitrification worse.  A large amount of direct radiant heat (as from top-mounted heating elements) appears to promote devitrification, as well.

The best strategy for starting recycled glass fusing is to find a source of crushed glass that is reliable and consistent.  Then develop a matrix for an experiment.  Combine different colors and different grades, and fire with different ramping temperatures from 1450°F to 1600°F.  Most of the early experiments will fail, but patience will be rewarded.

Either side of the fused glass product can be chosen as the featured side.  The side exposed to the air will have a graceful “slump” and will probably be glossier than the other side.  The side that was in the mold can be made to take on any type of relief.

Virtually any clean glass can be fused at the right kiln temperatures.  Although making fused glass products as artwork or in small-scale production has been growing over the years, most artists and product developers have stayed away from using recycled glass because it is a more challenging material than some of the glass designed and manufactured for fusing, like Bullseye Glass.  However, some small businesses, like this one in Seattle, are successfully manufacturing glass tiles using this type of process.

Implementation: These instructions can be used with the Best Practice Permanent Molds for Fusing Glass to begin experimenting with glass fusing.

Benefits: This process is accessible to anyone with a ceramics kiln capable of 1600° F.

Application Sites: Crafts shops, small businesses.

Contact:  For more information about this Best Practice, contact CWC, mailto:info@cwc.org.

References:

Richerson, David W., Modern Ceramic Engineering - Properties, Processing, and Use in Design, Mercel Dekker, Inc., 1982.

For a general discussion of glass fusing and moldmaking:
Lundstrum, Boyce, Glass Casting and Moldmaking, Vitreous Press, 1989. The book is available on Amazon.com or from many ceramics and glass supply stores.

For more about issues related to glass devitrification, see CWC report Densification, Crystallization,

and Sticking Behavior of Crushed Waste Glass Sintered in Refractory Molds with Release Agents, CWC, 2000.

Issue Date / Update: April 2000