Most contemporary fusing methods involve stacking, or layering thin sheets of glass, often using different colors to create patterns or simple images. The stack is then placed inside the kiln (which is almost always electric, but can be heated by gas or wood) and then heated through a series of ramps (rapid heating cycles) and soaks (holding the temperature at a specific point) until the separate pieces begin to bond together. The longer the kiln is held at the maximum temperature the more thoroughly the stack will fuse, eventually softening and rounding the edges of the original shape. Once the desired effect has been achieved at the maximum desired temperature, the kiln temperature will be brought down quickly through the temperature range of 815 °C (1,499 °F) to 573 °C (1,063 °F) in order to avoid devitrification. It is then allowed to cool slowly over a specified time, soaking at specified temperature ranges which are essential to the annealing process. This prevents uneven cooling and breakage and produces a strong finished product.
This cooling takes place normally for a period of 10–12 hours in 3 stages.
The first stage- the rapid cool period is meant to place the glass into the upper end of the annealing range 516 °C (961 °F). The second stage- the anneal soak at 516 °C (961 °F) is meant to equalize the temperature at the core and the surface of the glass at 516 °C (961 °F) relieving the stress between those areas. The last stage, once all areas have had time to reach a consistent temperature, is the final journey to room temperature. The kiln is slowly brought down over the course of 2 hours to 371 °C (700 °F), soaked for 2 hours at 371 °C (700 °F), down again to 260 °C (500 °F) which ends the firing schedule. The glass will remain in the unopened kiln until the pyrometer reads room temperature.
Note that these temperatures are not hard and fast rules. Depending on the kiln, the size of the project, the number of layers, the desired finished look, and even the brand of glass, ramp and soak temperatures and times may vary.
Fused glass is a term used to describe glass that has been fired (heat-processed) in a kiln at a range of high temperatures from 593 °C (1,099 °F) to 816 °C (1,501 °F). There are 3 main distinctions for temperature application and the resulting effect on the glass.
Firing in the lower ranges of these temperatures 593–677 °C (1099–1251 °F) is called slumping. Firing in the middle ranges of these temperatures 677–732 °C (1251–1350 °F) is considered “tack fusing”. Firing the glass at the higher spectrum of this range 732–816 °C (1350–1501 °F) is a “full fuse”.
All of these techniques can be applied to one glass work in separate firings to add depth, relief and shape.
While the precise origins of glass fusing techniques are not known with certainty, there is archeological evidence that the Egyptians were familiar with rudimentary techniques ca. 2000 BCE. Although this date is generally accepted by all researchers, some historians argue that the earliest fusing techniques were first developed by the Romans, who were much more prolific glassworkers. Fusing was the primary method of making small glass objects for approximately 2,000 years, until the development of the glass blowpipe.
Glassblowing largely supplanted fusing due to its greater efficiency and utility.
While glass working in general enjoyed a revival during the Renaissance, fusing was largely ignored during this period as well. Fusing began to regain popularity in the early part of the 20th century, particularly in the U.S. during the 1960s. Modern glass fusing is a widespread hobby but the technique is also gaining popularity in the world of fine art.
It is generally agreed that the Mesopotamians did the first fusing and casting in the 2nd millennium B.C.
Early warm glass processes evolved from ceramics/metalworking techniques.
Glass was classed as a precious material alongside gold and silver.
Next came the Egyptians, who by the later half of the 2nd millennium B.C. were proficient in both casting and fusing. They also developed the technique of working with glass rods (now known as lampworking).
Romans and Greeks adapted and improved on early techniques from the 3rd century B.C. till the birth of Christ.
The development of kiln forming was put on hold when a new approach – Glassblowing was developed by the Romans.
Blowing became popular due to its greater efficiency, repeatability and lower cost.
Warm glass techniques were forgotten until they were rediscovered in Europe during the 19th Century.
One of the first areas to be developed was the ‘Pate de Verre’ movement in France.
Henri Cros, Albert Dammouse and Gabriel Argy-Rousseau developed methods for casting with a paste made from small glass particles.
In the early 20th century The Studio Glass movement, led by Harvey Littleton and centered on blowing brought respectability to working with glass.
The Bullseye Glass Co. formed by 3 glassblowers, played a significant role in the development of ‘Warm glass’. They led the first major research in the development of ‘tested compatible’ glass made specifically for fusing.
Today, after nearly 2 centuries of re-discovery, warm glass continues to develop and grow as a viable artistic discipline. The increasing availability of better materials and the continued experimentation of artists leaves warm glass poised for continuing growth during the 21st Century and beyond.
Fusing glass in a kiln is a fascinating technique that enables artists to create unique and gorgeous projects. The following fusing rules and firing instructions should provide you with enough information to make a variety of projects, creating an appreciation for the complexities and potential of fused glass, and paving the way for more intricate designs and ideas.
Otherwise, when the glass cools, one glass will pull on the other and cause the piece to crack along the seam. Even if the glass survives the cooling process, there is still a risk it might still crack if you try to reheat it in a kiln or even place it in a sunny window. Stress from incompatible glass is always in the piece. Do not try to refire broken incompatible glass.
- All glass has a coefficient of expansion, or COE.
- Glass manufactured specifically for fusing is often “tested compatible,” or guaranteed to be a certain COE.
- The most popular fusing glasses are either 90 COE (Bullseye, Uroboros and Wasser) or 96 COE (Spectrum and Uroboros).
- Always use compatible glass, which is known to have the same COE.
Slower Is Better
- You can’t heat or cool glass too slowly. Going too fast can result in cracked glass or Thermal Shock ( see image to right).
- A safe rate to heat is 15° per minute (900° per hour), although stacked glass 2″ in diameter and smaller can be heated at a faster rate.
- Slow-cooling of glass or “annealing” depends on the thickness of the glass. If the glass breaks because it was heated too fast; turn off the kiln, allow the glass to cool, push it back together, and try again – at a slower rate. Breaks from thermal shock usually go straight across the piece and have a little hook near the edge. They can usually be repaired by refiring.
Glass Likes To Be ¼” Thick
When heating glass to full fuse, anything with less mass will shrink up, anything with more will spread out. This movement can be controlled somewhat by fusing slower, and not going to full fuse.
All Kilns Are Not Alike
- There are some variances between kilns, especially mini kilns. Sometimes pyrometers are slightly off, and sometimes current loads vary.
- Use firing schedules as a guide, but remember to check your piece frequently during fusing, and record changes in schedules as needed.
- Prepare your kiln by applying kiln wash with a kiln brush. Apply one thin coat in each direction. Don’t forget to apply kiln wash to molds, too.
Take Good Notes
- Use a project log to keep important information about your projects.
- Keep track of what glass was used, how thick the glass was, the firing schedule and the results.
- This helps repeat good performances and prevent bad ones.
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