Shotgun Annealing

 Shotgun annealing is chosen when the annealing temperature is unknown or uncertain. The name comes from the characteristic spread of the shot pellets to include the target.

To follow this process, pick highest relevant anneal temperature. We know soda lime glass has a range from about 540°C/1004°F to 470°C/878°F. Unless you are firing float glass (which anneals between 540°C/1004°F and 520°C/968°F), you can start the anneal cycle at 520°C/968°F and continue it to 470°C/878°F (a 50°C range). The rate to be used is determined by the amount of time required to anneal the piece according to thickness.

To be safe, a shotgun anneal will need double the time to go through the chosen range that a normal anneal soak requires.

• A 6mm/0.25” full fused piece would normally need an hour soak. So the shotgun anneal rate would be 25C/45F per hour over a 50°C/90°F range.
• A 12mm/0.5” full fused piece would normally need a two hour soak. This implies a rate of one quarter of the range or a cool rate of 12°C/22°F over the range.
• A 6mm/0.25” tack fused piece would need to be fired for twice its thickness, so as for 12mm/0.5”.

Annealing times for different profiles and thicknesses are given in this blog post:  and in this ebook.

If the glass is really unknown or older than fusing glass, a wider shotgun anneal range should be used. This gives a temperature range of 540°C/1004°F and goes to 470°C/878°F, or a range of 70°C/126°F. There is still a requirement for the shotgun process to be double the normal anneal soak.

• So for a 6mm/0.25” full fused piece two hours are required to go through the range, or 35°C/63°F per hour.
• A 12mm/0.5” full fused piece and a 6mm/0.25” tack fused piece will need a rate that takes 4 hours to go through the range, or 18°C/32°F per hour.

Once the slow fall of temperature through the range is complete, there should be a one hour soak to ensure the temperature has been equalised throughout the reduction in temperatures. This is applicable to pieces 12mm/0.5” thick. Thicker pieces need a longer soak at this point.

The final part of the anneal is cooling at a rate appropriate for the thickness and profile. E.g.:

• A 6mm/0.25” full fused piece would be cooled at 83°C/150°F to 427°C/800°F, and then at 150°C/270°F to 370°C/700°F or lower.
• A 12mm/0.5” full fused piece needs a two hour soak, so the cooling rates are determined by that, i.e., 55°C/99°F per hour to 427°C/800°F and then at 99°C/178°F per hour to 370°C/700°F or lower.

There is an alternative process which is used to determine the annealing temperature of an unknown glass. Once the anneal temperature is determined for a glass, there is no need for a shotgun anneal process. This is known as the slump point test. 

Much more on the principles and practices of annealing can be found in my e-book. Annealing Concepts, Principles and Practice from Bullseye, Etsy and stephen.richard43@gmail.com