Heatwork and Cadmium-Bearing Glasses | Kilnforming

Heatwork and Cadmium-Bearing Glasses

Cadmium-bearing Glasses

Bullseye’s red, orange, and yellow-hued glass styles contain the element, cadmium, which tends to be more sensitive to heatwork than other colorants. This document explores how to work successfully with Bullseye’s cadmium-bearing glass styles.

Heatwork within Bullseye’s Tested-Compatible Range

Bullseye tests all cadmium-bearing glasses within its fusible (Grade-F) glass styles to guarantee they will withstand up to three firings at 1500 °F (815 °C) with 15-minute soaks before annealing. Within that tested-compatible, triple-fired range, you can fuse these glasses successfully without giving it a second thought.

Heatwork Beyond the Guaranteed Range

Many of our glass styles can withstand soaks longer than 15 minutes and far more than three firings. Artists and makers within our community do so all the time with wonderful results. But when venturing beyond Bullseye’s tested heatwork parameters, the only way to ensure consistent success is to conduct your own testing.
If your firings take the glass outside the parameters of our test for compatibility, you’ll want to test all the glasses you intend to use through all the firing schedules you intend to use (basic steps below). This is especially true when using cadmium-bearing glasses. Generally speaking, these are more prone to shifts in color and compatibility than other glass styles. So before firing beyond tested limits, it is helpful to familiarize yourself with our cadmium-bearing glasses (list on the reverse page) and to keep in mind a simple guideline:
When firing any glass beyond the tested-compatible range, it is essential to conduct tests—and this is especially true when using cadmium-bearing glass(es).

Testing for Specific Conditions

  1. Determine all glass styles to be used in your project.
  2. Determine precise firing schedules for the project’s full range of heatwork.
  3. Make sure test pieces share the same production date as the glass to be used for your project.
  4. Create test samples for each glass style/production date.
  5. Fire the samples using the project’s intended firing schedules and full range of heatwork.
  6. Observe what works, what doesn’t, and take meticulous notes.
  7. If the transparent cadmium glass(es) that you test opalize, that’s a sign that they are also shifting in compatibility characteristics, and you are advised to change the process or use glass(es) from a different production date.

A Cautionary Example: Bubble Squeezes

Because many of our glasses remain stable well beyond their tested limits, it can be easy to mistake some low-temperature processes as falling within the risk-free range. Take performing a bubble squeeze, as an example. During a first full-fuse firing with layers of sheet glass, it often makes sense to hold at 1225 °F (663 °C) for 30–120 minutes to prevent large unwanted bubbles. If subsequent firings rotely repeat that hold, however, it may prove detrimental. The color quality and compatibility characteristics of cadmium-bearing glass can degrade over the course of multiple firings when held too long at a bubble squeeze temperature of 1225 °F (663 °C). It is possible, then, for processes that benefit a project during a first firing to begin undermining it when repeated. Again, testing is the best way to make such discoveries and fine tune your processes.

Tips for Minimizing Overall Heatwork

  1. Whenever possible, avoid longer bubble squeeze holds. (Shorten or omit them as your project allows.)
  2. Add cadmium-bearing styles in later firings.

More Information

Cadmium-Bearing Glass & Heatwork

Download the Heatwork & Cadmium-Bearing Glass PDF

Cadmium-Bearing Glass Styles

This list encompasses all forms of these glasses: sheet, frit, rod, etc. Be aware that some color mix styles may include one or more styles from the list.

000024 Tomato Red Opal 000025 Tangerine Orange Opal 000120 Canary Yellow Opal
000124 Red Opal 000125 Orange Opal 000126 Spring Green Opal
000203 Woodland Brown Opal 000220 Sunflower Yellow Opal 000221 Citronelle Opal
000222 Avocado Green Opal 000224 Deep Red Opal 000225 Pimento Red Opal
000227 Golden Green Opal 000309 Cinnabar Opal 000310 Umber Opal
000320 Marigold Yellow Opal 000321 Pumpkin Orange Opal 000329 Burnt Orange Opal
000337 Butterscotch Opal
001025 Light Orange 001119 Sienna 001120 Yellow
001122 Red 001125 Orange 001126 Chartreuse
001320 Marigold Yellow 001321 Carnelian 001322 Garnet Red
001422 Lemon Lime Green
002020 Clear, Sunflower Yellow Opal 002024 Clear, Red Opal 002026 Clear, Spring Green Opal
002121 Yellow, Deep Forest Green 002123 White, Orange Opal 002124 Red Opal, White
002125 Yellow, Red 003123 White, Orange Opal, Deep Forest Green 003203 Woodland Brown Opal, Ivory, Black