What to Expect from Bullseye Glass

Color Variation

While Bullseye strives for consistent colors, our glass is a handmade product and colors may vary slightly between production runs (and from images in the catalog). Some colors may change slightly upon repeated firing or with extensive heatwork. We recommend that you test samples of glass using the same firing cycles and processes to be used in finished pieces.

Colors That Strike

In order to provide the largest and most interesting palette of colors to kiln and torch workers, Bullseye produces some glasses that appear pale or colorless in the cold form but which “strike” or mature to target color upon firing. Catalog illustrations indicate which styles differ in color from cold form to struck form. Keep in mind that struck color may vary depending on temperature, atmosphere and amount of heatwork. For example, heating Ruby Red Tint Striker (001824-0030-F) too rapidly during the initial stages of a firing cycle can prevent the glass from striking correctly, resulting in a blue-brown cast (sapphirine effect) instead of a true ruby red color.

Unstruck colors on the left. Struck (mature) colors on the right.

Compatibility

Bullseye glasses are well known for reliable compatibility. But understanding the conditions of our factory testing is important, especially if you are firing glass under unusual or extreme conditions.

At Bullseye, glasses known to be fairly stable are tested by firing to a top temperature of 1500°F (815°C) and soaking for 15 minutes before annealing. Once cooled, these tests are viewed for stress through polarized light and graded accordingly. Other glasses known to be less stable are fired three times with this cycle to insure good performance under typical multiple fusing and slumping conditions, such as those used in making a simple plate.

If you are using a heat process that involves an extra-high temperature or an unusually long firing time, we recommend that you test the glass again, under the conditions specific to your project. For instance, imagine that you want to include some flameworked elements in a kilnformed project. Consider that flameworking takes glass to temperatures exceeding factory compatibility tests; also, the compatibility of some glasses is more sensitive than others to extensive work in the flame. Therefore, it will be important not to overwork your glass during flameworking and to test the flameworked components for compatibility using the full range of kilnforming processes planned for the finished project.

Some processes that may not immediately appear to exceed the parameters of the test for compatibility actually do. Holding some glasses for long times at temperatures around 1400°F (760°C), which is in the devitrification range, can cause the glass to change dramatically.

Many artists (Klaus Moje, for example) are able to push Bullseye glass to high temperatures for long times with exceptionally good results, but their success is insured by their own testing before making large or complex pieces. Testing is a wise practice with whatever glass you use. No manufacturer can guarantee glass to perform as expected under all imaginable working conditions.

Revised December, 2010.