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I have been working on a casting glass project using glass from normal household items I buy at the store. When I buy a glass cup at the store in violet or red color, make frit from it and burn it in the kelin the resulting color is different. for example I got a purple color glass made frit from it and place it in a mold in the kelin to make a perfume cover attached, the result did did not come in purple but rather it was marbled effect, tired the same thing in different color and also the same result, any idea on how I can get consistent colors from crushing glass bottle or cups

Several reasons. First, some glass colors aren't stable with prolonged heat treatment, and will change in the kiln. Cool reds and pinks can turn brown, for example.

Second, some transparent glass colors such as hot reds, yellows, oranges and some greens, can go opaque with the long heating cycles needed for casting.

Third, using crushed glass in a mold will result in air trapped between the particles of glass, making tiny bubbles. The bubbles trap the light coming into the piece so that your eye is stopped at the surface instead of seeing through. That can also change the apparent color, and because the bubbles probably aren't very evenly distributed, the piece will look mottled.

The third issue can be solved by putting the glass on a reservoir or rack outside the mold and letting it drip in. That eliminates most of the bubbles so that the color is more even, and gives a better chance for transparency.

The first two issues are more difficult to resolve. The only way I know to reliably prevent them is to fire test samples of the glass and avoid using the ones that change. Sometimes applying less heat can stop the change, sometimes not--it can be difficult to manage.

Thank you for your guidance I will do test runs on different glass

Morganica wrote:The third issue can be solved by putting the glass on a reservoir or rack outside the mold and letting it drip in. That eliminates most of the bubbles so that the color is more even, and gives a better chance for transparency.

OMG!OMG!OMG!

SO brilliant! I LOVE it.