From System96 website (www.system96.com):
Uroboros has created two unique glass formulas for System 96 that are designed to create an interface color, or a ‘reaction', when melted against certain other colors. This occurs when ions from the two adjacent glasses migrate and mingle when the glasses become hot and fluid. Given the right glass chemistry, the comingled ions form a very thin colored line, or 'reaction'. Since our formulas react to create a noticeably reddish hue we named them Red Reactives: as in Red Reactive Transparent, and Red Reactive Opal.
The primary ingredient needed in a glass to generate a reaction color with our Reactive glass is Copper. Copper is present in many blue and blue-green glass colors, but in some cases in too low a concentration to create a noticeable reaction.
The glass I am using for the test batch of buttons (above) is a layer of turquoise blue opal (bottom layer) with some areas coated so as to prevent color interaction and also with a few tiny pieces of amber raku frit (small glass pieces about 1 mm in this case) and top layer of clear reactive glass followed by blue stringers (glass element that has similar thickness of thin spaghetti).
Very happy with the result - am looking forward to creating a few larger pieces (large sushi tray or pasta bowl comes to mind) with this combination of colors.