Inside out
From Melting Pot Wiki
A technique where a clear glass tube is opened to allow colored glass to be applied to the interior of the tube, and then closed again. When properly executed, the colored glass is submerged completely into the clear outer layer to magnify and intensify the color's appearance. This is performed with nearly any color available to the glass artist, with exceptions due to COE compatibility which leads to cracking. This can also be done to great effect with combinations of metal fuming prior to or post application of color or clear glass.