Apparatus including a flow channel defined by a floor, roof, and sidewall structure connecting the floor and roof. One or more combustion burners is positioned in either the roof, the sidewall structure, or both, and transfer heat to a molten mass of glass containing bubbles having a bubble atmosphere flowing through the flow channel. The burners contribute to formation of a channel atmosphere above the molten glass. Apparatus includes a device, at least a portion of which is positionable under a level of the molten glass in the flow channel, configured to emit a composition into the molten glass under the level to intimately contact the composition with the molten glass and bubbles therein. The composition diffuses into the bubbles to form modified atmosphere bubbles sufficiently different from the channel atmosphere to increase diffusion of a species in the channel atmosphere into the modified atmosphere bubbles.