If black glass heats up, will it emit long-wave infrared radiation?

I am not totally clear on the concept of emissivity. I am wondering what will happen if I cover a flat panel heater with thin (3mm) black glass (touching the heater). Will some of the heat (near or medium-wave) transmit through, and some (long-wave) be absorbed and “re-emitted”? Will I lose efficiency from the heater?