Firefly Doesn't Understand What *Grainy* Means

Asked Firefly for a "grainy black and white photo" and the resulting images are all sharp and clear and not the least bit grainy. Does anybody know what labguage one could use in a prompt to obtain a grainy-looking image? I could download and use the film grain filter in Photoshop to add graininess, but it seems like Firefly should be able to handle that.