Go to the craft store & buy a piece of grey cardboard or foam core.
Lay the brass on this featureless grey surface to take the pictures.
Autofocus works by finding what it thinks is an edge, so if you only give it one edge that's where it will focus (within the limits of the camera that is).
Depending on your camera you may have either manual control of the settings, or be able to force the automatic to do what you want. If you have a "P-A-S-M" set of markings set it to "A", then set the highest number you can. If there is a "speed/action" & a "scenery" mode use scenery. Yes scenery even if you are taking a close up. This will give you as much extra depth as is possible, but may force you to add support to the camera as it will slow down the speed to the point you may get blur. If this happens you need more light.
Use macro, don't use flash. If you can take the pictures by a window that lights one side more than the other that helps as well.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”