Photographing enamels has always been difficult. The shiny surface makes the light reflections awkward. However I came across some useful tips on Saachti online for photographing artwork in general, here's a summary adapted for photographing enamel paintings:

• Ensure the camera lens is clean, a lint free cloth or compressed air jet can be used to clean it.
• Make sure the camera lens is directly facing the painting, that is, so the line of sight of the camera is normal to the surface of the painting. With the enamel box lid, it is not possible to get all of the painting facing the camera as the lid curves but aim to get the piece facing on average.
• Choose daylight to give the most true-to-life photograph of the work. Other types of light have their own colour and will bias the colour representation.
• Diffuse light is essential with enamels otherwise the surface will pick out the point light and reflect it. A bright but cloudy day is good. Also a light box can help to soften any sources of light. The light box can be closed at the front with a slit for the camera lens to poke through.
• Have a large window or photograph the piece outside. The light source needs to be facing the painting and not from behind.
• Use a low ISO number (100-200) to reduce the noise in the photograph.
• Have the zoom somewhere in the middle as most lenses are sharper when the zoom is not at either extreme.
• Have the iris somewhere in the middle too, f7 is good. Too wide and the photograph will not be as sharp. If I'm taking a picture of the box in perspective then I may need to stop down more to get a greater depth of field, i.e. more things in focus.
• Use the shutter timer so that the camera does not wobble then stay put until the camera has taken the shot. My camera fixes it's settings before the timer starts so if I move away during the timer the light levels are no longer the ones the camera expects.
• When taking a picture face-on the enamel may give a reflection image of the camera. Fortunately to avoid this problem the light box I use has a small slit cut into the fabric of one side so that the camera lens can peep through. Other alternative solutions to this problem are to hide the image of the camera lens somewhere in the picture, or to use a zoom lens and place the camera far enough back so that any image reflections are very small.
• Oh yes, I almost forgot, use a tripod!

Here's some photo's of my camera and light box setup:

This shows the tripod leaning slightly against the table to get closer to the subject. The light source is from a large window to the left. I'm using the camera to take this photo, but obviously during the shoot the camera sits on the tripod!

This is the set up inside the light box. I'm using an acrylic table to place the enamel on but I don't really need that since I'm not looking for any reflections in the table surface. There is a black drape to limit the light coming from in front of the camera and to darken the background.

You can see the results of the set up in the image gallery for more recent work. Here's a link to the Louis Armstrong Portrait and this one to the Space Shuttle Discovery, the other photograph showing the Shuttle was taken before I employed the light box technique.