Photographing artwork with a white background?

The camera's meter is set to meter an "average" scene. 18% gray. Modern cameras have "smarter' metering systems that can figure out what the scene is and try to correct the metering accordingly, but your basically white scene is going to come out gray anyway! I usually use an incident light meter that measures the actual intensity of the light rather than the intensity of the light reflected off the scene/artwork, but i think the ideal thing for you to do is go to manual metering and use the exposure you get if you place a gray card in front of your artwork. There are 18% gray cards for this! Simplest solution, though, is to simply bracket your exposures: since your camera is underexposing, manually try exposures of 1, 2, and 3 stops overexposed and one of them should be fine. Try using the next three slower shutter speeds.