I found this:
" Greetings Dimitris: I photograph a lot of artwork for reproduction in books and catalogs - mostly paintings, both oil and watercolor. I'll offer the whole technique; then you can pick and choose what aspect, if any, you want to try.
To do this right, you first need the painting at camera level, set on an easel or some kind of table that you can make plumb, square and level to the camera lens. Likewise, the camera has to be square, plumb and level to the painting. A view camera works best for this, although I've pulled it off using 35mm and medium formats as well. A tripod, as Laura mentioned, is pretty much mandatory, as is a lens hood or lens shade. A tape measure, or some kind of laser-measuring device for checking distances, is handy to have too.
Your lighting has to be balanced, and equal. Remember that angle of incidence is equal to angle of reflection. I usually use two studio lights of 1000 w/s each, having UV coated flash tubes or UV filter on your lens. Organic pigments used in oil colors tend to fluoresce under UV light, and that fluorescence causes parts of the painting to appear as though they're glowing and color shift.
The lights are positioned at 45 degrees to either side of the camera, set equal distance from the centerline of the picture.
Using a diffuser isn't much help, because all that does is soften the light output. If it's not set at a proper angle to the work, you'll still see hot spots or glare, or some areas of the painting may be washed out. And to avoid that you need balanced lighting anyway, from two sources.
If either your lights aren't set correctly, (and doing this well with an on camera flash is nearly impossible) or if your picture is out of square to the camera, then chances are you'll see some kind of glare or unwanted reflection in the final image. If the painting is framed in glass, it's still doable but much trickier. Most of the time, I have the gallery or artist remove the glass. Even a polarizing filter isn't much help photographing art work because the colors or contrast are inaccurate.
Also, in at least one frame of each painting, I attach a color card and Kodak gray scale card to the tops of the frame to help the printer match the color or find the right contrast for reproduction.
In situations where you can't move the painting, then you need to go to it. In a church, that may involve using something like a rolling scaffold with lights clamped to either side of the support rail. We did that recently for a series of churches in New Jersey. It also included shooting all the stained glass for Dow Corning. YIKES!!! Compared to that, the paintings were a piece of cake. It's all about knowing how to use, what type to use, how much to use, and how to control your lighting.
Take it errrr ... light ;>)"
