I do quite a bit of this kind of work in my commercial photography business. My clients for this work are artists, museums, galleries, art publications, collector and experts that evaluate and authenticate oil paintings.

To acheive accurate results you have to employ precise copying methods. Unless the lighting, exposure and reflectivity issues are under control, achieving good detail and color rendition could be difficult. I will attach a recent item I wrote about photographing artwork will glossy surfaces, however, the technique is exactly the same as to polarized light, CPL filter usage and plastic polarized screens on the lights. The cross-polarization method that is explained in the attached article will help in color reproduction in that it will cut through surface reflections and ensure good color saturation. Oftentimes varnishes that are apple to paintings can affect reproduction so it is important to mange this issue.

Before you start, you have to get all you ducks in line, otherwise it's like a cat chasing its tail. Your lighting, you can use electronic flash with modeling lamps or photographic type LED sources, must be of exactly the same color temperature. Set you white balance accordingly. Make certain that your monitor is calibrated so a Color Checker or similar kit is required. If that is not done, you have no standard of correctly judging color and density. When shooting, you should include a small a small color-bar with a 18% gray patch that can be placed in the frame- out of the field where it can be cropped out, to help obtain a proper color balance.

One fact that you should make your artist clients aware of is that 100% exact color reproduction is not always possible even under the best of circumstances with the best equipment. Fact is, the dyes and pigments in the paints and not EXACTLY the same as those in the in the inks or dyes in prints or in the physical makeup of a screen image. When adjusting the final color and density in post processing, the original should be on hand for color matching. A screen image is illuminated or trnas-illuminated and a paint is viewed by reflected light. The original should be viewed under color corrected light and perhaps a print should be made for comparison. If the artists are viewing you copies on their own screens, there is a good chance that they, he screens, are not calibrated.

I don't know what the final usage of you copies will be- making prints, lithographic reproductions, E-business, web sites etc.?
lens should be adequate unless you are experiencing a sharpness issue. Of course, this is also dependent on the size and usage of the copies.

Here's the "blog":

In my commercial business, I do quite a bit of art reproduction, some of which is of glossy, highly reflective pieces or paintings coated with very glossy varnishes. The only method that I have found practical and easily repeatable is CROSS POLARIZATION, that is, 2 LIGHTS ARE PLACED, EACH AT 45 DEGREES TO THE ARTWORK AND ARE BOTH FITTED WITH POLARIZATION GELS. A CPL filter is employed on the lens. Rotating the filter will visually indicate when all the unwanted reflections are eliminated. It is best to darken the room so the only the photographic ligh sources are turned on. I wear black clothing or trigger the camera remotely to make certain that ligh will not reflect from me and show up in the image.

You can not strictly depend on angle of incidence manipulations because the camer needs to be exactly centered and parallel to the artwork and the lights can not be shifted to where the do not cover the piece evenly. In pieces where you want to record surface textures or relief, you can go to interprivie copying techniques by moving the lights to slight more that 45 degrees, skimming the surface or by the use of only one light. The polarizati will still hod to minimize or negate bad reflections.

Make exposure readings from each corner of the artwork to make sure the lighting is even. A reading from the center my show a slightly elevated reading but that is normal and will not be problematic. Bracket your exposures.

I use electronic flas with modeling lamps so I can assess the polarization effect.

The polarizing gels need to be oriented in the same direction. The are usually supplied in cardboard frames with appropriate markings. This system also works with pictures behind glass, and those printed or painted on high gloss plastics, some metals (where the metal is entirely printed or painted over) and super-gloss photographic papers and printing materials.

The attached screenshots show my basic setup. The two paintings were coated with numerous layer or Damar varnish- almost like mirrors in certin light- the older painting also and some burnished and cracked areas on the surface. The polarization cut through it all.

The old diagram shows a view camera. The system works perfectly with a digital camera on a tripod. Just keep it centered, level, and parallel.

I hope this helps.

At the very start of this thread it was stated....I've been recruited by several artists to photograph their paintings where said photos will be used to enter competitions or used to publicize their work. The problem: color accuracy.....

Color accuracy will be a problem all the way to the end because it will only be as good as the final output device or medium.

At least the use of a color checker may allow the final output to be judged and/or adjusted if somebody is educated in what the standard may look like and how to adjust the output.