Dark dust III. The high-quality single-cloud reddening curve sample: Scrutinizing extinction curves in the Milky Way

The nature of dust in the diffuse interstellar medium can be best investigated by means of reddening curves where only a single interstellar cloud lies between the observer and the background source. Published reddening curves often suffer from various systematic uncertainties. We merged a sample of 820 reddening curves of stars for which both FORS2 polarization spectra and UVES high-resolution spectra are available. The resulting 111 sightlines towards OB-type stars have 175 reddening curves. For these stars, we derived their spectral-type from the UVES high-resolution spectroscopy. To obtain high-quality reddening curves, we excluded stars with composite spectra in the IUE/FUSE data due to multiple stellar systems. Likewise, we omitted stars that have uncertain spectral-type designations or stars with photometric variability. We neglected stars that show inconsistent parallaxes when comparing data releases two and three from Gaia. Finally, we identified stars that show differences in the space- and ground-based-derived reddening curves between 0.28 mu m and the U band or in R-V. In total, we find 53 stars with one or more reddening curves passing the rejection criteria. This provides the highest-quality Milky Way reddening curve sample available today. Averaging the curves from our high-quality sample, we find R-V = 3.1 +/- 0.4, confirming previous estimates. A future paper in this series will use the current sample of precise reddening curves and combine them with polarization data to study the properties of dark dust.