Neo coccine is a synthetic red dye belonging to the azo dye family that has been used in traditional photographic retouching as a staining agent for working on gelatin based photographic emulsions. When applied to the surface of a photographic negative, print, or other gelatin coated material, neo coccine is selectively absorbed by the gelatin of the emulsion in proportion to the concentration of the dye solution used and the duration of its application, building up a controllable degree of red stain that increases the local density and opacity of the treated area.
In photographic retouching practice, neo coccine staining is used primarily to reduce the printing density of localised areas of a negative by increasing their opacity, thereby lightening the corresponding areas in the finished print. Areas of a negative that are too thin - insufficiently dense - as a result of underexposure or uneven development can be selectively built up in density by careful application of neo coccine solution, bringing their printing characteristics closer to the surrounding tonal values and correcting the deficiency in the original negative without the need to resort to overall chemical intensification treatments that would affect the entire image indiscriminately.
Neo coccine is valued as a retouching dye because of several practically useful properties. It is water soluble, allowing it to be applied in controlled dilutions and built up gradually in thin layers to achieve precise degrees of local density increase. Crucially, it is also reversible - unwanted or excessive staining can be removed by washing the treated area with clean water, dissolving and carrying away the water soluble dye and allowing the retoucher to correct mistakes or reduce an area of staining that has been built up too heavily. This reversibility makes neo coccine a particularly forgiving retouching medium that allows a high degree of control and the ability to work iteratively towards the desired result without risk of permanently damaging the negative or print.
The red colour of neo coccine staining has specific practical advantages in the context of photographic retouching on orthochromatic and panchromatic black and white materials. Red dyes absorb green light strongly, and because photographic enlarging papers are typically sensitive to blue and green light, a red stain on a negative increases its opacity to the green component of the enlarger's light source, effectively increasing the printing density of the stained area and lightening the corresponding area in the print. On panchromatic papers and films, the red stain also absorbs red light, providing a useful increase in density across a broader range of the spectrum.