Flocking is tiny lengths of synthetic fibres (usually nylon or rayon) available in different lengths and largely used in conjunction with pigments to intrinsically pigment translucent materials such as silicone and gelatine.
When pigmenting silicone to match a performers skin tone, the majority of colour achieved is through using silicone pigments. However, a great deal can be achieved by the addition of different coloured flocking - usually for skin tones red, blue and yellow.
With most skin tones, there is some appearance of 'mottle', the appearance of a less than perfectly smooth colour made from layers of veins, hair follicles, moles, freckles and other things which 'break up' the skin surface.
When recreating realistic skin, this is incredibly useful and often prosthetics can look fake and 'mask-like' precisely because they are smooth and perfect like makeup foundation. Real skin looks real when it has those aberrations which are essentially small specks of other colours.
Adding flocking to the mix will help create that in the base material itself, so much of the colouring is IN the appliance, not ON the surface. This is one of the great things about using translucent materials such as silicone and gelatine for such appliance makeups.
For prosthetic appliances, usually the shortest flocking length is desirable, so they are not identifiable as observable lines - so usually 1mm or 2mm or less in length is best. Adding too much flock can cause the material you are colouring to thicken (you are adding fibres to the mix after all).
Using flocking is more like adding a spice to a recipe - it is something that will enhance the dish but isn't something that will make up the majority of the ingredients.
The colours mostly used to enhance skin tones are the three primaries - red, blue and yellow. Combinations of these can be used to create secondary colours, just as with painting. Red and yellow added will make an orange etc. Using basic colour theory, flocking is a great way to correct and modify skin tones in materials to better match the wearer's skin. Blue flocking, for example, is a great way to tone down orange.
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Flocking can be added a little at a time using a spoon or wooden stirrer as a means of dispensing. If you have a lot to mix up, consider using a clean spice container with sprinkler style lid, like a pepper pot with many small holes at the end. Shake out a little of what you need, stir into the mixture and add more as required. For larger quantities, consider a sugar shaker.
When using any products, materials or equipment, you should familiarise yourself with them and take appropriate health and safety measures to protect yourself and others around you from harm. Obtain and consult the relevant MSDS (Material Safety Data Sheet) from your supplier. Avoid breathing in fibres by using an appropriate respirator mask and wearing sensibly to avoid spills and strong draughts which may make the fibres airborne.