Oxides: Oxides are very common soap additives that you can purchase from specialist soap ingredient suppliers. Although oxides are more stable than plant dyes, they do still have a tendancy to fade in strong sunlight. The end colour result is also affected by the colours of the ingredients in your soap. For example if you are using a very green coloured olive oil this may inhibit a true blue oxide from producing a very blue soap.
Amount to use: Once again this depends upon the depth of colour you are looking for in the final product. Anything between ¼ tsp up to 2 teaspoons per kg of oil would be a good starting point.
A note about synthetic pigments These soap additives are definitely NOT natural, but they are very stable and provided you can measure exact amounts will always produce the same colour in the same formula. Since only very, very small amounts are required this can be difficult to measure when making small batches. These soap additives have no real place in with the other natural options listed above, but since they are very popular with soap makers and the nature of soap is to wash off - there is little chance of any component of a synthetic coloured soap additive being absorbed by the skin. The synthetic colours or lakes as they are often called are mixed with oil – never used in water soluble products and always used very sparingly. Lakes are Inorganic pigments, and do not usually bleed or leach colour.
There are 4 types of Lakes: Calcium Salt, Barium Salt, Aluminum Salt and a Sodium Salt. The colour from the dye is placed onto one of these as an insoluble base to hold the colour onto. Which Salt is used is determined by the colour they want to achieve, as the Salts impact on the result. Some lakes are also Rosinated. Rosin is derived from tree sap and gives a blue tone to reds.
Amount of coloured soap additive to use: 1/8 teaspoon per kg of oils. (for measuring tiny amounts use the end of a spoon handle).
Colouring TechniquesSolid Coloured SoapOnce you have chosen your colour and have prepared it accordingly you only need to add it to your batch of soap and mix well to obtain a uniform solid coloured soap. You may wish to add it at a light trace in order to give yourself plenty of time to mix it in properly before the soap starts to set up.Layering
SwirlingThere are several ways to swirl. The key to success however is in judging the level of trace so that the coloured soap suspends within the main batch. Using synthetic fragrances in a swirled soap adds an extra dimension of difficulty since they often accelorate the rate of trace. It is definitely advisable to begin swirling only those soaps that contain essential oils.
Swirling Technique a:When your soap has reached a light trace pour about a quarter of it into a separate container. Mix the essential oils and any other additives required with the remaining soap and then pour into the mould. Now add the colour to the quarter of soap you have left out and mix well. Pour this soap onto the soap in the mould in straight lines forming a grid pattern (a bit like naughts and crosses game) vary the height at which you pour the soap so that it falls right through to the bottom of the mould in places. Now take a knife or spatula and holding it upright trace lines through the soap running in the opposite direction to the grid pattern already made with the contrast colour. Cover the soap and insulate as normal.
Swirling Technique b: When your soap has reached a light trace pour about a quarter of it into a separate container to which you add the colour. Add the fragrance to the main soap pot. Return the coloured soap to the main soap pot and then swirl once with whisk or spatula. Pour the soap into the mould, cover and insulate as normal.
Swirling Technique c: When your soap has reached a light trace pour about a quarter of it into a separate container to which you add the colour. Add the fragrance to the main soap pot, stir and then pour into the mould. Stir the colour into the soap in the separate container and then pour this into a nozzled plastic ketchup container. Use this to swirl the coloured soap into the main batch. With this method you can get some very specific and delicate lines of colour throughout the soap. Finally cover and insulate as normal.