Indigo is one of the few natural botanicals that actually stays blue in Cold Process Soap. Although you can achieve a bluish colour from Alkanet root, it is very unstable in the high pH environment of Cold Process Soap and is likely to produce a grey or maroon coloured soap.
The easiest way to infuse a rich, denim type blue colour is to use Tub Tea Bags or Muslin Cotton Bags. I recommend a double or triple infusion for the strongest depth of colour. If you under-colour, the soap will be a grey-ish tone instead of any where from mid-blue to indigo – it is however wise to avoid going ‘as dark as crayon’ as this may stain towels etc.
The benefit of enclosing the powder inside bags (particularly the Corn Fibre ones), is that there is less mess when you need to squeeze out the last of the oil from the botanical material. If you are using a large proportion of oil infusion in Cold Process Soap, it should be included in your lye calculations. To avoid the need to do this, ensure the infusions you make are concentrated.
It is important not to overfill biodegradable Tub Tea Bags because when the material fills with oil it will expand and could split open. I double and triple infuse using either our Tea Bag with String (Corn Fibre) Medium or Tea Bags Heat Sealable (Filter), Large . Put approximately two tablespoons of dry Indigo Powder into the bags and then seal. Once used, simply remove from the warm oil, squeeze and take to the compost bin, then replace with a fresh bag for the next infusion. I STRONGLY RECOMMEND using gloves for squeezing the oil from the bags! These infusions will last well if you use fresh oils and if stored in a cool, dry place.
If you are not making a concentrated infusion you will need to add the oil content into your lye calculations.
Indigo Powder ready for Infusion
You can see this Blueberry Jam Cold Process Soap tutorial Here