Measure out the *epsom salts and sea salt into the tub or bucket.
Calculating the total weight of the mixture, measure out 2% in weight of your chosen essential oil/s or fragrance into just enough Cornstarch (or Tapioca Starch) to make a dry powder and then add to the mixture and stir in thoroughly. (It is worth remembering that the volume and weight of *epsom salts and coarse sea salt correspond quite closely, so 200g of the mixture will also fill nearly exactly 200ml in volume, so for every 100g of mixture, use 2ml of essential oil/s or fragrance).
Next, if desired, colour can be added. Colours that work well in this mixture are ultramarines and oxides as they are dry powders, so you have an immediate choice of pink, blue, green or purple. These are added a little at a time as powder and mixed thoroughly until the desired shade is achieved. Other colours that work well are Amaranth red, FCF blue, Tartrazine yellow and Sunset yellow, which come as powders but should be diluted in a little water and mixed to a dry powder with a little Cornstarch (or Tapioca Starch) and added a little at a time, mixing thoroughly, until the desired shade is achieved. When adding any 'wet' ingredient to a dry mix such as bath crystals it is important to mix them with an absorbant material to avoid the whole mixture getting damp and setting up in clumps.
Leave the mixture to stand, covered, for at least a day for the fragrance to thoroughly permeate all the salts.
Using a large funnel or suitable utensil, you can now decant a little at a time into a suitable bottle or jar for using as you please.For adding a little effervescence, mix together a little sodium bicarbonate and granular citric acid (50/50) and replace some of the sea salt weight-for-weight.