How can I tell if another soap is residue free? - Dreadlocks FAQ's
First, look at the label, if it talks about moisturizers or conditioners, or if they make claims about making hair more manageable or skin softer feeling, their soap is going to leave a residue behind. If it's classified as a Castile soap, a cold pressed soap or an handmade soap it will leave a precipitate as it cleans. This precipitate, commonly called soap scum, is a byproduct of the cleaning process for non-residue free soaps. This scummy byproducts is left in your dreads exactly as it's left as rings in your sink or bathtub.
There are very few soaps and shampoos available that are actually residue free. They are generally sold for special purposes - conditions where residues can not be tollerated for one reason or another and they tend to cost more than traditional soap formulas. There are two tests you can do to see if a specific soap or shampoo leaves residue. Additive residues are nearly always accompanied with a fragrance. If a soap or shampoo leaves a fragrance behind after you've taken care to rinse it away you can be certain that something was left behind. If only a fragrance is being left and you have included a plan for removing it in your washing routine this shouldn't be a problem. Unfortunately the residue left is rarely limited to a fragrance - more often the fragrance is just the easiest residue to identify. A quick way to test this is to wash your hands well with the soap or shampoo in question. As they are drying see if they feel oily or "soft". After they have been dried fully, smell them and see if the fragrance remains on your hands. If so, it's obvious the soap or shampoo failed to rinse away completely.
Nearly all soaps that are sold as homemade or natural soaps, whether they are liquid or (worse still) bar soaps, end up being traditional soap formulas that leave residue as they clean. If residue is being left it can be seen first hand with a simple experiment. Put two and a half gallons of warm water in a 5 gallon bucket. Add an ounce of greasy dirt (a bar-b-q grill is a good sorce) to a wash cloth. Spread it around and work it in. Add an ounce of soap to the wash cloth. Scrub the wash cloth to create lather and remove the dirt. Repeat as needed to get the wash cloth as close to clean as possible. When you're done, allow the water to sit in the bucket until the foam goes away. A "bath tub" ring will be left at the waters edge where the foam had been. If the soap is not residue free you'll see a sludgy white precipitate mixed in with the oil and dirt that was removed. For immediate satisfaction try doing the same with our residue free Dread Soap side by side and compare the results.
For more info on soap and shampoo residues and how they effect your dreads see our residue page.