This question came up at the Australian Science Festival and got me thinking.   Bar soap is one of the most efficient personal care products that you can buy. It is cheap to manufacture, it is easy to pack – in fact it required very little packaging, it is good at cutting through greases and a little goes a long way. Great for the green consumer!  However, while soap is a great staple of the environmentally conscious bathroom cabinet it can’t do everything and this is down to its chemistry.

Bar soap has been traditionally manufactured by taking a fat  (this is almost always vegetable derived these days) and reacting it with an alkali solution such as potassium hydroxide. This reaction is called saponification and results in the production of the soap plus glycerine and water.  Bar soaps made like this may have the produced glycerine scooped out before the bar is set or it may be left in to give some additional moisturising.

Soaps made like this are very cheap and effective and can be made at home with a bit of effort. However, this type of soap is not great for the hair as the bar will be very high in pH and the alkalinity will strip the oils from the hair leaving it dull. This type of soap will also react to with salts and oils in the hair to form a scum which is difficult to remove without copious rinsing. This may also leave the hair hard to manage and comb. The high pH and the rinsing issues may also irritate those people with sensitive scalps and that is why shampoo was developed, but more about that later!

The traditional bar soap can be made a little more nourishing by the addition of moisturising agents such as goats milk or other natural emollients.  While these may help to negate the drying effect on your scalp, your hair will still not glisten and shine as well as you may like.

Another way of making bar soap use synthetic detergents instead of using oils and saponifying them – The Dove bar would be one example. These have existed since the 1950’s and are formulated to be pH 4.5-5.5 to more closely match the skin. These bars contain surfactants that are great for the hair – one of the key ingredients of the original Dove bar – Sodium Cocoyl Isethionate has been found to help prevent dyed hair fading when used in place of SLES based shampoos.  These bars are harder to make at home due to their harder to find ingredients but would make a reasonable alternative to a liquid shampoo. The only down side is that syndet bars also contain emollients and moisturising ingredients which may again leave the hair shine free.  So, cleansing = YES, shine and good comb-ability = NO.

Liquid soaps can come in all shapes and sizes some of which are fine for hair while others are not. Liquid Castile soap is an old favorite in green circles. This is the name given to soap made by saponification of olive oil only the glycerine is left in. The Castile bit comes from the Spanish town where the practice originated.  This soap has very similar properties to the bar soaps made by the saponification process. It is very efficient at degreasing surfaces (hair and skin) and has a high pH so can leave the skin quite irritated. Also being a soap it will also be hard to rinse and can form complexes with dirt leaving your hair dull and hard to comb.  The one thing on liquid castile soaps side is the ability of the user to add some of their own bits to the mix. Because it is liquid you can add essential oils, some more nourishing bases oils or herbs as desired to create something special. Just keep in mind that the liquid soap does contain water so environmentally it will be less efficient to transport around than the bars.

Otherwise liquid soaps can be made from a wide range of other surfactants and end up resembling basic shampoos. These more mainstream liquid soaps can be made at home (with access to the right ingredients) and can work very well as basic shampoos as they foam, clean and rinse off easily.

So, can bar soap cover all of your personal care needs? Well, nearly but not quite.  I do know plenty of people who wash their hair with either traditional bar soaps or with Castile soap and have no problem.  Like with most things it is probably a case of try it, you might like it but I for one have not managed to find a bar shaped substitute for my favorite shampoo.  The hair is very different to the skin, it is dead once it comes out of the scalp so to keep it in tip top condition it needs some care.  The traditional soap bars may even damage the hair as they make it difficult to comb and leave it prone to tangling whereas shampoo’s are often developed to have some ‘wet combability’ benefit built in.  I would stick to a shampoo for the hair and soap for the rest of the body – horses for courses!