It can be a surprise to see a white powdery substance appear on top of your cold press soap. It looks like some-one has sprinkled the top with icing sugar, but this substance is called Sodium Carbonate. Although this is harmless and does not spoil your soap, cosmetically it does not look good especially if you are selling your bars.
What is soda ash and why does it form on your soap
It forms when the top of your soap has contact with air - it's the Sodium molecules from unsaponified lye which reacts with Carbonic Acid (carbon dioxide that is dissolved in water) and reacts with the Carbon Dioxide in the air.
For anyone who loves science this is quite fascinating. The chemical formula for Sodium Hydroxide which is used to make the lye in cold press soap is NaOH, the chemical formula for Carbon Dioxide is Co2, so the chemical formula for soda ash (Sodium Carbonate) is Na2CO3.
How to prevent Soda Ash forming on your soap
You'll find lots of advice on the internet about this, however my first suggestion would be to make sure your soap batter has actually reached trace before you pour into a mould. This might sound silly but sometimes people get a 'false' trace which would mean your batter has not reached saponification at all. When using a stick blender I teach my students to blend for a short while then stop, give the soap batter a good stir, continue to blend, stop and stir several times until you have reached the first, thin trace. The first trace is like custard that you pour over a pudding. You could continue to blend, stop and stir until you reach the second, medium trace which looks like blancmange in consistency and then pour into your mould.
Another simple idea is to cover your soap with a towel or blanket to incubate it and limit the air contact with the top of the soap.
Check your liquid content and make sure you do not use too much. You can discount your liquid and not use as much, however, this will probably mean your batter will get to trace quicker or you will get a 'false' trace.
Try and get your lye and oils/butters near enough the same temperature. Once you have made soap enough and know what to expect you can even soap at a higher temperature i.e. 100f. Again, you will probably get to trace quicker.
Use good quality, pure Sodium Hydroxide from a well known and trusted source so you know it will not contain any other substances i.e. Sodium Carbonate.
Always use distilled water or de-ionised water. Using tap water, especially in hard water areas can result in impurities like Sodium Carbonate going into your soap batter. This could result in soda ash appearing throughout your soap and not just on the top.
Spray the top with Isopropyl Alcohol after your pour your soap batter into the mould. This gives a protective layer so the air does not come into contact with the top of your soap.
A few simple rules could prevent soda ash forming, so choose a good soap recipe, use really good ingredients, soap at a good temperature and make sure you really have reached trace before pouring into your mould and finally, cover or incubate your soap.
If you do happen to get soda ash, don't worry it will disappear as soon as you use it. If you are selling your soap you can try to remove the soda ash by holding a steam iron over the top of the soap for 10-20 seconds as the water and steam will dissolve it, or wash the top with clean water and dry.
Happy soaping x Diane