Making soap

How to make soap with used oil

Mary Smith
By Mary Smith. Updated: January 16, 2017
How to make soap with used oil

There comes a point where we can no longer reuse cooking oil for frying foods and end up throwing it away. In the best case scenario, we can take it to a used oil collection point where it will be taken to the right place for recycling and be used to make biodiesel and, in the worst case scenario, we tip it down the sink, which is bad for the environment. But there is another alternative to get the most out of used cooking oil: making homemade soap! This is a technique which was used for generations and gives very good results. This article explains step by step how to make soap with used oil.

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  1. Equipment needed
  2. Protective measures
  3. Steps to make soap from used oil
  4. Things to Know

Equipment needed

  • 1 litre of oil used for frying
  • 1 litre of water (rainwater works best)
  • 160g of 90% caustic soda
  • 20-40ml of essential oil (optional)
  • Gloves
  • Goggles, if in an enclosed place
  • Utensils to stir the mixture
  • Container to mix ingredients
  • Mould

Protective measures

Before you begin, you have to put on gloves because caustic soda can cause skin damage and if you are in an enclosed space, it is recommended that you wear goggles so that the vapours do not irritate your eyes.

Steps to make soap from used oil

1. Put the water into a large plastic or stainless steel container.

2. Add the caustic soda and stir until it dissolves in the water. You should exercise extreme caution because the temperature will increase to around 80 ºC and generate vapours which should not be inhaled as they are toxic.

3. Leave to stand and cool to about 35°C if you want to add some essential oils, do so now.

4. Slowly pour the cooking oil onto the water and soda and stir the mixture constantly always in the same direction (so they do not cut), for about 30 to 40 minutes, until the mixture begins to thicken during the saponification process.

5. When the mixture has a paste-like texture, pour it into the moulds.

6. Leave the soap to dry for about 30 days.

Things to Know

  • You can find caustic soda in chemists and supermarkets. You must keep it out of reach of children, as it is a dangerous product.
  • It is recommended that you make the soap in an open or very well ventilated area, wear long-sleeved shirt and long pants to protect yourself from possible splashes or even where a plastic apron.
  • If you want to cut the soap into bars, you can do it with a knife the day after making the mixture, when it has acquired some consistency.
  • It is very important that you do not use soap during the 30 days of its development, as it may contain trace amounts of soda and irritate the skin.

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  • Use extreme precaution when handling caustic soda.
Write a comment
What did you think of this article?
Are the ingredients accurate? Because other soap recipes are measured in weight and not by volume. Iin this recipe only the caustic soda is in grams.
You replied to my email that I should go to this link to find the answer to my question; however, the above articles did not answer my question at all.
As it is a skin irritant, any alternative to using caustic soda for home-made soap? I'd like natural ingredients on my home-made soap, please.
OneHowTo Editor
Here's how to make soap with glycerin:
Hope this helps
fam christy
how to make lavender soap
OneHowTo Editor
Hi there,

Here you have a step by step article that may be useful:

Hope this helps!
Marissa G. Rosales
Thank you for this information. I want to receive more articles on how to add scent to soaps that i will make. Where can the scents be bought?
OneHowTo Editor
You may find them at drugstores or specialized craft shops.
How to make soap with used oil